base64

#include <stdio.h>
#include <stdint.h>

/* base64 charset for fast encoding */
static const uint8_t enc_ch[] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/* decode charset */
static const uint8_t dec_ch[] = {
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 62, 0, 0, 0, 63,
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 0, 0, 0, 0, 0, 0,
    0, 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, 0, 0, 0, 0, 0,
    0, 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 void b64enc_block(uint8_t *in, uint8_t *out, size_t len)
{
    out[0] = enc_ch[in[0] >> 2];
    out[1] = enc_ch[((in[0] & 0x03) << 4) | ((in[1] & 0xf0) >> 4)];
    out[2] = len > 1 ? enc_ch[((in[1] & 0x0f) << 2) | ((in[2] & 0xc0) >> 6)] : '=';
    out[3] = len > 2 ? enc_ch[in[2] & 0x3f] : '=';
}

static void b64dec_block(uint8_t *in, uint8_t *out)
{
    out[0] = dec_ch[in[0]] << 2 | dec_ch[in[1]] >> 4;
    out[1] = dec_ch[in[1]] << 4 | dec_ch[in[2]] >> 2;
    out[2] = ((dec_ch[in[2]] << 6) & 0xc0) | dec_ch[in[3]];
}

int b64enc(uint8_t *in, uint8_t *out, size_t len)
{
    int i, o;
    for(i = 0, o = 0; i < len; i += 3, o += 4)
        b64enc_block(in + i, out + o, len - i);
    return o;
}

int b64dec(uint8_t *in, uint8_t *out, size_t len)
{
    int i, o;
    for(i = 0, o = 0; i < len; i += 4, o += 3)
        b64dec_block(in + i, out + o);
    return o;
}

#ifdef STANDALONE

/* simple main to test the functions */
int main(int argc, char *argv[])
{
    /* usually base64 text is formatted in 76 characters line so
     * every 57 input bytes we get 76 characters (57 / 3 * 4 = 76)
     */
    uint8_t dec[58];
    uint8_t enc[77];
    size_t len;

    if(argc != 2 || argv[1][0] != '-' || (argv[1][1] != 'd' && argv[1][1] != 'e')) {
        fprintf(stderr, "usage: %s [-e|-d]\n", *argv);
        return 1;
    }

    if(argv[1][1] == 'e')
        while((len = fread(dec, 1, 57, stdin))) {
            len = b64enc(dec, enc, len);
            enc[len] = 0;
            puts((char *)enc);
        }
    else
        while((len = fread(enc, 1, 76, stdin))) {
            len = b64dec(enc, dec, len);
            dec[len] = 0;
            fputs((char *)dec, stdout);
            /* skip the \n */
            if(!fread(enc, 1, 1, stdin))
                return 0;
        }

    fclose(stdin);
    fclose(stdout);
    return 0;
}

#endif