private string InternalBuild(string payload, string alg, string enc, JsonWebKey jsonWebKey, byte[] hmacKey = null)
{
var contentEncryptionKeyHandler = _cekHandlers.FirstOrDefault(e => e.AlgName == alg);
var encEncryptionHandler = _encHandlers.FirstOrDefault(e => e.EncName == enc);
if (contentEncryptionKeyHandler == null)
{
throw new JwtException(ErrorCodes.INTERNAL_ERROR, string.Format(ErrorMessages.UNKNOWN_ALG, alg));
}
if (encEncryptionHandler == null)
{
throw new JwtException(ErrorCodes.INTERNAL_ERROR, string.Format(ErrorMessages.UNKNOWN_ENC, enc));
}
var cek = BitHelper.GenerateRandomBytes(encEncryptionHandler.KeyLength);
var encCEK = contentEncryptionKeyHandler.Encrypt(cek, jsonWebKey);
var splittedCEK = BitHelper.SplitInHalf(cek);
var iv = BitHelper.GenerateRandomBytes(128 /*encEncryptionHandler.KeyLength / 2*/);
if (hmacKey == null)
{
hmacKey = splittedCEK.First();
}
var key = splittedCEK.Last();
var cipherText = encEncryptionHandler.Encrypt(payload, key, iv);
var protectedHeader = JsonConvert.SerializeObject(new JweHeader(alg, enc, jsonWebKey.Kid), Formatting.None,
new JsonSerializerSettings
{
NullValueHandling = NullValueHandling.Ignore
});
var protectedHeaderJson = protectedHeader.ToString();
var aad = Encoding.ASCII.GetBytes(Encoding.UTF8.GetBytes(protectedHeaderJson).Base64EncodeBytes());
var al = BitHelper.LongToBytes(aad.Length * 8);
var hmacInput = BitHelper.Concat(aad, iv, cipherText, al);
var hmacValue = encEncryptionHandler.BuildHash(hmacKey, hmacInput);
var authTag = BitHelper.SplitInHalf(hmacValue).First();
var base64EncodedjweProtectedHeaderSerialized = protectedHeader.ToString().Base64Encode();
var base64EncodedJweEncryptedKey = encCEK.Base64EncodeBytes();
var base64EncodedIv = iv.Base64EncodeBytes();
var base64EncodedCipherText = cipherText.Base64EncodeBytes();
var base64EncodedAuthenticationTag = authTag.Base64EncodeBytes();
return(base64EncodedjweProtectedHeaderSerialized + "." +
base64EncodedJweEncryptedKey + "." +
base64EncodedIv + "." +
base64EncodedCipherText + "." +
base64EncodedAuthenticationTag);
}