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;
        // fall through
      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;
        // fall through
      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;
        // fall through
      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);
}