EVP_aes_256_gcm - Retrieving the MAC value

[Roar Lien]

Hi,
This is an API question on using OpenSSL and AES GCM. I am successfully able to 
perform encryption / decryption and I am also able to detect errors in the mac 
value. The code I am using for this is below. I want to retrieve the mac value 
to my own buffer, in addition to just doing the internal comparison. I tried 
this by providing a local buffer in the function call 'DecryptFinal(..)', but 
nothing is written to the buffer. What am I doing wrong?

- Roar

int decryptAes256Gcm(byte_t* pIv, int ivLen,
  byte_t* pKey, int keyLen,
                       byte_t* pAAD, int AADLen,
                       byte_t* pCipherText, int cipherTextLen,
                       byte_t* pClearText,
  byte_t* pMAC, int MACLen)
{
       int                  iResult = 0;
       EVP_CIPHER_CTX       ctx;
       int                  bytesProcessed;

       EVP_CIPHER_CTX_init(&ctx);
EVP_DecryptInit_ex(&ctx, EVP_aes_256_gcm(), NULL, pKey, pIv);
EVP_CIPHER_CTX_ctrl(&ctx, EVP_CTRL_GCM_SET_TAG, MACLen, pMAC);
EVP_DecryptUpdate(&ctx, NULL, &bytesProcessed, pAAD, AADLen);
EVP_DecryptUpdate(&ctx, pClearText, &bytesProcessed, pCipherText, 
cipherTextLen);

       if(NULL != pMAC)
       {
              byte_t localTag[GCM_BLOCK_SIZE];
              int localTagLen = GCM_BLOCK_SIZE;

              if(1 != EVP_DecryptFinal_ex(&ctx, localTag, &bytesProcessed))
              {
                     goto CleanUp;
              }
       }
       else
       {
              if(!EVP_DecryptFinal_ex(&ctx, NULL, &bytesProcessed))
              {
                     goto CleanUp;
              }
       }

       iResult = 1;

CleanUp:
       EVP_CIPHER_CTX_cleanup(&ctx);

       return iResult;
}