[openssh.git] / auth-rsa.c

/* $OpenBSD: auth-rsa.c,v 1.71 2006/08/03 03:34:41 deraadt Exp $ */
/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * RSA-based authentication.  This code determines whether to admit a login
 * based on RSA authentication.  This file also contains functions to check
 * validity of the host key.
 *
 * As far as I am concerned, the code I have written for this software
 * can be used freely for any purpose.  Any derived versions of this
 * software must be clearly marked as such, and if the derived work is
 * incompatible with the protocol description in the RFC file, it must be
 * called by a name other than "ssh" or "Secure Shell".
 */

#include "includes.h"

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

#include <openssl/rsa.h>
#include <openssl/md5.h>

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

#include "xmalloc.h"
#include "rsa.h"
#include "packet.h"
#include "ssh1.h"
#include "uidswap.h"
#include "match.h"
#include "buffer.h"
#include "auth-options.h"
#include "pathnames.h"
#include "log.h"
#include "servconf.h"
#include "key.h"
#include "hostfile.h"
#include "auth.h"
#ifdef GSSAPI
#include "ssh-gss.h"
#endif
#include "monitor_wrap.h"
#include "ssh.h"
#include "misc.h"

/* import */
extern ServerOptions options;

/*
 * Session identifier that is used to bind key exchange and authentication
 * responses to a particular session.
 */
extern u_char session_id[16];

/*
 * The .ssh/authorized_keys file contains public keys, one per line, in the
 * following format:
 *   options bits e n comment
 * where bits, e and n are decimal numbers,
 * and comment is any string of characters up to newline.  The maximum
 * length of a line is SSH_MAX_PUBKEY_BYTES characters.  See sshd(8) for a
 * description of the options.
 */

BIGNUM *
auth_rsa_generate_challenge(Key *key)
{
	BIGNUM *challenge;
	BN_CTX *ctx;

	if ((challenge = BN_new()) == NULL)
		fatal("auth_rsa_generate_challenge: BN_new() failed");
	/* Generate a random challenge. */
	BN_rand(challenge, 256, 0, 0);
	if ((ctx = BN_CTX_new()) == NULL)
		fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	BN_mod(challenge, challenge, key->rsa->n, ctx);
	BN_CTX_free(ctx);

	return challenge;
}

int
auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
{
	u_char buf[32], mdbuf[16];
	MD5_CTX md;
	int len;

	/* don't allow short keys */
	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
		error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
		    BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
		return (0);
	}

	/* The response is MD5 of decrypted challenge plus session id. */
	len = BN_num_bytes(challenge);
	if (len <= 0 || len > 32)
		fatal("auth_rsa_verify_response: bad challenge length %d", len);
	memset(buf, 0, 32);
	BN_bn2bin(challenge, buf + 32 - len);
	MD5_Init(&md);
	MD5_Update(&md, buf, 32);
	MD5_Update(&md, session_id, 16);
	MD5_Final(mdbuf, &md);

	/* Verify that the response is the original challenge. */
	if (memcmp(response, mdbuf, 16) != 0) {
		/* Wrong answer. */
		return (0);
	}
	/* Correct answer. */
	return (1);
}

/*
 * Performs the RSA authentication challenge-response dialog with the client,
 * and returns true (non-zero) if the client gave the correct answer to
 * our challenge; returns zero if the client gives a wrong answer.
 */

int
auth_rsa_challenge_dialog(Key *key)
{
	BIGNUM *challenge, *encrypted_challenge;
	u_char response[16];
	int i, success;

	if ((encrypted_challenge = BN_new()) == NULL)
		fatal("auth_rsa_challenge_dialog: BN_new() failed");

	challenge = PRIVSEP(auth_rsa_generate_challenge(key));

	/* Encrypt the challenge with the public key. */
	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);

	/* Send the encrypted challenge to the client. */
	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	packet_put_bignum(encrypted_challenge);
	packet_send();
	BN_clear_free(encrypted_challenge);
	packet_write_wait();

	/* Wait for a response. */
	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
	for (i = 0; i < 16; i++)
		response[i] = (u_char)packet_get_char();
	packet_check_eom();

	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
	BN_clear_free(challenge);
	return (success);
}

/*
 * check if there's user key matching client_n,
 * return key if login is allowed, NULL otherwise
 */

int
auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
{
	char line[SSH_MAX_PUBKEY_BYTES], *file;
	int allowed = 0;
	u_int bits;
	FILE *f;
	u_long linenum = 0;
	struct stat st;
	Key *key;

	/* Temporarily use the user's uid. */
	temporarily_use_uid(pw);

	/* The authorized keys. */
	file = authorized_keys_file(pw);
	debug("trying public RSA key file %s", file);

	/* Fail quietly if file does not exist */
	if (stat(file, &st) < 0) {
		/* Restore the privileged uid. */
		restore_uid();
		xfree(file);
		return (0);
	}
	/* Open the file containing the authorized keys. */
	f = fopen(file, "r");
	if (!f) {
		/* Restore the privileged uid. */
		restore_uid();
		xfree(file);
		return (0);
	}
	if (options.strict_modes &&
	    secure_filename(f, file, pw, line, sizeof(line)) != 0) {
		xfree(file);
		fclose(f);
		logit("Authentication refused: %s", line);
		restore_uid();
		return (0);
	}

	/* Flag indicating whether the key is allowed. */
	allowed = 0;

	key = key_new(KEY_RSA1);

	/*
	 * Go though the accepted keys, looking for the current key.  If
	 * found, perform a challenge-response dialog to verify that the
	 * user really has the corresponding private key.
	 */
	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
		char *cp;
		char *key_options;
		int keybits;

		/* Skip leading whitespace, empty and comment lines. */
		for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
			;
		if (!*cp || *cp == '\n' || *cp == '#')
			continue;

		/*
		 * Check if there are options for this key, and if so,
		 * save their starting address and skip the option part
		 * for now.  If there are no options, set the starting
		 * address to NULL.
		 */
		if (*cp < '0' || *cp > '9') {
			int quoted = 0;
			key_options = cp;
			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
				if (*cp == '\\' && cp[1] == '"')
					cp++;	/* Skip both */
				else if (*cp == '"')
					quoted = !quoted;
			}
		} else
			key_options = NULL;

		/* Parse the key from the line. */
		if (hostfile_read_key(&cp, &bits, key) == 0) {
			debug("%.100s, line %lu: non ssh1 key syntax",
			    file, linenum);
			continue;
		}
		/* cp now points to the comment part. */

		/* Check if the we have found the desired key (identified by its modulus). */
		if (BN_cmp(key->rsa->n, client_n) != 0)
			continue;

		/* check the real bits  */
		keybits = BN_num_bits(key->rsa->n);
		if (keybits < 0 || bits != (u_int)keybits)
			logit("Warning: %s, line %lu: keysize mismatch: "
			    "actual %d vs. announced %d.",
			    file, linenum, BN_num_bits(key->rsa->n), bits);

		/* We have found the desired key. */
		/*
		 * If our options do not allow this key to be used,
		 * do not send challenge.
		 */
		if (!auth_parse_options(pw, key_options, file, linenum))
			continue;

		/* break out, this key is allowed */
		allowed = 1;
		break;
	}

	/* Restore the privileged uid. */
	restore_uid();

	/* Close the file. */
	xfree(file);
	fclose(f);

	/* return key if allowed */
	if (allowed && rkey != NULL)
		*rkey = key;
	else
		key_free(key);
	return (allowed);
}

/*
 * Performs the RSA authentication dialog with the client.  This returns
 * 0 if the client could not be authenticated, and 1 if authentication was
 * successful.  This may exit if there is a serious protocol violation.
 */
int
auth_rsa(Authctxt *authctxt, BIGNUM *client_n)
{
	Key *key;
	char *fp;
	struct passwd *pw = authctxt->pw;

	/* no user given */
	if (!authctxt->valid)
		return 0;

	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
		auth_clear_options();
		return (0);
	}

	/* Perform the challenge-response dialog for this key. */
	if (!auth_rsa_challenge_dialog(key)) {
		/* Wrong response. */
		verbose("Wrong response to RSA authentication challenge.");
		packet_send_debug("Wrong response to RSA authentication challenge.");
		/*
		 * Break out of the loop. Otherwise we might send
		 * another challenge and break the protocol.
		 */
		key_free(key);
		return (0);
	}
	/*
	 * Correct response.  The client has been successfully
	 * authenticated. Note that we have not yet processed the
	 * options; this will be reset if the options cause the
	 * authentication to be rejected.
	 */
	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	verbose("Found matching %s key: %s",
	    key_type(key), fp);
	xfree(fp);
	key_free(key);

	packet_send_debug("RSA authentication accepted.");
	return (1);
}
Commit	Line	Data
31652869	1	/* $OpenBSD: auth-rsa.c,v 1.71 2006/08/03 03:34:41 deraadt Exp $ */
8efc0c15	2	/*
5260325f	3	* Author: Tatu Ylonen <ylo@cs.hut.fi>
5260325f	4	* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5260325f	5	* All rights reserved
5260325f	6	* RSA-based authentication. This code determines whether to admit a login
	7	* based on RSA authentication. This file also contains functions to check
	8	* validity of the host key.
6ae2364d	9	*
bcbf86ec	10	* As far as I am concerned, the code I have written for this software
	11	* can be used freely for any purpose. Any derived versions of this
	12	* software must be clearly marked as such, and if the derived work is
	13	* incompatible with the protocol description in the RFC file, it must be
	14	* called by a name other than "ssh" or "Secure Shell".
5260325f	15	*/
8efc0c15	16
8efc0c15	17	#include "includes.h"
4095f623	18
	19	#include <sys/types.h>
	20	#include <sys/stat.h>
42f11eb2	21
	22	#include <openssl/rsa.h>
	23	#include <openssl/md5.h>
8efc0c15	24
b1842393	25	#include <pwd.h>
cf851879	26	#include <stdio.h>
24436b92	27	#include <stdarg.h>
00146caa	28	#include <string.h>
b1842393	29
31652869	30	#include "xmalloc.h"
8efc0c15	31	#include "rsa.h"
8efc0c15	32	#include "packet.h"
42f11eb2	33	#include "ssh1.h"
8efc0c15	34	#include "uidswap.h"
4fe2af09	35	#include "match.h"
31652869	36	#include "buffer.h"
38c295d6	37	#include "auth-options.h"
42f11eb2	38	#include "pathnames.h"
	39	#include "log.h"
	40	#include "servconf.h"
31652869	41	#include "key.h"
46df736f	42	#include "hostfile.h"
31652869	43	#include "auth.h"
	44	#ifdef GSSAPI
	45	#include "ssh-gss.h"
	46	#endif
1853d1ef	47	#include "monitor_wrap.h"
82b00264	48	#include "ssh.h"
41feb690	49	#include "misc.h"
94ec8c6b	50
	51	/* import */
	52	extern ServerOptions options;
	53
aa3378df	54	/*
	55	* Session identifier that is used to bind key exchange and authentication
	56	* responses to a particular session.
	57	*/
1e3b8b07	58	extern u_char session_id[16];
8efc0c15	59
aa3378df	60	/*
	61	* The .ssh/authorized_keys file contains public keys, one per line, in the
	62	* following format:
	63	* options bits e n comment
	64	* where bits, e and n are decimal numbers,
	65	* and comment is any string of characters up to newline. The maximum
ea067773	66	* length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
aa3378df	67	* description of the options.
aa3378df	68	*/
8efc0c15	69
1853d1ef	70	BIGNUM *
46f1eece	71	auth_rsa_generate_challenge(Key *key)
	72	{
	73	BIGNUM *challenge;
	74	BN_CTX *ctx;
	75
	76	if ((challenge = BN_new()) == NULL)
	77	fatal("auth_rsa_generate_challenge: BN_new() failed");
	78	/* Generate a random challenge. */
	79	BN_rand(challenge, 256, 0, 0);
	80	if ((ctx = BN_CTX_new()) == NULL)
	81	fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	82	BN_mod(challenge, challenge, key->rsa->n, ctx);
	83	BN_CTX_free(ctx);
	84
	85	return challenge;
	86	}
	87
1853d1ef	88	int
46f1eece	89	auth_rsa_verify_response(Key key, BIGNUM challenge, u_char response[16])
	90	{
	91	u_char buf[32], mdbuf[16];
	92	MD5_CTX md;
	93	int len;
	94
52103b10	95	/* don't allow short keys */
82b00264	96	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
b7ced7d1	97	error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
b7ced7d1	98	BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
52103b10	99	return (0);
	100	}
	101
46f1eece	102	/* The response is MD5 of decrypted challenge plus session id. */
	103	len = BN_num_bytes(challenge);
	104	if (len <= 0 \|\| len > 32)
	105	fatal("auth_rsa_verify_response: bad challenge length %d", len);
	106	memset(buf, 0, 32);
	107	BN_bn2bin(challenge, buf + 32 - len);
	108	MD5_Init(&md);
	109	MD5_Update(&md, buf, 32);
	110	MD5_Update(&md, session_id, 16);
	111	MD5_Final(mdbuf, &md);
	112
	113	/* Verify that the response is the original challenge. */
	114	if (memcmp(response, mdbuf, 16) != 0) {
	115	/* Wrong answer. */
	116	return (0);
	117	}
	118	/* Correct answer. */
	119	return (1);
	120	}
	121
aa3378df	122	/*
	123	* Performs the RSA authentication challenge-response dialog with the client,
	124	* and returns true (non-zero) if the client gave the correct answer to
	125	* our challenge; returns zero if the client gives a wrong answer.
	126	*/
8efc0c15	127
8efc0c15	128	int
46f1eece	129	auth_rsa_challenge_dialog(Key *key)
8efc0c15	130	{
c8d54615	131	BIGNUM challenge, encrypted_challenge;
46f1eece	132	u_char response[16];
46f1eece	133	int i, success;
5260325f	134
b775c6f2	135	if ((encrypted_challenge = BN_new()) == NULL)
b775c6f2	136	fatal("auth_rsa_challenge_dialog: BN_new() failed");
5260325f	137
1853d1ef	138	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
5260325f	139
5260325f	140	/* Encrypt the challenge with the public key. */
46f1eece	141	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
5260325f	142
	143	/* Send the encrypted challenge to the client. */
	144	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	145	packet_put_bignum(encrypted_challenge);
	146	packet_send();
c8d54615	147	BN_clear_free(encrypted_challenge);
5260325f	148	packet_write_wait();
5260325f	149
c8d54615	150	/* Wait for a response. */
54a5250f	151	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
c8d54615	152	for (i = 0; i < 16; i++)
febd6f21	153	response[i] = (u_char)packet_get_char();
95500969	154	packet_check_eom();
c8d54615	155
1853d1ef	156	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
5260325f	157	BN_clear_free(challenge);
46f1eece	158	return (success);
8efc0c15	159	}
8efc0c15	160
aa3378df	161	/*
46f1eece	162	* check if there's user key matching client_n,
46f1eece	163	* return key if login is allowed, NULL otherwise
aa3378df	164	*/
8efc0c15	165
1853d1ef	166	int
46f1eece	167	auth_rsa_key_allowed(struct passwd pw, BIGNUM client_n, Key **rkey)
8efc0c15	168	{
ea067773	169	char line[SSH_MAX_PUBKEY_BYTES], *file;
1853d1ef	170	int allowed = 0;
1e3b8b07	171	u_int bits;
5260325f	172	FILE *f;
1e3b8b07	173	u_long linenum = 0;
5260325f	174	struct stat st;
46df736f	175	Key *key;
94ec8c6b	176
5260325f	177	/* Temporarily use the user's uid. */
63bd8c36	178	temporarily_use_uid(pw);
5260325f	179
5260325f	180	/* The authorized keys. */
c8445989	181	file = authorized_keys_file(pw);
c8445989	182	debug("trying public RSA key file %s", file);
5260325f	183
	184	/* Fail quietly if file does not exist */
	185	if (stat(file, &st) < 0) {
	186	/* Restore the privileged uid. */
	187	restore_uid();
c8445989	188	xfree(file);
159897f3	189	return (0);
8efc0c15	190	}
5260325f	191	/* Open the file containing the authorized keys. */
	192	f = fopen(file, "r");
	193	if (!f) {
	194	/* Restore the privileged uid. */
	195	restore_uid();
c8445989	196	xfree(file);
159897f3	197	return (0);
8efc0c15	198	}
c8445989	199	if (options.strict_modes &&
6978866a	200	secure_filename(f, file, pw, line, sizeof(line)) != 0) {
c8445989	201	xfree(file);
c8445989	202	fclose(f);
bbe88b6d	203	logit("Authentication refused: %s", line);
c8445989	204	restore_uid();
159897f3	205	return (0);
5260325f	206	}
46f1eece	207
	208	/* Flag indicating whether the key is allowed. */
	209	allowed = 0;
5260325f	210
46df736f	211	key = key_new(KEY_RSA1);
5260325f	212
aa3378df	213	/*
	214	* Go though the accepted keys, looking for the current key. If
	215	* found, perform a challenge-response dialog to verify that the
	216	* user really has the corresponding private key.
	217	*/
ea067773	218	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
5260325f	219	char *cp;
ca75d7de	220	char *key_options;
2ceb8101	221	int keybits;
5260325f	222
aa3378df	223	/* Skip leading whitespace, empty and comment lines. */
	224	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	225	;
5260325f	226	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	227	continue;
	228
aa3378df	229	/*
	230	* Check if there are options for this key, and if so,
	231	* save their starting address and skip the option part
	232	* for now. If there are no options, set the starting
	233	* address to NULL.
	234	*/
5260325f	235	if (cp < '0' \|\| cp > '9') {
5260325f	236	int quoted = 0;
ca75d7de	237	key_options = cp;
5260325f	238	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	239	if (*cp == '\\' && cp[1] == '"')
	240	cp++; /* Skip both */
	241	else if (*cp == '"')
	242	quoted = !quoted;
8efc0c15	243	}
5260325f	244	} else
ca75d7de	245	key_options = NULL;
5260325f	246
5260325f	247	/* Parse the key from the line. */
46df736f	248	if (hostfile_read_key(&cp, &bits, key) == 0) {
96a7b0cc	249	debug("%.100s, line %lu: non ssh1 key syntax",
42f11eb2	250	file, linenum);
5260325f	251	continue;
8efc0c15	252	}
5260325f	253	/* cp now points to the comment part. */
5260325f	254
5260325f	255	/* Check if the we have found the desired key (identified by its modulus). */
46df736f	256	if (BN_cmp(key->rsa->n, client_n) != 0)
aa3378df	257	continue;
5260325f	258
57112b5a	259	/* check the real bits */
2ceb8101	260	keybits = BN_num_bits(key->rsa->n);
2ceb8101	261	if (keybits < 0 \|\| bits != (u_int)keybits)
bbe88b6d	262	logit("Warning: %s, line %lu: keysize mismatch: "
57112b5a	263	"actual %d vs. announced %d.",
46df736f	264	file, linenum, BN_num_bits(key->rsa->n), bits);
57112b5a	265
5260325f	266	/* We have found the desired key. */
d343d900	267	/*
	268	* If our options do not allow this key to be used,
	269	* do not send challenge.
	270	*/
ca75d7de	271	if (!auth_parse_options(pw, key_options, file, linenum))
d343d900	272	continue;
5260325f	273
46f1eece	274	/* break out, this key is allowed */
46f1eece	275	allowed = 1;
33de75a3	276	break;
8efc0c15	277	}
8efc0c15	278
5260325f	279	/* Restore the privileged uid. */
5260325f	280	restore_uid();
8efc0c15	281
5260325f	282	/* Close the file. */
c8445989	283	xfree(file);
5260325f	284	fclose(f);
8efc0c15	285
46f1eece	286	/* return key if allowed */
	287	if (allowed && rkey != NULL)
	288	*rkey = key;
94ec8c6b	289	else
46f1eece	290	key_free(key);
	291	return (allowed);
	292	}
	293
	294	/*
	295	* Performs the RSA authentication dialog with the client. This returns
	296	* 0 if the client could not be authenticated, and 1 if authentication was
	297	* successful. This may exit if there is a serious protocol violation.
	298	*/
	299	int
fdaef11e	300	auth_rsa(Authctxt authctxt, BIGNUM client_n)
46f1eece	301	{
	302	Key *key;
	303	char *fp;
fdaef11e	304	struct passwd *pw = authctxt->pw;
46f1eece	305
46f1eece	306	/* no user given */
fdaef11e	307	if (!authctxt->valid)
46f1eece	308	return 0;
46f1eece	309
1853d1ef	310	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
94ec8c6b	311	auth_clear_options();
46f1eece	312	return (0);
	313	}
	314
	315	/* Perform the challenge-response dialog for this key. */
	316	if (!auth_rsa_challenge_dialog(key)) {
	317	/* Wrong response. */
	318	verbose("Wrong response to RSA authentication challenge.");
	319	packet_send_debug("Wrong response to RSA authentication challenge.");
	320	/*
	321	* Break out of the loop. Otherwise we might send
	322	* another challenge and break the protocol.
	323	*/
	324	key_free(key);
	325	return (0);
	326	}
	327	/*
	328	* Correct response. The client has been successfully
	329	* authenticated. Note that we have not yet processed the
	330	* options; this will be reset if the options cause the
	331	* authentication to be rejected.
	332	*/
	333	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	334	verbose("Found matching %s key: %s",
	335	key_type(key), fp);
	336	xfree(fp);
	337	key_free(key);
8efc0c15	338
46f1eece	339	packet_send_debug("RSA authentication accepted.");
46f1eece	340	return (1);
8efc0c15	341	}