crypt.c

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <assert.h>

int main(int argc, char *argv[]) {
	assert(argc == 3);
	int i;
	char key = tolower(argv[2][0]);
	char op = tolower(argv[1][0]);
	char buff[1000];
	
	assert(key >= 'a' && key <= 'z');

	while(fgets(buff, 1000, stdin)) {
		for(i = 0;  i < strlen(buff); i++) {
			if(tolower(buff[i]) >= 'a' && tolower(buff[i]) <= 'z') {
				char offset = buff[i] < 'a' ? 'A' : 'a';
				char buffKey = buff[i] < 'a' ? toupper(key) : key;
				// Obtém a chave simétrica
				if(op == 'd' && buffKey != offset) 
					buffKey = 2 * offset + 26 - buffKey;

				buff[i] = offset + (buff[i] + buffKey - 2 * offset) % 26;
			}
		}
		fputs(buff, stdout);
	}

	return 0;
}