t_otp.c

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

#define KEYFILE "otpkey.dat"
#define MAXMSGLEN 1024

char ktext[MAXMSGLEN] = {0};

void
genkey(const char *path)
{
	FILE *f, *r;
	size_t rbytes, wbytes;

	printf("Generating key ... \n");
	if ((r = fopen("/dev/random", "r")) == NULL)
		err(1, "Failed to open random(4)");

	if ((f = fopen(KEYFILE, "w+")) == NULL)
		err(1, "Failed to create key file");

	rbytes = fread(ktext, MAXMSGLEN, 1, r);
	wbytes = fwrite(ktext, MAXMSGLEN, 1, f);

	if (rbytes != wbytes)
		errx(1, "Failed to write key file");

	fclose(r);
	fclose(f);
}

void
loadkey(const char *path)
{
	FILE *f;
	size_t rbytes;

	printf("Loading key ... \n");
	f = fopen(KEYFILE, "rw");

	if (f == NULL)
		errx(1, "Failed to load key file");

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

	if (rbytes != 1)
		errx(1, "Failed to read key file. %jd bytes read", rbytes);

	fclose(f);
}

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

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

int
decrypt(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 [-ed]\n", progname);
}

int
main(int argc, char *argv[])
{
	FILE *kf;
	char c;
	char buf[MAXMSGLEN];
	char cbuf[MAXMSGLEN];

	int opt_encryt = 0;
	int opt_decrypt = 0;

	bzero(buf, MAXMSGLEN);
	bzero(cbuf, MAXMSGLEN);

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

	if ((kf = fopen(KEYFILE, "r")) == NULL)
		genkey(KEYFILE);
	else
		loadkey(KEYFILE);

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

	if (opt_encryt) {
		fgets(buf, MAXMSGLEN, stdin);
		encrypt(buf, cbuf, strlen(buf));
		fprintf(stdout, "Encrypted: %s\n", cbuf);
	} else if (opt_decrypt) {
		decrypt(cbuf, buf, strlen(cbuf));
		fprintf(stdout, "Decrypted: %s\n", buf);
	} else {
		usage(argv[0]);
	}

	return 0;
}