[openssh.git] / auth-rsa.c

/* $OpenBSD: auth-rsa.c,v 1.68 2006/07/06 16:03:53 stevesk 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 "rsa.h"
#include "packet.h"
#include "xmalloc.h"
#include "ssh1.h"
#include "uidswap.h"
#include "match.h"
#include "auth-options.h"
#include "pathnames.h"
#include "log.h"
#include "servconf.h"
#include "auth.h"
#include "hostfile.h"
#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
b1842393	1	/* $OpenBSD: auth-rsa.c,v 1.68 2006/07/06 16:03:53 stevesk 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>
b1842393	26
8efc0c15	27	#include "rsa.h"
	28	#include "packet.h"
	29	#include "xmalloc.h"
42f11eb2	30	#include "ssh1.h"
8efc0c15	31	#include "uidswap.h"
4fe2af09	32	#include "match.h"
38c295d6	33	#include "auth-options.h"
42f11eb2	34	#include "pathnames.h"
	35	#include "log.h"
	36	#include "servconf.h"
	37	#include "auth.h"
46df736f	38	#include "hostfile.h"
1853d1ef	39	#include "monitor_wrap.h"
82b00264	40	#include "ssh.h"
41feb690	41	#include "misc.h"
94ec8c6b	42
	43	/* import */
	44	extern ServerOptions options;
	45
aa3378df	46	/*
	47	* Session identifier that is used to bind key exchange and authentication
	48	* responses to a particular session.
	49	*/
1e3b8b07	50	extern u_char session_id[16];
8efc0c15	51
aa3378df	52	/*
	53	* The .ssh/authorized_keys file contains public keys, one per line, in the
	54	* following format:
	55	* options bits e n comment
	56	* where bits, e and n are decimal numbers,
	57	* and comment is any string of characters up to newline. The maximum
ea067773	58	* length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
aa3378df	59	* description of the options.
aa3378df	60	*/
8efc0c15	61
1853d1ef	62	BIGNUM *
46f1eece	63	auth_rsa_generate_challenge(Key *key)
	64	{
	65	BIGNUM *challenge;
	66	BN_CTX *ctx;
	67
	68	if ((challenge = BN_new()) == NULL)
	69	fatal("auth_rsa_generate_challenge: BN_new() failed");
	70	/* Generate a random challenge. */
	71	BN_rand(challenge, 256, 0, 0);
	72	if ((ctx = BN_CTX_new()) == NULL)
	73	fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	74	BN_mod(challenge, challenge, key->rsa->n, ctx);
	75	BN_CTX_free(ctx);
	76
	77	return challenge;
	78	}
	79
1853d1ef	80	int
46f1eece	81	auth_rsa_verify_response(Key key, BIGNUM challenge, u_char response[16])
	82	{
	83	u_char buf[32], mdbuf[16];
	84	MD5_CTX md;
	85	int len;
	86
52103b10	87	/* don't allow short keys */
82b00264	88	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
b7ced7d1	89	error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
b7ced7d1	90	BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
52103b10	91	return (0);
	92	}
	93
46f1eece	94	/* The response is MD5 of decrypted challenge plus session id. */
	95	len = BN_num_bytes(challenge);
	96	if (len <= 0 \|\| len > 32)
	97	fatal("auth_rsa_verify_response: bad challenge length %d", len);
	98	memset(buf, 0, 32);
	99	BN_bn2bin(challenge, buf + 32 - len);
	100	MD5_Init(&md);
	101	MD5_Update(&md, buf, 32);
	102	MD5_Update(&md, session_id, 16);
	103	MD5_Final(mdbuf, &md);
	104
	105	/* Verify that the response is the original challenge. */
	106	if (memcmp(response, mdbuf, 16) != 0) {
	107	/* Wrong answer. */
	108	return (0);
	109	}
	110	/* Correct answer. */
	111	return (1);
	112	}
	113
aa3378df	114	/*
	115	* Performs the RSA authentication challenge-response dialog with the client,
	116	* and returns true (non-zero) if the client gave the correct answer to
	117	* our challenge; returns zero if the client gives a wrong answer.
	118	*/
8efc0c15	119
8efc0c15	120	int
46f1eece	121	auth_rsa_challenge_dialog(Key *key)
8efc0c15	122	{
c8d54615	123	BIGNUM challenge, encrypted_challenge;
46f1eece	124	u_char response[16];
46f1eece	125	int i, success;
5260325f	126
b775c6f2	127	if ((encrypted_challenge = BN_new()) == NULL)
b775c6f2	128	fatal("auth_rsa_challenge_dialog: BN_new() failed");
5260325f	129
1853d1ef	130	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
5260325f	131
5260325f	132	/* Encrypt the challenge with the public key. */
46f1eece	133	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
5260325f	134
	135	/* Send the encrypted challenge to the client. */
	136	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	137	packet_put_bignum(encrypted_challenge);
	138	packet_send();
c8d54615	139	BN_clear_free(encrypted_challenge);
5260325f	140	packet_write_wait();
5260325f	141
c8d54615	142	/* Wait for a response. */
54a5250f	143	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
c8d54615	144	for (i = 0; i < 16; i++)
febd6f21	145	response[i] = (u_char)packet_get_char();
95500969	146	packet_check_eom();
c8d54615	147
1853d1ef	148	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
5260325f	149	BN_clear_free(challenge);
46f1eece	150	return (success);
8efc0c15	151	}
8efc0c15	152
aa3378df	153	/*
46f1eece	154	* check if there's user key matching client_n,
46f1eece	155	* return key if login is allowed, NULL otherwise
aa3378df	156	*/
8efc0c15	157
1853d1ef	158	int
46f1eece	159	auth_rsa_key_allowed(struct passwd pw, BIGNUM client_n, Key **rkey)
8efc0c15	160	{
ea067773	161	char line[SSH_MAX_PUBKEY_BYTES], *file;
1853d1ef	162	int allowed = 0;
1e3b8b07	163	u_int bits;
5260325f	164	FILE *f;
1e3b8b07	165	u_long linenum = 0;
5260325f	166	struct stat st;
46df736f	167	Key *key;
94ec8c6b	168
5260325f	169	/* Temporarily use the user's uid. */
63bd8c36	170	temporarily_use_uid(pw);
5260325f	171
5260325f	172	/* The authorized keys. */
c8445989	173	file = authorized_keys_file(pw);
c8445989	174	debug("trying public RSA key file %s", file);
5260325f	175
	176	/* Fail quietly if file does not exist */
	177	if (stat(file, &st) < 0) {
	178	/* Restore the privileged uid. */
	179	restore_uid();
c8445989	180	xfree(file);
159897f3	181	return (0);
8efc0c15	182	}
5260325f	183	/* Open the file containing the authorized keys. */
	184	f = fopen(file, "r");
	185	if (!f) {
	186	/* Restore the privileged uid. */
	187	restore_uid();
c8445989	188	xfree(file);
159897f3	189	return (0);
8efc0c15	190	}
c8445989	191	if (options.strict_modes &&
6978866a	192	secure_filename(f, file, pw, line, sizeof(line)) != 0) {
c8445989	193	xfree(file);
c8445989	194	fclose(f);
bbe88b6d	195	logit("Authentication refused: %s", line);
c8445989	196	restore_uid();
159897f3	197	return (0);
5260325f	198	}
46f1eece	199
	200	/* Flag indicating whether the key is allowed. */
	201	allowed = 0;
5260325f	202
46df736f	203	key = key_new(KEY_RSA1);
5260325f	204
aa3378df	205	/*
	206	* Go though the accepted keys, looking for the current key. If
	207	* found, perform a challenge-response dialog to verify that the
	208	* user really has the corresponding private key.
	209	*/
ea067773	210	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
5260325f	211	char *cp;
ca75d7de	212	char *key_options;
2ceb8101	213	int keybits;
5260325f	214
aa3378df	215	/* Skip leading whitespace, empty and comment lines. */
	216	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	217	;
5260325f	218	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	219	continue;
	220
aa3378df	221	/*
	222	* Check if there are options for this key, and if so,
	223	* save their starting address and skip the option part
	224	* for now. If there are no options, set the starting
	225	* address to NULL.
	226	*/
5260325f	227	if (cp < '0' \|\| cp > '9') {
5260325f	228	int quoted = 0;
ca75d7de	229	key_options = cp;
5260325f	230	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	231	if (*cp == '\\' && cp[1] == '"')
	232	cp++; /* Skip both */
	233	else if (*cp == '"')
	234	quoted = !quoted;
8efc0c15	235	}
5260325f	236	} else
ca75d7de	237	key_options = NULL;
5260325f	238
5260325f	239	/* Parse the key from the line. */
46df736f	240	if (hostfile_read_key(&cp, &bits, key) == 0) {
96a7b0cc	241	debug("%.100s, line %lu: non ssh1 key syntax",
42f11eb2	242	file, linenum);
5260325f	243	continue;
8efc0c15	244	}
5260325f	245	/* cp now points to the comment part. */
5260325f	246
5260325f	247	/* Check if the we have found the desired key (identified by its modulus). */
46df736f	248	if (BN_cmp(key->rsa->n, client_n) != 0)
aa3378df	249	continue;
5260325f	250
57112b5a	251	/* check the real bits */
2ceb8101	252	keybits = BN_num_bits(key->rsa->n);
2ceb8101	253	if (keybits < 0 \|\| bits != (u_int)keybits)
bbe88b6d	254	logit("Warning: %s, line %lu: keysize mismatch: "
57112b5a	255	"actual %d vs. announced %d.",
46df736f	256	file, linenum, BN_num_bits(key->rsa->n), bits);
57112b5a	257
5260325f	258	/* We have found the desired key. */
d343d900	259	/*
	260	* If our options do not allow this key to be used,
	261	* do not send challenge.
	262	*/
ca75d7de	263	if (!auth_parse_options(pw, key_options, file, linenum))
d343d900	264	continue;
5260325f	265
46f1eece	266	/* break out, this key is allowed */
46f1eece	267	allowed = 1;
33de75a3	268	break;
8efc0c15	269	}
8efc0c15	270
5260325f	271	/* Restore the privileged uid. */
5260325f	272	restore_uid();
8efc0c15	273
5260325f	274	/* Close the file. */
c8445989	275	xfree(file);
5260325f	276	fclose(f);
8efc0c15	277
46f1eece	278	/* return key if allowed */
	279	if (allowed && rkey != NULL)
	280	*rkey = key;
94ec8c6b	281	else
46f1eece	282	key_free(key);
	283	return (allowed);
	284	}
	285
	286	/*
	287	* Performs the RSA authentication dialog with the client. This returns
	288	* 0 if the client could not be authenticated, and 1 if authentication was
	289	* successful. This may exit if there is a serious protocol violation.
	290	*/
	291	int
fdaef11e	292	auth_rsa(Authctxt authctxt, BIGNUM client_n)
46f1eece	293	{
	294	Key *key;
	295	char *fp;
fdaef11e	296	struct passwd *pw = authctxt->pw;
46f1eece	297
46f1eece	298	/* no user given */
fdaef11e	299	if (!authctxt->valid)
46f1eece	300	return 0;
46f1eece	301
1853d1ef	302	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
94ec8c6b	303	auth_clear_options();
46f1eece	304	return (0);
	305	}
	306
	307	/* Perform the challenge-response dialog for this key. */
	308	if (!auth_rsa_challenge_dialog(key)) {
	309	/* Wrong response. */
	310	verbose("Wrong response to RSA authentication challenge.");
	311	packet_send_debug("Wrong response to RSA authentication challenge.");
	312	/*
	313	* Break out of the loop. Otherwise we might send
	314	* another challenge and break the protocol.
	315	*/
	316	key_free(key);
	317	return (0);
	318	}
	319	/*
	320	* Correct response. The client has been successfully
	321	* authenticated. Note that we have not yet processed the
	322	* options; this will be reset if the options cause the
	323	* authentication to be rejected.
	324	*/
	325	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	326	verbose("Found matching %s key: %s",
	327	key_type(key), fp);
	328	xfree(fp);
	329	key_free(key);
8efc0c15	330
46f1eece	331	packet_send_debug("RSA authentication accepted.");
46f1eece	332	return (1);
8efc0c15	333	}