FIDO2 CBOR Attestation Parser

FIDO2 Attestation Object Structure (CBOR map)

attestationObject = {
  "fmt": "packed" | "tpm" | "android-key" | "android-safetynet" | "apple" | "u2f" | "none",
  "attStmt": { ... },  // format-specific
  "authData": bytes     // CBOR byte string
}

authData Structure (binary)

 0-31:  RP ID Hash (SHA-256, 32 bytes)
32   :  Flags (1 byte)
       Bit 0 (0x01): UP — User Present
       Bit 1 (0x02): (reserved)
       Bit 2 (0x04): UV — User Verified
       Bit 3 (0x08): (reserved)
       Bit 4 (0x10): (reserved)
       Bit 5 (0x20): (reserved)
       Bit 6 (0x40): AT — Attested Credential Data included
       Bit 7 (0x80): ED — Extension Data included
33-36:  Signature Counter (4 bytes, big-endian uint32)

If AT flag set:
37-52:  AAGUID (16 bytes)
53-54:  Credential ID Length (2 bytes, big-endian uint16)
55+:    Credential ID (variable)
     :  COSE_Key (CBOR-encoded public key)

If ED flag set:
     :  Extension Data (CBOR map)

Attestation Statement Formats

packed

attStmt = {
  "alg": COSE AlgorithmIdentifier (int, e.g. -7=ES256, -257=RS256),
  "sig": signature bytes,
  "x5c": [cert1, cert2, ...] (optional, X.509 cert chain)
}

tpm

attStmt = {
  "ver": "2.0",
  "alg": COSE AlgorithmIdentifier,
  "x5c": [AIK cert, ...],
  "sig": TPMT_SIGNATURE,
  "certInfo": TPMS_ATTEST (CBOR)
}

u2f

attStmt = {
  "x5c": [attestation cert],
  "sig": ECDSA signature (ASN.1 DER)
}

Common AAGUIDs

COSE Key Types & Algorithms