Client to Authenticator Protocol (CTAP)

Proposed Standard,

This version:
https://fidoalliance.org/specs/fido-v2.0-ps-20190130/fido-client-to-authenticator-protocol-v2.0-ps-20190130.html
Previous Versions:
Issue Tracking:
GitHub
Editors:
(Google)
(Google)
(Yubico)
(Microsoft)
(Microsoft)
(Nok Nok Labs)
(FIDO Alliance)
(OneSpan)
Former Editors:
(Gemalto)
(Google)
(Microsoft)
(SurePassID)
Contributors:
Jeff Hodges (PayPal)

Abstract

This specification describes an application layer protocol for communication between a roaming 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.

1. Introduction

This section is not 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 a roaming authenticator (e.g., a smartphone).

In order to provide evidence of user interaction, a roaming 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 roaming 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.

1.1. Relationship to Other Specifications

This specification is part of the FIDO2 project which includes this CTAP and the [FIDOServerGuidelines] specifications, and is related to the W3C [WebAuthn] specification. This specification refers to two CTAP protocol versions:

  1. The CTAP1/U2F protocol, which is defined by the U2F Raw Messages specification [U2FRawMsgs]. CTAP1/U2F messages are recognizable by their APDU-like binary structure. CTAP1/U2F may also be referred to as CTAP 1.2 or U2F 1.2. The latter was the U2F specification version used as the basis for several portions of this specification. Authenticators implementing CTAP1/U2F are typically referred to as U2F authenticators or CTAP1 authenticators.

  2. The CTAP2 protocol, whose messages are encoded in the CTAP2 canonical CBOR encoding form. Authenticators implementing CTAP2 are referred to as CTAP2 authenticators, FIDO2 authenticators, or WebAuthn Authenticators.

Both CTAP1 and CTAP2 share the same underlying transports: USB Human Interface Device (USB HID), Near Field Communication (NFC), and Bluetooth Smart / Bluetooth Low Energy Technology (BLE).

The [U2FUsbHid], [U2FNfc], [U2FBle], and [U2FRawMsgs] specifications, specifically, are superseded by this specification.

Occasionally, the term "CTAP" may be used without clarifying whether it is referring to CTAP1 or CTAP2. In such cases, it should be understood to be referring to the entirety of this specification or portions of this specification that are not specific to either CTAP1 or CTAP2. For example, some error messages begin with the term "CTAP" without clarifying whether they are CTAP1- or CTAP2-specific because they are applicable to both CTAP protocol versions. CTAP protocol-specific error messages are prefixed with either "CTAP1" or "CTAP2" as appropriate.

Using CTAP2 with CTAP1/U2F authenticators is defined in Interoperating with CTAP1/U2F authenticators.

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", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119].

3. Protocol Structure

This protocol is specified in three parts:

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

4. Protocol Overview

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 (0x01) Byte Array Required Hash of the ClientData contextual binding specified by host. See [WebAuthn].
rp (0x02) 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 of type text string, (optionally) a human-friendly RP name of type text string, and (optionally) a URL of type text string, referencing 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. The RP name and URL are optional so that the RP can be more privacy friendly if it chooses to. For example, for authenticators with a display, RP may not want to display name/icon for single-factor scenarios.
user (0x03) 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 of type byte array, (optionally) a user name of type text string, (optionally) a user display name of type text string, and (optionally) a URL of type text string, referencing a user icon 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). The user name, display name, and URL are optional for privacy reasons for single-factor scenarios where only user presence is required. For example, in certain closed physical environments like factory floors, user presence only authenticators can satisfy RP’s productivity and security needs. In these environments, omitting user name, display name and URL makes the credential more privacy friendly. Although this information is not available without user verification, devices which support user verification but do not have it configured, can be tricked into releasing this information by configuring the user verification.
pubKeyCredParams (0x04) 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 (0x05) 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 (0x06) CBOR map of extension identifierauthenticator extension input values Optional Parameters to influence authenticator operation, as specified in [WebAuthn]. These parameters might be authenticator specific.
options (0x07) Map of authenticator options Optional Parameters to influence authenticator operation, as specified in in the table below.
pinAuth
(0x08)
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 (0x09) 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.

Note that the [WebAuthn] specification defines an abstract authenticatorMakeCredential operation, which corresponds to the operation described in this section. The parameters in the abstract [WebAuthn] authenticatorMakeCredential operation map to the above parameters as follows:

[WebAuthn] authenticatorMakeCredential operation CTAP authenticatorMakeCredential operation
hash clientDataHash
rpEntity rp
userEntity user
requireResidentKey options.rk
requireUserPresence Not present in the current version of CTAP. Authenticators are assumed to always check user presence.
requireUserVerification options.uv or pinAuth/pinProtocol
credTypesAndPubKeyAlgs pubKeyCredParams
excludeCredentialDescriptorList excludeList
extensions extensions

Note that icon values used with authenticators can employ [RFC2397] "data" URLs so that the image data is passed by value, rather than by reference. This can enable authenticators with a display but no Internet connection to display icons.

Note that a text string is a UTF-8 encoded string (CBOR major type 3).

When an authenticatorMakeCredential 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 and bound to the specified rpId, wait for user presence, then terminate this procedure and return error code CTAP2_ERR_CREDENTIAL_EXCLUDED. User presence check is required for CTAP2 authenticators before the RP gets told that the token is already registered to behave similarly to CTAP1/U2F authenticators.

  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 the options. If the option is known but not supported, terminate this procedure and return CTAP2_ERR_UNSUPPORTED_OPTION. If the option is known but not valid for this command, terminate this procedure and return CTAP2_ERR_INVALID_OPTION. Ignore any options that are not understood. Note that because this specification defines normative behaviors for them, all authenticators MUST understand the "rk", "up", and "uv" options.

  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.

  6. If pinAuth parameter is not present and clientPin been set on the authenticator, return CTAP2_ERR_PIN_REQUIRED error.

  7. If pinAuth parameter is present and the pinProtocol is not supported, return CTAP2_ERR_PIN_AUTH_INVALID.

  8. 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.

  9. Generate a new credential key pair for the algorithm specified.

  10. 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.

  11. 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 (0x01) Byte Array Required The authenticator data object.
fmt (0x02) String Required The attestation statement format identifier.
attStmt (0x03) Byte Array, 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 (0x01) String Required Relying party identifier. See [WebAuthn].
clientDataHash (0x02) Byte Array Required Hash of the serialized client data collected by the host. See [WebAuthn].
allowList (0x03) Sequence of PublicKeyCredentialDescriptors Optional A sequence of PublicKeyCredentialDescriptor structures, each denoting a credential, as specified in [WebAuthn]. If this parameter is present and has 1 or more entries, the authenticator MUST only generate an assertion using one of the denoted credentials.
extensions (0x04) CBOR map of extension identifierauthenticator extension input values Optional Parameters to influence authenticator operation. These parameters might be authenticator specific.
options (0x05) Map of authenticator options Optional Parameters to influence authenticator operation, as specified in the table below.
pinAuth
(0x06)
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 (0x07) 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.

Note that the [WebAuthn] specification defines an abstract authenticatorGetAssertion operation, which corresponds to the operation described in this section. The parameters in the abstract [WebAuthn] authenticatorGetAssertion operation map to the above parameters as follows:

[WebAuthn] authenticatorGetAssertion operation CTAP authenticatorGetAssertion operation
hash clientDataHash
rpId rpId
allowCredentialDescriptorList allowList
requireUserPresence options.up
requireUserVerification options.uv or pinAuth/pinProtocol
extensions extensions

When an authenticatorGetAssertion 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.

    • Let numberOfCredentials be the number of credentials found.

  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.

  3. If pinAuth parameter is present and the pinProtocol is not supported, return CTAP2_ERR_PIN_AUTH_INVALID.

  4. If pinAuth parameter is not present and clientPin has been set on the authenticator, set the "uv" bit to 0 in the response.

  5. If the options parameter is present, process all the options. If the option is known but not supported, terminate this procedure and return CTAP2_ERR_UNSUPPORTED_OPTION. If the option is known but not valid for this command, terminate this procedure and return CTAP2_ERR_INVALID_OPTION. Ignore any options that are not understood. Note that because this specification defines normative behaviors for them, all authenticators MUST understand the "rk", "up", and "uv" options.

  6. 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.

  7. 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_UNSUPPORTED_OPTION 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.

  8. If no credentials were located in step 1, return CTAP2_ERR_NO_CREDENTIALS.

  9. If more than one credential was located in step 1 and allowList is present and not empty, select any applicable credential and proceed to step 12. Otherwise, order the credentials by the time when they were created in reverse order. The first credential is the most recent credential that was created.

  10. If authenticator does not have a display:

    • Remember the authenticatorGetAssertion parameters.

    • Create a credential counter(credentialCounter) and set it 1. This counter signifies how many credentials are sent to the platform by the authenticator.

    • Start a timer. This is used during authenticatorGetNextAssertion command. This step is optional if transport is done over NFC.

    • Update the response to include the first credential’s publicKeyCredentialUserEntity information and numberOfCredentials. User identifiable information (name, DisplayName, icon) inside publicKeyCredentialUserEntity MUST not be returned if user verification is not done by the authenticator.

  11. 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.

  12. 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 (0x01) 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 (0x02) Byte Array Required The signed-over contextual bindings made by the authenticator, as specified in [WebAuthn].
signature (0x03) Byte Array Required The assertion signature produced by the authenticator, as specified in [WebAuthn].
user (0x04) PublicKeyCredentialUserEntity Optional PublicKeyCredentialUserEntity structure containing the user account information. User identifiable information (name, DisplayName, icon) MUST not be returned if user verification is not done by the authenticator.

U2F Devices: For U2F devices, this parameter is not returned as this user information is not present for U2F credentials.

FIDO Devices - server resident credentials: For server resident credentials on FIDO devices, this parameter is optional as server resident credentials behave same as U2F credentials where they are discovered given the user information on the RP. Authenticators optionally MAY store user information inside the credential ID.

FIDO devices - device resident credentials: For device resident keys on FIDO devices, at least user "id" is mandatory.

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 (0x05) 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.

Within the "flags" bits of the authenticator data structure returned, the authenticator will report what was actually done within the authenticator boundary. The meanings of the combinations of the User Present (UP) and User Verified (UV) flags are as follows:

Flags Meaning
"up"=0 "uv"=0 Silent authentication
"up"=1 "uv"=0 Physical user presence verified, but no user verification
"up"=0 "uv"=1 User verification performed, but physical user presence not verified (a typical "smartcard scenario")
"up"=1 "uv"=1 User verification performed and physical user presence verified

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 credentialCounter is equal to or greater than numberOfCredentials, 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. This step is optional if transport is done over NFC.

  4. Sign the clientDataHash along with authData with the credential using credentialCounter as index (e.g., credentials[n] assuming 0-based array), using the structure specified in [WebAuthn].

  5. Reset the timer. This step is optional if transport is done over NFC.

  6. Increment credentialCounter.

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

5.3.1. 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. 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 (0x01) Sequence of strings Required List of supported versions. Supported versions are: "FIDO_2_0" for CTAP2 / FIDO2 / Web Authentication authenticators and "U2F_V2" for CTAP1/U2F authenticators.
extensions (0x02) Sequence of strings Optional List of supported extensions.
aaguid (0x03) Byte String Required The claimed AAGUID. 16 bytes in length and encoded the same as MakeCredential AuthenticatorData, as specified in [WebAuthn].
options (0x04) Map Optional List of supported options.
maxMsgSize (0x05) Unsigned Integer Optional Maximum message size supported by the authenticator.
pinProtocols (0x06) 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.

Client PIN is one of the ways to do user verification.

Not supported
up user presence: Indicates that the device is capable of testing user presence. true
uv user verification: Indicates that the device is capable of verifying the user within itself. For example, devices with UI, biometrics fall into this category.

If present and set to true, it indicates that the device is capable of user verification within itself and has been configured.

If present and set to false, it indicates that the device is capable of user verification within itself and has not been yet configured. For example, a biometric device that has not yet been configured will return this parameter set to false.

If absent, it indicates that the device is not capable of user verification within itself.

A device that can only do Client PIN will not return the "uv" parameter.

If a device is capable of verifying the user within itself as well as able to do Client PIN, it will return both "uv" and the Client PIN option.

Not Supported

5.5. 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 (0x01) Unsigned Integer Required PIN protocol version chosen by the client. For this version of the spec, this SHALL be the number 1.
subCommand (0x02) Unsigned Integer Required The authenticator Client PIN sub command currently being requested
keyAgreement (0x03) COSE_Key Optional Public key of platformKeyAgreementKey. The COSE_Key-encoded public key MUST contain the optional "alg" parameter and MUST NOT contain any other optional parameters. The "alg" parameter MUST contain a COSEAlgorithmIdentifier value.
pinAuth (0x04) 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 (0x05) Byte Array Optional Encrypted new PIN using sharedSecret. Encryption is done over UTF-8 representation of new PIN.
pinHashEnc (0x06) Byte Array Optional Encrypted first 16 bytes of SHA-256 of PIN using sharedSecret.

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

subCommand Name subCommand Number
getRetries 0x01
getKeyAgreement 0x02
setPIN 0x03
changePIN 0x04
getPINToken 0x05

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

Parameter name Data type Required? Definition
KeyAgreement (0x01) 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. The COSE_Key-encoded public key MUST contain the optional "alg" parameter and MUST NOT contain any other optional parameters. The "alg" parameter MUST contain a COSEAlgorithmIdentifier value.
pinToken (0x02) Byte Array Optional Encrypted pinToken using sharedSecret to be used in subsequent authenticatorMakeCredential and authenticatorGetAssertion operations.
retries
(0x03)
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.

5.5.1. Client PIN Support Requirements

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

5.5.2. 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:

5.5.3. Getting Retries from Authenticator

Retries count is the number of attempts remaining before lockout. When the device is nearing authenticator lockout, the platform can optionally warn the user to be careful while entering the PIN.

Platform performs the following operations to get retries:

5.5.4. 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:

5.5.5. Setting a New PIN

Following operations are performed to set up a new PIN:

5.5.6. Changing existing PIN

Following operations are performed to change an existing PIN:

5.5.7. 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:

5.5.8. 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.

5.5.8.1. Using pinToken in authenticatorMakeCredential

Following operations are performed to use pinToken in authenticatorMakeCredential API:

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.

5.5.8.2. Using pinToken in authenticatorGetAssertion

Following operations are performed to use pinToken in authenticatorGetAssertion API:

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.

5.5.8.3. Without pinToken in authenticatorGetAssertion

Following operations are performed without using pinToken in authenticatorGetAssertion API:

Client PIN
Client PIN

5.6. 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 the CTAP2 canonical CBOR encoding form as specified below, which differs from the canonicalization suggested CTAP2 canonical CBOR encoding form as specified in Section 3.9 of [RFC7049]. All encoders MUST serialize CBOR in the CTAP2 canonical CBOR encoding form without duplicate map keys. All decoders SHOULD reject CBOR that is not validly encoded in the CTAP2 canonical CBOR encoding form and SHOULD reject messages with duplicate map keys.

The CTAP2 canonical CBOR encoding form uses the following rules:

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.

Authenticators SHOULD return the CTAP2_ERR_INVALID_CBOR error if received CBOR does not conform to the requirements above.

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
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.

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).

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 valid 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).
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 unsigned integer(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. The COSE_Key-encoded public key MUST contain the optional "alg" parameter and MUST NOT contain any other optional parameters. The "alg" parameter MUST contain a COSEAlgorithmIdentifier value.
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. Status codes

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, CTAP2_OK 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.
0x11 CTAP2_ERR_CBOR_UNEXPECTED_TYPE Invalid/unexpected CBOR error.
0x12 CTAP2_ERR_INVALID_CBOR Error when parsing CBOR.
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.
0x19 CTAP2_ERR_CREDENTIAL_EXCLUDED Valid credential found in the exclude list.
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.
0x2A CTAP2_ERR_NO_OPERATION_PENDING No outstanding operations.
0x2B CTAP2_ERR_UNSUPPORTED_OPTION Unsupported option.
0x2C CTAP2_ERR_INVALID_OPTION Not a valid option for current operation.
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 Invalid.
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.
0x3A CTAP2_ERR_ACTION_TIMEOUT The current operation has timed out.
0x3B CTAP2_ERR_UP_REQUIRED User presence is required for the requested operation.
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. Framing of U2F commands

The U2F protocol is based on a request-response mechanism, where a requester sends a request message to a U2F device, which always results in a response message being sent back from the U2F device to the requester.

The request message has to be "framed" to send to the lower layer. Taking the signature request as an example, the "framing" is a way for the FIDO client to tell the lower transport layer that it is sending a signature request and then send the raw message contents. The framing also specifies how the transport will carry back the response raw message and any meta-information such as an error code if the command failed.

In this current version of U2F, the framing is defined based on the ISO7816-4:2005 extended APDU format. This is very appropriate for the USB transport since devices are typically built around secure elements which understand this format already. This same argument may apply for futures such as Bluetooth based devices. For other futures based on other transports, such as a built-in u2f token on a mobile device TEE, this framing may not be appropriate, and a different framing may need to be defined.

7.1.1. U2F Request Message Framing

The raw request message is framed as a command APDU:

CLA INS P1 P2 LC1 LC2 LC3

Where:

CLA: Reserved to be used by the underlying transport protocol (if applicable). The host application shall set this byte to zero.

INS: U2F command code, defined in the following sections.

P1, P2: Parameter 1 and 2, defined by each command.

LC1-LC3: Length of the request data, big-endian coded, i.e. LC1 being MSB and LC3 LSB

7.1.2. U2F Response Message Framing

The raw response data is framed as a response APDU:

SW1 SW2

Where:

SW1, SW2: Status word bytes 1 and 2, forming a 16-bit status word, defined below. SW1 is MSB and SW2 LSB. Status Codes

The following ISO7816-4 defined status words have a special meaning in U2F:

SW_NO_ERROR: The command completed successfully without error.

SW_CONDITIONS_NOT_SATISFIED: The request was rejected due to test-of-user-presence being required.

SW_WRONG_DATA: The request was rejected due to an invalid key handle.

Each implementation may define any other vendor-specific status codes, providing additional information about an error condition. Only the error codes listed above will be handled by U2F FIDO clients, whereas others will be seen as general errors and logging of these is optional.

7.2. 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_UNSUPPORTED_OPTION.

        • 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: FIDO U2F Raw Message Formats v1.0 §registration-response-message-success) to a CTAP2 authenticatorMakeCredential response message:

    • Generate authenticatorData from the U2F registration response message (FIDO U2F Raw Message Formats v1.0 §registration-response-message-success) received from the authenticator:

      • Initialize attestedCredData:

        • 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 x9encodedUserPublicKeybe 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 attestedCredData 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 Attested credential data. Initialized with attestedCredData’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" 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" to authenticatorData.

      • Set "fmt" to "fido-u2f".

      • Set "attStmt" to attestationStatement.

  5. Return attestationObject to the caller.

CTAP1/U2F Register - authenticatorMakeCredential Compat.
Mapping: WebAuthn authenticatorMakeCredential to and from CTAP1/U2F Registration Messages.

7.3. 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_UNSUPPORTED_OPTION.

        • 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:

        • If "up" is set to false, set it to 0x08 (dont-enforce-user-presence-and-sign).

        • 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
        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.

        and let Control Byte be P1 of the framing.

  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:

      • Copy bits 0 (the UP bit) and bit 1 from the CTAP2/U2F response user presence byte to bits 0 and 1 of the CTAP2 flags, respectively. Set all other bits of flags to zero. Note: 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.

CTAP1/U2F Authenticate - authenticatorGetAssertion Compat.
Mapping: WebAuthn authenticatorGetAssertion to and from CTAP1/U2F Authentication Messages.

8. Transport-specific Bindings

8.1. USB Human Interface Device (USB HID)

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

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:

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 or aborted 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 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. The client starting a transaction may also abort it.

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 or the transaction is aborted. 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.

If an application wishes to abort a command after the request has been fully sent, e.g. while an authenticator is waiting for user presence, the application may do this by sending a CTAPHID_CANCEL command.

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 maintain 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

// 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 a 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.

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. Please note that keep-alive messages MAY be sent from the device to the client before the response message is returned.

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_CBOR
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. This version of the CTAPHID protocol is 2.

A 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 of the current version, but additional fields may be added.

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

The capability flags value is a bitfield where the following bits values are defined. Unused values are reserved for future use and must be set to zero by device vendors.

Name Value Description
CAPABILITY_WINK 0x01 If set to 1, authenticator implements CTAPHID_WINK function
CAPABILITY_CBOR 0x04 If set to 1, authenticator implements CTAPHID_CBOR function
CAPABILITY_NMSG 0x08 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. If there is an outstanding request that can be cancelled, the authenticator MUST cancel it and that cancelled request will reply with the error CTAP2_ERR_KEEPALIVE_CANCEL.

As the CTAPHID_CANCEL command is sent during an ongoing transaction, transaction semantics do not apply. Whether a request was cancelled or not, the authenticator MUST NOT reply to the CTAPHID_CANCEL message itself. The CTAPHID_CANCEL command MAY be sent by the client during ongoing processing of a CTAPHID_CBOR request. The CTAP2_ERR_KEEPALIVE_CANCEL response MUST be the response to that request, not an error response in the HID transport.

A CTAPHID_CANCEL received while no CTAPHID_CBOR request is being processed, or on a non-active CID SHALL be ignored by the authenticator.

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 0x01 The command in the request is invalid
ERR_INVALID_PAR 0x02 The parameter(s) in the request is invalid
ERR_INVALID_LEN 0x03 The length field (BCNT) is invalid for the request
ERR_INVALID_SEQ 0x04 The sequence does not match expected value
ERR_MSG_TIMEOUT 0x05 The message has timed out
ERR_CHANNEL_BUSY 0x06 The device is busy for the requesting channel
ERR_LOCK_REQUIRED 0x0A Command requires channel lock
ERR_INVALID_CHANNEL 0x0B CID is not valid.
ERR_OTHER 0x7F Unspecified error
Note: These values are identical to the BLE transport values.
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. A KEEPALIVE sent by an authenticator does not constitute a response and does therefore not end an ongoing transaction.

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. 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, while being CTAPHID compliant. Such commands, if implemented, must use 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 [ISO7816-4] 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

  5. Return to 3.

Because of timeouts that may otherwise occur on some platforms, it is RECOMMENDED that the Authenticators reply to APDU commands within 800 milliseconds.

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
PIX 0x2F0001

The command to select the FIDO applet is:

CLA INS P1 P2 Data In Le
0x00 0xA4 0x04 0x00 AID Variable

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

Clients and authenticators MAY support additional selection mechanisms. Clients MUST fall back to the previously defined selection process if the additional selection mechanisms fail to select the applet. Authenticators MUST at least support the previously defined selection process.

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

8.2.4. Framing

Conceptually, framing defines an encapsulation of FIDO2 commands. This encapsulation is done in an APDU following [ISO7816-4]. Authenticators MUST support short and extended length encoding for this APDU. 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 CTAP Status code || Response data "9000" - Success
Status update Status data "9100" - OK
When receiving this, the ISO transport layer will immediately issue an NFCCTAP_GETREPONSE command unless a cancel was issued. The ISO transport layer will provide the status data to the higher layers.
Errors See [ISO7816-4]

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:

Short APDU Chaining commands SHALL have the following format:

CLA INS P1 P2 Data In
0x90 0x10 0x00 0x00 CTAP Payload

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

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.

8.2.6.2. 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.

8.3. Bluetooth Smart / Bluetooth Low Energy Technology

8.3.1. 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].

8.3.2. 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.

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.

8.3.4. 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.

8.3.4.1. 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 §6 Message Encoding.

8.3.4.2. 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 §6 Message Encoding.

8.3.4.3. 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
ERR_BUSY 0x06 The device is busy and can’t accept commands at this time.
NA 0x0a Value reserved (HID)
NA 0x0b Value reserved (HID)
ERR_OTHER 0x7f Other, unspecified error
Note: These values are identical to the HID transport values.

8.3.5. GATT Service Description

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

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

FIDO mandatory service and characteristics
Mandatory GATT services and characteristics that MUST be offered by a FIDO Authenticator. Note that the Generic Access Profile 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.

8.3.5.1. 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 FIDO2
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.

8.3.5.2. Device Information Service

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

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.

8.3.5.3. Generic Access Profile Service

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

8.3.6. 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. Optionally, the client writes a CANCEL command to the fidoControlPoint characteristic to cancel the pending request.

  10. Authenticator evaluates the request and responds by sending notifications over fidoStatus characteristic.

  11. 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.

8.3.7. 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.

8.3.8. Requests

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

Upon receiving a CANCEL request, if there is an outstanding request that can be cancelled, the authenticator MUST cancel it and that cancelled request will reply with the error CTAP2_ERR_KEEPALIVE_CANCEL. Whether a request was cancelled or not, the authenticator MUST NOT reply to the cancel message itself.

8.3.9. 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.

8.3.10. 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 wraparound 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]

8.3.11. 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.

8.3.12. Implementation Considerations

8.3.12.1. Bluetooth pairing: Client considerations

As noted in §8.3.2 Pairing, 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 that 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.

8.3.12.2. 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].

8.3.13. 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:

Constant Value
kMaxCommandTransmitDelayMillis 1500 milliseconds
kErrorWaitMillis 2000 milliseconds
kKeepAliveMillis 500 milliseconds

8.3.14. 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:

8.3.15. 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.

8.3.16. 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.

9. Defined Extensions

This section defines an authenticator extension and corresponding WebAuthn extension.

9.1. HMAC Secret Extension (hmac-secret)

Extension identifier

hmac-secret

This extension is used by the platform to retrieve a symmetric secret from the authenticator when it needs to encrypt or decrypt data using that symmetric secret. This symmetric secret is scoped to a credential. The authenticator and the platform each only have the part of the complete secret to prevent offline attacks. This extension can be used to maintain different secrets on different machines.

Client extension input

create() : A boolean value to indicate that this extension is requested by the Relying Party.

partial dictionary AuthenticationExtensionsClientInputs {
  bool hmacCreateSecret;
};

get() : A JavaScript object defined as follows:

dictionary HMACGetSecretInput {
  required ArrayBuffer salt1;   // 32-byte random data
  ArrayBuffer salt2;  // Optional additional 32-byte random data
};

partial dictionary AuthenticationExtensionsClientInputs {
  HMACGetSecretInput hmacGetSecret;
};

The salt2 input is optional. It can be used when the platform wants to roll over the symmetric secret in one operation.

Client extension processing
  1. If present in a create():

    1. If set to true, pass a CBOR true value as the authenticator extension input.

    2. If set to false, do not process this extension.

  2. If present in a get():

    1. Verify that salt1 is a 32-byte ArrayBuffer.

    2. If salt2 is present, verify that it is a 32-byte ArrayBuffer.

    3. Pass salt1 and, if present, salt2 as the authenticator extension input.

Client extension output

create(): Boolean true value indicating that the authenticator has processed the extension.

partial dictionary AuthenticationExtensionsClientOutputs {
  bool hmacCreateSecret;
};

get(): A dictionary with the following data:

dictionary HMACGetSecretOutput {
  required ArrayBuffer output1;
  ArrayBuffer output2;
};

partial dictionary AuthenticationExtensionsClientOutputs {
  HMACGetSecretOutput hmacGetSecret;
};
Authenticator extension input

Same as the client extension input, except represented in CBOR.

Authenticator extension processing
Authenticator extension output

Same as the client extension output, except represented in CBOR.

10. IANA Considerations

10.1. WebAuthn Extension Identifier Registrations

This section registers the extension identifier values defined in Section §9 Defined Extensions in the IANA "WebAuthn Extension Identifier" registry.

11. Security Considerations

See FIDO Security Reference document [FIDOSecRef].

Index

Terms defined by this specification

Terms defined by reference

References

Normative 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.org/docman/handlers/downloaddoc.ashx?doc_id=229737
[BTCORE]
Bluetooth Core Specification 4.0. URL: https://www.bluetooth.org/docman/handlers/downloaddoc.ashx?doc_id=229737
[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.org/docman/handlers/downloaddoc.ashx?doc_id=229737
[BTGAS]
Generic Access Profile service. Bluetooth Core Specification 4.0, Volume 3, Part C, Section 12. URL: https://www.bluetooth.org/docman/handlers/downloaddoc.ashx?doc_id=229737
[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.org/docman/handlers/downloaddoc.ashx?doc_id=229737
[BTXPLAD]
Bluetooth TX Power AD Type. Bluetooth Core Specification 4.0, Volume 3, Part C, Section 11. URL: https://www.bluetooth.org/docman/handlers/downloaddoc.ashx?doc_id=229737
[CREDENTIAL-MANAGEMENT-1]
Mike West. Credential Management Level 1. 4 August 2017. WD. URL: https://www.w3.org/TR/credential-management-1/
[FIDOSecRef]
R. Lindemann; D. Baghdasaryan; B. Hill. FIDO Security Reference. Implementation Draft. URL: https://fidoalliance.org/specs/fido-v2.0-id-20180227/fido-security-ref-v2.0-id-20180227.html
[FIDOServerGuidelines]
FIDO2 Server Guidelines. URL: https://fidoalliance.org/specs/fido-v2.0-rd-20180702/fido-server-v2.0-rd-20180702.html
[IANA-COSE-ALGS-REG]
Jim Schaad; et al. IANA CBOR Object Signing and Encryption (COSE) Algorithms Registry. URL: https://www.iana.org/assignments/cose/cose.xhtml#algorithms
[ISO7816-4]
ISO 7816-4: Identification cards - Integrated circuit cards; Part 4: Organization, security and commands for interchange. 2013-04. URL: https://www.iso.org/standard/54550.html
[RFC2397]
L. Masinter. The "data" URL scheme. August 1998. Proposed Standard. URL: https://tools.ietf.org/html/rfc2397
[RFC6090]
D. McGrew; K. Igoe; M. Salter. Fundamental Elliptic Curve Cryptography Algorithms. February 2011. Informational. URL: https://tools.ietf.org/html/rfc6090
[RFC7049]
C. Bormann; P. Hoffman. Concise Binary Object Representation (CBOR). October 2013. Proposed Standard. URL: https://tools.ietf.org/html/rfc7049
[RFC8152]
J. Schaad. CBOR Object Signing and Encryption (COSE). July 2017. Proposed Standard. URL: https://tools.ietf.org/html/rfc8152
[SEC1V2]
SEC1: Elliptic Curve Cryptography, Version 2.0. May 2009. URL: http://secg.org/download/aid-780/sec1-v2.pdf
[SP800-56A]
Elaine Barker; et al. Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography. May 2013. URL: http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf
[U2FBle]
D. Balfanz. FIDO Bluetooth® Specification. Proposed Standard. URL: https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-bt-protocol-v1.2-ps-20170411.html
[U2FNfc]
D. Balfanz. FIDO NFC Protocol Specification. Proposed Standard. URL: https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-nfc-protocol-v1.2-ps-20170411.html
[U2FRawMsgs]
D. Balfanz. FIDO U2F Raw Message Formats v1.0. Proposed Standard. URL: https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-raw-message-formats-v1.2-ps-20170411.html
[U2FUsbHid]
D. Balfanz. FIDO U2F HID Protocol Specification. Proposed Standard. URL: https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-u2f-hid-protocol-v1.2-ps-20170411.html
[WebAuthn]
Dirk Balfanz; et al. Web Authentication: An API for accessing Public Key Credentials Level 1. March 2018. CR. URL: https://www.w3.org/TR/webauthn/
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. 15 December 2016. ED. URL: https://heycam.github.io/webidl/

Informative 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

IDL Index

partial dictionary AuthenticationExtensionsClientInputs {
  bool hmacCreateSecret;
};

dictionary HMACGetSecretInput {
  required ArrayBuffer salt1;   // 32-byte random data
  ArrayBuffer salt2;  // Optional additional 32-byte random data
};

partial dictionary AuthenticationExtensionsClientInputs {
  HMACGetSecretInput hmacGetSecret;
};

partial dictionary AuthenticationExtensionsClientOutputs {
  bool hmacCreateSecret;
};

dictionary HMACGetSecretOutput {
  required ArrayBuffer output1;
  ArrayBuffer output2;
};

partial dictionary AuthenticationExtensionsClientOutputs {
  HMACGetSecretOutput hmacGetSecret;
};