Question

In that intercepted function below, I have buff as a pointer to buffer, how can I change data without creating new my own buffer?

int WINAPI OwnRecv(SOCKET s, char FAR *buff, int len, int flags)
{
    if(s == ServerSocket)
    {
        int received = pTrampolineRecv(s, buff, len, flags);

        if(received <= 0)
        {
            return received;
        }

            unsigned char ReceiveBuffer[1024]; // how can i avoid creatin this by changing buff directly?
        do_decrypt((const unsigned char *) buff, ReceiveBuffer, received, KeyTest, NULL);
        buff = (char *) ReceiveBuffer;

        return received;
    }
    return pTrampolineRecv(s, buff, len, flags);
}

upd

I added my do_decrypt function

bool do_decrypt(const unsigned char *in, unsigned char *out, int inlen)
{
  int buflen, tmplen;

  if(!EVP_DecryptUpdate(&Decrypt_ctx, out, &buflen, in, inlen))
  {
        return false;
  }

  if(!EVP_DecryptFinal_ex(&Decrypt_ctx, out + buflen, &tmplen))
    {
        return false;
  }

  return true;
}

Answer 1

You should definitely take a look at C++ tutorial about pointers before jumping straight on WinAPI code.

Also, FAR keyword is deprecated.

The simplest idea would be

do_decrypt ((const unsigned char*) buff, (unsigned char *) buff, received, KeyTest, NULL);

So basically – putting buff as both input and output variable.

But I DO NOT recommend using it, as I don’t know how do_decrypt function works – you might end messing up your own data when decrypting. Only use above if do_decrypt is using some sort of (possibly smaller) internal buffer.

Maybe try to clarify the question a bit more, as now I won’t be able to give you any more advice on the question.

EDIT: So, thanks to other answers, you can use solution above when:

1. Result data will fit in the input buffer (size of output <= size of input)
2. do_decrypt function allows in situ operation – thus, modyfing data that’s being read.