base64

base64.cpp

/**

ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/

00, 01, 02, 03, 04, 05, 06, 07, 08, 09, 0A, 0B, 0C, 0D, 0E, 0F,
10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E, 1F,
20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 2A, 2B, 2C, 2D, 2E, 2F,
30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 3A, 3B, 3C, 3D, 3E, 3F,

The two functions just avoid having to use a lookup table for both encoding and decoding.

*/
#include <cstdio>
#include <cstdint>
#include <cstdlib>
#include <cstring>

char 
base64_encode(int c) {
    return (c < 26) ? c + 'A' :
           (c < 52) ? (c - 26) + 'a' :
           (c < 62) ? (c - 52) + '0' :
           (c == 63) ? '/' : '+';
}

char 
base64_decode(int c) {
    return (c == '+') ? 62 :
           (c == '/') ? 63 :
           (c <= '9') ? (c - '0') + 52 :
           (c <= 'Z') ? (c - 'A') : (c - 'a') + 26;
}

void
base64(size_t outlen, void *inbuf, void *outbuf) {
    uint8_t *out = (uint8_t*)outbuf;
    uint8_t *in = (uint8_t*)inbuf;
    uint32_t x = 0, z = 0;
 
    while (outlen) {
        x = (x << 8) | *in++;
        z += 8;
        while (z >= 6) {
            z -= 6;
            uint8_t c = (x >> z) & 63;
            *out++ = (c < 26) ? c + 'A' : (c < 52) ? (c - 26) + 'a' : (c < 62) ? (c - 52) + '0' : (c == 63) ? '/' : '+';
            outlen--;
        }
    }
}

int
main(void) {
    uint8_t data[64]={0}, enc[64]={0}, dec[64]={0};

    for (int i=0; i<sizeof(data); i++) {
        data[i] = i;
        enc[i] = base64_encode(data[i]);
        printf("%c", enc[i]);
    }
    
    printf("\n");
    
    for (int i=0; i<sizeof(data); i++) {
        printf("%02X, ", base64_decode(enc[i]));
        if ((i & 15) == 15) putchar('\n');
    }
    return 0;
}