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"
#include <stdint.h>

static int base64_decode_value_signed(int8_t value_in){
  static const int8_t 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 int8_t 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;
}

static int base64_decode_block_signed(const int8_t* code_in, const int length_in, int8_t* plaintext_out, base64_decodestate* state_in){
  const int8_t* codechar = code_in;
  int8_t* plainchar = plaintext_out;
  int8_t 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 = (int8_t)base64_decode_value_signed(*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 = (int8_t)base64_decode_value_signed(*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 = (int8_t)base64_decode_value_signed(*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 = (int8_t)base64_decode_value_signed(*codechar++);
        } while (fragment < 0);
        *plainchar++   |= (fragment & 0x03f);
    }
  }
  /* control should not reach here */
  return plainchar - plaintext_out;
}

static int base64_decode_chars_signed(const int8_t* code_in, const int length_in, int8_t* plaintext_out){
  base64_decodestate _state;
  base64_init_decodestate(&_state);
  int len = base64_decode_block_signed(code_in, length_in, plaintext_out, &_state);
  if(len > 0) plaintext_out[len] = 0;
  return len;
}

int base64_decode_value(char value_in){
  return base64_decode_value_signed(*((int8_t *) &value_in));
}

int base64_decode_block(const char* code_in, const int length_in, char* plaintext_out, base64_decodestate* state_in){
  return base64_decode_block_signed((int8_t *) code_in, length_in, (int8_t *) plaintext_out, state_in);
}

int base64_decode_chars(const char* code_in, const int length_in, char* plaintext_out){
  return base64_decode_chars_signed((int8_t *) code_in, length_in, (int8_t *) plaintext_out);
}