public override (AttestationType, X509Certificate2[]) Verify()
{
// 1. Verify that attStmt is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract the contained fields.
// (handled in base class)
if (null == Sig || CBORType.ByteString != Sig.Type || 0 == Sig.GetByteString().Length)
{
throw new Fido2VerificationException("Invalid TPM attestation signature");
}
if ("2.0" != attStmt["ver"].AsString())
{
throw new Fido2VerificationException("FIDO2 only supports TPM 2.0");
}
// 2. Verify that the public key specified by the parameters and unique fields of pubArea
// is identical to the credentialPublicKey in the attestedCredentialData in authenticatorData
PubArea pubArea = null;
if (null != attStmt["pubArea"] &&
CBORType.ByteString == attStmt["pubArea"].Type &&
0 != attStmt["pubArea"].GetByteString().Length)
{
pubArea = new PubArea(attStmt["pubArea"].GetByteString());
}
if (null == pubArea || null == pubArea.Unique || 0 == pubArea.Unique.Length)
{
throw new Fido2VerificationException("Missing or malformed pubArea");
}
var coseKty = CredentialPublicKey[CBORObject.FromObject(COSE.KeyCommonParameter.KeyType)].AsInt32();
if (3 == coseKty) // RSA
{
var coseMod = CredentialPublicKey[CBORObject.FromObject(COSE.KeyTypeParameter.N)].GetByteString(); // modulus
var coseExp = CredentialPublicKey[CBORObject.FromObject(COSE.KeyTypeParameter.E)].GetByteString(); // exponent
if (!coseMod.ToArray().SequenceEqual(pubArea.Unique.ToArray()))
{
throw new Fido2VerificationException("Public key mismatch between pubArea and credentialPublicKey");
}
if ((coseExp[0] + (coseExp[1] << 8) + (coseExp[2] << 16)) != pubArea.Exponent)
{
throw new Fido2VerificationException("Public key exponent mismatch between pubArea and credentialPublicKey");
}
}
else if (2 == coseKty) // ECC
{
var curve = CredentialPublicKey[CBORObject.FromObject(COSE.KeyTypeParameter.Crv)].AsInt32();
var X = CredentialPublicKey[CBORObject.FromObject(COSE.KeyTypeParameter.X)].GetByteString();
var Y = CredentialPublicKey[CBORObject.FromObject(COSE.KeyTypeParameter.Y)].GetByteString();
if (pubArea.EccCurve != CoseCurveToTpm[curve])
{
throw new Fido2VerificationException("Curve mismatch between pubArea and credentialPublicKey");
}
if (!pubArea.ECPoint.X.SequenceEqual(X))
{
throw new Fido2VerificationException("X-coordinate mismatch between pubArea and credentialPublicKey");
}
if (!pubArea.ECPoint.Y.SequenceEqual(Y))
{
throw new Fido2VerificationException("Y-coordinate mismatch between pubArea and credentialPublicKey");
}
}
// 3. Concatenate authenticatorData and clientDataHash to form attToBeSigned
// See Data field of base class
// 4. Validate that certInfo is valid
CertInfo certInfo = null;
if (null != attStmt["certInfo"] &&
CBORType.ByteString == attStmt["certInfo"].Type &&
0 != attStmt["certInfo"].GetByteString().Length)
{
certInfo = new CertInfo(attStmt["certInfo"].GetByteString());
}
if (null == certInfo)
{
throw new Fido2VerificationException("CertInfo invalid parsing TPM format attStmt");
}
// 4a. Verify that magic is set to TPM_GENERATED_VALUE
// Handled in CertInfo constructor, see CertInfo.Magic
// 4b. Verify that type is set to TPM_ST_ATTEST_CERTIFY
// Handled in CertInfo constructor, see CertInfo.Type
// 4c. Verify that extraData is set to the hash of attToBeSigned using the hash algorithm employed in "alg"
if (null == Alg || true != Alg.IsNumber)
{
throw new Fido2VerificationException("Invalid TPM attestation algorithm");
}
using (var hasher = CryptoUtils.GetHasher(CryptoUtils.HashAlgFromCOSEAlg(Alg.AsInt32())))
{
if (!hasher.ComputeHash(Data).SequenceEqual(certInfo.ExtraData))
{
throw new Fido2VerificationException("Hash value mismatch extraData and attToBeSigned");
}
}
// 4d. Verify that attested contains a TPMS_CERTIFY_INFO structure, whose name field contains a valid Name for pubArea, as computed using the algorithm in the nameAlg field of pubArea
using (var hasher = CryptoUtils.GetHasher(CryptoUtils.HashAlgFromCOSEAlg(certInfo.Alg)))
{
if (false == hasher.ComputeHash(pubArea.Raw).SequenceEqual(certInfo.AttestedName))
{
throw new Fido2VerificationException("Hash value mismatch attested and pubArea");
}
}
// 4e. Note that the remaining fields in the "Standard Attestation Structure" [TPMv2-Part1] section 31.2, i.e., qualifiedSigner, clockInfo and firmwareVersion are ignored. These fields MAY be used as an input to risk engines.
// 5. If x5c is present, this indicates that the attestation type is not ECDAA
if (null != X5c && CBORType.Array == X5c.Type && 0 != X5c.Count)
{
if (null == X5c.Values || 0 == X5c.Values.Count ||
CBORType.ByteString != X5c.Values.First().Type ||
0 == X5c.Values.First().GetByteString().Length)
{
throw new Fido2VerificationException("Malformed x5c in TPM attestation");
}
// 5a. Verify the sig is a valid signature over certInfo using the attestation public key in aikCert with the algorithm specified in alg.
var aikCert = new X509Certificate2(X5c.Values.First().GetByteString());
var cpk = new CredentialPublicKey(aikCert, Alg.AsInt32());
if (true != cpk.Verify(certInfo.Raw, Sig.GetByteString()))
{
throw new Fido2VerificationException("Bad signature in TPM with aikCert");
}
// 5b. Verify that aikCert meets the TPM attestation statement certificate requirements
// https://www.w3.org/TR/webauthn/#tpm-cert-requirements
// 5bi. Version MUST be set to 3
if (3 != aikCert.Version)
{
throw new Fido2VerificationException("aikCert must be V3");
}
// 5bii. Subject field MUST be set to empty - they actually mean subject name
if (0 != aikCert.SubjectName.Name.Length)
{
throw new Fido2VerificationException("aikCert subject must be empty");
}
// 5biii. The Subject Alternative Name extension MUST be set as defined in [TPMv2-EK-Profile] section 3.2.9.
// https://www.w3.org/TR/webauthn/#tpm-cert-requirements
(string tpmManufacturer, string tpmModel, string tpmVersion) = SANFromAttnCertExts(aikCert.Extensions);
// From https://www.trustedcomputinggroup.org/wp-content/uploads/Credential_Profile_EK_V2.0_R14_published.pdf
// "The issuer MUST include TPM manufacturer, TPM part number and TPM firmware version, using the directoryName
// form within the GeneralName structure. The ASN.1 encoding is specified in section 3.1.2 TPM Device
// Attributes. In accordance with RFC 5280[11], this extension MUST be critical if subject is empty
// and SHOULD be non-critical if subject is non-empty"
// Best I can figure to do for now?
if (string.Empty == tpmManufacturer ||
string.Empty == tpmModel ||
string.Empty == tpmVersion)
{
throw new Fido2VerificationException("SAN missing TPMManufacturer, TPMModel, or TPMVersion from TPM attestation certificate");
}
if (false == TPMManufacturers.Contains(tpmManufacturer))
{
throw new Fido2VerificationException("Invalid TPM manufacturer found parsing TPM attestation");
}
// 5biiii. The Extended Key Usage extension MUST contain the "joint-iso-itu-t(2) internationalorganizations(23) 133 tcg-kp(8) tcg-kp-AIKCertificate(3)" OID.
// OID is 2.23.133.8.3
var EKU = EKUFromAttnCertExts(aikCert.Extensions, "2.23.133.8.3");
if (!EKU)
{
throw new Fido2VerificationException("aikCert EKU missing tcg-kp-AIKCertificate OID");
}
// 5biiiii. The Basic Constraints extension MUST have the CA component set to false.
if (IsAttnCertCACert(aikCert.Extensions))
{
throw new Fido2VerificationException("aikCert Basic Constraints extension CA component must be false");
}
// 5biiiiii. An Authority Information Access (AIA) extension with entry id-ad-ocsp and a CRL Distribution Point extension [RFC5280]
// are both OPTIONAL as the status of many attestation certificates is available through metadata services. See, for example, the FIDO Metadata Service [FIDOMetadataService].
var trustPath = X5c.Values
.Select(x => new X509Certificate2(x.GetByteString()))
.ToArray();
// 5c. If aikCert contains an extension with OID 1.3.6.1.4.1.45724.1.1.4 (id-fido-gen-ce-aaguid) verify that the value of this extension matches the aaguid in authenticatorData
var aaguid = AaguidFromAttnCertExts(aikCert.Extensions);
if ((null != aaguid) &&
(!aaguid.SequenceEqual(Guid.Empty.ToByteArray())) &&
(0 != AttestedCredentialData.FromBigEndian(aaguid).CompareTo(AuthData.AttestedCredentialData.AaGuid)))
{
throw new Fido2VerificationException(string.Format("aaguid malformed, expected {0}, got {1}", AuthData.AttestedCredentialData.AaGuid, new Guid(aaguid)));
}
return(AttestationType.AttCa, trustPath);
}
// If ecdaaKeyId is present, then the attestation type is ECDAA
else if (null != EcdaaKeyId)
{
// Perform ECDAA-Verify on sig to verify that it is a valid signature over certInfo
// https://www.w3.org/TR/webauthn/#biblio-fidoecdaaalgorithm
throw new Fido2VerificationException("ECDAA support for TPM attestation is not yet implemented");
// If successful, return attestation type ECDAA and the identifier of the ECDAA-Issuer public key ecdaaKeyId.
//attnType = AttestationType.ECDAA;
//trustPath = ecdaaKeyId;
}
else
{
throw new Fido2VerificationException("Neither x5c nor ECDAA were found in the TPM attestation statement");
}
}
