b64.c

#include <stdint.h>
#include <stdlib.h>

#define BLANK 64
static char *enc =
	"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
static uint8_t dec[256];

static void make_dec(void)
{
	char *s;
	if (dec[enc[3]] == 3) /* stupid test, but it works.. */
		return;
	for (s=enc; *s; s++)
		dec[*s] = (s - enc) & 0x3f;
}

static size_t encode_triplet(char *dst, uint8_t *src, size_t n)
{
	uint8_t s[3];

	s[0] = n > 0 ? src[0] : 0;
	s[1] = n > 1 ? src[1] : 0;
	s[2] = n > 2 ? src[2] : 0;

	dst[0] = enc[((s[0]       ) >> 2)                       ];
	dst[1] = enc[((s[0] & 0x03) << 4) | ((s[1] & 0xf0) >> 4)];
	dst[2] = enc[((s[1] & 0x0f) << 2) | ((s[2] & 0xc0) >> 6)];
	dst[3] = enc[((s[2] & 0x3f)     )                       ];

	if (n < 2) dst[2] = enc[BLANK];
	if (n < 3) dst[3] = enc[BLANK];

	return 4;
}

static size_t decode_triplet(char *s, uint8_t *dst)
{
	dst[0] = (dec[s[0]] << 2) | (dec[s[1]] >> 4);
	dst[1] = (dec[s[1]] << 4) | (dec[s[2]] >> 2);
	dst[2] = (dec[s[2]] << 6) | (dec[s[3]]     );

	return 1 + (s[2] != enc[BLANK]) + (s[3] != enc[BLANK]);
}

size_t b64_encode(char *dst, uint8_t *src, size_t n)
{
	char *d = dst;
	uint8_t *s = src;
	for (; s < src + n; s += 3)
		d += encode_triplet(d, s, src + n - s);
	return d - dst;
}

size_t b64_decode(char *src, uint8_t *dst)
{
	uint8_t *d = dst;
	char *s = src;
	make_dec();
	for (; s[0] && s[1] && s[2] && s[3]; s += 4)
		d += decode_triplet(s, d);
	return d - dst;
}

test.c

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

extern size_t b64_encode(char *dst, uint8_t *src, size_t n);
extern size_t b64_decode(char *src, uint8_t *dst);

char *message =
	"Man is distinguished, not only by his reason, but by this singular "
	"passion from other animals, which is a lust of the mind, that by a "
	"perseverance of delight in the continued and indefatigable generation "
	"of knowledge, exceeds the short vehemence of any carnal pleasure.";
char encd[65536];
char decd[65536];

int main(int argc, char *argv[])
{
	size_t n;

	n = b64_encode(encd, message, strlen(message));
	encd[n] = '\0';
	puts(encd);

	n = b64_decode(encd, decd);
	decd[n] = '\0';
	puts(decd);
}