decode.c

C语言实现的一个简易的SMPT服务器代码

/*
 *  Embedded SMTP Server Base64 Decoder
 *
 *  ./software/ch5/emsmtpd/decode.c
 *
 *  [e-mail]
 *
 *  Copyright 2001,2002 M. Tim Jones <[e-mail]>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the
 * Free Software Foundation; either version 2, or (at your option) any
 * later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 */

#include <stdio.h>
#include <fcntl.h>
#include "server.h"

/*
 *  b64decode()
 *
 *  Performs Base64 content decoding.  'src' pointer contains the
 *  input data reference, and 'dest' pointer where the decoded
 *  attachment should be stored.
 *
 *  Returns -1 on error or the number of octets decoded into 'dest'.
 *
 */

int b64decode(char *src, char *dest)
{
  unsigned char c;
  int state = 0, attachlen = 0;

  while (1) {
    c = *src++;
    if (isupper(c)) c = c - 'A';
    else if (isdigit(c)) c -= '0' - 52;
    else if (islower(c)) c -= 'a' - 26;
    else if (c == '/') c = 63;
    else if (c == '+') c = 62;
    else if (c == '=') {

      switch(state++) {
        case 3:
          if (*src == 0x0d) goto fini;
          state = 0;
          break;
        case 2:
          if ((*src == '=') || (*src == 0x0d) ||
              (*src == 0x0a)) goto fini;
        default:
          return(-1);
      }
      continue;
    } else continue;

    switch(state++) {
        case 0:
          dest[attachlen] = c << 2;
          break;
        case 1:
          dest[attachlen++] |= c >> 4;
          dest[attachlen] = c << 4;
          break;
        case 2:
          dest[attachlen++] |= c >> 2;
          dest[attachlen] = c << 6;
          break;
        case 3:
          dest[attachlen++] |= c;
          state = 0;
          break;

    }

    if (attachlen >= MAX_ATTACHMENT) return -1;
  }

fini:

  return(attachlen);
}