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Implement Elliptic Curve Cryptography modes for key exchange (ECDH) and
host/user keys (ECDSA) as specified by RFC5656. ECDH and ECDSA offer
better performance than plain DH and DSA at the same equivalent symmetric
key length, as well as much shorter keys.

Only the mandatory sections of RFC5656 are implemented, specifically the
three REQUIRED curves nistp256, nistp384 and nistp521 and only ECDH and
ECDSA. Point compression (optional in RFC5656 is NOT implemented).

Certificate host and user keys using the new ECDSA key types are supported.

Note that this code has not been tested for interoperability and may be
subject to change.

feedback and ok markus@

/* $OpenBSD: auth2-jpake.c,v 1.4 2010/08/31 11:54:45 djm Exp $ */
/*
 * Copyright (c) 2008 Damien Miller.  All rights reserved.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

/*
 * Server side of zero-knowledge password auth using J-PAKE protocol
 * as described in:
 *
 * F. Hao, P. Ryan, "Password Authenticated Key Exchange by Juggling",
 * 16th Workshop on Security Protocols, Cambridge, April 2008
 *
 * http://grouper.ieee.org/groups/1363/Research/contributions/hao-ryan-2008.pdf
 */

#ifdef JPAKE

#include <sys/types.h>
#include <sys/param.h>

#include <pwd.h>
#include <stdio.h>
#include <string.h>
#include <login_cap.h>

#include <openssl/bn.h>
#include <openssl/evp.h>

#include "xmalloc.h"
#include "ssh2.h"
#include "key.h"
#include "hostfile.h"
#include "auth.h"
#include "buffer.h"
#include "packet.h"
#include "dispatch.h"
#include "log.h"
#include "servconf.h"
#include "auth-options.h"
#include "canohost.h"
#ifdef GSSAPI
#include "ssh-gss.h"
#endif
#include "monitor_wrap.h"

#include "schnorr.h"
#include "jpake.h"

/*
 * XXX options->permit_empty_passwd (at the moment, they will be refused
 * anyway because they will mismatch on fake salt.
 */

/* Dispatch handlers */
static void input_userauth_jpake_client_step1(int, u_int32_t, void *);
static void input_userauth_jpake_client_step2(int, u_int32_t, void *);
static void input_userauth_jpake_client_confirm(int, u_int32_t, void *);

static int auth2_jpake_start(Authctxt *);

/* import */
extern ServerOptions options;
extern u_char *session_id2;
extern u_int session_id2_len;

/*
 * Attempt J-PAKE authentication.
 */
static int
userauth_jpake(Authctxt *authctxt)
{
	int authenticated = 0;

	packet_check_eom();

	debug("jpake-01@openssh.com requested");

	if (authctxt->user != NULL) {
		if (authctxt->jpake_ctx == NULL)
			authctxt->jpake_ctx = jpake_new();
		if (options.zero_knowledge_password_authentication)
			authenticated = auth2_jpake_start(authctxt);
	}

	return authenticated;
}

Authmethod method_jpake = {
	"jpake-01@openssh.com",
	userauth_jpake,
	&options.zero_knowledge_password_authentication
};

/* Clear context and callbacks */
void
auth2_jpake_stop(Authctxt *authctxt)
{
	/* unregister callbacks */
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_STEP1, NULL);
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_STEP2, NULL);
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_CONFIRM, NULL);
	if (authctxt->jpake_ctx != NULL) {
		jpake_free(authctxt->jpake_ctx);
		authctxt->jpake_ctx = NULL;
	}
}

/* Returns 1 if 'c' is a valid crypt(3) salt character, 0 otherwise */
static int
valid_crypt_salt(int c)
{
	if (c >= 'A' && c <= 'Z')
		return 1;
	if (c >= 'a' && c <= 'z')
		return 1;
	if (c >= '.' && c <= '9')
		return 1;
	return 0;
}

/*
 * Derive fake salt as H(username || first_private_host_key)
 * This provides relatively stable fake salts for non-existent
 * users and avoids the jpake method becoming an account validity
 * oracle.
 */
static void
derive_rawsalt(const char *username, u_char *rawsalt, u_int len)
{
	u_char *digest;
	u_int digest_len;
	Buffer b;
	Key *k;

	buffer_init(&b);
	buffer_put_cstring(&b, username);
	if ((k = get_hostkey_by_index(0)) == NULL ||
	    (k->flags & KEY_FLAG_EXT))
		fatal("%s: no hostkeys", __func__);
	switch (k->type) {
	case KEY_RSA1:
	case KEY_RSA:
		if (k->rsa->p == NULL || k->rsa->q == NULL)
			fatal("%s: RSA key missing p and/or q", __func__);
		buffer_put_bignum2(&b, k->rsa->p);
		buffer_put_bignum2(&b, k->rsa->q);
		break;
	case KEY_DSA:
		if (k->dsa->priv_key == NULL)
			fatal("%s: DSA key missing priv_key", __func__);
		buffer_put_bignum2(&b, k->dsa->priv_key);
		break;
	case KEY_ECDSA:
		if (EC_KEY_get0_private_key(k->ecdsa) == NULL)
			fatal("%s: ECDSA key missing priv_key", __func__);
		buffer_put_bignum2(&b, EC_KEY_get0_private_key(k->ecdsa));
		break;
	default:
		fatal("%s: unknown key type %d", __func__, k->type);
	}
	if (hash_buffer(buffer_ptr(&b), buffer_len(&b), EVP_sha256(),
	    &digest, &digest_len) != 0)
		fatal("%s: hash_buffer", __func__);
	buffer_free(&b);
	if (len > digest_len)
		fatal("%s: not enough bytes for rawsalt (want %u have %u)",
		    __func__, len, digest_len);
	memcpy(rawsalt, digest, len);
	bzero(digest, digest_len);
	xfree(digest);
}

/* ASCII an integer [0, 64) for inclusion in a password/salt */
static char
pw_encode64(u_int i64)
{
	const u_char e64[] =
	    "./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
	return e64[i64 % 64];
}

/* Generate ASCII salt bytes for user */
static char *
makesalt(u_int want, const char *user)
{
	u_char rawsalt[32];
	static char ret[33];
	u_int i;

	if (want > sizeof(ret) - 1)
		fatal("%s: want %u", __func__, want);

	derive_rawsalt(user, rawsalt, sizeof(rawsalt));
	bzero(ret, sizeof(ret));
	for (i = 0; i < want; i++)
		ret[i] = pw_encode64(rawsalt[i]);
	bzero(rawsalt, sizeof(rawsalt));

	return ret;
}

/*
 * Select the system's default password hashing scheme and generate
 * a stable fake salt under it for use by a non-existent account.
 * Prevents jpake method being used to infer the validity of accounts.
 */
static void
fake_salt_and_scheme(Authctxt *authctxt, char **salt, char **scheme)
{
	char *rounds_s, *style;
	long long rounds;
	login_cap_t *lc;


	if ((lc = login_getclass(authctxt->pw->pw_class)) == NULL &&
	    (lc = login_getclass(NULL)) == NULL)
		fatal("%s: login_getclass failed", __func__);
	style = login_getcapstr(lc, "localcipher", NULL, NULL);
	if (style == NULL)
		style = xstrdup("blowfish,6");
	login_close(lc);
	
	if ((rounds_s = strchr(style, ',')) != NULL)
		*rounds_s++ = '\0';
	rounds = strtonum(rounds_s, 1, 1<<31, NULL);
	
	if (strcmp(style, "md5") == 0) {
		xasprintf(salt, "$1$%s$", makesalt(8, authctxt->user));
		*scheme = xstrdup("md5");
	} else if (strcmp(style, "old") == 0) {
		*salt = xstrdup(makesalt(2, authctxt->user));
		*scheme = xstrdup("crypt");
	} else if (strcmp(style, "newsalt") == 0) {
		rounds = MAX(rounds, 7250);
		rounds = MIN(rounds, (1<<24) - 1);
		xasprintf(salt, "_%c%c%c%c%s",
		    pw_encode64(rounds), pw_encode64(rounds >> 6),
		    pw_encode64(rounds >> 12), pw_encode64(rounds >> 18),
		    makesalt(4, authctxt->user));
		*scheme = xstrdup("crypt-extended");
	} else {
		/* Default to blowfish */
		rounds = MAX(rounds, 3);
		rounds = MIN(rounds, 31);
		xasprintf(salt, "$2a$%02lld$%s", rounds,
		    makesalt(22, authctxt->user));
		*scheme = xstrdup("bcrypt");
	}
	xfree(style);
	debug3("%s: fake %s salt for user %s: %s",
	    __func__, *scheme, authctxt->user, *salt);
}

/*
 * Fetch password hashing scheme, password salt and derive shared secret
 * for user. If user does not exist, a fake but stable and user-unique
 * salt will be returned.
 */
void
auth2_jpake_get_pwdata(Authctxt *authctxt, BIGNUM **s,
    char **hash_scheme, char **salt)
{
	char *cp;
	u_char *secret;
	u_int secret_len, salt_len;

#ifdef JPAKE_DEBUG
	debug3("%s: valid %d pw %.5s...", __func__,
	    authctxt->valid, authctxt->pw->pw_passwd);
#endif

	*salt = NULL;
	*hash_scheme = NULL;
	if (authctxt->valid) {
		if (strncmp(authctxt->pw->pw_passwd, "$2$", 3) == 0 &&
		    strlen(authctxt->pw->pw_passwd) > 28) {
			/*
			 * old-variant bcrypt:
			 *     "$2$", 2 digit rounds, "$", 22 bytes salt
			 */
			salt_len = 3 + 2 + 1 + 22 + 1;
			*salt = xmalloc(salt_len);
			strlcpy(*salt, authctxt->pw->pw_passwd, salt_len);
			*hash_scheme = xstrdup("bcrypt");
		} else if (strncmp(authctxt->pw->pw_passwd, "$2a$", 4) == 0 &&
		    strlen(authctxt->pw->pw_passwd) > 29) {
			/*
			 * current-variant bcrypt:
			 *     "$2a$", 2 digit rounds, "$", 22 bytes salt
			 */
			salt_len = 4 + 2 + 1 + 22 + 1;
			*salt = xmalloc(salt_len);
			strlcpy(*salt, authctxt->pw->pw_passwd, salt_len);
			*hash_scheme = xstrdup("bcrypt");
		} else if (strncmp(authctxt->pw->pw_passwd, "$1$", 3) == 0 &&
		    strlen(authctxt->pw->pw_passwd) > 5) {
			/*
			 * md5crypt:
			 *     "$1$", salt until "$"
			 */
			cp = strchr(authctxt->pw->pw_passwd + 3, '$');
			if (cp != NULL) {
				salt_len = (cp - authctxt->pw->pw_passwd) + 1;
				*salt = xmalloc(salt_len);
				strlcpy(*salt, authctxt->pw->pw_passwd,
				    salt_len);
				*hash_scheme = xstrdup("md5crypt");
			}
		} else if (strncmp(authctxt->pw->pw_passwd, "_", 1) == 0 &&
		    strlen(authctxt->pw->pw_passwd) > 9) {
			/*
			 * BSDI extended crypt:
			 *     "_", 4 digits count, 4 chars salt
			 */
			salt_len = 1 + 4 + 4 + 1;
			*salt = xmalloc(salt_len);
			strlcpy(*salt, authctxt->pw->pw_passwd, salt_len);
			*hash_scheme = xstrdup("crypt-extended");
		} else if (strlen(authctxt->pw->pw_passwd) == 13  &&
		    valid_crypt_salt(authctxt->pw->pw_passwd[0]) &&
		    valid_crypt_salt(authctxt->pw->pw_passwd[1])) {
			/*
			 * traditional crypt:
			 *     2 chars salt
			 */
			salt_len = 2 + 1;
			*salt = xmalloc(salt_len);
			strlcpy(*salt, authctxt->pw->pw_passwd, salt_len);
			*hash_scheme = xstrdup("crypt");
		}
		if (*salt == NULL) {
			debug("%s: unrecognised crypt scheme for user %s",
			    __func__, authctxt->pw->pw_name);
		}
	}
	if (*salt == NULL)
		fake_salt_and_scheme(authctxt, salt, hash_scheme);

	if (hash_buffer(authctxt->pw->pw_passwd,
	    strlen(authctxt->pw->pw_passwd), EVP_sha256(),
	    &secret, &secret_len) != 0)
		fatal("%s: hash_buffer", __func__);
	if ((*s = BN_bin2bn(secret, secret_len, NULL)) == NULL)
		fatal("%s: BN_bin2bn (secret)", __func__);
#ifdef JPAKE_DEBUG
	debug3("%s: salt = %s (len %u)", __func__,
	    *salt, (u_int)strlen(*salt));
	debug3("%s: scheme = %s", __func__, *hash_scheme);
	JPAKE_DEBUG_BN((*s, "%s: s = ", __func__));
#endif
	bzero(secret, secret_len);
	xfree(secret);
}

/*
 * Begin authentication attempt.
 * Note, sets authctxt->postponed while in subprotocol
 */
static int
auth2_jpake_start(Authctxt *authctxt)
{
	struct jpake_ctx *pctx = authctxt->jpake_ctx;
	u_char *x3_proof, *x4_proof;
	u_int x3_proof_len, x4_proof_len;
	char *salt, *hash_scheme;

	debug("%s: start", __func__);

	PRIVSEP(jpake_step1(pctx->grp,
	    &pctx->server_id, &pctx->server_id_len,
	    &pctx->x3, &pctx->x4, &pctx->g_x3, &pctx->g_x4,
	    &x3_proof, &x3_proof_len,
	    &x4_proof, &x4_proof_len));

	PRIVSEP(auth2_jpake_get_pwdata(authctxt, &pctx->s,
	    &hash_scheme, &salt));

	if (!use_privsep)
		JPAKE_DEBUG_CTX((pctx, "step 1 sending in %s", __func__));

	packet_start(SSH2_MSG_USERAUTH_JPAKE_SERVER_STEP1);
	packet_put_cstring(hash_scheme);
	packet_put_cstring(salt);
	packet_put_string(pctx->server_id, pctx->server_id_len);
	packet_put_bignum2(pctx->g_x3);
	packet_put_bignum2(pctx->g_x4);
	packet_put_string(x3_proof, x3_proof_len);
	packet_put_string(x4_proof, x4_proof_len);
	packet_send();
	packet_write_wait();

	bzero(hash_scheme, strlen(hash_scheme));
	bzero(salt, strlen(salt));
	xfree(hash_scheme);
	xfree(salt);
	bzero(x3_proof, x3_proof_len);
	bzero(x4_proof, x4_proof_len);
	xfree(x3_proof);
	xfree(x4_proof);

	/* Expect step 1 packet from peer */
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_STEP1,
	    input_userauth_jpake_client_step1);

	authctxt->postponed = 1;
	return 0;
}

/* ARGSUSED */
static void
input_userauth_jpake_client_step1(int type, u_int32_t seq, void *ctxt)
{
	Authctxt *authctxt = ctxt;
	struct jpake_ctx *pctx = authctxt->jpake_ctx;
	u_char *x1_proof, *x2_proof, *x4_s_proof;
	u_int x1_proof_len, x2_proof_len, x4_s_proof_len;

	/* Disable this message */
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_STEP1, NULL);

	/* Fetch step 1 values */
	if ((pctx->g_x1 = BN_new()) == NULL ||
	    (pctx->g_x2 = BN_new()) == NULL)
		fatal("%s: BN_new", __func__);
	pctx->client_id = packet_get_string(&pctx->client_id_len);
	packet_get_bignum2(pctx->g_x1);
	packet_get_bignum2(pctx->g_x2);
	x1_proof = packet_get_string(&x1_proof_len);
	x2_proof = packet_get_string(&x2_proof_len);
	packet_check_eom();

	if (!use_privsep)
		JPAKE_DEBUG_CTX((pctx, "step 1 received in %s", __func__));

	PRIVSEP(jpake_step2(pctx->grp, pctx->s, pctx->g_x3,
	    pctx->g_x1, pctx->g_x2, pctx->x4,
	    pctx->client_id, pctx->client_id_len,
	    pctx->server_id, pctx->server_id_len,
	    x1_proof, x1_proof_len,
	    x2_proof, x2_proof_len,
	    &pctx->b,
	    &x4_s_proof, &x4_s_proof_len));

	bzero(x1_proof, x1_proof_len);
	bzero(x2_proof, x2_proof_len);
	xfree(x1_proof);
	xfree(x2_proof);

	if (!use_privsep)
		JPAKE_DEBUG_CTX((pctx, "step 2 sending in %s", __func__));

	/* Send values for step 2 */
	packet_start(SSH2_MSG_USERAUTH_JPAKE_SERVER_STEP2);
	packet_put_bignum2(pctx->b);
	packet_put_string(x4_s_proof, x4_s_proof_len);
	packet_send();
	packet_write_wait();

	bzero(x4_s_proof, x4_s_proof_len);
	xfree(x4_s_proof);

	/* Expect step 2 packet from peer */
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_STEP2,
	    input_userauth_jpake_client_step2);
}

/* ARGSUSED */
static void
input_userauth_jpake_client_step2(int type, u_int32_t seq, void *ctxt)
{
	Authctxt *authctxt = ctxt;
	struct jpake_ctx *pctx = authctxt->jpake_ctx;
	u_char *x2_s_proof;
	u_int x2_s_proof_len;

	/* Disable this message */
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_STEP2, NULL);

	if ((pctx->a = BN_new()) == NULL)
		fatal("%s: BN_new", __func__);

	/* Fetch step 2 values */
	packet_get_bignum2(pctx->a);
	x2_s_proof = packet_get_string(&x2_s_proof_len);
	packet_check_eom();

	if (!use_privsep)
		JPAKE_DEBUG_CTX((pctx, "step 2 received in %s", __func__));

	/* Derive shared key and calculate confirmation hash */
	PRIVSEP(jpake_key_confirm(pctx->grp, pctx->s, pctx->a,
	    pctx->x4, pctx->g_x3, pctx->g_x4, pctx->g_x1, pctx->g_x2,
	    pctx->server_id, pctx->server_id_len,
	    pctx->client_id, pctx->client_id_len,
	    session_id2, session_id2_len,
	    x2_s_proof, x2_s_proof_len,
	    &pctx->k,
	    &pctx->h_k_sid_sessid, &pctx->h_k_sid_sessid_len));

	bzero(x2_s_proof, x2_s_proof_len);
	xfree(x2_s_proof);

	if (!use_privsep)
		JPAKE_DEBUG_CTX((pctx, "confirm sending in %s", __func__));

	/* Send key confirmation proof */
	packet_start(SSH2_MSG_USERAUTH_JPAKE_SERVER_CONFIRM);
	packet_put_string(pctx->h_k_sid_sessid, pctx->h_k_sid_sessid_len);
	packet_send();
	packet_write_wait();

	/* Expect confirmation from peer */
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_CONFIRM,
	    input_userauth_jpake_client_confirm);
}

/* ARGSUSED */
static void
input_userauth_jpake_client_confirm(int type, u_int32_t seq, void *ctxt)
{
	Authctxt *authctxt = ctxt;
	struct jpake_ctx *pctx = authctxt->jpake_ctx;
	int authenticated = 0;

	/* Disable this message */
	dispatch_set(SSH2_MSG_USERAUTH_JPAKE_CLIENT_CONFIRM, NULL);

	pctx->h_k_cid_sessid = packet_get_string(&pctx->h_k_cid_sessid_len);
	packet_check_eom();

	if (!use_privsep)
		JPAKE_DEBUG_CTX((pctx, "confirm received in %s", __func__));

	/* Verify expected confirmation hash */
	if (PRIVSEP(jpake_check_confirm(pctx->k,
	    pctx->client_id, pctx->client_id_len,
	    session_id2, session_id2_len,
	    pctx->h_k_cid_sessid, pctx->h_k_cid_sessid_len)) == 1)
		authenticated = authctxt->valid ? 1 : 0;
	else
		debug("%s: confirmation mismatch", __func__);
		
	/* done */
	authctxt->postponed = 0;
	jpake_free(authctxt->jpake_ctx);
	authctxt->jpake_ctx = NULL;
	userauth_finish(authctxt, authenticated, method_jpake.name);
}

#endif /* JPAKE */