#include <stdio.h>
#include <stdlib.h>

int caesar(int key, char ch)
{
	return (ch + key) % 256;
}

int encrypt_file(int key, FILE* file)
{
	int ch = 0;
	while ( (ch = fgetc(file)) != EOF )
		fputc( caesar(key, ch), stdout);

	return 0;
}

int main(int argc, char* argv[])
{
	if ( argc <= 1 )
	{
		printf("Usage: caesar <key> [FILES]\n");
		return 0;
	}

	int key = atoi(argv[1]);

	if ( argc == 2 )
		return encrypt_file(key, stdin);

	for (int i = 2; i < argc; ++i )
	{
		FILE* file = fopen(argv[i], "r");
		if ( ! file ) return fprintf(stderr, "caesar: could not open %s\n", argv[i]);
		encrypt_file(key, file);
		fclose(file);
	}
}