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 (0 == attStmt.Keys.Count || 0 == attStmt.Values.Count)
{
throw new Fido2VerificationException("Attestation format android-key must have attestation statement");
}
if (null == Sig || CBORType.ByteString != Sig.Type || 0 == Sig.GetByteString().Length)
{
throw new Fido2VerificationException("Invalid android-key attestation signature");
}
// 2. Verify that sig is a valid signature over the concatenation of authenticatorData and clientDataHash
// using the attestation public key in attestnCert with the algorithm specified in alg
if (null == X5c || CBORType.Array != X5c.Type || 0 == X5c.Count)
{
throw new Fido2VerificationException("Malformed x5c in android-key attestation");
}
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 android-key attestation");
}
X509Certificate2 androidKeyCert;
ECDsa androidKeyPubKey;
try
{
androidKeyCert = new X509Certificate2(X5c.Values.First().GetByteString());
androidKeyPubKey = androidKeyCert.GetECDsaPublicKey(); // attestation public key
}
catch (Exception ex)
{
throw new Fido2VerificationException("Failed to extract public key from android key: " + ex.Message, ex);
}
if (null == Alg || true != Alg.IsNumber)
{
throw new Fido2VerificationException("Invalid android key attestation algorithm");
}
byte[] ecsig;
try
{
ecsig = CryptoUtils.SigFromEcDsaSig(Sig.GetByteString(), androidKeyPubKey.KeySize);
}
catch (Exception ex)
{
throw new Fido2VerificationException("Failed to decode android key attestation signature from ASN.1 encoded form", ex);
}
if (true != androidKeyPubKey.VerifyData(Data, ecsig, CryptoUtils.HashAlgFromCOSEAlg(Alg.AsInt32())))
{
throw new Fido2VerificationException("Invalid android key attestation signature");
}
// 3. Verify that the public key in the first certificate in x5c matches the credentialPublicKey in the attestedCredentialData in authenticatorData.
if (true != AuthData.AttestedCredentialData.CredentialPublicKey.Verify(Data, Sig.GetByteString()))
{
throw new Fido2VerificationException("Incorrect credentialPublicKey in android key attestation");
}
// 4. Verify that the attestationChallenge field in the attestation certificate extension data is identical to clientDataHash
var attExtBytes = AttestationExtensionBytes(androidKeyCert.Extensions);
if (null == attExtBytes)
{
throw new Fido2VerificationException("Android key attestation certificate contains no AttestationRecord extension");
}
try
{
var attestationChallenge = GetAttestationChallenge(attExtBytes);
if (false == clientDataHash.SequenceEqual(attestationChallenge))
{
throw new Fido2VerificationException("Mismatch between attestationChallenge and hashedClientDataJson verifying android key attestation certificate extension");
}
}
catch (Exception)
{
throw new Fido2VerificationException("Malformed android key AttestationRecord extension verifying android key attestation certificate extension");
}
// 5. Verify the following using the appropriate authorization list from the attestation certificate extension data
// 5a. The AuthorizationList.allApplications field is not present, since PublicKeyCredential MUST be bound to the RP ID
if (true == FindAllApplicationsField(attExtBytes))
{
throw new Fido2VerificationException("Found all applications field in android key attestation certificate extension");
}
// 5bi. The value in the AuthorizationList.origin field is equal to KM_ORIGIN_GENERATED ( which == 0).
if (false == IsOriginGenerated(attExtBytes))
{
throw new Fido2VerificationException("Found origin field not set to KM_ORIGIN_GENERATED in android key attestation certificate extension");
}
// 5bii. The value in the AuthorizationList.purpose field is equal to KM_PURPOSE_SIGN (which == 2).
if (false == IsPurposeSign(attExtBytes))
{
throw new Fido2VerificationException("Found purpose field not set to KM_PURPOSE_SIGN in android key attestation certificate extension");
}
var trustPath = X5c.Values
.Select(x => new X509Certificate2(x.GetByteString()))
.ToArray();
return(AttestationType.Basic, trustPath);
}
public override (AttestationType, X509Certificate2[]) Verify()
{
// verify that aaguid is 16 empty bytes (note: required by fido2 conformance testing, could not find this in spec?)
if (0 != AuthData.AttestedCredentialData.AaGuid.CompareTo(Guid.Empty))
{
throw new Fido2VerificationException("Aaguid was not empty parsing fido-u2f atttestation statement");
}
// https://www.w3.org/TR/webauthn/#fido-u2f-attestation
// 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 == X5c || CBORType.Array != X5c.Type || X5c.Count != 1)
{
throw new Fido2VerificationException("Malformed x5c in fido - u2f attestation");
}
// 2a. Check that x5c has exactly one element and let attCert be that element.
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 fido-u2f attestation");
}
var attCert = new X509Certificate2(X5c.Values.First().GetByteString());
// TODO : Check why this variable isn't used. Remove it or use it.
var u2ftransports = U2FTransportsFromAttnCert(attCert.Extensions);
// 2b. If certificate public key is not an Elliptic Curve (EC) public key over the P-256 curve, terminate this algorithm and return an appropriate error
var pubKey = attCert.GetECDsaPublicKey();
var keyParams = pubKey.ExportParameters(false);
if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
{
if (!keyParams.Curve.Oid.FriendlyName.Equals(ECCurve.NamedCurves.nistP256.Oid.FriendlyName))
{
throw new Fido2VerificationException("Attestation certificate public key is not an Elliptic Curve (EC) public key over the P-256 curve");
}
}
else
{
if (!keyParams.Curve.Oid.Value.Equals(ECCurve.NamedCurves.nistP256.Oid.Value))
{
throw new Fido2VerificationException("Attestation certificate public key is not an Elliptic Curve (EC) public key over the P-256 curve");
}
}
// 3. Extract the claimed rpIdHash from authenticatorData, and the claimed credentialId and credentialPublicKey from authenticatorData
// see rpIdHash, credentialId, and credentialPublicKey members of base class AuthenticatorData (AuthData)
// 4. Convert the COSE_KEY formatted credentialPublicKey (see Section 7 of [RFC8152]) to CTAP1/U2F public Key format (Raw ANSI X9.62 public key format)
// 4a. Let x be the value corresponding to the "-2" key (representing x coordinate) in credentialPublicKey, and confirm its size to be of 32 bytes. If size differs or "-2" key is not found, terminate this algorithm and return an appropriate error
var x = CredentialPublicKey[CBORObject.FromObject(COSE.KeyTypeParameter.X)].GetByteString();
// 4b. Let y be the value corresponding to the "-3" key (representing y coordinate) in credentialPublicKey, and confirm its size to be of 32 bytes. If size differs or "-3" key is not found, terminate this algorithm and return an appropriate error
var y = CredentialPublicKey[CBORObject.FromObject(COSE.KeyTypeParameter.Y)].GetByteString();
// 4c.Let publicKeyU2F be the concatenation 0x04 || x || y
var publicKeyU2F = new byte[1] {
0x4
}.Concat(x).Concat(y).ToArray();
// 5. Let verificationData be the concatenation of (0x00 || rpIdHash || clientDataHash || credentialId || publicKeyU2F)
var verificationData = new byte[1] {
0x00
};
verificationData = verificationData
.Concat(AuthData.RpIdHash)
.Concat(clientDataHash)
.Concat(AuthData.AttestedCredentialData.CredentialID)
.Concat(publicKeyU2F.ToArray())
.ToArray();
// 6. Verify the sig using verificationData and certificate public key
if (null == Sig || CBORType.ByteString != Sig.Type || 0 == Sig.GetByteString().Length)
{
throw new Fido2VerificationException("Invalid fido-u2f attestation signature");
}
byte[] ecsig;
try
{
ecsig = CryptoUtils.SigFromEcDsaSig(Sig.GetByteString(), pubKey.KeySize);
}
catch (Exception ex)
{
throw new Fido2VerificationException("Failed to decode fido-u2f attestation signature from ASN.1 encoded form", ex);
}
var coseAlg = CredentialPublicKey[CBORObject.FromObject(COSE.KeyCommonParameter.Alg)].AsInt32();
var hashAlg = CryptoUtils.HashAlgFromCOSEAlg(coseAlg);
if (true != pubKey.VerifyData(verificationData, ecsig, hashAlg))
{
throw new Fido2VerificationException("Invalid fido-u2f attestation signature");
}
// 7. Optionally, inspect x5c and consult externally provided knowledge to determine whether attStmt conveys a Basic or AttCA attestation
var trustPath = X5c.Values
.Select(x => new X509Certificate2(x.GetByteString()))
.ToArray();
return(AttestationType.AttCa, trustPath);
}
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");
}
}
