base64.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "base64.h"

size_t b64_encoded_size(size_t inlen)
{
    size_t ret;

    ret = inlen;
    if (inlen % 3 != 0)
        ret += 3 - (inlen % 3);
    ret /= 3;
    ret *= 4;

    return ret;
}

char *b64_encode(char *in)
{
    const char b64chars[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    size_t len = strlen(in);
    char   *out;
    size_t  elen;
    size_t  i;
    size_t  j;
    size_t  v;

    if (in == NULL || len == 0)
        return NULL;

    elen = b64_encoded_size(len);
    out  = malloc(elen+1);
    out[elen] = '\0';

    for (i=0, j=0; i<len; i+=3, j+=4) {
        v = in[i];
        v = i+1 < len ? v << 8 | in[i+1] : v << 8;
        v = i+2 < len ? v << 8 | in[i+2] : v << 8;

        out[j]   = b64chars[(v >> 18) & 0x3F];
        out[j+1] = b64chars[(v >> 12) & 0x3F];
        if (i+1 < len) {
            out[j+2] = b64chars[(v >> 6) & 0x3F];
        } else {
            out[j+2] = '=';
        }
        if (i+2 < len) {
            out[j+3] = b64chars[v & 0x3F];
        } else {
            out[j+3] = '=';
        }
    }

    return out;
}

int main(int argc, char **argv)
{
    (void)argc;
    (void)argv;
    char *data = "123456SO+PA=SOPA!";
    char       *enc;

    printf("data:    '%s'\n", data);

    enc = b64_encode(data);
    printf("encoded: '%s'\n", enc);

    free(enc);

    return 0;
}