cdecode.c

/*
cdecoder.c - c source to a base64 decoding algorithm implementation

This is part of the libb64 project, and has been placed in the public domain.
For details, see http://sourceforge.net/projects/libb64
*/

#include <cdecode.h>

int
base64_decode_value (char value_in)
{
  static const char decoding[] =
    { 62, -1, -1, -1, 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1,
    -2, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
    17, 18, 19, 20, 21, 22, 23,
    24, 25, -1, -1, -1, -1, -1, -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
    36, 37, 38, 39, 40, 41, 42,
    43, 44, 45, 46, 47, 48, 49, 50, 51
  };
  static const char decoding_size = sizeof (decoding);
  value_in -= 43;
  if (value_in < 0 || value_in >= decoding_size)
    return -1;
  return decoding[(int) value_in];
}

void
base64_init_decodestate (base64_decodestate * state_in)
{
  state_in->step = step_a;
  state_in->plainchar = 0;
}

int
base64_decode_block (const char *code_in, const int length_in,
		     char *plaintext_out, base64_decodestate * state_in)
{
  const char *codechar = code_in;
  char *plainchar = plaintext_out;
  signed char fragment;

  *plainchar = state_in->plainchar;

  switch (state_in->step)
    {
      while (1)
	{
    case step_a:
	  do
	    {
	      if (codechar == code_in + length_in)
		{
		  state_in->step = step_a;
		  state_in->plainchar = *plainchar;
		  return plainchar - plaintext_out;
		}
	      fragment = (char) base64_decode_value (*codechar++);
	    }
	  while (fragment < 0);
	  *plainchar = (fragment & 0x03f) << 2;
    case step_b:
	  do
	    {
	      if (codechar == code_in + length_in)
		{
		  state_in->step = step_b;
		  state_in->plainchar = *plainchar;
		  return plainchar - plaintext_out;
		}
	      fragment = (char) base64_decode_value (*codechar++);
	    }
	  while (fragment < 0);
	  *plainchar++ |= (fragment & 0x030) >> 4;
	  *plainchar = (fragment & 0x00f) << 4;
    case step_c:
	  do
	    {
	      if (codechar == code_in + length_in)
		{
		  state_in->step = step_c;
		  state_in->plainchar = *plainchar;
		  return plainchar - plaintext_out;
		}
	      fragment = (char) base64_decode_value (*codechar++);
	    }
	  while (fragment < 0);
	  *plainchar++ |= (fragment & 0x03c) >> 2;
	  *plainchar = (fragment & 0x003) << 6;
    case step_d:
	  do
	    {
	      if (codechar == code_in + length_in)
		{
		  state_in->step = step_d;
		  state_in->plainchar = *plainchar;
		  return plainchar - plaintext_out;
		}
	      fragment = (char) base64_decode_value (*codechar++);
	    }
	  while (fragment < 0);
	  *plainchar++ |= (fragment & 0x03f);
	}
    }
  /* control should not reach here */
  return plainchar - plaintext_out;
}