based on http://stackoverflow.com/questions/342409/how-do-i-base64-encode-decode-in-c, UTF-8 is not supported. (require libc)

base64.c

#include "base64.h"

void build_decoding_table() 
{
	int i;
	decode_table = (char*) malloc(128);

	for (i = 0; i < 64; i++)
		decode_table[encode_table[i]] = i;
}

char* base64_encode(const char* src)
{
	int len, outLen, a, b, c, triple, i, j;

	len = strlen(src);
	outLen = 4 * ((len+2) / 3);
	
	char* pool = (char*) malloc(outLen + 1);

	if (!pool)
		return NULL;

	for (i = 0; i < outLen + 1; i++)
		pool[i] = 0;

	for (i = 0, j = 0; i < len; ) 
	{
		a = i < len ? src[i++] : 0;
		b = i < len ? src[i++] : 0;
	    c = i < len ? src[i++] : 0;

		triple = (a << 0x10) + (b << 0x08) + c;

		pool[j++] = encode_table[(triple >> 3 * 6) & 0x3F];
		pool[j++] = encode_table[(triple >> 2 * 6) & 0x3F];
		pool[j++] = encode_table[(triple >> 1 * 6) & 0x3F];
		pool[j++] = encode_table[(triple >> 0 * 6) & 0x3F];
	}

	for (i = 0; i < mod_table[len % 3]; i++)
		pool[outLen - 1 - i] = '=';

	return pool;
}

char* base64_decode(const char* src)
{
	if (!decode_table) 
		build_decoding_table();

	int len, outLen, a, b, c, d, triple, i, j;

	len = strlen(src);
	outLen = len / 4 * 3;

	if (len % 4 != 0) 
		return NULL;

	if (src[len - 1] == '=' || src[len - 2] == '=') 
		outLen--;

	char* pool = (char*) malloc(outLen + 1);

	if (!pool)
		return NULL;

	for (i = 0; i < outLen + 1; i++)
		pool[i] = 0;

	for (i = 0, j = 0; i < len;)
	{
		a = src[i] == '=' ? 0 & i++ : decode_table[src[i++]];
		b = src[i] == '=' ? 0 & i++ : decode_table[src[i++]];
		c = src[i] == '=' ? 0 & i++ : decode_table[src[i++]];
		d = src[i] == '=' ? 0 & i++ : decode_table[src[i++]];

		triple = (a << 3 * 6) + (b << 2 * 6) + (c << 1 * 6) + (d << 0 * 6);

		if (j < outLen) pool[j++] = (triple >> 2 * 8) & 0xFF;
		if (j < outLen) pool[j++] = (triple >> 1 * 8) & 0xFF;
		if (j < outLen) pool[j++] = (triple >> 0 * 8) & 0xFF;
	}

	return pool;

}

base64.h

#ifndef _BASE64_H
#define _BASE64_H_

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

static char encode_table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

static char* decode_table = NULL;
static int mod_table[] = {0, 2, 1};

void build_decoding_table();

char* base64_encode(const char* src);
char* base64_decode(const char* src);

#endif