private string InternalDecrypt(string payload, JsonWebKey jsonWebKey, byte[] hmacKey = null)
{
IEnumerable <string> parts = null;
if (!IsValid(payload, out parts))
{
throw new JwtException(ErrorCodes.INTERNAL_ERROR, ErrorMessages.INVALID_JWE);
}
var serializedProtectedHeader = parts.ElementAt(0).Base64Decode();
var encryptedContentEncryptionKeyBytes = parts.ElementAt(1).Base64DecodeBytes();
var iv = parts.ElementAt(2).Base64DecodeBytes();
var cipherText = parts.ElementAt(3).Base64DecodeBytes();
var authenticationTag = parts.ElementAt(4).Base64DecodeBytes();
var protectedHeader = JsonConvert.DeserializeObject(serializedProtectedHeader) as JObject;
var alg = protectedHeader["alg"].ToString();
var enc = protectedHeader["enc"].ToString();
var contentEncryptionKeyHandler = _cekHandlers.FirstOrDefault(e => e.AlgName == alg);
var encEncryptionHandler = _encHandlers.FirstOrDefault(e => e.EncName == enc);
if (contentEncryptionKeyHandler == null || encEncryptionHandler == null)
{
return(null);
}
var cek = contentEncryptionKeyHandler.Decrypt(encryptedContentEncryptionKeyBytes, jsonWebKey);
var splittedCEK = BitHelper.SplitInHalf(cek);
if (hmacKey == null)
{
hmacKey = splittedCEK.First();
}
var decrypted = encEncryptionHandler.Decrypt(cipherText, splittedCEK.Last(), iv);
var aad = Encoding.ASCII.GetBytes(Encoding.UTF8.GetBytes(serializedProtectedHeader).Base64EncodeBytes());
var al = BitHelper.LongToBytes(aad.Length * 8);
var hmacInput = BitHelper.Concat(aad, iv, cipherText, al);
var hmacValue = encEncryptionHandler.BuildHash(hmacKey, hmacInput);
var newAuthenticationTag = BitHelper.SplitInHalf(hmacValue)[0];
if (!BitHelper.ConstantTimeEquals(newAuthenticationTag, authenticationTag))
{
return(null);
}
return(decrypted);
}