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. if (0 == attStmt.Keys.Count || 0 == attStmt.Values.Count) { throw new Fido2VerificationException("Attestation format packed must have attestation statement"); } if (null == Sig || CBORType.ByteString != Sig.Type || 0 == Sig.GetByteString().Length) { throw new Fido2VerificationException("Invalid packed attestation signature"); } if (null == Alg || true != Alg.IsNumber) { throw new Fido2VerificationException("Invalid packed attestation algorithm"); } // 2. If x5c is present, this indicates that the attestation type is not ECDAA if (null != X5c) { if (CBORType.Array != X5c.Type || 0 == X5c.Count || null != EcdaaKeyId) { throw new Fido2VerificationException("Malformed x5c array in packed attestation statement"); } var enumerator = X5c.Values.GetEnumerator(); while (enumerator.MoveNext()) { if (null == enumerator || null == enumerator.Current || CBORType.ByteString != enumerator.Current.Type || 0 == enumerator.Current.GetByteString().Length) { throw new Fido2VerificationException("Malformed x5c cert found in packed attestation statement"); } var x5ccert = new X509Certificate2(enumerator.Current.GetByteString()); // X509Certificate2.NotBefore/.NotAfter return LOCAL DateTimes, so // it's correct to compare using DateTime.Now. if (DateTime.Now < x5ccert.NotBefore || DateTime.Now > x5ccert.NotAfter) { throw new Fido2VerificationException("Packed signing certificate expired or not yet valid"); } } // The attestation certificate attestnCert MUST be the first element in the array. var attestnCert = new X509Certificate2(X5c.Values.First().GetByteString()); // 2a. 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 var cpk = new CredentialPublicKey(attestnCert, Alg.AsInt32()); if (true != cpk.Verify(Data, Sig.GetByteString())) { throw new Fido2VerificationException("Invalid full packed signature"); } // Verify that attestnCert meets the requirements in https://www.w3.org/TR/webauthn/#packed-attestation-cert-requirements // 2bi. Version MUST be set to 3 if (3 != attestnCert.Version) { throw new Fido2VerificationException("Packed x5c attestation certificate not V3"); } // 2bii. Subject field MUST contain C, O, OU, CN // OU must match "Authenticator Attestation" if (true != IsValidPackedAttnCertSubject(attestnCert.Subject)) { throw new Fido2VerificationException("Invalid attestation cert subject"); } // 2biii. If the related attestation root certificate is used for multiple authenticator models, // the Extension OID 1.3.6.1.4.1.45724.1.1.4 (id-fido-gen-ce-aaguid) MUST be present, containing the AAGUID as a 16-byte OCTET STRING // verify that the value of this extension matches the aaguid in authenticatorData var aaguid = AaguidFromAttnCertExts(attestnCert.Extensions); // 2biiii. The Basic Constraints extension MUST have the CA component set to false if (IsAttnCertCACert(attestnCert.Extensions)) { throw new Fido2VerificationException("Attestation certificate has CA cert flag present"); } // 2c. If attestnCert 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 if (aaguid != null) { if (0 != AttestedCredentialData.FromBigEndian(aaguid).CompareTo(AuthData.AttestedCredentialData.AaGuid)) { throw new Fido2VerificationException("aaguid present in packed attestation cert exts but does not match aaguid from authData"); } } // id-fido-u2f-ce-transports var u2ftransports = U2FTransportsFromAttnCert(attestnCert.Extensions); // 2d. 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); } // 3. If ecdaaKeyId is present, then the attestation type is ECDAA else if (null != EcdaaKeyId) { // 3a. Verify that sig is a valid signature over the concatenation of authenticatorData and clientDataHash // using ECDAA-Verify with ECDAA-Issuer public key identified by ecdaaKeyId // https://www.w3.org/TR/webauthn/#biblio-fidoecdaaalgorithm throw new Fido2VerificationException("ECDAA is not yet implemented"); // 3b. If successful, return attestation type ECDAA and attestation trust path ecdaaKeyId. // attnType = AttestationType.ECDAA; // trustPath = ecdaaKeyId; } // 4. If neither x5c nor ecdaaKeyId is present, self attestation is in use else { // 4a. Validate that alg matches the algorithm of the credentialPublicKey in authenticatorData if (false == AuthData.AttestedCredentialData.CredentialPublicKey.IsSameAlg((COSE.Algorithm)Alg.AsInt32())) { throw new Fido2VerificationException("Algorithm mismatch between credential public key and authenticator data in self attestation statement"); } // 4b. Verify that sig is a valid signature over the concatenation of authenticatorData and // clientDataHash using the credential public key with alg if (true != AuthData.AttestedCredentialData.CredentialPublicKey.Verify(Data, Sig.GetByteString())) { throw new Fido2VerificationException("Failed to validate signature"); } return(AttestationType.Self, null); } }
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"); } }