Greetings, We are struggling to implement device binding according to your documentation. We are generation a nonce value in backend like this: public static String generateNonce(int byteLength) { byte[] randomBytes = new byte[byteLength]; new SecureRandom().nextBytes(randomBytes); return Base64.getUrlEncoder().withoutPadding().encodeToString(randomBytes); } And our mobile client implement the attestation flow like this: @implementation AppAttestModule - (NSData *)sha256FromString:(NSString *)input { const char *str = [input UTF8String]; unsigned char result[CC_SHA256_DIGEST_LENGTH]; CC_SHA256(str, (CC_LONG)strlen(str), result); return [NSData dataWithBytes:result length:CC_SHA256_DIGEST_LENGTH]; } RCT_EXPORT_MODULE(); RCT_EXPORT_METHOD(generateAttestation:(NSString *)nonce resolver:(RCTPromiseResolveBlock)resolve rejecter:(RCTPromiseRejectBlock)reject) { if (@available(iOS 14.0, *)) { DCAppAttestService *service = [DCAppAttestService sharedService]; if (![service isSupported]) { reject(@"not_supported", @"App Attest is not supported on this device.", nil); return; } NSData *nonceData = [self sha256FromString:nonce]; NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults]; NSString *savedKeyId = [defaults stringForKey:@"AppAttestKeyId"]; NSString *savedAttestation = [defaults stringForKey:@"AppAttestAttestationData"]; void (^resolveWithValues)(NSString *keyId, NSData *assertion, NSString *attestationB64) = ^(NSString *keyId, NSData *assertion, NSString *attestationB64) { NSString *assertionB64 = [assertion base64EncodedStringWithOptions:0]; resolve(@{ @"nonce": nonce, @"signature": assertionB64, @"deviceType": @"IOS", @"attestationData": attestationB64 ?: @"", @"keyId": keyId }); }; void (^handleAssertion)(NSString *keyId, NSString *attestationB64) = ^(NSString *keyId, NSString *attestationB64) { [service generateAssertion:keyId clientDataHash:nonceData completionHandler:^(NSData *assertion, NSError *assertError) { if (!assertion) { reject(@"assertion_error", @"Failed to generate assertion", assertError); return; } resolveWithValues(keyId, assertion, attestationB64); }]; }; if (savedKeyId && savedAttestation) { handleAssertion(savedKeyId, savedAttestation); } else { [service generateKeyWithCompletionHandler:^(NSString *keyId, NSError *keyError) { if (!keyId) { reject(@"keygen_error", @"Failed to generate key", keyError); return; } [service attestKey:keyId clientDataHash:nonceData completionHandler:^(NSData *attestation, NSError *attestError) { if (!attestation) { reject(@"attestation_error", @"Failed to generate attestation", attestError); return; } NSString *attestationB64 = [attestation base64EncodedStringWithOptions:0]; [defaults setObject:keyId forKey:@"AppAttestKeyId"]; [defaults setObject:attestationB64 forKey:@"AppAttestAttestationData"]; [defaults synchronize]; handleAssertion(keyId, attestationB64); }]; }]; } } else { reject(@"ios_version", @"App Attest requires iOS 14+", nil); } } @end For validation we are extracting the nonce from the certificate like this: private static byte[] extractNonceFromAttestationCert(X509Certificate certificate) throws IOException { byte[] extensionValue = certificate.getExtensionValue("1.2.840.113635.100.8.2"); if (Objects.isNull(extensionValue)) { throw new IllegalArgumentException("Apple App Attest nonce extension not found in certificate."); } ASN1Primitive extensionPrimitive = ASN1Primitive.fromByteArray(extensionValue); ASN1OctetString outerOctet = ASN1OctetString.getInstance(extensionPrimitive); ASN1Sequence sequence = (ASN1Sequence) ASN1Primitive.fromByteArray(outerOctet.getOctets()); ASN1TaggedObject taggedObject = (ASN1TaggedObject) sequence.getObjectAt(0); ASN1OctetString nonceOctet = ASN1OctetString.getInstance(taggedObject.getObject()); return nonceOctet.getOctets(); } And for the verification we are using this method: private OptionalMethodResult<Void> verifyNonce(X509Certificate certificate, String expectedNonce, byte[] authData) { byte[] expectedNonceHash; try { byte[] nonceBytes = MessageDigest.getInstance("SHA-256").digest(expectedNonce.getBytes()); byte[] combined = ByteBuffer.allocate(authData.length + nonceBytes.length).put(authData).put(nonceBytes).array(); expectedNonceHash = MessageDigest.getInstance("SHA-256").digest(combined); } catch (NoSuchAlgorithmException e) { log.error("Error while validations iOS attestation: {}", e.getMessage(), e); return OptionalMethodResult.ofError(deviceBindError.getChallengeNotMatchedError()); } byte[] actualNonceFromCert; try { actualNonceFromCert = extractNonceFromAttestationCert(certificate); } catch (Exception e) { log.error("Error while extracting nonce from certificate: {}", e.getMessage(), e); return OptionalMethodResult.ofError(deviceBindError.getChallengeNotMatchedError()); } if (!Arrays.equals(expectedNonceHash, actualNonceFromCert)) { return OptionalMethodResult.ofError(deviceBindError.getChallengeNotMatchedError()); } return OptionalMethodResult.empty(); } But the values did not matched. What are we doing wrong here? Thanks.