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Use the safe idiom of cleaning sensitive data from memory with explicit_bzero,
instead of relying on other methods, after readpassphrase. Some programs on
this diff won't benefit that much since it happens near the terminal path, but
someone might copy the unsafe idiom to another program and place it where it
may leak sensitive data.

Discussed aeons ago with tb@, OK deraadt@ and beck@

/*	$OpenBSD: skey.c,v 1.34 2017/05/03 09:51:39 mestre Exp $	*/
/*
 * OpenBSD S/Key (skey.c)
 *
 * Authors:
 *          Neil M. Haller <nmh@thumper.bellcore.com>
 *          Philip R. Karn <karn@chicago.qualcomm.com>
 *          John S. Walden <jsw@thumper.bellcore.com>
 *          Scott Chasin <chasin@crimelab.com>
 *          Todd C. Miller <Todd.Miller@courtesan.com>
 *
 *
 * Stand-alone program for computing responses to S/Key challenges.
 * Takes the iteration count and seed as command line args, prompts
 * for the user's key, and produces both word and hex format responses.
 *
 * Usage example:
 *	>skey 88 ka9q2
 *	Enter password:
 *	OMEN US HORN OMIT BACK AHOY
 *	>
 *
 */

#include <err.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <readpassphrase.h>
#include <skey.h>

void usage();

extern char *__progname;

int
main(int argc, char *argv[])
{
	int     n, i, cnt = 1, pass = 0, hexmode = 0;
	char    passwd[SKEY_MAX_PW_LEN+1], key[SKEY_BINKEY_SIZE];
	char	buf[33], *seed, *slash, *algo;
	const char *errstr;

	if (pledge("stdio tty", NULL) == -1)
		err(1, "pledge");

	/* If we were called as otp-METHOD, set algorithm based on that */
	if (strncmp(__progname, "otp-", 4) == 0) {
		algo = __progname + 4;
		if (skey_set_algorithm(algo) == NULL)
			errx(1, "Unknown hash algorithm %s", algo);
	}

	for (i = 1; i < argc && argv[i][0] == '-' && strcmp(argv[i], "--");) {
		if (argv[i][2] == '\0') {
			/* Single character switch */
			switch (argv[i][1]) {
			case 'n':
				if (++i == argc)
					usage();
				cnt = strtonum(argv[i], 1, SKEY_MAX_SEQ -1, &errstr);
				if (errstr)
					usage();
				break;
			case 'p':
				if (++i == argc)
					usage();
				if (strlcpy(passwd, argv[i], sizeof(passwd)) >=
				    sizeof(passwd))
					errx(1, "Password too long");
				pass = 1;
				break;
			case 'x':
				hexmode = 1;
				break;
			default:
				usage();
			}
		} else {
			/* Multi character switches are hash types */
			if (skey_set_algorithm(&argv[i][1]) == NULL) {
				warnx("Unknown hash algorithm %s", &argv[i][1]);
				usage();
			}
		}
		i++;
	}

	if (argc > i + 2)
		usage();

	/* Could be in the form <number>/<seed> */
	if (argc <= i + 1) {
		/* look for / in it */
		if (argc <= i)
			usage();
		slash = strchr(argv[i], '/');
		if (slash == NULL)
			usage();
		*slash++ = '\0';
		seed = slash;

		n = strtonum(argv[i], 0, SKEY_MAX_SEQ, &errstr);
		if (errstr) {
			warnx("%s: %s", argv[i], errstr);
			usage();
		}
	} else {
		n = strtonum(argv[i], 0, SKEY_MAX_SEQ, &errstr);
		if (errstr) {
			warnx("%s: %s", argv[i], errstr);
			usage();
		}
		seed = argv[++i];
	}

	/* Get user's secret passphrase */
	if (!pass && (readpassphrase("Enter secret passphrase: ", passwd,
	    sizeof(passwd), 0) == NULL || passwd[0] == '\0'))
		exit(1);

	/* Crunch seed and passphrase into starting key */
	if (keycrunch(key, seed, passwd) != 0) {
		explicit_bzero(passwd, sizeof(passwd));
		errx(1, "key crunch failed");
	}

	explicit_bzero(passwd, sizeof(passwd));

	if (cnt == 1) {
		while (n-- != 0)
			f(key);
		(void)puts(hexmode ? put8(buf, key) : btoe(buf, key));
	} else {
		for (i = 0; i <= n - cnt; i++)
			f(key);
		for (; i <= n; i++) {
			if (hexmode)
				(void)printf("%d: %s\n", i, put8(buf, key));
			else
				(void)printf("%d: %-29s\n", i, btoe(buf, key));
			f(key);
		}
	}
	exit(0);
}

void
usage(void)
{
	fprintf(stderr,
	    "usage: %s [-x] [-md5 | -rmd160 | -sha1] [-n count]\n\t"
	    "[-p passphrase] <sequence#>[/] key\n", __progname);
	exit(1);
}