## Abstract

This specification describes an application layer protocol for communication between an external authenticator and another client/platform, as well as bindings of this application protocol to a variety of transport protocols using different physical media. The application layer protocol defines requirements for such transport protocols. Each transport binding defines the details of how such transport layer connections should be set up, in a manner that meets the requirements of the application layer protocol.

## Status of This Document

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## Table of Contents

## 1. Overview

This section is non-normative.

This protocol is intended to be used in scenarios where a user interacts with a relying party (a website or native app) on some platform (e.g., a PC) which prompts the user to interact with an external authenticator (e.g., a smartphone).

In order to provide evidence of user interaction, an external authenticator implementing this protocol is expected to have a mechanism to obtain a user gesture. Possible examples of user gestures include: as a consent button, password, a PIN, a biometric or a combination of these.

Prior to executing this protocol, the client/platform (referred to as host hereafter) and external authenticator (referred to as authenticator hereafter) must establish a confidential and mutually authenticated data transport channel. This specification does not specify the details of how such a channel is established, nor how transport layer security must be achieved.

## 2. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this specification are to be interpreted as described in [RFC2119].

## 3. Protocol Structure

This section is non-normative.

This protocol is specified in three parts:

- Authenticator API: At this level of abstraction, each authenticator operation is defined similarly to an API call - it accepts input parameters and returns either an output or error code. Note that this API level is conceptual and does not represent actual APIs. The actual APIs will be provided by each implementing platform.
- Message Encoding: In order to invoke a method in the authenticator API, the host must construct and encode a request and send it to the authenticator over the chosen transport protocol. The authenticator will then process the request and return an encoded response.
- Transport-specific Binding: Requests and responses are conveyed to external authenticators over specific transports (e.g., USB, NFC, Bluetooth). For each transport technology, message bindings are specified for this protocol.

This document specifies all three of the above pieces for external FIDO2 authenticators.

## 4. Protocol Overview

This section is non-normative.

The general protocol between a platform and an authenticator is as follows:

1. Platform establishes the connection with the authenticator.
2. Platform gets information about the authenticator using authenticatorGetInfo command which helps it determine the capabilities of the authenticator.
3. Platform sends a command for an operation if the authenticator is capable of supporting it.
4. Authenticator replies with response data or error.

## 5. Authenticator API

Each operation in the authenticator API can be performed independently of the others, and all operations are asynchronous. The authenticator may enforce a limit on outstanding operations to limit resource usage - in this case, the authenticator is expected to return a busy status and the host is expected to retry the operation later. Additionally, this protocol does not enforce in-order or reliable delivery of requests and responses; if these properties are desired, they must be provided by the underlying transport protocol or implemented at a higher layer by applications.

Note that this API level is conceptual and does not represent actual APIs. The actual APIs will be provided by each implementing platform.

The authenticator API has the following methods and data structures.

### 5.1 authenticatorMakeCredential(0x01)

This method is invoked by the host to request generation of a new credential in the authenticator. It takes the following input parameters, which explicitly correspond to those defined in The authenticatorMakeCredential operation section of the Web Authentication specification:

Parameter name | Data type | Required? | Definition
--- | --- | --- | ---
clientDataHash | Byte Array | Required | Hash of the ClientData contextual binding specified by host. See [WebAuthN].
rp | PublicKeyCredentialRpEntity | Required | This PublicKeyCredentialRpEntity data structure describes a Relying Party with which the new public key credential will be associated. It contains the Relying party identifier, (optionally) a human-friendly RP name, and (optionally) a serialized URL pointing to a RP icon image. The RP name is to be used by the authenticator when displaying the credential to the user for selection and usage authorization.
user | PublicKeyCredentialUserEntity | Required | This PublicKeyCredentialUserEntity data structure describes the user account to which the new public key credential will be associated at the RP. It contains an RP-specific user account identifier, (optionally) a user name, (optionally) a user display name, and (optionally) a URL pointing to an image (of a user avatar, for example). The authenticator associates the created public key credential with the account identifier, and MAY also associate any or all of the user name, user display name, and image data (pointed to by the URL, if any).
pubKeyCredParams | CBOR Array | Required | A sequence of CBOR maps consisting of pairs of PublicKeyCredentialType (a string) and cryptographic algorithm (a positive or negative integer), where algorithm identifiers are values that SHOULD be registered in the IANA COSE Algorithms registry [IANA-COSE-ALGS-REG]. This sequence is ordered from most preferred (by the RP) to least preferred.
excludeList | Sequence of PublicKeyCredentialDescriptors | Optional | A sequence of PublicKeyCredentialDescriptor structures, as specified in [WebAuthN]. The authenticator returns an error if the authenticator already contains one of the credentials enumerated in this sequence. This allows RPs to limit the creation of multiple credentials for the same account on a single authenticator.
extensions | CBOR map of extension identifier → authenticator extension input values | Optional | Parameters to influence authenticator operation, as specified in [WebAuthN]. These parameters might be authenticator specific.
options | Sequence of authenticator options | Optional | Parameters to influence authenticator operation, as specified in in the table below.
pinAuth | Byte Array | Optional | First 16 bytes of HMAC-SHA-256 of clientDataHash using pinToken which platform got from the authenticator: `HMAC-SHA-256(pinToken, clientDataHash)`.
pinProtocol | Unsigned Integer | Optional | PIN protocol version chosen by the Client

The following values are defined for use in the options parameter. All options are booleans.

Key | Default value | Definition
--- | --- | ---
rk | false | resident key: Instructs the authenticator to store the key material on the device.
uv | false | user verification: Instructs the authenticator to require a gesture that verifies the user to complete the request. Examples of such gestures are fingerprint scan or a PIN.

When such a request is received, the authenticator performs the following procedure:

1. If the excludeList parameter is present and contains a credential ID that is present on this authenticator, terminate this procedure and return error code CTAP2_ERR_CREDENTIAL_EXCLUDED.
2. If the pubKeyCredParams parameter does not contain a valid COSEAlgorithmIdentifier value that is supported by the authenticator, terminate this procedure and return error code CTAP2_ERR_UNSUPPORTED_ALGORITHM.
3. If the options parameter is present, process all options and if any of the requested options can't be satisfied, terminate this procedure and return the CTAP2_ERR_OPTION_NOT_SUPPORTED error.
4. Optionally, if the extensions parameter is present, process any extensions that this authenticator supports. Authenticator extension outputs generated by the authenticator extension processing are returned in the authenticator data.
5. If pinAuth parameter is present and pinProtocol is 1, verify it by matching it against first 16 bytes of HMAC-SHA-256 of clientDataHash parameter using pinToken: `HMAC-SHA-256(pinToken, clientDataHash)`.
   - If the verification succeeds, set the "uv" bit to 1 in the response.
   - If the verification fails, return CTAP2_ERR_PIN_AUTH_INVALID error.
   If pinAuth parameter is not present and clientPin been set on the authenticator, return CTAP2_ERR_PIN_REQUIRED error.
6. If the authenticator has a display, show the items contained within the user and rp parameter structures to the user. Alternatively, request user interaction in an authenticator-specific way (e.g., flash the LED light). Request permission to create a credential. If the user declines permission, return the CTAP2_ERR_OPERATION_DENIED error.
7. Generate a new credential key pair for the algorithm specified.
8. If "rk" in options parameter is set to true:
   - If a credential for the same RP ID and account ID already exists on the authenticator, overwrite that credential.
   - Store the user parameter along the newly-created key pair.
   - If authenticator does not have enough internal storage to persist the new credential, return CTAP2_ERR_KEY_STORE_FULL.
9. Generate an attestation statement for the newly-created key using clientDataHash.

On success, the authenticator returns an attestation object in its response as defined in [WebAuthN]:

Member name | Data type | Required? | Definition
--- | --- | --- | ---
authData | Sequence of bytes | Required | The authenticator data object.
fmt | String | Required | The attestation statement format identifier.
attStmt | Sequence of bytes, the structure of which depends on the attestation statement format identifier | Required | The attestation statement, whose format is identified by the "fmt" object member. The client treats it as an opaque object.

### 5.2 authenticatorGetAssertion(0x02)

This method is used by a host to request cryptographic proof of user authentication as well as user consent to a given transaction, using a previously generated credential that is bound to the authenticator and relying party identifier. It takes the following input parameters, which explicitly correspond to those defined in The authenticatorGetAssertion operation section of the Web Authentication specification:

Parameter name | Data type | Required? | Definition
--- | --- | --- | ---
rpId | String | Required | Relying party identifier. See [WebAuthN].
clientDataHash | Byte Array | Required | Hash of the serialized client data collected by the host. See [WebAuthN].
allowList | Sequence of PublicKeyCredentialDescriptors | Optional | A sequence of PublicKeyCredentialDescriptor structures, each denoting a credential, as specified in [WebAuthN]. The authenticator is requested to only generate an assertion using one of the denoted credentials.
extensions | CBOR map of extension identifier → authenticator extension input values | Optional | Parameters to influence authenticator operation. These parameters might be authenticator specific.
options | Sequence of authenticator options | Optional | Parameters to influence authenticator operation, as specified in the table below.
pinAuth | Byte Array | Optional | First 16 bytes of HMAC-SHA-256 of clientDataHash using pinToken which platform got from the authenticator: `HMAC-SHA-256(pinToken, clientDataHash)`.
pinProtocol | Unsigned Integer | Optional | PIN protocol version selected by Client.

The following values are defined for use in the options parameter. All options are booleans.

Key | Default value | Definition
--- | --- | ---
up | true | user presence: Instructs the authenticator to require user consent to complete the operation.
uv | false | user verification: Instructs the authenticator to require a gesture that verifies the user to complete the request. Examples of such gestures are fingerprint scan or a PIN.

When such a request is received, the authenticator performs the following procedure:

1. Locate all credentials that are eligible for retrieval under the specified criteria:
   - If an allowList is present and is non-empty, locate all denoted credentials present on this authenticator and bound to the specified rpId.
   - If an allowList is not present, locate all credentials that are present on this authenticator and bound to the specified rpId.
2. If pinAuth parameter is present and pinProtocol is 1, verify it by matching it against first 16 bytes of HMAC-SHA-256 of clientDataHash parameter using pinToken: `HMAC-SHA-256(pinToken, clientDataHash)`.
   - If the verification succeeds, set the "uv" bit to 1 in the response.
   - If the verification fails, return CTAP2_ERR_PIN_AUTH_INVALID error.
   If pinAuth parameter is not present and clientPin has been set on the authenticator, set the "uv" bit to 0 in the response.
3. Optionally, if the extensions parameter is present, process any extensions that this authenticator supports. Authenticator extension outputs generated by the authenticator extension processing are returned in the authenticator data.
4. Collect user consent if required. This step MUST happen before the following steps due to privacy reasons (i.e., authenticator cannot disclose existence of a credential until the user interacted with the device):
   - If the "uv" option was specified and set to true:
     - If device doesn't support user-identifiable gestures, return the CTAP2_ERR_OPTION_NOT_SUPPORTED error.
     - Collect a user-identifiable gesture. If gesture validation fails, return the CTAP2_ERR_OPERATION_DENIED error.
   - If the "up" option was specified and set to true, collect the user's consent.
     - If no consent is obtained and a timeout occurs, return the CTAP2_ERR_OPERATION_DENIED error.
5. If no credentials were located in step 1, return CTAP2_ERR_NO_CREDENTIALS.
6. If only one credential was located in step 1, go to step 9.
7. Order the credentials by the time when they were created. The first credential is the most recent credential that was created.
8. If authenticator does not have a display:
   - Remember the authenticatorGetAssertion parameters.
   - Create a counter and set it to the total number of credentials.
   - Start a timer. This is used during authenticatorGetNextAssertion command.
   - Update the response to include the first credential's publicKeyCredentialUserEntity information and numberOfCredentials.
9. If authenticator has a display:
   - Display all these credentials to the user, using their friendly name along with other stored account information.
   - Also, display the rpId of the requester (specified in the request) and ask the user to select a credential.
   - If the user declines to select a credential or takes too long (as determined by the authenticator), terminate this procedure and return the CTAP2_ERR_OPERATION_DENIED error.
10. Sign the clientDataHash along with authData with the selected credential, using the structure specified in [WebAuthN].

On success, the authenticator returns the following structure in its response:

Member name | Data type | Required? | Definition
--- | --- | --- | ---
credential | PublicKeyCredentialDescriptor | Optional | PublicKeyCredentialDescriptor structure containing the credential identifier whose private key was used to generate the assertion. May be omitted if the allowList has exactly one Credential.
authData | Byte Array | Required | The signed-over contextual bindings made by the authenticator, as specified in [WebAuthN].
signature | Byte Array | Required | The assertion signature produced by the authenticator, as specified in [WebAuthN].
user | PublicKeyCredentialUserEntity | Required | PublicKeyCredentialUserEntity structure containing the user account information. For single account per RP case, authenticator returns "id" field to the platform which will be returned to the [WebAuthN] layer. For multiple accounts per RP case, where the authenticator does not have a display, authenticator returns "id" as well as other fields to the platform. Platform will use this information to show the account selection UX to the user and for the user selected account, it will ONLY return "id" back to the [WebAuthN] layer and discard other user details.
numberOfCredentials | Integer | Optional | Total number of account credentials for the RP. This member is required when more than one account for the RP and the authenticator does not have a display. Omitted when returned for the authenticatorGetNextAssertion method.

### 5.3 authenticatorGetNextAssertion(0x08)

The client calls this method when the authenticatorGetAssertion response contains the numberOfCredentials member and the number of credentials exceeds 1. This method is used to obtain the next per-credential signature for a given authenticatorGetAssertion request.

This method takes no arguments as it is always follows a call to authenticatorGetAssertion or authenticatorGetNextAssertion.

When such a request is received, the authenticator performs the following procedure:

1. If authenticator does not remember any authenticatorGetAssertion parameters, return CTAP2_ERR_NOT_ALLOWED.
2. If the credential counter is 0, return CTAP2_ERR_NOT_ALLOWED.
3. If timer since the last call to authenticatorGetAssertion/authenticatorGetNextAssertion is greater than 30 seconds, discard the current authenticatorGetAssertion state and return CTAP2_ERR_NOT_ALLOWED.
4. Sign the clientDataHash with the credential using credential counter as index (e.g., credentials[n] assuming 1-based array), using the structure specified in [WebAuthN].
5. Reset the timer.
6. Decrement the credential counter.

On success, the authenticator returns the same structure as returned by the authenticatorGetAssertion method. The numberOfCredentials member is omitted.

### Client Logic

If client receives numberOfCredentials member value exceeding 1 in response to the authenticatorGetAssertion call:

1. Call authenticatorGetNextAssertion numberOfCredentials minus 1 times.
   - Make sure ‘rp’ member matches the current request.
   - Remember the ‘response’ member.
   - Add credential user information to the ‘credentialInfo’ list.
2. Draw a UX that displays credentialInfo list.
3. Let user select which credential to use.
4. Return the value of the ‘response’ member associated with the user choice.
5. Discard all other responses.

### 5.4 authenticatorCancel(0x03)

Using this method, the host can request the authenticator to cancel all ongoing operations are return to a ready state. It takes no input parameters and returns success or failure.

### 5.5 authenticatorGetInfo(0x04)

Using this method, the host can request that the authenticator report a list of all supported protocol versions, supported extensions, AAGUID of the device, and its capabilities. This method takes no inputs.

On success, the authenticator returns:

Member name | Data type | Required? | Definition
--- | --- | --- | ---
versions | Sequence of strings | Required | List of supported versions.
extensions | Sequence of strings | Optional | List of supported extensions.
aaguid | Byte String | Required | The claimed AAGUID. 16 bytes in length and encoded the same as MakeCredential AuthenticatorData, as specified in [WebAuthN].
options | Map | Optional | List of supported options.
maxMsgSize | Unsigned Integer | Optional | Maximum message size supported by the authenticator.
pinProtocols | Array of Unsigned Integers | Optional | List of supported PIN Protocol versions.

All options are in the form key-value pairs with string IDs and boolean values. When an option is not present, the default is applied per table below. The following is a list of supported options:

Option ID | Definition | Default
--- | --- | ---
plat | platform device: Indicates that the device is attached to the client and therefore can't be removed and used on another client. | false
rk | resident key: Indicates that the device is capable of storing keys on the device itself and therefore can satisfy the authenticatorGetAssertion request with allowList parameter not specified or empty. | false
clientPin | Client PIN: If present and set to true, it indicates that the device is capable of accepting a PIN from the client and PIN has been set. If present and set to false, it indicates that the device is capable of accepting a PIN from the client and PIN has not been set yet. If absent, it indicates that the device is not capable of accepting a PIN from the client. | Not supported
up | user presence: Indicates that the device is capable of testing user presence as part of the authenticatorGetAssertion request. | true
uv | user verification: Indicates that the device is capable of verifying the user as part of the authenticatorGetAssertion request. | false

### 5.6 authenticatorClientPIN(0x06)

One of the design goals of this command is to have minimum burden on the authenticator and to not send actual encrypted PIN to the authenticator in normal authenticator usage scenarios to have more security. Hence, below design only sends PIN in encrypted format while setting or changing a PIN. On normal PIN usage scenarios, design uses randomized pinToken which gets generated every power cycle.

This command is used by the platform to establish key agreement with Authenticator and getting sharedSecret, setting a new PIN on the Authenticator, changing existing PIN on the Authenticator and getting "pinToken" from the Authenticator which can be used in subsequent authenticatorMakeCredential and authenticatorGetAssertion operations.

It takes the following input parameters:

Parameter name | Data type | Required? | Definition
--- | --- | --- | ---
pinProtocol | Integer | Required | PIN protocol version chosen by the Client. For this version of the spec, this SHALL be the number 1.
subCommand | Integer | Required | The authenticator client PIN sub command currently being requested
keyAgreement | COSE_KEY | Optional | Public key of platformKeyAgreementKey.
pinAuth | Byte Array | Optional | First 16 bytes of HMAC-SHA-256 of encrypted contents using sharedSecret. See Setting a new PIN, Changing existing PIN and Getting pinToken from the authenticator for more details.
newPinEnc | Byte Array | Optional | Encrypted new PIN using sharedSecret. Encryption is done over UTF-8 representation of new PIN.
pinHashEnc | Byte Array | Optional | Encrypted first 16 bytes of SHA-256 of PIN using sharedSecret.
getKeyAgreement | Boolean | Optional | Asks authenticator to return public key of its authenticatorKeyAgreementKey for getting SharedSecret from the authenticator.
getRetries | Boolean | Optional | Asks authenticator to return number of PIN attempts remaining before lockout.

The list of sub commands for PIN Protocol Version 1 is:

Subcommand Name | Subcommand Number
--- | ---
Get Retries | 1
Get Key Agreement | 2
Set PIN | 3
Change PIN | 4
Get PIN token | 5

On success, Authenticator returns the following structure in its response.

Parameter name | Data type | Required? | Definition
--- | --- | --- | ---
KeyAgreement | COSE_KEY | Optional | Authenticator key agreement public key in COSE_KEY format. This will be used to establish a sharedSecret between platform and the authenticator.
pinToken | Byte Array | Optional | Encrypted pinToken using sharedSecret to be used in subsequent authenticatorMakeCredential and authenticatorGetAssertion operations.
retries | Unsigned Integer | Optional | Number of PIN attempts remaining before lockout. This is optionally used to show in UI when collecting the PIN in Setting a new PIN, Changing existing PIN and Getting pinToken from the authenticator flows.

#### 5.6.1 Client PIN support requirements

- Platform has to fulfill following PIN support requirements while gathering input from the user:
  - Minimum PIN Length: 4 Unicode characters
  - Maximum PIN Length: UTF-8 representation must not exceed 255 bytes
- Authenticator has to fulfill following PIN support requirements:
  - Minimum PIN Length: 4 bytes
  - Maximum PIN Length: 255 bytes
  - Maximum incorrect PIN retry count: 8
    - Each correct PIN entry resets retries counter.
    - Once the authenticator reaches the maximum incorrect PIN retry count, the authenticator has to be reset before any further operations with requires PIN.
  - PIN storage on the device has to be of the same or better security assurances as of private keys on the device.

Note: Authenticators can implement minimum PIN lengths that are longer than 4 characters.

#### 5.6.2 Authenticator Configuration Operations Upon Power Up

Authenticator generates following configuration at power up. This is to have less burden on the Authenticator as key agreement is an expensive operation. This also ensures randomness across power cycles.

Following are the operations Authenticator performs on each powerup:

- Generate "authenticatorKeyAgreementKey":
  - Generate a ECDH P-256 key pair called "authenticatorKeyAgreementKey" denoted by `(a, aG)` where `"a"` denotes the private key and `"aG"` denotes the public key.
    - See [RFC6090] Section 4.1 and [SP800-56A] for more ECDH key agreement protocol details.
- Generate "pinToken":
  - Generate a random integer of length which is multiple of 16 bytes (AES block length).
  - "pinToken" is used so that there is minimum burden on the authenticator and platform does not have to not send actual encrypted PIN to the authenticator in normal authenticator usage scenarios. This also provides more security as we are not sending actual PIN even in encrypted form. "pinToken" will be given to the platform upon verification of the PIN to be used in subsequent authenticatorMakeCredential and authenticatorGetAssertion operations.

#### 5.6.3 Getting sharedSecret from Authenticator

Platform does the ECDH key agreement to arrive at sharedSecret to be used only during that transaction. Authenticator does not have to keep a list of sharedSecrets for all active sessions. If there are subsequent authenticatorClientPIN transactions, a new sharedSecret is generated every time.

Platform performs the following operations to arrive at the sharedSecret:

- Platform sends authenticatorClientPIN command by setting getKeyAgreement parameter to true.
  - Platform optionally can set getRetries parameter to true to get the retries count. Retries count is the number of attempts remaining before lockout so when device is near authenticator lockout stage, platform can optionally warn the user to be careful while entering PIN.
- Authenticator responds back with public key of authenticatorKeyAgreementKey, "aG".
  - Authenticator optionally also sends retires count if getRetries parameter is set to true.
- Platform generates "platformKeyAgreementKey":
  - Platform generates ECDH P-256 key pair called "platformKeyAgreementKey" denoted by `(b, bG)` where `"b"` denotes the private key and `"bG"` denotes the public key.
- Platform generates "sharedSecret"
  - Platform generates "sharedSecret" using SHA-256 over ECDH key agreement protocol using private key of platformKeyAgreementKey, "b" and public key of authenticatorKeyAgreementKey, "aG": `SHA-256((baG).x)`.
    - SHA-256 is done over only "x" curve point of baG.
    - See [RFC6090] Section 4.1 and appendix (C.2) of [SP800-56A] for more ECDH key agreement protocol details and key representation.

#### 5.6.4 Setting a New PIN

Following operations are performed to set up a new PIN:

- Platform gets sharedSecret from the authenticator.
- Platform collects new PIN ("newPinUnicode") from the user in Unicode format.
  - Platform checks the Unicode character length of "newPinUnicode" against the minimum 4 Unicode character requirement and returns CTAP2_ERR_PIN_POLICY_VIOLATION if the check fails.
  - Let "newPin" be the UTF-8 representation of "newPinUnicode".
    - Platform checks the byte length of "newPin" against the max UTF-8 representation limit of 255 bytes and returns CTAP2_ERR_PIN_POLICY_VIOLATION if the check fails.
- Platform sends authenticatorClientPIN command with following parameters to the authenticator:
  - keyAgreement: public key of platformKeyAgreementKey, "bG".
  - newPinEnc: Encrypted newPin using sharedSecret: `AES256-CBC(sharedSecret, IV=0, newPin)`.
    - During encryption, newPin is padded with trailing 0x00 bytes and is of minimum 64 bytes length. This is to prevent leak of PIN length while communicating to the authenticator. There is no PKCS #7 padding used in this scheme.
  - pinAuth: `LEFT(HMAC-SHA-256(sharedSecret, newPinEnc), 16)`.
    - The platform sends the first 16 bytes of the HMAC-SHA-256 result.
- Authenticator performs following operations upon receiving the request:
  - Authenticator generates "sharedSecret": `SHA-256((abG).x)` using private key of authenticatorKeyAgreementKey, "a" and public key of platformKeyAgreementKey, "bG".
    - SHA-256 is done over only `"x"` curve point of `"abG"`
    - See [RFC6090] Section 4.1 and appendix (C.2) of [SP800-56A] for more ECDH key agreement protocol details and key representation.
  - Authenticator verifies pinAuth by generating `LEFT(HMAC-SHA-256(sharedSecret, newPinEnc), 16)` and matching against input pinAuth parameter.
    - If pinAuth verification fails, authenticator returns CTAP2_ERR_PIN_AUTH_INVALID error.
  - Authenticator decrypts newPinEnc using above "sharedSecret" producing newPin and checks newPin length against minimum PIN length of 4 characters.
    - The decrypted padded newPin should be of at least 64 bytes length and authenticator determines actual PIN length by looking for first 0x00 byte which terminates the PIN.
    - If minimum PIN length check fails, authenticator returns CTAP2_ERR_PIN_POLICY_VIOLATION error.
    - Authenticator may have additional constraints for PIN policy. The current spec only enforces minimum length of 4 characters.
  - Authenticator stores `LEFT(SHA-256(newPin), 16)` on the device and returns CTAP2_OK.

#### 5.6.5 Changing existing PIN

Following operations are performed to change an existing PIN:

- Platform gets sharedSecret from the authenticator.
- Platform collects current PIN ("curPinUnicode") and new PIN ("newPinUnicode") from the user.
  - Platform checks the Unicode character length of "newPinUnicode" against the minimum 4 Unicode character requirement and returns CTAP2_ERR_PIN_POLICY_VIOLATION if the check fails.
  - Let "curPin" be the UTF-8 representation of "curPinUnicode" and "newPin" be the UTF-8 representation of "newPinUnicode"
    - Platform checks the byte length of "curPin" and "newPin" against the max UTF-8 representation limit of 255 bytes and returns CTAP2_ERR_PIN_POLICY_VIOLATION if the check fails.
- Platform sends authenticatorClientPIN command with following parameters to the authenticator:
  - keyAgreement: public key of platformKeyAgreementKey, "bG".
  - pinHashEnc: Encrypted first 16 bytes of SHA-256 hash of curPin using sharedSecret: `AES256-CBC(sharedSecret, IV=0, LEFT(SHA-256(curPin),16))`.
  - newPinEnc: Encrypted "newPin" using sharedSecret: `AES256-CBC(sharedSecret, IV=0, newPin)`.
    - During encryption, newPin is padded with trailing 0x00 bytes and is of minimum 64 bytes length. This is to prevent leak of PIN length while communicating to the authenticator. There is no PKCS #7 padding used in this scheme.
  - pinAuth: `LEFT(HMAC-SHA-256(sharedSecret, newPinEnc || pinHashEnc), 16)`.
    - The platform sends the first 16 bytes of the HMAC-SHA-256 result.
- Authenticator performs following operations upon receiving the request:
  - Authenticator generates "sharedSecret": SHA-256((abG).x) using private key of authenticatorKeyAgreementKey, "a" and public key of platformKeyAgreementKey, "bG".
    - SHA-256 is done over only `"x"` curve point of `"abG"`
    - See [RFC6090] Section 4.1 and appendix (C.2) of [SP800-56A] for more ECDH key agreement protocol details and key representation.
  - Authenticator verifies pinAuth by generating `LEFT(HMAC-SHA-256(sharedSecret, newPinEnc || pinHashEnc), 16)` and matching against input pinAuth parameter.
    - If pinAuth verification fails, authenticator returns CTAP2_ERR_PIN_AUTH_INVALID error.
  - Authenticator decrypts pinHashEnc and verifies against its internal stored `LEFT(SHA-256(curPin), 16)`.
    - If a mismatch is detected, authenticator generate new "authenticatorKeyAgreementKey" first and then returns CTAP2_ERR_PIN_INVALID error.
      - Generate a new ECDH P-256 key pair called "authenticatorKeyAgreementKey" denoted by `(a, aG)` where `"a"` denotes the private key and `"aG"` denotes the public key.
        - See [RFC6090] Section 4.1 and [SP800-56A] for more ECDH key agreement protocol details.
  - Authenticator decrypts newPinEnc using above "sharedSecret" producing newPin and checks newPin length against minimum PIN length of 4 characters.
    - The decrypted padded newPin should be of at least 64 bytes length and authenticator determines actual PIN length by looking for first 0x00 byte which terminates the PIN.
    - If minimum PIN length check fails, authenticator returns CTAP2_ERR_PIN_POLICY_VIOLATION error.
    - Authenticator may have additional constraints for PIN policy. The current spec only enforces minimum length of 4 characters.
  - Authenticator stores `LEFT(SHA-256(newPin), 16)` on the device and returns CTAP2_OK.

#### 5.6.6 Getting pinToken from the Authenticator

This step only has to be performed once for the lifetime of the authenticator/platform handle. Getting pinToken once provides allows high security without any additional roundtrips every time (except for the first key-agreement phase) and its overhead is minimal.

Following operations are performed to get pinToken which will be used in subsequent authenticatorMakeCredential and authenticatorGetAssertion operations:

- Platform gets sharedSecret from the authenticator.
- Platform collects PIN from the user.
- Platform sends authenticatorClientPIN command with following parameters to the authenticator:
  - keyAgreement: public key of platformKeyAgreementKey, "bG".
  - pinHashEnc: `AES256-CBC(sharedSecret, IV=0, LEFT(SHA-256(PIN),16))`.
- Authenticator performs following operations upon receiving the request:
  - Authenticator generates "sharedSecret": `SHA-256((abG).x)` using private key of authenticatorKeyAgreementKey, "a" and public key of platformKeyAgreementKey, "bG".
    - SHA-256 is done over only `"x"` curve point of `"abG"`
    - See [RFC6090] Section 4.1 and appendix (C.2) of [SP800-56A] for more ECDH key agreement protocol details and key representation.
  - Authenticator decrypts pinHashEnc and verifies against its internal stored `LEFT(SHA-256(curPin), 16)`.
    - If a mismatch is detected, authenticator generate new "authenticatorKeyAgreementKey" first and then returns CTAP2_ERR_PIN_INVALID error.
      - Generate a new ECDH P-256 key pair called "authenticatorKeyAgreementKey" denoted by `(a, aG)` where `"a"` denotes the private key and `"aG"` denotes the public key.
        - See [RFC6090] Section 4.1 and [SP800-56A] for more ECDH key agreement protocol details.
  - Authenticator returns encrypted pinToken using "sharedSecret": `AES256-CBC(sharedSecret, IV=0, pinToken)`.
    - pinToken should be a multiple of 16 bytes (AES block length) without any padding or IV. There is no PKCS #7 padding used in this scheme.

#### 5.6.7 Using pinToken

Platform has the flexibility to manage the lifetime of pinToken based on the scenario however it should get rid of the pinToken as soon as possible when not required. Authenticator also can expire pinToken based on certain conditions like changing a PIN, timeout happening on authenticator, machine waking up from a suspend state etc. If pinToken has expired, authenticator will return CTAP2_ERR_PIN_TOKEN_EXPIRED and platform can act on the error accordingly.

##### 5.6.7.1 Using pinToken in authenticatorMakeCredential

Following operations are performed to use pinToken in authenticatorMakeCredential API:

- Platform gets pinToken from the authenticator.
- Platform sends authenticatorMakeCredential command with following additional optional parameter:
  - pinAuth: `LEFT(HMAC-SHA-256(pinToken, clientDataHash), 16)`.
    - The platform sends the first 16 bytes of the HMAC-SHA-256 result.
- Authenticator verifies pinAuth by generating `LEFT(HMAC-SHA-256(pinToken, clientDataHash), 16)` and matching against input pinAuth parameter.
- Authenticator returns authenticatorMakeCredential response with "uv" bit set to 1.

If platform sends zero length pinAuth, authenticator needs to wait for user touch and then returns either CTAP2_ERR_PIN_NOT_SET if pin is not set or CTAP2_ERR_PIN_INVALID if pin has been set. This is done for the case where multiple authenticators are attached to the platform and the platform wants to enforce clientPin semantics, but the user has to select which authenticator to send the pinToken to.

##### 5.6.7.2 Using pinToken in authenticatorGetAssertion

Following operations are performed to use pinToken in authenticatorGetAssertion API:

- Platform gets pinToken from the authenticator.
- Platform sends authenticatorGetAssertion command with following additional optional parameter:
  - pinAuth: `LEFT(HMAC-SHA-256(pinToken, clientDataHash), 16)`.
- Authenticator verifies pinAuth by generating `LEFT(HMAC-SHA-256(pinToken, clientDataHash), 16)` and matching against input pinAuth parameter.
- Authenticator returns authenticatorGetAssertion response with "uv" bit set to 1.

If platform sends zero length pinAuth, authenticator needs to wait for user touch and then returns either CTAP2_ERR_PIN_NOT_SET if pin is not set or CTAP2_ERR_PIN_INVALID if pin has been set. This is done for the case where multiple authenticators are attached to the platform and the platform wants to enforce clientPin semantics, but the user has to select which authenticator to send the pinToken to.

##### 5.6.7.3 Without pinToken in authenticatorGetAssertion

Following operations are performed without using pinToken in authenticatorGetAssertion API:

- Platform sends authenticatorGetAssertion command without pinAuth optional parameter.
- Authenticator returns authenticatorGetAssertion response with "uv" bit set to 0.

Fig. 1 Client Pin

### 5.7 authenticatorReset(0x07)

This method is used by the client to reset an authenticator back to a factory default state, invalidating all generated credentials. In order to prevent accidental trigger of this mechanism, some form of user approval MAY be performed on the authenticator itself, meaning that the client will have to poll the device until the reset has been performed. The actual user-flow to perform the reset will vary depending on the authenticator and it outside the scope of this specification.

## 6. Message encoding

Many transports (e.g., Bluetooth Smart) are bandwidth constrained, and serialization formats such as JSON are too heavy-weight for such environments. For this reason, all encoding is done using the concise binary encoding CBOR [RFC7049].

To reduce the complexity of the messages and the resources required to parse and validate them, all messages MUST use Canonical CBOR as specified below. All encoders MUST generate Canonical CBOR without duplicate map keys. All decoders SHOULD enforce Canonical CBOR and SHOULD reject messages with duplicate map keys. Canonical CBOR for CTAP uses the following rules:

- Integers must be encoded as small as possible.
  - 0 to 23 and -1 to -24 must be expressed in the same byte as the major type;
  - 24 to 255 and -25 to -256 must be expressed only with an additional uint8_t;
  - 256 to 65535 and -257 to -65536 must be expressed only with an additional uint16_t;
  - 65536 to 4294967295 and -65537 to -4294967296 must be expressed only with an additional uint32_t.
- The expression of lengths in major types 2 through 5 must be as short as possible. The rules for these lengths follow the above rule for integers.
- Indefinite-length items must be made into definite-length items.
- The keys in every map must be sorted lowest value to highest. Sorting is performed on the bytes of the representation of the key data items without paying attention to the 3/5 bit splitting for major types. The sorting rules are:
  - If the major types are different, the one with the lower value in numerical order sorts earlier.
  - If two keys have different lengths, the shorter one sorts earlier;
  - If two keys have the same length, the one with the lower value in (byte-wise) lexical order sorts earlier.

Because some authenticators are memory constrained, the depth of nested CBOR structures used by all message encodings is limited to at most four (4) levels of any combination of CBOR maps and/or CBOR arrays. Authenticators MUST support at least 4 levels of CBOR nesting. Clients, platforms, and servers MUST NOT use more than 4 levels of CBOR nesting.

Likewise, because some authenticators are memory constrained, the maximum message size supported by an authenticator MAY be limited. By default, authenticators MUST support messages of at least 1024 bytes. Authenticators MAY declare a different maximum message size supported using the maxMsgSize authenticatorGetInfo result parameter. Clients, platforms, and servers MUST NOT send messages larger than 1024 bytes unless the authenticator's maxMsgSize indicates support for the larger message size. Authenticators MAY return the CTAP2_ERR_REQUEST_TOO_LARGE error if size or memory constraints are exceeded.

If map keys are present that an implementation does not understand, they MUST be ignored. Note that this enables additional fields to be used as new features are added without breaking existing implementations.

Messages from the host to authenticator are called "commands" and messages from authenticator to host are called "replies". All values are big endian encoded.

### 6.1 Commands

All commands are structured as:

Name | Length | Required? | Definition
--- | --- | --- | ---
Command Value | 1 byte | Required | The value of the command to execute
Command Parameters | variable | Optional | CBOR [RFC7049] encoded set of parameters. Some commands have parameters, while others do not (see below)

The assigned values for commands and their descriptions are:

Command Name | Command Value | Has parameters?
--- | --- | ---
authenticatorMakeCredential | 0x01 | yes
authenticatorGetAssertion | 0x02 | yes
authenticatorCancel | 0x03 | no
authenticatorGetInfo | 0x04 | no
authenticatorClientPIN | 0x06 | yes
authenticatorReset | 0x07 | no
authenticatorGetNextAssertion | 0x08 | no
authenticatorVendorFirst | 0x40 | NA
authenticatorVendorLast | 0xBF | NA

Command codes in the range between authenticatorVendorFirst and authenticatorVendorLast may be used for vendor-specific implementations. For example, the vendor may choose to put in some testing commands. Note that the FIDO client will never generate these commands. All other command codes are reserved for future use and may not be used.

Command parameters are encoded using a CBOR map (CBOR major type 5). The CBOR map must be encoded using the definite length variant.

Some commands have optional parameters. Therefore, the length of the parameter map for these commands may vary. For example, authenticatorMakeCredential may have 4, 5, 6, or 7 parameters, while authenticatorGetAssertion may have 2, 3, 4, or 5 parameters.

All command parameters are CBOR encoded following the JSON to CBOR conversion procedures as per the CBOR specification [RFC7049]. Specifically, parameters that are represented as DOM objects in the Authenticator API layers (formally defined in the Web API [WebAuthN]) are converted first to JSON and subsequently to CBOR.

Example 1
A PublicKeyCredentialRpEntity DOM object defined as follows:

      var rp = {
            name: "Acme"
          };

would be CBOR encoded as follows:

      a1                                              # map(1)
          64                                          # text(4)
              6e616d65                                # "name"
          64                                          # text(4)
              41636d65                                # "Acme"

Example 2
A PublicKeyCredentialUserEntity DOM object defined as follows:

      var user = {
            id: Uint8Array.from(window.atob("MIIBkzCCATigAwIBAjCCAZMwggE4oAMCAQIwggGTMII="), c=>c.charCodeAt(0)),
            icon: "https://pics.acme.com/00/p/aBjjjpqPb.png",
            name: "johnpsmith@example.com",
            displayName: "John P. Smith"
          };

would be CBOR encoded as follows:

      a4                                              # map(4)
          62                                          # text(2)
              6964                                    # "id"
          58 20                                       # bytes(32)
              3082019330820138a003020102              # userid
              3082019330820138a003020102              # ...
              308201933082                            # ...
          64                                          # text(4)
              69636f6e                                # "icon"
          7828                                        # text(40)
              68747470733a2f2f706963732e61636d        # "https://pics.acme.com/00/p/aBjjjpqPb.png"
              652e636f6d2f30302f702f61426a6a6a        # ...
              707150622e706e67                        # ...
          64                                          # text(4)
              6e616d65                                # "name"
          76                                          # text(22)
              6a6f686e70736d697468406578616d70        # "johnpsmith@example.com"
              6c652e636f6d                            # ...
          6b                                          # text(11)
              646973706c61794e616d65                  # "displayName"
          6d                                          # text(13)
              4a6f686e20502e20536d697468              # "John P. Smith"

Example 3
A DOM object that is a sequence of PublicKeyCredentialParameters defined as follows:

          var pubKeyCredParams = [
              {
                type: "public-key",
                alg: -7 // "ES256" as registered in the IANA COSE Algorithms registry
              },
              {
                type: "public-key",
                alg: -257 // "RS256" as registered by WebAuthn
              }
            ];

would be CBOR encoded as:

      82                                              # array(2)
          a2                                          # map(2)
              63                                      # text(3)
                  616c67                              # "alg"
              26                                      # -7 (ES256)
              64                                      # text(4)
                  74797065                            # "type"
              6a                                      # text(10)
                  7075626C69632D6B6579                # "public-key"
          a2                                          # map(2)
              63                                      # text(3)
                  616c67                              # "alg"
              390100                                  # -257 (RS256)
              64                                      # text(4)
                  74797065                            # "type"
              6a                                      # text(10)
                  7075626C69632D6B6579                # "public-key"

For each command that contains parameters, the parameter map keys and value types are specified below:

Command | Parameter Name | Key | Value type
--- | --- | --- | ---
authenticatorMakeCredential | clientDataHash | 0x01 | byte string (CBOR major type 2).
| rp | 0x02 | CBOR definite length map (CBOR major type 5).
| user | 0x03 | CBOR definite length map (CBOR major type 5).
| pubKeyCredParams | 0x04 | CBOR definite length array (CBOR major type 4) of CBOR definite length maps (CBOR major type 5).
| excludeList | 0x05 | CBOR definite length array (CBOR major type 4) of CBOR definite length maps (CBOR major type 5).
| extensions | 0x06 | CBOR definite length map (CBOR major type 5).
| options | 0x07 | CBOR definite length map (CBOR major type 5).
| pinAuth | 0x08 | byte string (CBOR major type 2).
| pinProtocol | 0x09 | PIN protocol version chosen by the Client. For this version of the spec, this SHALL be the number 1.
authenticatorGetAssertion | rpId | 0x01 | UTF-8 encoded text string (CBOR major type 3).
| clientDataHash | 0x02 | byte string (CBOR major type 2).
| allowList | 0x03 | CBOR definite length array (CBOR major type 4) of CBOR definite length maps (CBOR major type 5).
| extensions | 0x04 | CBOR definite length map (CBOR major type 5).
| options | 0x05 | CBOR definite length map (CBOR major type 5).
| pinAuth | 0x06 | byte string (CBOR major type 2).
| pinProtocol | 0x07 | PIN protocol version chosen by the Client. For this version of the spec, this SHALL be the number 1.
authenticatorClientPIN | pinProtocol | 0x01 | Unsigned Integer. (CBOR major type 0)
| subCommand | 0x02 | Unsigned Integer. (CBOR major type 0)
| keyAgreement | 0x03 | COSE_KEY
| pinAuth | 0x04 | byte string (CBOR major type 2).
| newPinEnc | 0x05 | byte string (CBOR major type 2). It is UTF-8 representation of encrypted input PIN value.
| pinHashEnc | 0x06 | byte string (CBOR major type 2).
| getKeyAgreement | 0x07 | Boolean. (CBOR major type 7, additional simple value information 20(False)/21(True)).
| getRetries | 0x08 | Boolean. (CBOR major type 7, additional simple value information 20(False)/21(True)).

Example 4
The following is a complete encoding example of the `authenticatorMakeCredential` command (using same account and crypto parameters as above) and the corresponding `authenticatorMakeCredential_Response` response:

              01                                      # authenticatorMakeCredential command
              a5                                      # map(5)
                 01                                   # unsigned(1) - clientDataHash
                 58 20                                # bytes(32)
                    687134968222ec17202e42505f8ed2b1  # h'687134968222ec17202e42505f8ed2b16ae22f16bb05b88c25db9e602645f141'
                    6ae22f16bb05b88c25db9e602645f141  #
                 02                                   # unsigned(2) - rp
                 a2                                   # map(2)
                    62                                # text(2)
                       6964                           # "id"
                    68                                # text(8)
                       61636d652e636f6d               # "acme.com"
                    64                                # text(4)
                       6e616d65                       # "name"
                    64                                # text(4)
                       41636d65                       # "Acme"
                 03                                   # unsigned(3) - user
                 a4                                   # map(4)
                    62                                # text(2)
                       6964                           # "id"
                    58 20                             # bytes(32)
                       3082019330820138a003020102     # userid
                       3082019330820138a003020102     # ...
                       308201933082                   # ...
                    64                                # text(4)
                       69636f6e                       # "icon"
                    78 28                             # text(40)
                       68747470733a2f2f706963732e616  # "https://pics.acme.com/00/p/aBjjjpqPb.png"
                       36d652e636f6d2f30302f702f6142  #
                       6a6a6a707150622e706e67         #
                    64                                # text(4)
                       6e616d65                       # "name"
                    76                                # text(22)
                       6a6f686e70736d697468406578616  # "johnpsmith@example.com"
                       d706c652e636f6d                #
                    6b                                # text(11)
                       646973706c61794e616d65         # "displayName"
                    6d                                # text(13)
                       4a6f686e20502e20536d697468     # "John P. Smith"
                 04                                   # unsigned(4) - pubKeyCredParams
                 82                                   # array(2)
                    a2                                # map(2)
                       63                             # text(3)
                          616c67                      # "alg"
                       26                             # -7 (ES256)
                       64                             # text(4)
                          74797065                    # "type"
                       6a                             # text(10)
                          7075626C69632D6B6579        # "public-key"
                    a2                                # map(2)
                       63                             # text(3)
                          616c67                      # "alg"
                       390100                         # -257 (RS256)
                       64                             # text(4)
                          74797065                    # "type"
                       6a                             # text(10)
                          7075626C69632D6B6579        # "public-key"
                 07                                   # unsigned(7) - options
                 a1                                   # map(1)
                    70                                # text(16)
                       6b657953746f72616765446576696  # "keyStorageDevice"
                       365                            #
                    f5                                # primitive(21)

`authenticatorMakeCredential_Response` response:

              00                                      # status = success
              a3                                      # map(3)
                 01                                   # unsigned(1)
                 66                                   # text(6)
                    7061636b6564                      # "packed"
                 02                                   # unsigned(2)
                 58 9a                                # bytes(154)
                    c289c5ca9b0460f9346ab4e42d842743  # authData
                    404d31f4846825a6d065be597a87051d  # ...
                    410000000bf8a011f38c0a4d15800617  # ...
                    111f9edc7d00108959cead5b5c48164e  # ...
                    8abcd6d9435c6fa363616c6765455332  # ...
                    353661785820f7c4f4a6f1d79538dfa4  # ...
                    c9ac50848df708bc1c99f5e60e51b42a  # ...
                    521b35d3b69a61795820de7b7d6ca564  # ...
                    e70ea321a4d5d96ea00ef0e2db89dd61  # ...
                    d4894c15ac585bd23684              # ...
                 03                                   # unsigned(3)
                 a3                                   # map(3)
                    63                                # text(3)
                       616c67                         # "alg"
                    26                                # -7 (ES256)
                    63                                # text(3)
                       736967                         # "sig"
                    58 47                             # bytes(71)
                       3045022013f73c5d9d530e8cc15cc  # signature...
                       9bd96ad586d393664e462d5f05612  # ...
                       35e6350f2b728902210090357ff91  # ...
                       0ccb56ac5b596511948581c8fddb4  # ...
                       a2b79959948078b09f4bdc6229     # ...
                    63                                # text(3)
                       783563                         # "x5c"
                    81                                # array(1)
                       59 0197                        # bytes(407)
                          3082019330820138a003020102  # certificate...
                          020900859b726cb24b4c29300a  # ...
                          06082a8648ce3d040302304731  # ...
                          0b300906035504061302555331  # ...
                          143012060355040a0c0b597562  # ...
                          69636f20546573743122302006  # ...
                          0355040b0c1941757468656e74  # ...
                          696361746f7220417474657374  # ...
                          6174696f6e301e170d31363132  # ...
                          30343131353530305a170d3236  # ...
                          313230323131353530305a3047  # ...
                          310b3009060355040613025553  # ...
                          31143012060355040a0c0b5975  # ...
                          6269636f205465737431223020  # ...
                          060355040b0c1941757468656e  # ...
                          74696361746f72204174746573  # ...
                          746174696f6e3059301306072a  # ...
                          8648ce3d020106082a8648ce3d  # ...
                          03010703420004ad11eb0e8852  # ...
                          e53ad5dfed86b41e6134a18ec4  # ...
                          e1af8f221a3c7d6e636c80ea13  # ...
                          c3d504ff2e76211bb44525b196  # ...
                          c44cb4849979cf6f896ecd2bb8  # ...
                          60de1bf4376ba30d300b300906  # ...
                          03551d1304023000300a06082a  # ...
                          8648ce3d040302034900304602  # ...
                          2100e9a39f1b03197525f7373e  # ...
                          10ce77e78021731b94d0c03f3f  # ...
                          da1fd22db3d030e7022100c4fa  # ...
                          ec3445a820cf43129cdb00aabe  # ...
                          fd9ae2d874f9c5d343cb2f113d  # ...
                          a23723f3                    # ...

Example 5
The following is a complete encoding example of the `authenticatorGetAssertion` command and the corresponding `authenticatorGetAssertion_Response` response:

              02                                      # authenticatorGetAssertion command
              a4                                      # map(4)
                 01                                   # unsigned(1)
                 68                                   # text(8)
                    61636d652e636f6d                  # "acme.com"
                 02                                   # unsigned(2)
                 58 20                                # bytes(32)
                    687134968222ec17202e42505f8ed2b1  # clientDataHash
                    6ae22f16bb05b88c25db9e602645f141  # ...
                 03                                   # unsigned(3)
                 82                                   # array(2)
                    a2                                # map(2)
                       62                             # text(2)
                          6964                        # "id"
                       58 40                          # bytes(64)
                          f22006de4f905af68a43942f02  # credential ID
                          4f2a5ece603d9c6d4b3df8be08  # ...
                          ed01fc442646d034858ac75bed  # ...
                          3fd580bf9808d94fcbee82b9b2  # ...
                          ef6677af0adcc35852ea6b9e    # ...
                       64                             # text(4)
                          74797065                    # "type"
                       6a                             # text(10)
                          7075626C69632D6B6579        # "public-key"
                    a2                                # map(2)
                       62                             # text(2)
                          6964                        # "id"
                       58 32                          # bytes(50)
                          03030303030303030303030303  # credential ID
                          03030303030303030303030303  # ...
                          03030303030303030303030303  # ...
                          0303030303030303030303      # ...
                       64                             # text(4)
                          74797065                    # "type"
                       6a                             # text(10)
                          7075626C69632D6B6579        # "public-key"
                 05                                   # unsigned(5)
                 a1                                   # map(1)
                    62                                # text(2)
                       747569                         # "uv"
                    f5                                # true

`authenticatorGetAssertion_Response` response:

              00                                      # status = success
              a3                                      # map(5)
                 01                                   # unsigned(1) - Credential
                 a2                                   # map(2)
                    62                                # text(2)
                       6964                           # "id"
                    58 40                             # bytes(64)
                       f22006de4f905af68a43942f024f2  # credentialId
                       a5ece603d9c6d4b3df8be08ed01fc  # ...
                       442646d034858ac75bed3fd580bf9  # ...
                       808d94fcbee82b9b2ef6677af0adc  # ...
                       c35852ea6b9e                   # ...
                    64                                # text(4)
                       74797065                       # "type"
                    6a                                # text(10)
                       7075626C69632D6B6579           # "public-key"
                 02                                   # unsigned(2)
                 58 25                                # bytes(37)
                    625ddadf743f5727e66bba8c2e387922  # authData
                    d1af43c503d9114a8fba104d84d02bfa  # ...
                    0100000011                        # ...
                 03                                   # unsigned(3)
                 58 47                                # bytes(71)
                    304502204a5a9dd39298149d904769b5  # signature
                    1a451433006f182a34fbdf66de5fc717  # ...
                    d75fb350022100a46b8ea3c3b933821c  # ...
                    6e7f5ef9daae94ab47f18db474c74790  # ...
                    eaabb14411e7a0                    # ...
                 04                                   # unsigned(4) - publicKeyCredentialUserEntity
                 a4                                   # map(4)
                   6b                                 # text(11)
                      646973706c61794e616d65          # "displayName"
                   6d                                 # text(13)
                      4a6f686e20502e20536d697468      # "John P. Smith"
                   64                                 # text(4)
                      6e616d65                        # "name"
                   76                                 # text(22)
                      6a6f686e70736d697468406578616d  # "johnpsmith@example.com"
                      706c652e636f6d                  # ...
                   62                                 # text(2)
                      6964                            # "id"
                    58 20                             # bytes(32)
                      3082019330820138a003020102      # userid
                      3082019330820138a003020102      # ...
                      308201933082                    # ...
                    64                                # text(4)
                      69636f6e                        # "icon"
                    7828                              # text(40)
                      68747470733a2f2f706963732e6163  # "https://pics.acme.com/00/p/aBjjjpqPb.png"
                      6d652e636f6d2f30302f702f61426a  # ...
                      6a6a707150622e706e67            # ...
                 05                                   # unsigned(5) - numberofCredentials
                 01                                   # unsigned(1)

### 6.2 Responses

All responses are structured as:

Name | Length | Required? | Definition
--- | --- | --- | ---
Status | 1 byte | Required | The status of the response. 0x00 means success; all other values are errors. See the table in the next section for error values.
Response Data | variable | Optional | CBOR encoded set of values.

Response data is encoded using a CBOR map (CBOR major type 5). The CBOR map must be encoded using the definite length variant.

For each response message, the map keys and value types are specified below:

Response Message | Member Name | Key | Value type
--- | --- | --- | ---
authenticatorMakeCredential_Response | fmt | 0x01 | text string (CBOR major type 3).
| authData | 0x02 | byte string (CBOR major type 2).
| attStmt | 0x03 | definite length map (CBOR major type 5).
authenticatorGetAssertion_Response | credential | 0x01 | definite length map (CBOR major type 5).
| authData | 0x02 | byte string (CBOR major type 2).
| signature | 0x03 | byte string (CBOR major type 2).
| publicKeyCredentialUserEntity | 0x04 | definite length map (CBOR major type 5). MUST not be present if UV bit is not set.
| numberOfCredentials | 0x05 | unsigned integer(CBOR major type 0).
authenticatorGetNextAssertion_Response | credential | 0x01 | definite length map (CBOR major type 5).
| authData | 0x02 | byte string (CBOR major type 2).
| signature | 0x03 | byte string (CBOR major type 2).
| publicKeyCredentialUserEntity | 0x04 | definite length map (CBOR major type 5).
authenticatorGetInfo_Response | versions | 0x01 | definite length array (CBOR major type 4) of UTF-8 encoded strings (CBOR major type 3).
| extensions | 0x02 | definite length array (CBOR major type 4) of UTF-8 encoded strings (CBOR major type 3).
| aaguid | 0x03 | byte string (CBOR major type 2). 16 bytes in length and encoded the same as MakeCredential AuthenticatorData, as specified in [WebAuthN].
| options | 0x04 | Definite length map (CBOR major type 5) of key-value pairs where keys are UTF8 strings (CBOR major type 3) and values are booleans (CBOR simple value 21).
| maxMsgSize | 0x05 | CBOR definite length array (CBOR major type 4) of CBOR unsigned integers (CBOR major type 0) This is the maximum message size supported by the authenticator.
| pinProtocols | 0x06 | array of unsigned integers (CBOR major type). This is the list of pinProtocols supported by the Authenticator.
authenticatorClientPIN_Response | keyAgreement | 0x01 | Authenticator public key in COSE_KEY format.
| pinToken | 0x02 | byte string (CBOR major type 2).
| retries | 0x03 | Unsigned integer (CBOR major type 0). This is number of retries left before lockout.

### 6.3 Error Responses

The error response values range from 0x01 - 0xff. This range is split based on error type.

Error response values in the range between CTAP2_OK and CTAP2_ERR_SPEC_LAST are reserved for spec purposes.

Error response values in the range between CTAP2_ERR_VENDOR_FIRST and CTAP2_ERR_VENDOR_LAST may be used for vendor-specific implementations. All other response values are reserved for future use and may not be used. These vendor specific error codes are not interoperable and the platform should treat these errors as any other unknown error codes.

Error response values in the range between CTAP2_ERR_EXTENSION_FIRST and CTAP2_ERR_EXTENSION_LAST may be used for extension-specific implementations. These errors need to be interoperable for vendors who decide to implement such optional extension.

Code | Name | Description
--- | --- | ---
0x00 | CTAP1_ERR_SUCCESS | Indicates successful response.
0x01 | CTAP1_ERR_INVALID_COMMAND | The command is not a valid CTAP command.
0x02 | CTAP1_ERR_INVALID_PARAMETER | The command included an invalid parameter.
0x03 | CTAP1_ERR_INVALID_LENGTH | Invalid message or item length.
0x04 | CTAP1_ERR_INVALID_SEQ | Invalid message sequencing.
0x05 | CTAP1_ERR_TIMEOUT | Message timed out.
0x06 | CTAP1_ERR_CHANNEL_BUSY | Channel busy.
0x0A | CTAP1_ERR_LOCK_REQUIRED | Command requires channel lock.
0x0B | CTAP1_ERR_INVALID_CHANNEL | Command not allowed on this cid.
0x10 | CTAP2_ERR_CBOR_PARSING | Error while parsing CBOR.
0x11 | CTAP2_ERR_CBOR_UNEXPECTED_TYPE | Invalid/unexpected CBOR error.
0x12 | CTAP2_ERR_INVALID_CBOR | Error when parsing CBOR.
0x13 | CTAP2_ERR_INVALID_CBOR_TYPE | Invalid or unexpected CBOR type.
0x14 | CTAP2_ERR_MISSING_PARAMETER | Missing non-optional parameter.
0x15 | CTAP2_ERR_LIMIT_EXCEEDED | Limit for number of items exceeded.
0x16 | CTAP2_ERR_UNSUPPORTED_EXTENSION | Unsupported extension.
0x17 | CTAP2_ERR_TOO_MANY_ELEMENTS | Limit for number of items exceeded.
0x18 | CTAP2_ERR_EXTENSION_NOT_SUPPORTED | Unsupported extension.
0x19 | CTAP2_ERR_CREDENTIAL_EXCLUDED | Valid credential found in the exludeList.
0x20 | CTAP2_ERR_CREDENTIAL_NOT_VALID | Credential not valid for authenticator.
0x21 | CTAP2_ERR_PROCESSING | Processing (Lengthy operation is in progress).
0x22 | CTAP2_ERR_INVALID_CREDENTIAL | Credential not valid for the authenticator.
0x23 | CTAP2_ERR_USER_ACTION_PENDING | Authentication is waiting for user interaction.
0x24 | CTAP2_ERR_OPERATION_PENDING | Processing, lengthy operation is in progress.
0x25 | CTAP2_ERR_NO_OPERATIONS | No request is pending.
0x26 | CTAP2_ERR_UNSUPPORTED_ALGORITHM | Authenticator does not support requested algorithm.
0x27 | CTAP2_ERR_OPERATION_DENIED | Not authorized for requested operation.
0x28 | CTAP2_ERR_KEY_STORE_FULL | Internal key storage is full.
0x29 | CTAP2_ERR_NOT_BUSY | Authenticator cannot cancel as it is not busy.
0x2A | CTAP2_ERR_NO_OPERATION_PENDING | No outstanding operations.
0x2B | CTAP2_ERR_UNSUPPORTED_OPTION | Unsupported option.
0x2C | CTAP2_ERR_INVALID_OPTION | Unsupported option.
0x2D | CTAP2_ERR_KEEPALIVE_CANCEL | Pending keep alive was cancelled.
0x2E | CTAP2_ERR_NO_CREDENTIALS | No valid credentials provided.
0x2F | CTAP2_ERR_USER_ACTION_TIMEOUT | Timeout waiting for user interaction.
0x30 | CTAP2_ERR_NOT_ALLOWED | Continuation command, such as, authenticatorGetNextAssertion not allowed.
0x31 | CTAP2_ERR_PIN_INVALID | PIN Blocked.
0x32 | CTAP2_ERR_PIN_BLOCKED | PIN Blocked.
0x33 | CTAP2_ERR_PIN_AUTH_INVALID | PIN authentication,pinAuth, verification failed.
0x34 | CTAP2_ERR_PIN_AUTH_BLOCKED | PIN authentication,pinAuth, blocked. Requires power recycle to reset.
0x35 | CTAP2_ERR_PIN_NOT_SET | No PIN has been set.
0x36 | CTAP2_ERR_PIN_REQUIRED | PIN is required for the selected operation.
0x37 | CTAP2_ERR_PIN_POLICY_VIOLATION | PIN policy violation. Currently only enforces minimum length.
0x38 | CTAP2_ERR_PIN_TOKEN_EXPIRED | pinToken expired on authenticator.
0x39 | CTAP2_ERR_REQUEST_TOO_LARGE | Authenticator cannot handle this request due to memory constraints.
0x7F | CTAP1_ERR_OTHER | Other unspecified error.
0xDF | CTAP2_ERR_SPEC_LAST | CTAP 2 spec last error.
0xE0 | CTAP2_ERR_EXTENSION_FIRST | Extension specific error.
0xEF | CTAP2_ERR_EXTENSION_LAST | Extension specific error.
0xF0 | CTAP2_ERR_VENDOR_FIRST | Vendor specific error.
0xFF | CTAP2_ERR_VENDOR_LAST | Vendor specific error.

## 7. Interoperating with CTAP1/U2F authenticators

This section defines how a platform maps CTAP2 requests to CTAP1/U2F requests and CTAP1/U2F responses to CTAP2 responses in order to support CTAP1/U2F authenticators via CTAP2. CTAP2 requests can be mapped to CTAP1/U2F requests provided the CTAP2 request does not have parameters that only CTAP2 authenticators can fulfill. The processes for RPs to use to verify CTAP1/U2F based authenticatorMakeCredential and authenticatorGetAssertion responses are also defined below. Platform may choose to skip this feature and work only with CTAP devices.

### 7.1 Using the CTAP2 authenticatorMakeCredential Command with CTAP1/U2F authenticators

Platform follows the following procedure (Fig: Mapping: WebAuthn authenticatorMakeCredential to and from CTAP1/U2F Registration Messages):

1. Platform tries to get information about the authenticator by sending authenticatorGetInfo command as specified in CTAP2 protocol overview.
   - CTAP1/U2F authenticator returns a command error or improperly formatted CBOR response. For any failure, platform may fall back to CTAP1/U2F protocol.
2. Map CTAP2 authenticatorMakeCredential request to U2F_REGISTER request.
   - Platform verifies that CTAP2 request does not have any parameters that CTAP1/U2F authenticators cannot fulfill.
     - All of the below conditions must be true for the platform to proceed to next step. If any of the below conditions is not true, platform errors out with CTAP2_ERR_OPTION_NOT_SUPPORTED.
       - pubKeyCredParams must use the ES256 algorithm (-7).
       - Options must not include "rk" set to true.
       - Options must not include "uv" set to true.
     - If excludeList is not empty:
       - If the excludeList is not empty, the platform must send signing request with check-only control byte to the CTAP1/U2F authenticator using each of the credential ids (key handles) in the excludeList. If any of them does not result in an error, that means that this is a known device. Afterwards, the platform must still send a dummy registration request (with a dummy appid and invalid challenge) to CTAP1/U2F authenticators that it believes are excluded. This makes it so the user still needs to touch the CTAP1/U2F authenticator before the RP gets told that the token is already registered.
   - Use clientDataHash parameter of CTAP2 request as CTAP1/U2F challenge parameter (32 bytes).
   - Let `rpIdHash` be a byte array of size 32 initialized with SHA-256 hash of `rp.id` parameter as CTAP1/U2F application parameter (32 bytes).
3. Send the U2F_REGISTER request to the authenticator as specified in [U2FRawMsgs] spec.
4. Map the U2F registration response message (see the "Registration Response Message: Success" section of [U2FRawMsgs]) to a CTAP2 authenticatorMakeCredential response message:
   - Generate `authenticatorData` from the U2F registration response message received from the authenticator:
     - Initialize `attestationData`:
       - Let `credentialIdLength` be a 2-byte unsigned big-endian integer representing length of the Credential ID initialized with CTAP1/U2F response key handle length.
       - Let `credentialID` be a `credentialIdLength` byte array initialized with CTAP1/U2F response key handle bytes.
       - Let `x9encodedUserPublicKey`be the `user public key` returned in the U2F registration response message [U2FRawMsgs]. Let `coseEncodedCredentialPublicKey` be the result of converting `x9encodedUserPublicKey`'s value from ANS X9.62 / Sec-1 v2 uncompressed curve point representation [SEC1V2] to COSE_Key representation ([RFC8152] Section 7).
       - Let `attestationData` be a byte array with following structure:

         Length (in bytes) | Description | Value
         --- | --- | ---
         16 | The AAGUID of the authenticator. | Initialized with all zeros.
         2 | Byte length L of Credential ID | Initialized with `credentialIdLength` bytes.
         `credentialIdLength` | Credential ID. | Initialized with `credentialID` bytes.
         77 | The credential public key. | Initialized with `coseEncodedCredentialPublicKey` bytes.

     - Initialize `authenticatorData`:
       - Let `flags` be a byte whose zeroth bit (bit 0, UP) is set, and whose sixth bit (bit 6, AT) is set, and all other bits are zero (bit zero is the least significant bit). See also Authenticator Data section of [WebAuthN].
       - Let `signCount` be a 4-byte unsigned integer initialized to zero.
       - Let `authenticatorData` be a byte array with the following structure:

         Length (in bytes) | Description | Value
         --- | --- | ---
         32 | SHA-256 hash of the `rp.id`. | Initialized with `rpIdHash` bytes.
         1 | Flags | Initialized with `flags`' value.
         4 | Signature counter (signCount). | Initialized with `signCount` bytes.
         Variable Length | Attestation Data. | Initialized with `attestationData`'s value.

   - Let `attestationStatement` be a CBOR map (see "attStmtTemplate" in Generating an Attestation Object [WebAuthN]) with the following keys whose values are as follows:
     - Set "x5c" as an array of the one attestation cert extracted from CTAP1/U2F response.
     - Set "sig"'s value to be the "signature" bytes from the U2F registration response message [U2FRawMsgs].
   - Let `attestationObject` be a CBOR map (see "attObj" in Attestation object [WebAuthN]) with the following keys whose values are as follows:
     - Set "authData"'s value to `authenticatorData`.
     - Set "fmt"'s value to "fido-u2f".
     - Set "attStmt"'s value to `attestationStatement`.
5. Return `attestationObject` to the caller.

Example 6
Sample CTAP2 authenticatorMakeCredential Request (CBOR):

    {1: h'687134968222EC17202E42505F8ED2B16AE22F16BB05B88C25DB9E602645F141',
     2: {"id": "acme.com",
         "name": "acme.com"},
     3: {"id": "1098237235409872",
         "name": "johnpsmith@example.com",
         "icon": "https://pics.acme.com/00/p/aBjjjpqPb.png",
         "displayName": "John P. Smith"},
     4: [{"type": "public-key", "alg": -7},
         {"type": "public-key", "alg": -257}]}

CTAP1/U2F Request from above CTAP2 authenticatorMakeCredential request

    687134968222EC17202E42505F8ED2B16AE22F16BB05B88C25DB9E602645F141    # clientdatahash
    1194228DA8FDBDEEFD261BD7B6595CFD70A50D70C6407BCF013DE96D4EFB17DE    # rpidhash

Sample CTAP1/U2F Response from the device

    05                                                                  # Reserved Byte (1 Byte)
    04E87625896EE4E46DC032766E8087962F36DF9DFE8B567F3763015B1990A60E    # User Public Key (65 Bytes)
    1427DE612D66418BDA1950581EBC5C8C1DAD710CB14C22F8C97045F4612FB20C    # ...
    91                                                                  # ...
    40                                                                  # Key Handle Length (1 Byte)
    3EBD89BF77EC509755EE9C2635EFAAAC7B2B9C5CEF1736C3717DA48534C8C6B6    # Key Handle (Key Handle Length Bytes)
    54D7FF945F50B5CC4E78055BDD396B64F78DA2C5F96200CCD415CD08FE420038    # ...
    3082024A30820132A0030201020204046C8822300D06092A864886F70D01010B    # X.509 Cert (Variable length Cert)
    0500302E312C302A0603550403132359756269636F2055324620526F6F742043    # ...
    412053657269616C203435373230303633313020170D31343038303130303030    # ...
    30305A180F32303530303930343030303030305A302C312A302806035504030C    # ...
    2159756269636F205532462045452053657269616C2032343931383233323437    # ...
    37303059301306072A8648CE3D020106082A8648CE3D030107034200043CCAB9    # ...
    2CCB97287EE8E639437E21FCD6B6F165B2D5A3F3DB131D31C16B742BB476D8D1    # ...
    E99080EB546C9BBDF556E6210FD42785899E78CC589EBE310F6CDB9FF4A33B30    # ...
    39302206092B0601040182C40A020415312E332E362E312E342E312E34313438    # ...
    322E312E323013060B2B0601040182E51C020101040403020430300D06092A86    # ...
    4886F70D01010B050003820101009F9B052248BC4CF42CC5991FCAABAC9B651B    # ...
    BE5BDCDC8EF0AD2C1C1FFB36D18715D42E78B249224F92C7E6E7A05C49F0E7E4    # ...
    C881BF2E94F45E4A21833D7456851D0F6C145A29540C874F3092C934B43D222B    # ...
    8962C0F410CEF1DB75892AF116B44A96F5D35ADEA3822FC7146F6004385BCB69    # ...
    B65C99E7EB6919786703C0D8CD41E8F75CCA44AA8AB725AD8E799FF3A8696A6F    # ...
    1B2656E631B1E40183C08FDA53FA4A8F85A05693944AE179A1339D002D15CABD    # ...
    810090EC722EF5DEF9965A371D415D624B68A2707CAD97BCDD1785AF97E258F3    # ...
    3DF56A031AA0356D8E8D5EBCADC74E071636C6B110ACE5CC9B90DFEACAE640FF    # ...
    1BB0F1FE5DB4EFF7A95F060733F5                                        # ...
    30450220324779C68F3380288A1197B6095F7A6EB9B1B1C127F66AE12A99FE85    # Signature (variable Length)
    32EC23B9022100E39516AC4D61EE64044D50B415A6A4D4D84BA6D895CB5AB7A1    # ...
    AA7D081DE341FA                                                      # ...

Authenticator Data from CTAP1/U2F Response

    1194228DA8FDBDEEFD261BD7B6595CFD70A50D70C6407BCF013DE96D4EFB17DE    # rpidhash
    41                                                                  # flags
    00000000                                                            # Sign Count
    00000000000000000000000000000000                                    # AAGUID
    0040                                                                # Key Handle Length (1 Byte)
    3EBD89BF77EC509755EE9C2635EFAAAC7B2B9C5CEF1736C3717DA48534C8C6B6    # Key Handle (Key Handle Length Bytes)
    54D7FF945F50B5CC4E78055BDD396B64F78DA2C5F96200CCD415CD08FE420038    # ...
    A5010203262001215820E87625896EE4E46DC032766E8087962F36DF9DFE8B56    # Public Key
    7F3763015B1990A60E1422582027DE612D66418BDA1950581EBC5C8C1DAD710C    # ...
    B14C22F8C97045F4612FB20C91                                          # ...

Mapped CTAP2 authenticatorMakeCredential response(CBOR)

    {"fmt": "fido-u2f",
     "authData": h'1194228DA8FDBDEEFD261BD7B6595CFD70A50D70C6407BCF013DE96D4EFB17DE
                   41000000000000000000000000000000000000000000403EBD89BF77EC509755
                   EE9C2635EFAAAC7B2B9C5CEF1736C3717DA48534C8C6B654D7FF945F50B5CC4E
                   78055BDD396B64F78DA2C5F96200CCD415CD08FE420038A50102032620012158
                   20E87625896EE4E46DC032766E8087962F36DF9DFE8B567F3763015B1990A60E
                   1422582027DE612D66418BDA1950581EBC5C8C1DAD710CB14C22F8C97045F461
                   2FB20C91',
     "attStmt": {"sig": h'30450220324779C68F3380288A1197B6095F7A6EB9B1B1C127F66AE12A99FE85
                  32EC23B9022100E39516AC4D61EE64044D50B415A6A4D4D84BA6D895CB5AB7A1
                  AA7D081DE341FA',
                 "x5c": [h'3082024A30820132A0030201020204046C8822300D06092A864886F70D01010B
                           0500302E312C302A0603550403132359756269636F2055324620526F6F742043
                           412053657269616C203435373230303633313020170D31343038303130303030
                           30305A180F32303530303930343030303030305A302C312A302806035504030C
                           2159756269636F205532462045452053657269616C2032343931383233323437
                           37303059301306072A8648CE3D020106082A8648CE3D030107034200043CCAB9
                           2CCB97287EE8E639437E21FCD6B6F165B2D5A3F3DB131D31C16B742BB476D8D1
                           E99080EB546C9BBDF556E6210FD42785899E78CC589EBE310F6CDB9FF4A33B30
                           39302206092B0601040182C40A020415312E332E362E312E342E312E34313438
                           322E312E323013060B2B0601040182E51C020101040403020430300D06092A86
                           4886F70D01010B050003820101009F9B052248BC4CF42CC5991FCAABAC9B651B
                           BE5BDCDC8EF0AD2C1C1FFB36D18715D42E78B249224F92C7E6E7A05C49F0E7E4
                           C881BF2E94F45E4A21833D7456851D0F6C145A29540C874F3092C934B43D222B
                           8962C0F410CEF1DB75892AF116B44A96F5D35ADEA3822FC7146F6004385BCB69
                           B65C99E7EB6919786703C0D8CD41E8F75CCA44AA8AB725AD8E799FF3A8696A6F
                           1B2656E631B1E40183C08FDA53FA4A8F85A05693944AE179A1339D002D15CABD
                           810090EC722EF5DEF9965A371D415D624B68A2707CAD97BCDD1785AF97E258F3
                           3DF56A031AA0356D8E8D5EBCADC74E071636C6B110ACE5CC9B90DFEACAE640FF
                           1BB0F1FE5DB4EFF7A95F060733F5']}}

Fig. 2 Mapping: WebAuthn authenticatorMakeCredential to and from CTAP1/U2F Registration Messages.

### 7.2 Using the CTAP2 authenticatorGetAssertion Command with CTAP1/U2F authenticators

Platform follows the following procedure (Fig: Mapping: WebAuthn authenticatorGetAssertion to and from CTAP1/U2F Authentication Messages) :

1. Platform tries to get information about the authenticator by sending authenticatorGetInfo command as specified in CTAP2 protocol overview.
   - CTAP1/U2F authenticator returns a command error or improperly formatted CBOR response. For any failure, platform may fall back to CTAP1/U2F protocol.
2. Map CTAP2 authenticatorGetAssertion request to U2F_AUTHENTICATE request:
   - Platform verifies that CTAP2 request does not have any parameters that CTAP1/U2F authenticators cannot fulfill:
     - All of the below conditions must be true for the platform to proceed to next step. If any of the below conditions is not true, platform errors out with CTAP2_ERR_OPTION_NOT_SUPPORTED.
       - Options must not include "uv" set to true.
       - allowList must have at least one credential.
   - If allowList has more than one credential, platform has to loop over the list and send individual different U2F_AUTHENTICATE commands to the authenticator. For each credential in credential list, map CTAP2 authenticatorGetAssertion request to U2F_AUTHENTICATE as below:
     - Let `controlByte` be a byte initialized as follows:
       - For USB, set it to 0x07 (check-only). This should prevent call getting blocked on waiting for user input. If response returns success, then call again setting the enforce-user-presence-and-sign.
       - For NFC, set it to 0x03 (enforce-user-presence-and-sign). The tap has already provided the presence and won’t block.
     - Use clientDataHash parameter of CTAP2 request as CTAP1/U2F challenge parameter (32 bytes).
     - Let `rpIdHash` be a byte array of size 32 initialized with SHA-256 hash of `rp.id` parameter as CTAP1/U2F application parameter (32 bytes).
     - Let `credentialID` is the byte array initialized with the id for this PublicKeyCredentialDescriptor.
     - Let `keyHandleLength` be a byte initialized with length of `credentialID` byte array.
     - Let `u2fAuthenticateRequest` be a byte array with the following structure:

       Length (in bytes) | Description | Value
       --- | --- | ---
       1 | Control Byte | Initialized with `controlByte`'s value.
       32 | Challenge parameter | Initialized with clientDataHash parameter bytes.
       32 | Application parameter | Initialized with `rpIdHash` bytes.
       1 | Key handle length | Initialized with `keyHandleLength`'s value.
       `keyHandleLength` | Key handle | Initialized with `credentialID` bytes.

3. Send `u2fAuthenticateRequest` to the authenticator.
4. Map the U2F authentication response message (see the "Authentication Response Message: Success" section of [U2FRawMsgs]) to a CTAP2 authenticatorGetAssertion response message:
   - Generate `authenticatorData` from the U2F authentication response message received from the authenticator:
     - Let `flags` be a byte whose zeroth bit (bit 0, UP) is set to 1 if CTAP1/U2F response user presence byte is set to 1, and all other bits are zero (bit zero is the least significant bit). See also Authenticator Data section of [WebAuthN].
     - Let `signCount` be a 4-byte unsigned integer initialized with CTAP1/U2F response counter field.
     - Let `authenticatorData` is a byte array of following structure:

       Length (in bytes) | Description | Value
       --- | --- | ---
       32 | SHA-256 hash of the `rp.id`. | Initialized with `rpIdHash` bytes.
       1 | Flags | Initialized with `flags`' value.
       4 | Signature counter (signCount) | Initialized with `signCount` bytes.

   - Let `authenticatorGetAssertionResponse` be a CBOR map with the following keys whose values are as follows:
     - Set 0x01 with the credential from allowList that whose response succeeded.
     - Set 0x02 with `authenticatorData` bytes.
     - Set 0x03 with signature field from CTAP1/U2F authentication response message.

Example 7
Sample CTAP2 authenticatorGetAssertion Request (CBOR):

    {1: "acme.com",
     2: h'687134968222EC17202E42505F8ED2B16AE22F16BB05B88C25DB9E602645F141',
     3: [{"type": "public-key",
          "id": h'3EBD89BF77EC509755EE9C2635EFAAAC7B2B9C5CEF1736C3717DA48534C8C6B6
                  54D7FF945F50B5CC4E78055BDD396B64F78DA2C5F96200CCD415CD08FE420038'}],
     5: {"up": true}}

CTAP1/U2F Request from above CTAP2 authenticatorGetAssertion request

    687134968222EC17202E42505F8ED2B16AE22F16BB05B88C25DB9E602645F141    # clientdatahash
    1194228DA8FDBDEEFD261BD7B6595CFD70A50D70C6407BCF013DE96D4EFB17DE    # rpidhash
    40                                                                  # Key Handle Length (1 Byte)
    3EBD89BF77EC509755EE9C2635EFAAAC7B2B9C5CEF1736C3717DA48534C8C6B6    # Key Handle (Key Handle Length Bytes)
    54D7FF945F50B5CC4E78055BDD396B64F78DA2C5F96200CCD415CD08FE420038    # ...

Sample CTAP1/U2F Response from the device

    01                                                                  # User Presence (1 Byte)
    0000003B                                                            # Sign Count (4 Bytes)
    304402207BDE0A52AC1F4C8B27E003A370CD66A4C7118DD22D5447835F45B99C    # Signature (variable Length)
    68423FF702203C517B47877F85782DE10086A783D1E7DF4E3639E771F5F6AFA3    # ...
    5AAD5373858E                                                        # ...

Authenticator Data from CTAP1/U2F Response

    1194228DA8FDBDEEFD261BD7B6595CFD70A50D70C6407BCF013DE96D4EFB17DE    # rpidhash
    01                                                                  # User Presence (1 Byte)
    0000003B                                                            # Sign Count (4 Bytes)

Mapped CTAP2 authenticatorGetAssertion response(CBOR)

    {1: {"type": "public-key",
         "id": h'3EBD89BF77EC509755EE9C2635EFAAAC7B2B9C5CEF1736C3717DA48534C8C6B6
                 54D7FF945F50B5CC4E78055BDD396B64F78DA2C5F96200CCD415CD08FE420038'},
     2: h'1194228DA8FDBDEEFD261BD7B6595CFD70A50D70C6407BCF013DE96D4EFB17DE
          010000003B',
     3: h'304402207BDE0A52AC1F4C8B27E003A370CD66A4C7118DD22D5447835F45B99C
          68423FF702203C517B47877F85782DE10086A783D1E7DF4E3639E771F5F6AFA3
          5AAD5373858E'}

Fig. 3 Mapping: WebAuthn authenticatorGetAssertion to and from CTAP1/U2F Authentication Messages.

## 8. Transport-specific Bindings

### 8.1 USB

#### 8.1.1 Design rationale

CTAP messages are framed for USB transport using the HID (Human Interface Device) protocol. We henceforth refer to the protocol as CTAPHID. The CTAPHID protocol is designed with the following design objectives in mind

- Driver-less installation on all major host platforms
- Multi-application support with concurrent application access without the need for serialization and centralized dispatching.
- Fixed latency response and low protocol overhead
- Scalable method for CTAPHID device discovery

Since HID data is sent as interrupt packets and multiple applications may access the HID stack at once, a non-trivial level of complexity has to be added to handle this.

#### 8.1.2 Protocol structure and data framing

The CTAP protocol is designed to be concurrent and state-less in such a way that each performed function is not dependent on previous actions. However, there has to be some form of "atomicity" that varies between the characteristics of the underlying transport protocol, which for the CTAPHID protocol introduces the following terminology:

- Transaction
- Message
- Packet

A transaction is the highest level of aggregated functionality, which in turn consists of a request, followed by a response message. Once a request has been initiated, the transaction has to be entirely completed before a second transaction can take place and a response is never sent without a previous request. Transactions exist only at the highest CTAP protocol layer.

Request and response messages are in turn divided into individual fragments, known as packets. The packet is the smallest form of protocol data unit, which in the case of CTAPHID are mapped into HID reports.

#### 8.1.3 Concurrency and channels

Additional logic and overhead is required to allow a CTAPHID device to deal with multiple "clients", i.e. multiple applications accessing the single resource through the HID stack. Each client communicates with a CTAPHID device through a logical channel, where each application uses a unique 32-bit channel identifier for routing and arbitration purposes.

A channel identifier is allocated by the FIDO authenticator device to ensure its system-wide uniqueness. The actual algorithm for generation of channel identifiers is vendor specific and not defined by this specification.

Channel ID 0 is reserved and `0xffffffff` is reserved for broadcast commands, i.e. at the time of channel allocation.

#### 8.1.4 Message and packet structure

Packets are one of two types, initialization packets and continuation packets. As the name suggests, the first packet sent in a message is an initialization packet, which also becomes the start of a transaction. If the entire message does not fit into one packet (including the CTAPHID protocol overhead), one or more continuation packets have to be sent in strict ascending order to complete the message transfer.

A message sent from a host to a device is known as a request and a message sent from a device back to the host is known as a response. A request always triggers a response and response messages are never sent ad-hoc, i.e. without a prior request message. However, a keep-alive message can be sent between a request and a response message.

The request and response messages have an identical structure. A transaction is started with the initialization packet of the request message and ends with the last packet of the response message.

Packets are always fixed size (defined by the endpoint and HID report descriptors) and although all bytes may not be needed in a particular packet, the full size always has to be sent. Unused bytes SHOULD be set to zero.

An initialization packet is defined as

Offset | Length | Mnemonic | Description
--- | --- | --- | ---
0 | 4 | CID | Channel identifier
4 | 1 | CMD | Command identifier (bit 7 always set)
5 | 1 | BCNTH | High part of payload length
6 | 1 | BCNTL | Low part of payload length
7 | (s - 7) | DATA | Payload data (s is equal to the fixed packet size)

The command byte has always the highest bit set to distinguish it from a continuation packet, which is described below.

A continuation packet is defined as

Offset | Length | Mnemonic | Description
--- | --- | --- | ---
0 | 4 | CID | Channel identifier
4 | 1 | SEQ | Packet sequence 0x00..0x7f (bit 7 always cleared)
5 | (s - 5) | DATA | Payload data (s is equal to the fixed packet size)

With this approach, a message with a payload less or equal to (s - 7) may be sent as one packet. A larger message is then divided into one or more continuation packets, starting with sequence number 0, which then increments by one to a maximum of 127.

With a packet size of 64 bytes (max for full-speed devices), this means that the maximum message payload length is 64 - 7 + 128 * (64 - 5) = 7609 bytes.

#### 8.1.5 Arbitration

In order to handle multiple channels and clients concurrency, the CTAPHID protocol has to maintain certain internal states, block conflicting requests and maintain protocol integrity. The protocol relies on each client application (channel) behaves politely, i.e. does not actively act to destroy for other channels. With this said, a malign or malfunctioning application can cause issues for other channels. Expected errors and potentially stalling applications should however, be handled properly.

##### 8.1.5.1 Transaction atomicity, idle and busy states.

A transaction always consists of three stages:

1. A message is sent from the host to the device
2. The device processes the message
3. A response is sent back from the device to the host

The protocol is built on the assumption that a plurality of concurrent applications may try ad-hoc to perform transactions at any time, with each transaction being atomic, i.e. it cannot be interrupted by another application once started.

The application channel that manages to get through the first initialization packet when the device is in idle state will keep the device locked for other channels until the last packet of the response message has been received. The device then returns to idle state, ready to perform another transaction for the same or a different channel. Between two transactions, no state is maintained in the device and a host application must assume that any other process may execute other transactions at any time.

If an application tries to access the device from a different channel while the device is busy with a transaction, that request will immediately fail with a busy-error message sent to the requesting channel.

##### 8.1.5.2 Transaction timeout

A transaction has to be completed within a specified period of time to prevent a stalling application to cause the device to be completely locked out for access by other applications. If for example an application sends an initialization packet that signals that continuation packets will follow and that application crashes, the device will back out that pending channel request and return to an idle state.

##### 8.1.5.3 Transaction abort and re-synchronization

If an application for any reason "gets lost", gets an unexpected response or error, it may at any time issue an abort-and-resynchronize command. If the device detects an INIT command during a transaction that has the same channel id as the active transaction, the transaction is aborted (if possible) and all buffered data flushed (if any). The device then returns to idle state to become ready for a new transaction.

##### 8.1.5.4 Packet sequencing

The device keeps track of packets arriving in correct and ascending order and that no expected packets are missing. The device will continue to assemble a message until all parts of it has been received or that the transaction times out. Spurious continuation packets appearing without a prior initialization packet will be ignored.

#### 8.1.6 Channel locking

In order to deal with aggregated transactions that may not be interrupted, such as tunneling of vendor-specific commands, a channel lock command may be implemented. By sending a channel lock command, the device prevents other channels from communicating with the device until the channel lock has timed out or been explicitly unlocked by the application.

This feature is optional and has not to be considered by general CTAP HID applications.

#### 8.1.7 Protocol version and compatibility

The CTAPHID protocol is designed to be extensible, yet maintaining backwards compatibility to the extent it is applicable. This means that a CTAPHID host shall support any version of a device with the command set available in that particular version.

#### 8.1.8 HID device implementation

This description assumes knowledge of the USB and HID specifications and is intended to provide the basics for implementing a CTAPHID device. There are several ways to implement USB devices and reviewing these different methods is beyond the scope of this document. This specification targets the interface part, where a device is regarded as either a single or multiple interface (composite) device.

The description further assumes (but is not limited to) a full-speed USB device (12 Mbit/s). Although not excluded per se, USB low-speed devices are not practical to use given the 8-byte report size limitation together with the protocol overhead.

##### 8.1.8.1 Interface and endpoint descriptors

The device implements two endpoints (except the control endpoint 0), one for IN and one for OUT transfers. The packet size is vendor defined, but the reference implementation assumes a full-speed device with two 64-byte endpoints.

Interface Descriptor

Mnemonic | Value | Description
--- | --- | ---
bNumEndpoints | 2 | One IN and one OUT endpoint
bInterfaceClass | 0x03 | HID
bInterfaceSubClass | 0x00 | No interface subclass
bInterfaceProtocol | 0x00 | No interface protocol

Endpoint 1 descriptor

Mnemonic | Value | Description
--- | --- | ---
bmAttributes | 0x03 | Interrupt transfer
bEndpointAdresss | 0x01 | 1, OUT
bMaxPacketSize | 64 | 64-byte packet max
bInterval | 5 | Poll every 5 millisecond

Endpoint 2 descriptor

Mnemonic | Value | Description
--- | --- | ---
bmAttributes | 0x03 | Interrupt transfer
bEndpointAdresss | 0x81 | 1, IN
bMaxPacketSize | 64 | 64-byte packet max
bInterval | 5 | Poll every 5 millisecond

The actual endpoint order, intervals, endpoint numbers and endpoint packet size may be defined freely by the vendor and the host application is responsible for querying these values and handle these accordingly. For the sake of clarity, the values listed above are used in the following examples.

##### 8.1.8.2 HID report descriptor and device discovery

A HID report descriptor is required for all HID devices, even though the reports and their interpretation (scope, range, etc.) makes very little sense from an operating system perspective. The CTAPHID just provides two "raw" reports, which basically map directly to the IN and OUT endpoints. However, the HID report descriptor has an important purpose in CTAPHID, as it is used for device discovery.

For the sake of clarity, a bit of high-level C-style abstraction is provided

Example 8

    // HID report descriptor

    const uint8_t HID_ReportDescriptor[] = {
      HID_UsagePage ( FIDO_USAGE_PAGE ),
      HID_Usage ( FIDO_USAGE_CTAPHID ),
      HID_Collection ( HID_Application ),
      HID_Usage ( FIDO_USAGE_DATA_IN ),
      HID_LogicalMin ( 0 ),
      HID_LogicalMaxS ( 0xff ),
      HID_ReportSize ( 8 ),
      HID_ReportCount ( HID_INPUT_REPORT_BYTES ),
      HID_Input ( HID_Data | HID_Absolute | HID_Variable ),
      HID_Usage ( FIDO_USAGE_DATA_OUT ),
      HID_LogicalMin ( 0 ),
      HID_LogicalMaxS ( 0xff ),
      HID_ReportSize ( 8 ),
      HID_ReportCount ( HID_OUTPUT_REPORT_BYTES ),
      HID_Output ( HID_Data | HID_Absolute | HID_Variable ),
    HID_EndCollection
    };

A unique Usage Page is defined (0xF1D0) for the FIDO alliance and under this realm, a CTAPHID Usage is defined as well (0x01). During CTAPHID device discovery, all HID devices present in the system are examined and devices that match this usage pages and usage are then considered to be CTAPHID devices.

The length values specified by the `HID_INPUT_REPORT_BYTES` and the `HID_OUTPUT_REPORT_BYTES` should typically match the respective endpoint sizes defined in the endpoint descriptors.

#### 8.1.9 CTAPHID commands

The CTAPHID protocol implements the following commands.

##### 8.1.9.1 Mandatory commands

The following list describes the minimum set of commands required by an CTAPHID device. Optional and vendor-specific commands may be implemented as described in respective sections of this document.

###### 8.1.9.1.1 CTAPHID_MSG (0x03)

This command sends an encapsulated CTAP1/U2F message to the device. The semantics of the data message is defined in the U2F Raw Message Format encoding specification. Please note that keep-alive messages MAY be sent from the device to the client before the response message is returned.

Request

CMD | CTAPHID_MSG
BCNT | 1..(n + 1)
DATA | U2F command byte
DATA + 1 | n bytes of data

Response at success

CMD | CTAPHID_MSG
BCNT | 1..(n + 1)
DATA | U2F status code
DATA + 1 | n bytes of data

###### 8.1.9.1.2 CTAPHID_CBOR (0x10)

This command sends an encapsulated CTAP CBOR encoded message. The semantics of the data message is defined in the CTAP Message encoding specification.

Request

CMD | CTAPHID_CBOR
BCNT | 1..(n + 1)
DATA | CTAP command byte
DATA + 1 | n bytes of CBOR encoded data

Response at success

CMD | CTAPHID_MSG
BCNT | 1..(n + 1)
DATA | CTAP status code
DATA + 1 | n bytes of CBOR encoded data

###### 8.1.9.1.3 CTAPHID_INIT (0x06)

This command has two functions.

If sent on an allocated CID, it synchronizes a channel, discarding the current transaction, buffers and state as quickly as possible. It will then be ready for a new transaction. The device then responds with the CID of the channel it received the INIT on, using that channel.

If sent on the broadcast CID, it requests the device to allocate a unique 32-bit channel identifier (CID) that can be used by the requesting application during its lifetime. The requesting application generates a nonce that is used to match the response. When the response is received, the application compares the sent nonce with the received one. After a positive match, the application stores the received channel id and uses that for subsequent transactions.

To allocate a new channel, the requesting application shall use the broadcast channel CTAPHID_BROADCAST_CID (0xFFFFFFFF). The device then responds with the newly allocated channel in the response, using the broadcast channel.

Request

CMD | CTAPHID_INIT
BCNT | 8
DATA | 8-byte nonce

Response at success

CMD | CTAPHID_INIT
BCNT | 17 (see note below)
DATA | 8-byte nonce
DATA+8 | 4-byte channel ID
DATA+12 | CTAPHID protocol version identifier
DATA+13 | Major device version number
DATA+14 | Minor device version number
DATA+15 | Build device version number
DATA+16 | Capabilities flags

The protocol version identifies the protocol version implemented by the device. An CTAPHID host shall accept a response size that is longer than the anticipated size to allow for future extensions of the protocol, yet maintaining backwards compatibility. Future versions will maintain the response structure to this current version, but additional fields may be added.

The meaning and interpretation of the version number is vendor defined.

The following device capabilities flags are defined. Unused values are reserved for future use and must be set to zero by device vendors.

CAPABILITY_WINK | If set to 1, authenticator implements CTAPHID_WINK function
CAPABILITY_CBOR | If set to 1, authenticator implements CTAPHID_CBOR function
CAPABILITY_NMSG | If set to 1, authenticator DOES NOT implement CTAPHID_MSG function

###### 8.1.9.1.4 CTAPHID_PING (0x01)

Sends a transaction to the device, which immediately echoes the same data back. This command is defined to be a uniform function for debugging, latency and performance measurements.

Request

CMD | CTAPHID_PING
BCNT | 0..n
DATA | n bytes

Response at success

CMD | CTAPHID_PING
BCNT | n
DATA | N bytes

###### 8.1.9.1.5 CTAPHID_CANCEL (0x11)

Cancel any outstanding requests on this CID.

Request

CMD | CTAPHID_CANCEL
BCNT | 0

Response at success

CMD | CTAPHID_CANCEL
BCNT | 0

###### 8.1.9.1.6 CTAPHID_ERROR (0x3F)

This command code is used in response messages only.

CMD | CTAPHID_ERROR
BCNT | 1
DATA | Error code

The following error codes are defined

ERR_INVALID_CMD | The command in the request is invalid
ERR_INVALID_PAR | The parameter(s) in the request is invalid
ERR_INVALID_LEN | The length field (BCNT) is invalid for the request
ERR_INVALID_SEQ | The sequence does not match expected value
ERR_MSG_TIMEOUT | The message has timed out
ERR_CHANNEL_BUSY | The device is busy for the requesting channel

###### 8.1.9.1.7 CTAPHID_KEEPALIVE (0x3B)

This command code is sent while processing a CTAPHID_MSG. It should be sent at least every 100ms and whenever the status changes.

CMD | CTAPHID_KEEPALIVE
BCNT | 1
DATA | Status code

The following status codes are defined

STATUS_PROCESSING | 1 | The authenticator is still processing the current request.
STATUS_UPNEEDED | 2 | The authenticator is waiting for user presence.

##### 8.1.9.2 Optional commands

The following commands are defined by this specification but are optional and does not have to be implemented.

###### 8.1.9.2.1 CTAPHID_WINK (0x08)

The wink command performs a vendor-defined action that provides some visual or audible identification a particular authenticator device. A typical implementation will do a short burst of flashes with a LED or something similar. This is useful when more than one device is attached to a computer and there is confusion which device is paired with which connection.

Request

CMD | CTAPHID_WINK
BCNT | 0
DATA | N/A

Response at success

CMD | CTAPHID_WINK
BCNT | 0
DATA | N/A

###### 8.1.9.2.2 CTAPHID_LOCK (0x04)

The lock command places an exclusive lock for one channel to communicate with the device. As long as the lock is active, any other channel trying to send a message will fail. In order to prevent a stalling or crashing application to lock the device indefinitely, a lock time up to 10 seconds may be set. An application requiring a longer lock has to send repeating lock commands to maintain the lock.

Request

CMD | CTAPHID_LOCK
BCNT | 1
DATA | Lock time in seconds 0..10. A value of 0 immediately releases the lock

Response at success

CMD | CTAPHID_LOCK
BCNT | 0
DATA | N/A

##### 8.1.9.3 Vendor specific commands

A CTAPHID may implement additional vendor specific commands that are not defined in this specification, yet being CTAPHID compliant. Such commands, if implemented must have a command in the range between CTAPHID_VENDOR_FIRST (0x40) and CTAPHID_VENDOR_LAST (0x7F).

### 8.2 ISO7816, ISO14443 and Near Field Communication (NFC)

#### 8.2.1 Conformance

Please refer to [ISOIEC-7816-4-2013] for APDU definition.

#### 8.2.2 Protocol

The general protocol between a FIDO2 client and an authenticator over ISO7816/ISO14443 is as follows:

1. Client sends an applet selection command
2. Authenticator replies with success if the applet is present
3. Client sends a command for an operation
4. Authenticator replies with response data or error

#### 8.2.3 Applet selection

A successful Select allows the client to know that the applet is present and active. A client SHALL send a Select to the authenticator before any other command.

The FIDO2 AID consists of the following fields:

Field | Value
--- | ---
RID | 0xA000000647
AC | 0x2f
AX | 0x0001

The command to select the FIDO applet is:

CLA | INS | P1 | P2 | Lc | Data In | Le
--- | --- | --- | --- | --- | --- | ---
0x00 | 0xA4 | 0x04 | 0x0C | 0x08 | AID | TBD (version string length)

In response to the applet selection command, the FIDO authenticator replies with its version information string in the successful response.

Given legacy support for CTAP1/U2F, the client must determine the capabilities of the device at the selection stage.

- If the authenticator implements CTAP1/U2F, the version information SHALL be the string U2F_V2 to maintain backwards-compatibility with CTAP1/U2F-only clients.
- If the authenticator ONLY implements CTAP2, the device SHALL respond with data that is NOT U2F_V2.
- If the authenticator implements both CTAP1/U2F and CTAP2, the version information SHALL be the string U2F_V2 to maintain backwards-compatibility with CTAP1/U2F-only clients. CTAP2-aware clients may then issue a CTAP authenticatorGetInfo command to determine if the device supports CTAP2 or not.

#### 8.2.4 Framing

Conceptually, framing defines an encapsulation of FIDO2 commands. In NFC, this encapsulation is done in an APDU following [ISOIEC-7816-4-2013]. Fragmentation, if needed, is discussed in the following paragraph.

##### 8.2.4.1 Commands

Commands SHALL have the following format:

CLA | INS | P1 | P2 | Data In | Le
--- | --- | --- | --- | --- | ---
0x80 | 0x10 | 0x00 | 0x00 | CTAP Command Byte || CBOR Encoded Data | Variable

##### 8.2.4.2 Response

Response SHALL have the following format in case of success:

Case | Data | Status word
--- | --- | ---
Success | Response data | "9000" - Success
Status update | Status data | "9100" - OK When receiving this, CTAP will immediately issue an NFCCTAP_GETREPONSE command unless a cancel was issued. CTAP will provide the status data to the higher layers.
Errors |  | See [ISOIEC-7816-4-2013]

#### 8.2.5 Fragmentation

APDU command may hold up to 255 or 65535 bytes of data using short or extended length encoding respectively. APDU response may hold up to 256 or 65536 bytes of data using short or extended length encoding respectively.

Some requests may not fit into a short APDU command, or the expected response may not fit in a short APDU response. For this reason, FIDO2 client MAY encode APDU command in the following way:

- The request may be encoded using extended length APDU encoding.
- The request may be encoded using short APDU encoding. If the request does not fit a short APDU command, the client MUST use ISO 7816-4 APDU chaining.

Some responses may not fit into a short APDU response. For this reason, FIDO2 authenticators MUST respond in the following way:

- If the request was encoded using extended length APDU encoding, the authenticator MUST respond using the extended length APDU response format.
- If the request was encoded using short APDU encoding, the authenticator MUST respond using ISO 7816-4 APDU chaining.

#### 8.2.6 Commands

##### 8.2.6.1 NFCCTAP_MSG (0x10)

The NFCCTAP_MSG command send a CTAP message to the authenticator. This command SHALL return as soon as processing is done. If the operation was not completed, it MAY return a 0x9100 result to trigger NFCCTAP_GETRESPONSE functionality if the client indicated support by setting the relevant bit in P1.

The values for P1 for the NFCCTAP_MSG command are:

P1 Bits | Meaning
--- | ---
0x80 | The client supports NFCCTAP_GETRESPONSE
0x7F | RFU, must be 0x00

Values for P2 are all RFU and MUST be set to 0.

##### NFCCTAP_GETRESPONSE (0x11)

The NFCCTAP_GETRESPONSE command is issued up to receiving 0x9100 unless a cancel was issued. This command SHALL return a 0x9100 result with a status indication if it has a status update, the reply to the request with a 0x9000 result code to indicate success or an error value.

All values for P1 and P2 are RFU and MUST be set to 0x00.

#### 8.2.7 Bluetooth Smart / Bluetooth Low Energy Technology

##### 8.2.7.1 Conformance

Authenticator and Client devices using Bluetooth Low Energy Technology SHALL conform to Bluetooth Core Specification 4.0 or later [BTCORE]

Bluetooth SIG specified UUID values SHALL be found on the Assigned Numbers website [BTASSNUM]

##### 8.2.7.2 Pairing

Bluetooth Low Energy Technology is a long-range wireless protocol and thus has several implications for privacy, security, and overall user-experience. Because it is wireless, Bluetooth Low Energy Technology may be subject to monitoring, injection, and other network-level attacks.

For these reasons, Clients and Authenticators MUST create and use a long-term link key (LTK) and SHALL encrypt all communications. Authenticator MUST never use short term keys.

Because Bluetooth Low Energy Technology has poor ranging (i.e., there is no good indication of proximity), it may not be clear to a FIDO Client with which Bluetooth Low Energy Technology Authenticator it should communicate. Pairing is the only mechanism defined in this protocol to ensure that FIDO Clients are interacting with the expected Bluetooth Low Energy Technology Authenticator. As a result, Authenticator manufacturers SHOULD instruct users to avoid performing Bluetooth pairing in a public space such as a cafe, shop or train station.

One disadvantage of using standard Bluetooth pairing is that the pairing is "system-wide" on most operating systems. That is, if an Authenticator is paired to a FIDO Client which resides on an operating system where Bluetooth pairing is "system-wide", then any application on that device might be able to interact with an Authenticator. This issue is discussed further in Implementation Considerations.

##### 8.2.7.3 Link Security

For Bluetooth Low Energy Technology connections, the Authenticator SHALL enforce `Security Mode 1, Level 2` (unauthenticated pairing with encryption) or `Security Mode 1, Level 3` (authenticated pairing with encryption) before any FIDO messages are exchanged.

##### 8.2.7.4 Framing

Conceptually, framing defines an encapsulation of FIDO raw messages responsible for correct transmission of a single request and its response by the transport layer.

All requests and their responses are conceptually written as a single frame. The format of the requests and responses is given first as complete frames. Fragmentation is discussed next for each type of transport layer.

###### 8.2.7.4.1 Request from Client to Authenticator

Request frames must have the following format

Offset | Length | Mnemonic | Description
--- | --- | --- | ---
0 | 1 | `CMD` | Command identifier
1 | 1 | `HLEN` | High part of data length
2 | 1 | `LLEN` | Low part of data length
3 | s | `DATA` | Data (s is equal to the length)

Supported commands are `PING`, `MSG` and `CANCEL`. The constant values for them are described below.

The `CANCEL` command cancels any outstanding `MSG` commands.

The data format for the `MSG` command is defined in the Message Encoding section of this document.

###### 8.2.7.4.2 Response from Authenticator to Client

Response frames must have the following format, which share a similar format to the request frames:

Offset | Length | Mnemonic | Description
--- | --- | --- | ---
0 | 1 | `STAT` | Response status
1 | 1 | `HLEN` | High part of data length
2 | 1 | `LLEN` | Low part of data length
3 | s | `DATA` | Data (s is equal to the length)

When the status byte in the response is the same as the command byte in the request, the response is a successful response. The value `ERROR` indicates an error, and the response data contains an error code as a variable-length, big-endian integer. The constant value for `ERROR` is described below.

Note that the errors sent in this response are errors at the encapsulation layer, e.g., indicating an incorrectly formatted request, or possibly an error communicating with the Authenticator’s FIDO message processing layer. Errors reported by the FIDO message processing layer itself are considered a success from the encapsulation layer’s point of view, and are reported as a complete `MSG` response.

Data format is defined in the Message Encoding section of this document.

###### 8.2.7.4.3 Command, Status, and Error constants

The COMMAND constants and values are:

Constant | Value
--- | ---
`PING` | 0x81
`KEEPALIVE` | 0x82
`MSG` | 0x83
`CANCEL` | 0xbe
`ERROR` | 0xbf

The KEEPALIVE command contains a single byte with the following possible values:

Status Constant | Value
--- | ---
`PROCESSING` | 0x01
`UP_NEEDED` | 0x02
`RFU` | 0x00, 0x03-0xFF

The ERROR constants and values are:

Error Constant | Value | Meaning
--- | --- | ---
`ERR_INVALID_CMD` | 0x01 | The command in the request is unknown/invalid
`ERR_INVALID_PAR` | 0x02 | The parameter(s) of the command is/are invalid or missing
`ERR_INVALID_LEN` | 0x03 | The length of the request is invalid
`ERR_INVALID_SEQ` | 0x04 | The sequence number is invalid
`ERR_REQ_TIMEOUT` | 0x05 | The request timed out
`NA` | 0x06 | Value reserved (HID)
`NA` | 0x0a | Value reserved (HID)
`NA` | 0x0b | Value reserved (HID)
`ERR_OTHER` | 0x7f | Other, unspecified error

##### 8.2.7.5 GATT Service Description

This profile defines two roles: FIDO Authenticator and FIDO Client.

- The FIDO Client shall be a GATT Client
- The FIDO Authenticator shall be a GATT Server

The following figure illustrates the mandatory services and characteristics that SHALL be offered by a FIDO Authenticator as part of its GATT server:

Fig. 4 Mandatory GATT services and characteristics that MUST be offered by a FIDO Authenticator. Note that the Generic Access Service ([BTGAS] is not present as it is already mandatory for any Bluetooth Low Energy Technology compliant device.

The table below summarizes additional GATT sub-procedure requirements for a FIDO Authenticator (GATT Server) beyond those required by all GATT Servers.

GATT Sub-Procedure | Requirements
--- | ---
Write Characteristic Value | `Mandatory`
Notifications | `Mandatory`
Read Characteristic Descriptors | `Mandatory`
Write Characteristic Descriptors | `Mandatory`

The table below summarizes additional GATT sub-procedure requirements for a FIDO Client (GATT Client) beyond those required by all GATT Clients.

GATT Sub-Procedure | Requirements
--- | ---
Discover All Primary Services | `(*)`
Discover Primary Services by Service UUID | `(*)`
Discover All Characteristics of a Service | `(**)`
Discover Characteristics by UUID | `(**)`
Discover All Characteristic Descriptors | `Mandatory`
Read Characteristic Value | `Mandatory`
Write Characteristic Value | `Mandatory`
Notification | `Mandatory`
Read Characteristic Descriptors | `Mandatory`
Write Characteristic Descriptors | `Mandatory`

(*): Mandatory to support at least one of these sub-procedures.

(**): Mandatory to support at least one of these sub-procedures.

Other GATT sub-procedures may be used if supported by both client and server.

Specifics of each service are explained below. In the following descriptions: all values are big-endian coded, all strings are in UTF-8 encoding, and any characteristics not mentioned explicitly are optional.

###### 8.2.7.5.1 FIDO Service

An Authenticator SHALL implement the FIDO Service described below. The UUID for the FIDO GATT service is `0xFFFD`, it shall be declared as a Primary Service. The service contains the following characteristics:

Characteristic Name | Mnemonic | Property | Length | UUID
--- | --- | --- | --- | ---
FIDO Control Point | `fidoControlPoint` | Write | Defined by Vendor (20-512 bytes) | F1D0FFF1-DEAA-ECEE-B42F-C9BA7ED623BB
FIDO Status | `fidoStatus` | Notify | N/A | F1D0FFF2-DEAA-ECEE-B42F-C9BA7ED623BB
FIDO Control Point Length | `fidoControlPointLength` | Read | 2 bytes | F1D0FFF3-DEAA-ECEE-B42F-C9BA7ED623BB
FIDO Service Revision Bitfield | `fidoServiceRevisionBitfield` | Read/Write | Defined by Vendor (1+ bytes) | F1D0FFF4-DEAA-ECEE-B42F-C9BA7ED623BB
FIDO Service Revision | `fidoServiceRevision` | Read | Defined by Vendor (20-512 bytes) | 0x2A28

`fidoControlPoint` is a write-only command buffer.

`fidoStatus` is a notify-only response attribute. The Authenticator will send a series of notifications on this attribute with a maximum length of (ATT_MTU-3) using the response frames defined above. This mechanism is used because this results in a faster transfer speed compared to a notify-read combination.

`fidoControlPointLength` defines the maximum size in bytes of a single write request to `fidoControlPoint`. This value SHALL be between 20 and 512.

`fidoServiceRevision` is a deprecated field that is only relevant to U2F 1.0 support. It defines the revision of the U2F Service. The value is a UTF-8 string. For version 1.0 of the specification, the value `fidoServiceRevision` SHALL be `1.0` or in raw bytes: `0x312e30`. This field SHALL be omitted if protocol version 1.0 is not supported.

The `fidoServiceRevision` Characteristic MAY include a Characteristic Presentation Format descriptor with format value 0x19, `UTF-8 String`.

`fidoServiceRevisionBitfield` defines the revision of the FIDO Service. The value is a bit field which each bit representing a version. For each version bit the value is 1 if the version is supported, 0 if it is not. The length of the bitfield is 1 or more bytes. All bytes that are 0 are omitted if all the following bytes are 0 too. The byte order is big endian. The client SHALL write a value to this characteristic with exactly 1 bit set before sending any FIDO commands unless u2fServiceRevision is present and U2F 1.0 compatibility is desired. If only U2F version 1.0 is supported, this characteristic SHALL be omitted.

Byte (left to right) | Bit | Version
--- | --- | ---
0 | 7 | U2F 1.1
0 | 6 | U2F 1.2
0 | 5 | FIDO 2.0
0 | 4-0 | Reserved

For example, a device that only supports FIDO2 Rev 1 will only have a fidoServiceRevisionBitfield characteristic of length 1 with value 0x20.

###### 8.2.7.5.2 Device Information Service

An Authenticator SHALL implement the Device Information Service [BTDIS] with the following characteristics:

- Manufacturer Name String
- Model Number String
- Firmware Revision String

All values for the Device Information Service are left to the vendors. However, vendors should not create uniquely identifiable values so that Authenticators do not become a method of tracking users.

###### 8.2.7.5.3 Generic Access Profile Service

Every Authenticator SHALL implement the Generic Access Profile Service [BTGAS] with the following characteristics:

- Device Name
- Appearance

##### 8.2.7.6 Protocol Overview

The general overview of the communication protocol follows:

1. Authenticator advertises the FIDO Service.
2. Client scans for Authenticator advertising the FIDO Service.
3. Client performs characteristic discovery on the Authenticator.
4. If not already paired, the Client and Authenticator SHALL perform BLE pairing and create a LTK. Authenticator SHALL only allow connections from previously bonded Clients without user intervention.
5. Client checks if the `fidoServiceRevisionBitfield` characteristic is present. If so, the client selects a supported version by writing a value with a single bit set.
6. Client reads the `fidoControlPointLength` characteristic.
7. Client registers for notifications on the `fidoStatus` characteristic.
8. Client writes a request (e.g., an enroll request) into the `fidoControlPoint` characteristic.
9. Authenticator evaluates the request and responds by sending notifications over `fidoStatus` characteristic.
10. The protocol completes when either:
    - The Client unregisters for notifications on the `fidoStatus` characteristic, or:
    - The connection times out and is closed by the Authenticator.

##### 8.2.7.7 Authenticator Advertising Format

When advertising, the Authenticator SHALL advertise the FIDO service UUID.

When advertising, the Authenticator MAY include the TxPower value in the advertisement (see [BTXPLAD]).

When advertising in pairing mode, the Authenticator SHALL either: (1) set the LE Limited Mode bit to zero and the LE General Discoverable bit to one OR (2) set the LE Limited Mode bit to one and the LE General Discoverable bit to zero. When advertising in non-pairing mode, the Authenticator SHALL set both the LE Limited Mode bit and the LE General Discoverable Mode bit to zero in the Advertising Data Flags.

The advertisement MAY also carry a device name which is distinctive and user-identifiable. For example, "ACME Key" would be an appropriate name, while "XJS4" would not be.

The Authenticator SHALL also implement the Generic Access Profile [BTGAP] and Device Information Service [BTDIS], both of which also provide a user-friendly name for the device that could be used by the Client.

It is not specified when or how often an Authenticator should advertise, instead that flexibility is left to manufacturers.

##### 8.2.7.8 Requests

Clients SHOULD make requests by connecting to the Authenticator and performing a write into the `fidoControlPoint` characteristic.

##### 8.2.7.9 Responses

Authenticators SHOULD respond to Clients by sending notifications on the `fidoStatus` characteristic.

Some Authenticators might alert users or prompt them to complete the test of user presence (e.g., via sound, light, vibration) Upon receiving any request, the Authenticators SHALL send KEEPALIVE commands every `kKeepAliveMillis` milliseconds until completing processing the commands. While the Authenticator is processing the request the KEEPALIVE command will contain status `PROCESSING`. If the Authenticator is waiting to complete the Test of User Presence, the KEEPALIVE command will contains status `UP_NEEDED`. While waiting to complete the Test of User Presence, the Authenticator MAY alert the user (e.g., by flashing) in order to prompt the user to complete the test of user presence. As soon the Authenticator has completed processing and confirmed user presence, it SHALL stop sending KEEPALIVE commands and send the reply.

Upon receiving a KEEPALIVE command, the Client SHALL assume the Authenticator is still processing the command; the Client SHALL not resend the command. The Authenticator SHALL continue sending KEEPALIVE messages at least every `kKeepAliveMillis` to indicate that it is still handling the request. Until a client-defined timeout occurs, the Client SHALL NOT move on to other devices when it receives a KEEPALIVE with `UP_NEEDED` status, as it knows this is a device that can satisfy its request.

##### 8.2.7.10 Framing fragmentation

A single request/response sent over Bluetooth Low Energy Technology MAY be split over multiple writes and notifications, due to the inherent limitations of Bluetooth Low Energy Technology which is not currently meant for large messages. Frames are fragmented in the following way:

A frame is divided into an initialization fragment and one or more continuation fragments.

An initialization fragment is defined as:

Offset | Length | Mnemonic | Description
--- | --- | --- | ---
0 | 1 | `CMD` | Command identifier
1 | 1 | `HLEN` | High part of data length
2 | 1 | `LLEN` | Low part of data length
3 | 0 to (maxLen - 3) | `DATA` | Data

where `maxLen` is the maximum packet size supported by the characteristic or notification.

In other words, the start of an initialization fragment is indicated by setting the high bit in the first byte. The subsequent two bytes indicate the total length of the frame, in big-endian order. The first `maxLen` - 3 bytes of data follow.

Continuation fragments are defined as:

Offset | Length | Mnemonic | Description
--- | --- | --- | ---
0 | 1 | `SEQ` | Packet sequence 0x00..0x7f (high bit always cleared)
1 | 0 to (maxLen - 1) | `DATA` | Data

where `maxLen` is the maximum packet size supported by the characteristic or notification.

In other words, continuation fragments begin with a sequence number, beginning at 0, implicitly with the high bit cleared. The sequence number must wrap around to 0 after reaching the maximum sequence number of 0x7f.

Example for sending a `PING` command with 40 bytes of data with a `maxLen` of 20 bytes:

Frame | Bytes
--- | ---
0 | `[810028] [17 bytes of data]`
1 | `[00] [19 bytes of data]`
2 | `[01] [4 bytes of data]`

Example for sending a ping command with 400 bytes of data with a `maxLen` of 512 bytes:

Frame | Bytes
--- | ---
0 | `[810190] [400 bytes of data]`

##### 8.2.7.11 Notifications

A client needs to register for notifications before it can receive them. Bluetooth Core Specification 4.0 or later [BTCORE] forces a device to remember the notification registration status over different connections [BTCCC]. Unless a client explicitly unregisters for notifications, the registration will be automatically restored when reconnecting. A client MAY therefor check the notification status upon connection and only register if notifications aren't already registered. Please note that some clients MAY disable notifications from a power management point of view (see below) and the notification registration is remembered per bond, not per client. A client MUST NOT remember the notification status in its own data storage.

##### 8.2.7.12 Implementation Considerations

###### 8.2.7.12.1 Bluetooth pairing: Client considerations

As noted in the Pairing section, a disadvantage of using standard Bluetooth pairing is that the pairing is "system-wide" on most operating systems. That is, if an Authenticator is paired to a FIDO Client which resides on an operating system where Bluetooth pairing is "system-wide", then any application on that device might be able to interact with an Authenticator. This poses both security and privacy risks to users.

While Client operating system security is partly out of FIDO's scope, further revisions of this specification MAY propose mitigations for this issue.

###### 8.2.7.12.2 Bluetooth pairing: Authenticator considerations

The method to put the Authenticator into Pairing Mode should be such that it is not easy for the user to do accidentally especially if the pairing method is Just Works. For example, the action could be pressing a physically recessed button or pressing multiple buttons. A visible or audible cue that the Authenticator is in Pairing Mode should be considered. As a counter example, a silent, long press of a single non-recessed button is not advised as some users naturally hold buttons down during regular operation.

Note that at times, Authenticators may legitimately receive communication from an unpaired device. For example, a user attempts to use an Authenticator for the first time with a new Client: he turns it on, but forgets to put the Authenticator into pairing mode. In this situation, after connecting to the Authenticator, the Client will notify the user that he needs to pair his Authenticator. The Authenticator should make it easy for the user to do so, e.g., by not requiring the user to wait for a timeout before being able to enable pairing mode.

Some Client platforms (most notably iOS) do not expose the AD Flag LE Limited and General Discoverable Mode bits to applications. For this reason, Authenticators are also strongly recommended to include the Service Data field [BTSD] in the Scan Response. The Service Data field is 3 or more octets long. This allows the Flags field to be extended while using the minimum number of octets within the data packet. All octets that are 0x00 are not transmitted as long as all other octets after that octet are also 0x00 and it is not the first octet after the service UUID. The first 2 bytes contain the FIDO Service UUID, the following bytes are flag bytes.

To help Clients show the correct UX, Authenticators can use the Service Data field to specify whether or not Authenticators will require a Passkey (PIN) during pairing.

Service Data Bit | Meaning (if set)
--- | ---
7 | Device is in pairing mode.
6 | Device requires Passkey Entry [BTPESTK].

##### 8.2.7.13 Handling command completion

It is important for low-power devices to be able to conserve power by shutting down or switching to a lower-power state when they have satisfied a Client's requests. However, the FIDO protocol makes this hard as it typically includes more than one command/response. This is especially true if a user has more than one key handle associated with an account or identity, multiple key handles may need to be tried before getting a successful outcome. Furthermore, Clients that fail to send follow-up commands in a timely fashion may cause the Authenticator to drain its battery by staying powered up anticipating more commands.

A further consideration is to ensure that a user is not confused about which command she is confirming by completing the test of user presence. That is, if a user performs the test of user presence, that action should perform exactly one operation.

We combine these considerations into the following series of recommendations:

- Upon initial connection to an Authenticator, and upon receipt of a response from an Authenticator, if a Client has more commands to issue, the Client MUST transmit the next command or fragment within `kMaxCommandTransmitDelayMillis` milliseconds.
- Upon final response from an Authenticator, if the Client decides it has no more commands to send it should indicate this by disabling notifications on the `fidoStatus` characteristic. When the notifications are disabled the Authenticator may enter a low power state or disconnect and shut down.
- Any time the Client wishes to send a FIDO message, it must have first enabled notifications on the `fidoStatus` characteristic and wait for the ATT acknowledgement to be sure the Authenticator is ready to process messages.
- Upon successful completion of a command which required a test of user presence, e.g. upon a successful authentication or registration command, the Authenticator can assume the Client is satisfied, and MAY reset its state or power down.
- Upon sending a command response that did not consume a test of user presence, the Authenticator MUST assume that the Client may wish to initiate another command, and leave the connection open until the Client closes it or until a timeout of at least `kErrorWaitMillis` elapses. Examples of command responses that do not consume user presence include failed authenticate or register commands, as well as get version responses, whether successful or not. After `kErrorWaitMillis` milliseconds have elapsed without further commands from a Client, an Authenticator MAY reset its state or power down.

Constant | Value
--- | ---
`kMaxCommandTransmitDelayMillis` | 1500 milliseconds
`kErrorWaitMillis` | 2000 milliseconds
`kKeepAliveMillis` | 500 milliseconds

##### 8.2.7.14 Data throughput

Bluetooth Low Energy Technology does not have particularly high throughput, this can cause noticeable latency to the user if request/responses are large. Some ways that implementers can reduce latency are:

- Support the maximum MTU size allowable by hardware (up to the 512-byte max from the BLE specifications).
- Make the attestation certificate as small as possible; do not include unnecessary extensions.

##### 8.2.7.15 Advertising

Though the standard does not appear to mandate it (in any way that we’ve found thus far), advertising and device discovery seems to work better when the Authenticators advertise on all 3 advertising channels and not just one.

##### 8.2.7.16 Authenticator Address Type

In order to enhance the user's privacy and specifically to guard against tracking, it is recommended that Authenticators use Resolvable Private Addresses (RPAs) instead of static addresses.

## A. References

### A.1 Normative references

[RFC2119]
  S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[RFC7049]
  C. Bormann; P. Hoffman. Concise Binary Object Representation (CBOR). October 2013. Proposed Standard. URL: https://tools.ietf.org/html/rfc7049
[SEC1V2]
  SEC1: Elliptic Curve Cryptography, Version 2.0. May 2009. URL: http://secg.org/download/aid-780/sec1-v2.pdf
[U2FRawMsgs]
  D. Balfanz. FIDO U2F Raw Message Formats v1.0. Draft. URL: https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-raw-message-formats-v1.2-ps-20170411.html
[WebAuthN]
  Dirk Balfanz; Alexei Czeskis; Jeff Hodges; J.C. Jones; Michael Jones; Akshay Kumar; Huakai Liao; Rolf Lindemann; Emil Lundberg. Web Authentication: An API for accessing Public Key Credentials Level 1. 20 March 2018. W3C Candidate Recommendation. URL: https://www.w3.org/TR/webauthn/

### A.2 Informative references

[BTASSNUM]
  Bluetooth Assigned Numbers. URL: https://www.bluetooth.org/en-us/specification/assigned-numbers
[BTCCC]
  Client Characteristic Configuration. Bluetooth Core Specification 4.0, Volume 3, Part G, Section 3.3.3.3. URL: https://www.bluetooth.com/specifications/adopted-specifications
[BTCORE]
  Bluetooth Core Specification 4.0. URL: https://www.bluetooth.com/specifications/adopted-specifications
[BTDIS]
  Device Information Service v1.1. URL: https://www.bluetooth.com/specifications/adopted-specifications
[BTGAP]
  Generic Access Profile. Bluetooth Core Specification 4.0, Volume 3, Part C, Section 12. URL: https://www.bluetooth.com/specifications/adopted-specifications
[BTGAS]
  Generic Access Profile service. Bluetooth Core Specification 4.0, Volume 3, Part C, Section 12. URL: https://developer.bluetooth.org/gatt/services/Pages/ServiceViewer.aspx?u=org.bluetooth.service.generic_access.xml
[BTPESTK]
  Passkey Entry. Bluetooth Core Specification 4.0, Volume 3, Part H, Section 2.3.5.3. URL: https://www.bluetooth.com/specifications/adopted-specifications
[BTSD]
  Bluetooth Service Data AD Type. Bluetooth Core Specification 4.0, Volume 3, Part C, Section 11. URL: https://www.bluetooth.com/specifications/adopted-specifications
[BTXPLAD]
  Bluetooth TX Power AD Type. Bluetooth Core Specification 4.0, Volume 3, Part C, Section 11. URL: https://www.bluetooth.com/specifications/adopted-specifications
[IANA-COSE-ALGS-REG]
  Jim Schaad; Göran Selander; Derek Atkins; Sean Turner. IANA CBOR Object Signing and Encryption (COSE) Algorithms Registry. URL: https://www.iana.org/assignments/cose/cose.xhtml#algorithms
[ISOIEC-7816-4-2013]
  ISO 7816-4: Identification cards – Integrated circuit cards; Part 4 : Organization, security and commands for interchange. URL:
[RFC6090]
  D. McGrew; K. Igoe; M. Salter. Fundamental Elliptic Curve Cryptography Algorithms. February 2011. Informational. URL: https://tools.ietf.org/html/rfc6090
[RFC8152]
  J. Schaad. CBOR Object Signing and Encryption (COSE). July 2017. Proposed Standard. URL: https://tools.ietf.org/html/rfc8152
[SP800-56A]
  NIST Special Publication 800-56A: Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography (Revised). March 2007 URL: https://csrc.nist.gov/publications/nistpubs/800-56A/SP800-56A_Revision1_Mar08-2007.pdf