t_otp.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <err.h>
#include <unistd.h>

#define MAXMSGLEN 1024

char ktext[MAXMSGLEN] = {0};

void
genkey(void)
{
	FILE *r;
	size_t rbytes;

	if ((r = fopen("/dev/random", "r")) == NULL)
		err(1, "Failed to open random(4)");

	rbytes = fread(ktext, MAXMSGLEN, 1, r);

	if (rbytes != 1)
		err(1, "Failed to generate the key");

	fclose(r);
}

void
enc(const char *msg, char *ctext, size_t len)
{
	int i;

	for (i = 0; i < len; i++)
		ctext[i] = msg[i] ^ ktext[i];
}

void
dec(const char *ctext, char *msg, size_t len)
{
	int i;

	for (i = 0; i < len; i++)
		msg[i] = ctext[i] ^ ktext[i];
}

void
usage(const char *progname)
{
	fprintf(stdout, "usage: %s [-edh]\n", progname);
}

void
print_hexa(char *label, char *buf, int len)
{
	int i;

	printf("%s : ", label);
	for (i = 0; i < len; i++)
		printf("%.2hhx", buf[i]);
	printf("\n");

}

void
hex2char(char *inbuf, char *outbuf, int len)
{
	int i, j;
	char pair[2];

	for (i = 0, j = 0; i < len; i += 2, j++) {
		pair[0] = inbuf[i];
		pair[1] = inbuf[i+1];
		outbuf[j] = (char)strtol(pair, NULL, 16);
	}
}

int
main(int argc, char *argv[])
{
	char c;
	char buf[MAXMSGLEN] = {0};
	char cbuf[MAXMSGLEN] = {0};
	char t1buf[MAXMSGLEN] = {0};
	char t2buf[MAXMSGLEN] = {0};

	int opt_encryt = 0;
	int opt_decrypt = 0;
	int opt_hexa = 0;

	while ((c = getopt(argc, argv, "edh")) != -1) {
		switch(c) {
		case 'e':
			opt_encryt = 1;
			break;
		case 'd':
			opt_decrypt = 1;
			break;
		case 'h':
			opt_hexa = 1;
			break;
		default:
			usage(argv[0]);
		}
	}

	if (opt_encryt && opt_decrypt)
		errx(1, "You can't encrypt and decrypt at once");

	if (opt_encryt) {
		genkey();

		printf("Message to encrypt:\n");
		fgets(buf, MAXMSGLEN, stdin);	/* Get the msg to encrypt */

		enc(buf, cbuf, strlen(buf));

		if (opt_hexa) {
			print_hexa("Key generated", ktext, strlen(buf));
			print_hexa("Encrypted message", cbuf, strlen(buf));
		}
	} else if (opt_decrypt) {
		printf("Enter key:\n");
		fgets(t1buf, MAXMSGLEN, stdin);
		printf("Enter ciphertext:\n");
		fgets(t2buf, MAXMSGLEN, stdin);

		hex2char(t1buf, ktext, MAXMSGLEN);
		hex2char(t2buf, cbuf, MAXMSGLEN);

		dec(cbuf, buf, strlen(ktext));
		fprintf(stdout, "Decrypted: %s\n", buf);
		if (opt_hexa) {
			print_hexa("Key input", ktext, strlen(buf));
			print_hexa("Encrypted message", cbuf, strlen(buf));
		}
	} else {
		usage(argv[0]);
	}

	return 0;
}