b64.c

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

static const char encode[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
			     "abcdefghijklmnopqrstuvwxyz0123456789+/";
static const char decode[] = "|$$$}rstuvwxyz{$$$$$$$>?@ABCDEFGHIJKLMNOPQRSTUVW"
			     "$$$$$$XYZ[\\]^_`abcdefghijklmnopq";

int b64_encode(const uint8_t *in, int in_len, char *out, int out_size)
{
	uint8_t triple[3];
	int i;
	int len;
	int line = 0;
	int done = 0;

	while(in_len){
		len = 0;
		for(i = 0; i < 3; i++){
			if(in_len){
				triple[i] = *in++;
				len++;
				in_len--;
			}else
				triple[i] = 0;
		}
		if(!len)
			continue;

		if(done + 4 >= out_size)
			return -1;

		*out++ = encode[triple[0] >> 2];
		*out++ = encode[((triple[0] & 0x03) << 4) |
					     ((triple[1] & 0xf0) >> 4)];
		*out++ = (len > 1 ? encode[((triple[1] & 0x0f) << 2) |
					     ((triple[2] & 0xc0) >> 6)] : '=');
		*out++ = (len > 2 ? encode[triple[2] & 0x3f] : '=');

		done += 4;
		line += 4;
	}

	if(done + 1 >= out_size)
		return -1;

	*out++ = '\0';

	return done;
}

int b64_decode(const char *in, uint8_t *out, int out_size)
{
	int len;
	int i;
	int done = 0;
	uint8_t v;
	uint8_t quad[4];

	while(*in){

		len = 0;
		for(i = 0; i < 4 && *in; i++){

			v = 0;
			while(*in && !v){

				v = *in++;
				v = (v < 43 || v > 122) ? 0 : decode[v - 43];
				if(v)
					v = (v == '$') ? 0 : v - 61;
				if(*in){
					len++;
					if(v)
						quad[i] = v - 1;
				}else
					quad[i] = 0;
			}
		}
		if(!len)
			continue;

		if(out_size < (done + len - 1))
			/* out buffer is too small */
			return -1;

		if(len >= 2)
			*out++ = quad[0] << 2 | quad[1] >> 4;
		if(len >= 3)
			*out++ = quad[1] << 4 | quad[2] >> 2;
		if(len >= 4)
			*out++ = ((quad[2] << 6) & 0xc0) | quad[3];

		done += len - 1;
	}

	if(done + 1 >= out_size)
		return -1;

	*out++ = '\0';

	return done;
}