Encrypt in JS and Decrypt in C# with ECIES

Question

I'm trying to encrypt data in javascript and pass to C# for decryption and I got error mac check in GCM failed. This backend code is provided by backend guy and I have to use javascript to encrypt data to accomodate the backend, hence I cannot change any code in the backend. I have no idea what went wrong and hope someone could point me the correct direction.

In C#

public static string Decrypt(string privateKey, byte[] cliperText)
    {
        
        X9ECParameters ecParams = ECNamedCurveTable.GetByName(_curveName);           
        PemReader pr = new(new StringReader(privateKey));
        AsymmetricCipherKeyPair keyPairServer = (AsymmetricCipherKeyPair) pr.ReadObject();
        ECPrivateKeyParameters serverECPrivateKeyParameters = (ECPrivateKeyParameters) keyPairServer.Private;

        int uncompressedPointKeyLengthBytes = 2 * (serverECPrivateKeyParameters.Parameters.N.BitLength + 8 - 1) / 8;
        byte[] sharedInfo = cliperText[0..uncompressedPointKeyLengthBytes];
        byte[] encryptedDataAndGcmTag = cliperText[uncompressedPointKeyLengthBytes..cliperText.Length];

        int keySizeLengthBytes = (sharedInfo.Length - 1) / 2;
        BigInteger x =  new(1, sharedInfo[1..(keySizeLengthBytes + 1)]);
        BigInteger y = new(1, sharedInfo[(keySizeLengthBytes + 1)..sharedInfo.Length]);

        ECDomainParameters domainParameters = new(ecParams.Curve, ecParams.G, ecParams.N, ecParams.H, ecParams.GetSeed());
        ECPublicKeyParameters pubkeyParam = new(ecParams.Curve.CreatePoint(x, y), domainParameters);

        ECDHCBasicAgreement keyAgreement = new();
        keyAgreement.Init(serverECPrivateKeyParameters);
        BigInteger symmetricKey = keyAgreement.CalculateAgreement(pubkeyParam);
        
        var counterData = new byte[] { 0x00, 0x00, 0x00, 0x01 };
        var preHashKey = symmetricKey.ToByteArrayUnsigned().Concat(counterData).Concat(sharedInfo).Select(i => i).ToArray();
        var hashedKey = SHA256.HashData(preHashKey);

        AeadParameters parameters = new(new KeyParameter(hashedKey[0..16]), 128, hashedKey[16..32], null);
        GcmBlockCipher cipher = new(new AesEngine());
        cipher.Init(false, parameters);
        byte[] plainBytes = new byte[cipher.GetOutputSize(encryptedDataAndGcmTag.Length)];

        // This is where i get Mac check error. plainBytes is 21bytes but all are 0s
        cipher.DoFinal(plainBytes, cipher.ProcessBytes(encryptedDataAndGcmTag, 0, encryptedDataAndGcmTag.Length, plainBytes, 0));

        return Encoding.UTF8.GetString(plainBytes).TrimEnd("
\0".ToCharArray());
    }

string privateKeyServerASN1 = @"
-----BEGIN EC PRIVATE KEY-----
some private keys
-----END EC PRIVATE KEY-----
";

// This js encrypted data is generated from my javascript code 
    var js = "BArRePhFlOuJkzMyydytWrFmlsy8sbAJBU5b/bxNimQ5hkwkz0RjK4dGVrdK9WinpDjjoJ5DnWcP4+zp2RvgSCfPBbSHC4ZaHBXuDKhxVbKc9Fk6Cp6aE/awVLUj4Tjyv/l2R8bY";
    
    string decryptedDataAndroid = ECIES.Decrypt(privateKeyServerASN1, Convert.FromBase64String(js));

In javascript:

async function performEncryption() {
  const publicKeyBase64 = "public key";
  
  const plaintextBytes = new TextEncoder().encode("Hello");

  // Convert the base64 public key to ArrayBuffer
  const publicKeyBytes = Uint8Array.from(atob(publicKeyBase64), c => c.charCodeAt(0)).buffer;

  // Import the public key
  const importedPublicKey = await crypto.subtle.importKey(
    "spki",
    publicKeyBytes,
    {
      name: "ECDH",
      namedCurve: "P-256",
    },
    true,
    []
  );

  // Generate an ephemeral EC key pair
  const ephemeralEcKeyPair = await crypto.subtle.generateKey(
    {
      name: "ECDH",
      namedCurve: "P-256",
    },
    true,
    ["deriveKey"]
  );

  // Use ECDH to generate a symmetric key
  const sharedSecret = await crypto.subtle.deriveKey(
    {
      name: "ECDH",
      public: importedPublicKey,
    },
    ephemeralEcKeyPair.privateKey,
    {
      name: "AES-GCM",
      length: 256,
    },
    true,
    ["encrypt", "decrypt"]
  );

  // Export the raw public key (x coordinate) as sharedInfo
  const rawPublicKey = await crypto.subtle.exportKey("raw", importedPublicKey);
  const sharedInfo = new Uint8Array(rawPublicKey);

  // Use SHA-256 to hash the shared secret
  const symmetricKey = await crypto.subtle.digest("SHA-256", new Uint8Array(await crypto.subtle.exportKey("raw", sharedSecret)));

  // Use the first 16 bytes as an AES-GCM key
  const aesGcmKey = await crypto.subtle.importKey(
    "raw",
    symmetricKey.slice(0, 16),
    {
      name: "AES-GCM",
      length: 256,
    },
    true,
    ["encrypt", "decrypt"]
  );

  // Use the second 16 bytes as the initialization vector (IV)
  const iv = symmetricKey.slice(16, 32);

  // Use AES/GCM/NoPadding to encrypt the plaintext and generate a 16-byte GCM tag
  const encryptedData = await crypto.subtle.encrypt(
    {
      name: "AES-GCM",
      iv: iv,
    },
    aesGcmKey,
    plaintextBytes
  );

  // Concatenate sharedInfo, IV, and encrypted data
  const combinedData = concatBuffers(concatBuffers(sharedInfo, iv), encryptedData);

    console.log("combinedData", combinedData)

  // Convert combinedData to base64
  const base64CombinedData = btoa(String.fromCharCode.apply(null, new Uint8Array(combinedData)));

  console.log("Encrypted Data (Base64):", base64CombinedData);
}

// Utility function to concatenate two ArrayBuffers
function concatBuffers(buffer1, buffer2) {
  const result = new Uint8Array(buffer1.byteLength + buffer2.byteLength);
  result.set(new Uint8Array(buffer1), 0);
  result.set(new Uint8Array(buffer2), buffer1.byteLength);
  return result;
}

performEncryption();

Encrypt in JS and Decrypt in C# with ECIES

Answers (1)

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