Introduction

The example 'C' program below shows how to encode and decode a string using the Base64 algorithm. Base64 encoding is used to convert binary data into a text-like format to be transported in environments that handle only text safely. For example, encoding UID's for use in HTTP URL's or to encode encryption keys to make them safely portable through e-mail, display them in HTML pages and use them with copy and paste.

The example below is a raw implementation to illustrate the algorithm. For programming convenience, the C libaries below provide functions for base64 handling: 

/* ------------------------------------------------------------------------ *
 * file:        base64_stringencode.c v1.0                                  *
 * purpose:     tests encoding/decoding strings with base64                 *
 * author:      02/23/2009 Frank4DD                                         *
 *                                                                          *
 * source:      http://base64.sourceforge.net/b64.c for encoding            *
 *              http://en.literateprograms.org/Base64_(C) for decoding      *
 * ------------------------------------------------------------------------ */
#include <stdio.h>
#include <string.h>

/* ---- Base64 Encoding/Decoding Table --- */
char b64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/* decodeblock - decode 4 '6-bit' characters into 3 8-bit binary bytes */
void decodeblock(unsigned char in[], char *clrstr) {
  unsigned char out[4];
  out[0] = in[0] << 2 | in[1] >> 4;
  out[1] = in[1] << 4 | in[2] >> 2;
  out[2] = in[2] << 6 | in[3] >> 0;
  out[3] = '\0';
  strncat(clrstr, out, sizeof(out));
}

void b64_decode(char *b64src, char *clrdst) {
  int c, phase, i;
  unsigned char in[4];
  char *p;

  clrdst[0] = '\0';
  phase = 0; i=0;
  while(b64src[i]) {
    c = (int) b64src[i];
    if(c == '=') {
      decodeblock(in, clrdst); 
      break;
    }
    p = strchr(b64, c);
    if(p) {
      in[phase] = p - b64;
      phase = (phase + 1) % 4;
      if(phase == 0) {
        decodeblock(in, clrdst);
        in[0]=in[1]=in[2]=in[3]=0;
      }
    }
    i++;
  }
}

/* encodeblock - encode 3 8-bit binary bytes as 4 '6-bit' characters */
void encodeblock( unsigned char in[], char b64str[], int len ) {
    unsigned char out[5];
    out[0] = b64[ in[0] >> 2 ];
    out[1] = b64[ ((in[0] & 0x03) << 4) | ((in[1] & 0xf0) >> 4) ];
    out[2] = (unsigned char) (len > 1 ? b64[ ((in[1] & 0x0f) << 2) |
             ((in[2] & 0xc0) >> 6) ] : '=');
    out[3] = (unsigned char) (len > 2 ? b64[ in[2] & 0x3f ] : '=');
    out[4] = '\0';
    strncat(b64str, out, sizeof(out));
}

/* encode - base64 encode a stream, adding padding if needed */
void b64_encode(char *clrstr, char *b64dst) {
  unsigned char in[3];
  int i, len = 0;
  int j = 0;

  b64dst[0] = '\0';
  while(clrstr[j]) {
    len = 0;
    for(i=0; i<3; i++) {
     in[i] = (unsigned char) clrstr[j];
     if(clrstr[j]) {
        len++; j++;
      }
      else in[i] = 0;
    }
    if( len ) {
      encodeblock( in, b64dst, len );
    }
  }
}

int main() {
  //char mysrc[] = "My bonnie is over the ocean...";
  char mysrc[] = "My bonnie is over the          ";
  char myb64[1024] = "";
  char mydst[1024] = "";

  b64_encode(mysrc, myb64);
  printf("The string\n[%s]\nencodes into base64 as:\n[%s]\n", mysrc, myb64);
  printf("\n");
  b64_decode(myb64, mydst);
  printf("The string\n[%s]\ndecodes from base64 as:\n[%s]\n", myb64, mydst);

  return 0;
}

A run of this test program returns the following output: 

# ./base64_stringencode
The string
[My bonnie is over the          ] 
encodes into base64 as:
[TXkgYm9ubmllIGlzIG92ZXIgdGhlICAgICAgICAgIA==]

The string
[TXkgYm9ubmllIGlzIG92ZXIgdGhlICAgICAgICAgIA==]
decodes from base64 as:
[My bonnie is over the          ]

Sample Code:

See Also: