base64.c

#include <string.h>

char b64string[] =
  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

long base64_encode(to, from, len)
     char *to, *from;
     unsigned int len;
{
  char *fromp = from;
  char *top = to;
  unsigned char cbyte;
  unsigned char obyte;
  char end[3];

  for (; len >= 3; len -= 3) {
    cbyte = *fromp++;
    *top++ = b64string[(int) (cbyte >> 2)];
    obyte = (cbyte << 4) & 0x30;        /* 0011 0000 */

    cbyte = *fromp++;
    obyte |= (cbyte >> 4);      /* 0000 1111 */
    *top++ = b64string[(int) obyte];
    obyte = (cbyte << 2) & 0x3C;        /* 0011 1100 */

    cbyte = *fromp++;
    obyte |= (cbyte >> 6);      /* 0000 0011 */
    *top++ = b64string[(int) obyte];
    *top++ = b64string[(int) (cbyte & 0x3F)];   /* 0011 1111 */
  }

  if (len) {
    end[0] = *fromp++;
    if (--len)
      end[1] = *fromp++;
    else
      end[1] = 0;
    end[2] = 0;

    cbyte = end[0];
    *top++ = b64string[(int) (cbyte >> 2)];
    obyte = (cbyte << 4) & 0x30;        /* 0011 0000 */

    cbyte = end[1];
    obyte |= (cbyte >> 4);
    *top++ = b64string[(int) obyte];
    obyte = (cbyte << 2) & 0x3C;        /* 0011 1100 */

    if (len)
      *top++ = b64string[(int) obyte];
    else
      *top++ = '=';
    *top++ = '=';
  }
  *top = 0;
  return top - to;
}

/* badchar(): check if c is decent; puts either the */
/* location of c or null into p.                  */
#define badchar(c,p) (!(p = memchr(b64string, c, 64)))

long base64_decode(to, from, len)
     char *to, *from;
     unsigned int len;
{
  char *fromp = from;
  char *top = to;
  char *p;
  unsigned char cbyte;
  unsigned char obyte;
  int padding = 0;

  for (; len >= 4; len -= 4) {
    if ((cbyte = *fromp++) == '=')
      cbyte = 0;
    else {
      if (badchar(cbyte, p))
        return -1;
      cbyte = (p - b64string);
    }
    obyte = cbyte << 2;         /* 1111 1100 */

    if ((cbyte = *fromp++) == '=')
      cbyte = 0;
    else {
      if (badchar(cbyte, p))
        return -1;
      cbyte = p - b64string;
    }
    obyte |= cbyte >> 4;        /* 0000 0011 */
    *top++ = obyte;

    obyte = cbyte << 4;         /* 1111 0000 */
    if ((cbyte = *fromp++) == '=') {
      cbyte = 0;
      padding++;
    } else {
      padding = 0;
      if (badchar(cbyte, p))
        return -1;
      cbyte = p - b64string;
    }
    obyte |= cbyte >> 2;        /* 0000 1111 */
    *top++ = obyte;

    obyte = cbyte << 6;         /* 1100 0000 */
    if ((cbyte = *fromp++) == '=') {
      cbyte = 0;
      padding++;
    } else {
      padding = 0;
      if (badchar(cbyte, p))
        return -1;
      cbyte = p - b64string;
    }
    obyte |= cbyte;             /* 0011 1111 */
    *top++ = obyte;
  }

  *top = 0;
  if (len)
    return -1;
  return (top - to) - padding;
}