How to verify a signature in .net generated in Android

Question

The problem is the following:

• I generate the key in Android (Xamarin.Droid):

   public IPublicKey CreateKey(string keyID)
   {
      /*KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance(
               KeyProperties.KEY_ALGORITHM_RSA, "AndroidKeyStore");
      keyPairGenerator.initialize(
              new KeyGenParameterSpec.Builder(
                      "key1",
                      KeyProperties.PURPOSE_SIGN)
                      .setDigests(KeyProperties.DIGEST_SHA256, KeyProperties.DIGEST_SHA512)
                      .setSignaturePaddings(KeyProperties.SIGNATURE_PADDING_RSA_PSS)
                      .build());
      KeyPair keyPair = keyPairGenerator.generateKeyPair();
      Signature signature = Signature.getInstance("SHA256withRSA/PSS");
      signature.initSign(keyPair.getPrivate());
  
       // The key pair can also be obtained from the Android Keystore any time as follows:
       KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
                  keyStore.load(null);
                  PrivateKey privateKey = (PrivateKey)keyStore.getKey("key1", null);
                  PublicKey publicKey = keyStore.getCertificate("key1").getPublicKey();*/
  
  
      //App.Current.MainPage.DisplayAlert("Info", "Creating a new key pair", "Ok");
      // UTILIZANDO RSA
      KeyPairGenerator kpg = 
      KeyPairGenerator.GetInstance(KeyProperties.KeyAlgorithmRsa, KEYSTORE_NAME);
      kpg.Initialize(
          new KeyGenParameterSpec.Builder(keyID,
          KeyStorePurpose.Sign)
          .SetSignaturePaddings(KeyProperties.SignaturePaddingRsaPss)
          .SetDigests(KeyProperties.DigestSha1)
          .Build()
          );
  
      KeyPair keyPair = kpg.GenerateKeyPair();
  
      Log.Debug(TAG, "New key created for fingerprint authentication");
  
      return keyPair.Public;
  }
  

• Then i generate a signature:

      KeyStore.PrivateKeyEntry PKentry = 
      (KeyStore.PrivateKeyEntry)_keystore.GetEntry(keyID, null);
      IPublicKey pk = (IPublicKey)PKentry.Certificate.PublicKey;
      //this.pk = pk;
      privKey = PKentry.PrivateKey;
  
      //cipher.Init(Cipher.EncryptMode, privKey);
      //byte[] output = cipher.DoFinal(Encoding.UTF8.GetBytes(input));
      //String s = new string(cipher.DoFinal(input));
  
      // signature
      Signature sig = Signature.GetInstance("SHA1withRSA/PSS");
      sig.InitSign(privKey);
      byte[] inputDataToSign = Encoding.UTF8.GetBytes(input);
      sig.Update(inputDataToSign);
      byte[] signatureBytes = sig.Sign();
  

• And i send the key and the signature to a ASP.net wep API 2 server. Client side response generation:

   RegistrationResponse registrationResponse = new RegistrationResponse();
   string fcparams = Utils.Base64Encode(JsonConvert.SerializeObject(finalChallengeParams));
    registrationResponse.fcParams = fcparams;
    byte[] signedData = sign(fcparams, registrationRequest.username, facetID);
   registrationResponse.signedData = signedData;
   registrationResponse.Base64key = convertPublicKeyToString(publicKey);
   ...
   ...
  private string convertPublicKeyToString(IPublicKey publicKey)
  {
      string publicKeyString = Base64.EncodeToString(publicKey.GetEncoded(), 0);
  
   return publicKeyString;
    }
  

I send it using Refit Nugget. And this is the code i use when i receive the HTTPRequest on server side:

[Route("regResponse/")]
    [HttpPost]
    public IHttpActionResult ProcessClientRegistrationResponse([FromBody] RegistrationResponse registrationResponse) 
    {


        //byte[] publicKeyBytes = Convert.FromBase64String(registrationResponse.Base64key);
        byte[] publicKeyBytes = registrationResponse.Base64key;
        AsymmetricKeyParameter asymmetricKeyParameter = PublicKeyFactory.CreateKey(publicKeyBytes);

        RsaKeyParameters rsaKeyParameters = (RsaKeyParameters)asymmetricKeyParameter;

        RSAParameters rsaParameters = new RSAParameters();
        rsaParameters.Modulus = rsaKeyParameters.Modulus.ToByteArrayUnsigned();
        rsaParameters.Exponent = rsaKeyParameters.Exponent.ToByteArrayUnsigned();

        RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
        rsa.ImportParameters(rsaParameters);

        /*****/

        string alg = rsa.SignatureAlgorithm;
        byte[] signedData = registrationResponse.signedData;
        byte[] fcParamsBytes = Encoding.UTF8.GetBytes(registrationResponse.fcParams);

        RSACng rsaCng = new RSACng();
        rsaCng.ImportParameters(rsaParameters);

        SHA1Managed hash = new SHA1Managed();
        byte[] hashedData;
        hashedData = hash.ComputeHash(signedData);


        /*********/

        bool rsaCngDataOk1 = rsaCng.VerifyData(fcParamsBytes, signedData, HashAlgorithmName.SHA1, RSASignaturePadding.Pss);
        bool rsaCngDataOk2 = rsaCng.VerifyData(fcParamsBytes, signedData, HashAlgorithmName.SHA1, RSASignaturePadding.Pss);
        bool rsaCngDataOk3 = rsaCng.VerifyData(hashedData, signedData, HashAlgorithmName.SHA1, RSASignaturePadding.Pss);
        bool rsaCngDataOk4 = rsaCng.VerifyData(hashedData, signedData, HashAlgorithmName.SHA1, RSASignaturePadding.Pss);

        bool rsaCngHashOk1 = rsaCng.VerifyHash(hashedData, signedData, HashAlgorithmName.SHA1, RSASignaturePadding.Pss);

        bool dataOK1 = rsa.VerifyData(fcParamsBytes, new SHA1CryptoServiceProvider(), signedData);
        bool dataOk2 = rsa.VerifyData(fcParamsBytes, signedData, HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);

        bool hashOk = rsa.VerifyHash(hashedData, CryptoConfig.MapNameToOID("SHA1"), signedData);

        return Ok(true);

    }

EVERY bool is wrong. I think the problem is clearly on the public key. The questions are,

1. does the method publickey.encode() do what i think? I think it converts my public key to a byte[] representation (source: Android developer Key Info)

2. do i convert the received byte[] representation of the key to a correct RSA key?

3. Is there any problem on algorithms? I don't think so but we never know...

I don't find the solution. I searched for ways to import public keys from strings in .net or c# and for ways to export Android Public key to string or byte[] but there's no much help for this concrete questions...

Answer 1

@James K Polk gave me the solution. Apparently C# doesn't work well with PSS padding. I just had to change it to PKCS1. And i changed to digest algorithm too to SHA512.