base64.c

static const char *BASE64STR = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int getBase64Length(int len)
{
	return (len+2)/3*4;
}

static void b64(char in1, char in2, char in3, char *out)
{
	/*
	bytes	012345 67 0123 4567 01 234567
	base64	234567 23 4567 2345 67 234567
	*/
	char base64[4] = {0,};

	base64[0] =   ( in1 >> 2 ) & 0x3F;
	base64[1] = ( ( in1 << 4 ) & 0x30 ) | ( ( in2 >> 4 ) & 0x0F );
	base64[2] = ( ( in2 << 2 ) & 0x3C ) | ( ( in3 >> 6 ) & 0x03 );
	base64[3] =   ( in3      ) & 0x3F;

	out[0] = BASE64STR[base64[0]];
	out[1] = BASE64STR[base64[1]];
	out[2] = BASE64STR[base64[2]];
	out[3] = BASE64STR[base64[3]];
}

int getBase64(const char *in, int in_len, char *out, int out_len, char padding)
{
	int cursor = 0;

    if( in_len < 0) return 0;
	if(out_len < getBase64Length(in_len)) return -1;

	// three bytes bundle
	for(cursor = 2; cursor < in_len; cursor += 3) {
		b64(in[cursor-2], in[cursor-1], in[cursor], out);
		out += 4;
	}

	// tail bytes
	if(cursor == in_len   ) { b64(in[cursor-2], in[cursor-1], 0, out); out[3] = padding; }
	if(cursor == in_len +1) { b64(in[cursor-2],            0, 0, out); out[2] = out[3] = padding; }
	if(cursor == in_len +2) { /* do nothing */ }

	return 0;
}