Bug in XSCryptCryptoBase64.cpp

[David Landwehr]

Hi,

The XSCryptCryptoBase64 has a bug in the base 64 encoding routine (encode).
The problem is the following type of logic:

 t = ((m_inputBuffer[i++] << 4) & 0x30) | (m_inputBuffer[i] >> 4);

Normally i will only be increment after the entire expression, so it is
identical to:
  t = ((m_inputBuffer[i] << 4) & 0x30) | (m_inputBuffer[i] >> 4);
  i++;

However because Microsoft Visual C Compiler seems to have a problem with
overloaded [] operator, your code works as you original expected when you
compile in Debug mode. That is it will increment i just before evaluating
the r-value of the |. In Release mode however the compiler will evaluate the
expression in the correct form:
  t = ((m_inputBuffer[i] << 4) & 0x30) | (m_inputBuffer[i] >> 4);

And this makes an incorrect base 64 encoding. 

To solve the problem you should write the code as:
  t = ((m_inputBuffer[i] << 4) & 0x30) | (m_inputBuffer[i+1] >> 4);
  i++;

Best regards,
David

PS for fun, this is a piece of code demonstrating the problem in MVC (try
compile it in Release and Debug mode and see the difference in output):
class test {
  int* b;
public:
  test() {
    b = new int[2];
    b[0] = 0x0f;
    b[1] = 0xf0;
  }


  int operator [](int i) {
    return b[i];
  }
};

int _tmain(int argc, _TCHAR* argv[])
{
  test b;
  int i=0;
  //t = ((m_inputBuffer[i++] << 2) & 0x3C) | (m_inputBuffer[i] >> 6);
 
  int k = ((b[i++]<<4) & 0xff ) | (b[i] >> 4);

  printf("The correct value should be 0x0f=0x%x, i=%d\n", k, i);

  return 0;
}