b64.c

/*
 * base64
 * with no arguments read from stdin and write base64 encoded output to stdout
 * with argument -d read base64 encoded input from stdin and write decoded output to stdout
 * exit status 1 on any error
 * ignores newlines in encoded input
 * any illegal character in encoded input causes error
 * no newlines in encoded output
 * does not require padding (=) but if padding is present it must be correct
 * after padding more encoded input may follow
 */

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

char code[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int encode(int c)
{
    static unsigned char s[3] = { 0 };
    static int i = 0;

    if (c == EOF)
        for (int j = i; j < 3; j++)
            s[j] = 0;
    else
        s[i++] = c;

    if (i == 3 || (i && c == EOF)) {
        putchar(        code[(s[0] >> 2            )       ]      );
        putchar(        code[(s[0] << 4 | s[1] >> 4) & 0x3f]      );
        putchar(i > 1 ? code[(s[1] << 2 | s[2] >> 6) & 0x3f] : '=');
        putchar(i > 2 ? code[(s[2]                 ) & 0x3f] : '=');
        i = 0;
    }
    return c != EOF;
}

int decode(int c)
{
    static char s[4] = { 0 };
    static int i = 0;
    char *p = strchr(code, c);

    if (c == '\n')
        return 1;

    if ((c == '=' && i < 2)          ||
        (c == EOF && i && i < 2)     ||
        (c != EOF && c != '=' && !p) ||
        (c == '=' && i == 2 && (c = getchar()) != '='))
           exit(1);

    if (p)
        s[i++] = p - code;

    if (i == 4 || c == '=' || c == EOF) {
        if (i > 1) putchar((s[0] << 2 | s[1] >> 4) & 0xff);
        if (i > 2) putchar((s[1] << 4 | s[2] >> 2) & 0xff);
        if (i > 3) putchar((s[2] << 6 | s[3]     ) & 0xff);
        i = 0;
    }
    return c != EOF;
}

int main(int argc, char **argv)
{
    int (*fn)(int) = encode;

    if (argc == 2 && !strcmp(argv[1], "-d"))
        fn = decode;
    else if (argc != 1)
        exit(1);

    while (fn(getchar()))
        ;
    return 0;
}