base64.c

#include <stdio.h>
#include <stdlib.h>  // malloc
#include <string.h>
#include <stdarg.h>  // va_start, va_end

#define zmem(mem, n) memset(mem, 0, n)
#define STR_LEN 1024

static int b64enc_internal(const unsigned char *data, int len, char *dest)
{
    /* base64 alphabet, taken from rfc3548 */
    char *b64alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    char *buf = dest;

    /* Encode three bytes per iteration a la rfc3548. */
    while (len >= 3) {
        buf[0] = b64alpha[(data[0] >> 2) & 0x3f];
        buf[1] = b64alpha[((data[0] << 4) & 0x30) | ((data[1] >> 4) & 0xf)];
        buf[2] = b64alpha[((data[1] << 2) & 0x3c) | ((data[2] >> 6) & 0x3)];
        buf[3] = b64alpha[data[2] & 0x3f];
        data += 3;
        buf += 4;
        len -= 3;
    }

    /* Pad the remaining bytes. len is 0, 1, or 2 here. */
    if (len > 0) {
        buf[0] = b64alpha[(data[0] >> 2) & 0x3f];
        if (len > 1) {
            buf[1] = b64alpha[((data[0] << 4) & 0x30) | ((data[1] >> 4) & 0xf)];
            buf[2] = b64alpha[(data[1] << 2) & 0x3c];
        } else {
            buf[1] = b64alpha[(data[0] << 4) & 0x30];
            buf[2] = '=';
        }
        buf[3] = '=';
        buf += 4;
    }

    /*
     * As mentioned in rfc3548, we need to be careful about
     * how we null terminate and handle embedded null-termination.
     */
    *buf = '\0';

    return (buf - dest);
}

void fatal(char *fmt, ...) {
  va_list ap;

  va_start(ap, fmt);
  fflush(stdout);
  vfprintf(stderr, fmt, ap);
  fprintf(stderr, "\nQUITTING!\n");
  va_end(ap);
  exit(1);
}

void *safe_malloc(size_t size) {
  void *mymem;

  if ((int)size < 0)            /* Catch caller errors */
    fatal("Tried to malloc negative amount of memory!!!");
  mymem = malloc(size);
  if (mymem == NULL)
    fatal("Malloc Failed! Probably out of space.");
  return mymem;
}

/* Take in plain text and encode into base64. */
char *b64enc(const unsigned char *data, int len)
{
    char *dest;

    /* malloc enough space to do something useful */
    dest = (char *) safe_malloc(4 * len / 3 + 4);

    dest[0] = '\0';

    /* Call internal function to base64 encode data */
    b64enc_internal(data, len, dest);

    return (dest);
}

// https://c.runoob.com/front-end/693/
int main(int argc, char *argv[]) {
	char s[STR_LEN];
	char *t;
	
	zmem(s, STR_LEN);
	strcpy(s, "runoob");
	t = b64enc(s, strlen(s));
	fprintf(stdout, "%s\n", t); // cnVub29i
	free(t);
	
	return 0;
}