Expected output `Y2lhbyBjb21lIHN0YWk/`

base64.c

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

void encode(unsigned char data[21])
{
	unsigned char base[15];
	memcpy(base, data, 15);
	for(int x = 0; x < 20; x++) {
		unsigned char temp = 0b11111100;
		temp &= base[0];
		temp = temp >> 2;
		if(temp < 26) {
			data[x] = temp + 'A';
		} else if(temp >= 26 && temp <= 51) {
			data[x] = temp - 26 + 'a';
		} else if(temp >= 52 && temp <= 61) {
			data[x] = temp - 52 + '0';
		} else if(temp == 62) {
			data[x] = '+';
		} else if(temp == 63) {
			data[x] = '/';
		}

		// Shift everything left
		for(int y = 0; y < 14; y++) {
			base[y] = base[y] << 6;
			temp = 0b11111100;
			temp &= base[y + 1];
			temp = temp >> 2;
			base[y] |= temp;
		}
		base[14] = base[14] << 6;
	}
	data[20] = '\0';
}

int main()
{
	unsigned char data[21] = "ciao come stai?";
	encode(data);
	printf("%s\n", data);
}