go-ctap

go-ctap is an idiomatic Go library for interacting with FIDO2 authenticators using CTAP. It exposes several abstraction levels, from raw CTAPHID transport framing to ergonomic authenticator workflows.

Warning

Work in progress! API may change during v0.x!

Current Status

The library implements the CTAP 2.1 core command set over CTAPHID, with NFC and BLE transports out of scope for now. It also includes selected CTAP 2.2 and CTAP 2.3 features and extensions, such as largeBlobKey and hmac-secret-mc. The hmac-secret-mc implementation has not yet been tested against a physical authenticator with support for it, and the dedicated largeBlob extension is still pending.

My current priorities are to ensure complete FIDO2/CTAP 2.1 support, prepare the library for FIDO2/CTAP 2.3, and fix bugs.

Key Features and Architecture

The library exposes several abstraction levels, allowing you to choose the API that best suits your needs:

1. Transport Layer (transport/ctaphid)

   Direct access to the raw CTAPHID transport protocol. If you need maximum control, you can communicate with devices at the frame level.

2. Client Layer (client)

   Implements CTAP command messaging atop the transport, letting you call authenticator commands while still managing the device handle, channel ID, PIN/UV auth tokens, and command inputs yourself.

3. Authenticator Layer (authenticator)

   Provides a convenient wrapper over the client package, managing the HID device descriptor, channel ID (CID), cached authenticator info, and common CTAP flows.

4. Discovery Helpers (discover)

   A set of helpers for finding and selecting authenticators, including user-presence based selection when several authenticators are connected.

5. Crypto Helpers (crypto)

   Public helpers for CTAP-specific cryptography, including PIN/UV Auth Protocol One and Two, and LargeBlob encryption/decryption. The lower-level crypto/protocolone and crypto/protocoltwo packages are available for callers that need direct access to the protocol primitives.

6. Protocol Model (protocol)

   CTAP command constants, request/response wire structures, options, permissions, parsed authenticator data, and CTAP extension wire inputs/outputs.

7. Domain Types (credential, attestation, extension, webauthn)

   Shared public-key credential primitives, attestation statement formats, extension identifiers/policies, and WebAuthn-shaped extension input/output structures used across the lower-level and higher-level APIs.

8. Vendor Extensions (yubico)

   Yubico-specific commands and response types, including YubiKey Device Information over the vendor CTAPHID command 0xc2.

Highlights

• Implements major FIDO2 commands: MakeCredential, GetAssertion, ClientPIN (with both PIN/UV methods), Reset, CredentialManagement, and more.
• Both low-level access and ergonomic, high-level APIs.
• YubiKey Device Information, including firmware version, serial number, form factor, USB/NFC capabilities, configuration-lock state, and device timeouts.
• Modern Go design, making use of language features like iterators.
• HID access uses the go-ctap/hid cgo-free backend.

Feature Matrix

CTAP 2.3

• MakeCredential
  • attestationFormatsPreference
  • unsignedExtensionOutputs
  • credential-store state invalidation for discoverable credentials
• GetAssertion / GetNextAssertion
  • unsignedExtensionOutputs
• GetInfo
  • attestationFormats
  • uvCountSinceLastPinEntry
  • longTouchForReset
  • encIdentifier
  • encCredStoreState
  • transportsForReset
  • pinComplexityPolicy
  • pinComplexityPolicyURL
  • maxPINLength
  • authenticatorConfigCommands
  • perCredMgmtRO option
• ClientPIN
  • getPINRetries
  • getKeyAgreement
  • setPIN
  • changePIN
  • getPinToken
  • getPinUvAuthTokenUsingUvWithPermissions
  • getUVRetries
  • getPinUvAuthTokenUsingPinWithPermissions
  • persistent PIN/UV auth token state
  • pcmr permission
  • perCredMgmtRO flow
• Reset
  • transportsForReset handling
  • long-touch reset handling
  • reset unsupported / alternate reset handling
  • credential-store cache invalidation after reset
• BioEnrollment
  • enrollBegin
  • enrollCaptureNextSample
  • cancelCurrentEnrollment
  • enumerateEnrollments
  • setFriendlyName
  • removeEnrollment
  • getFingerprintSensorInfo
• CredentialManagement
  • getCredsMetadata
  • enumerateRPsBegin / enumerateRPsGetNextRP
  • enumerateCredentialsBegin / enumerateCredentialsGetNextCredential
  • deleteCredential
  • updateUserInformation
  • read-only persistent credential management via pcmr
  • encCredStoreState-based cache invalidation
• Selection
• LargeBlobs
  • raw get
  • raw set
  • get serialized large-blob array
  • set serialized large-blob array
  • largeBlob extension integration
  • unsigned largeBlob extension outputs
• Config
  • enableEnterpriseAttestation
  • toggleAlwaysUv
  • setMinPINLength
  • enableLongTouchForReset
  • authenticatorConfigCommands feature detection
  • setMinPINLength CTAP 2.3 refinements
  • PIN complexity policy CTAP 2.3 refinements
• Hybrid Transports
  • QR-initiated transactions
  • state-assisted transactions
  • post-handshake getInfo
  • post-handshake supported features: ctap
  • post-handshake supported features: dc
  • WebSocket data transfer channel
  • BLE data transfer channel
  • multiple data transfer channels / QR key 6
• JSON-based Messages / Digital Credentials
  • tunnel message type 3
  • JSON-based request
  • JSON-based response
• NFC / ISO7816 refinements
  • ISO7816 contact smart-card interface
  • explicit FIDO applet selection
  • applet deselection handling
  • NFCCTAP_GETRESPONSE timeout handling
  • NFCCTAP_GETRESPONSE cancel handling
• Prototype BioEnrollment
• Prototype CredentialManagement

Extensions

CTAP

• credProtect
• credBlob
• largeBlobKey
• largeBlob
  • MakeCredential support
  • MakeCredential supported output
  • GetAssertion read
  • GetAssertion write
• minPinLength
• pinComplexityPolicy
• hmac-secret
• hmac-secret-mc (not tested)
• thirdPartyPayment

WebAuthn

• credProps
• prf
• largeBlob

Crypto

• PIN/UV Auth Protocol One
• PIN/UV Auth Protocol Two
• Encrypt/Decrypt using LargeBlobsKey extension
• persistent PIN/UV auth token support
• Decrypt GetInfo.encIdentifier
• Decrypt GetInfo.encCredStoreState

Yubico Device Information

YubiKey 5 Series devices and Security Keys by Yubico expose firmware, serial number, form factor, capabilities, and other device information through GetYubiKeyDeviceInfo:

info, err := device.GetYubiKeyDeviceInfo()
if err != nil {
	return err
}

if info.Serial != nil {
	fmt.Printf("YubiKey serial: %d\n", *info.Serial)
}
version := info.FirmwareVersion
fmt.Printf("Firmware: %d.%d.%d\n", version.Major, version.Minor, version.Build)

The implementation uses Yubico's vendor-specific CTAPHID command 0xc2. Lower-level callers can use yubico.GetDeviceInfo with an io.ReadWriter and channel ID directly.

Planned Improvements

• CTAP 2.2/2.3 support