[openssh.git] / auth-rsa.c

/*
 * 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"
RCSID("$OpenBSD: auth-rsa.c,v 1.62 2004/12/11 01:48:56 dtucker Exp $");

#include <openssl/rsa.h>
#include <openssl/md5.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] = 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;

		/* 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  */
		if (bits != BN_num_bits(key->rsa->n))
			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
8efc0c15	1	/*
5260325f	2	* Author: Tatu Ylonen <ylo@cs.hut.fi>
5260325f	3	* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5260325f	4	* All rights reserved
5260325f	5	* RSA-based authentication. This code determines whether to admit a login
	6	* based on RSA authentication. This file also contains functions to check
	7	* validity of the host key.
6ae2364d	8	*
bcbf86ec	9	* As far as I am concerned, the code I have written for this software
	10	* can be used freely for any purpose. Any derived versions of this
	11	* software must be clearly marked as such, and if the derived work is
	12	* incompatible with the protocol description in the RFC file, it must be
	13	* called by a name other than "ssh" or "Secure Shell".
5260325f	14	*/
8efc0c15	15
8efc0c15	16	#include "includes.h"
41feb690	17	RCSID("$OpenBSD: auth-rsa.c,v 1.62 2004/12/11 01:48:56 dtucker Exp $");
42f11eb2	18
	19	#include <openssl/rsa.h>
	20	#include <openssl/md5.h>
8efc0c15	21
	22	#include "rsa.h"
	23	#include "packet.h"
	24	#include "xmalloc.h"
42f11eb2	25	#include "ssh1.h"
8efc0c15	26	#include "uidswap.h"
4fe2af09	27	#include "match.h"
38c295d6	28	#include "auth-options.h"
42f11eb2	29	#include "pathnames.h"
	30	#include "log.h"
	31	#include "servconf.h"
	32	#include "auth.h"
46df736f	33	#include "hostfile.h"
1853d1ef	34	#include "monitor_wrap.h"
82b00264	35	#include "ssh.h"
41feb690	36	#include "misc.h"
94ec8c6b	37
	38	/* import */
	39	extern ServerOptions options;
	40
aa3378df	41	/*
	42	* Session identifier that is used to bind key exchange and authentication
	43	* responses to a particular session.
	44	*/
1e3b8b07	45	extern u_char session_id[16];
8efc0c15	46
aa3378df	47	/*
	48	* The .ssh/authorized_keys file contains public keys, one per line, in the
	49	* following format:
	50	* options bits e n comment
	51	* where bits, e and n are decimal numbers,
	52	* and comment is any string of characters up to newline. The maximum
ea067773	53	* length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
aa3378df	54	* description of the options.
aa3378df	55	*/
8efc0c15	56
1853d1ef	57	BIGNUM *
46f1eece	58	auth_rsa_generate_challenge(Key *key)
	59	{
	60	BIGNUM *challenge;
	61	BN_CTX *ctx;
	62
	63	if ((challenge = BN_new()) == NULL)
	64	fatal("auth_rsa_generate_challenge: BN_new() failed");
	65	/* Generate a random challenge. */
	66	BN_rand(challenge, 256, 0, 0);
	67	if ((ctx = BN_CTX_new()) == NULL)
	68	fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	69	BN_mod(challenge, challenge, key->rsa->n, ctx);
	70	BN_CTX_free(ctx);
	71
	72	return challenge;
	73	}
	74
1853d1ef	75	int
46f1eece	76	auth_rsa_verify_response(Key key, BIGNUM challenge, u_char response[16])
	77	{
	78	u_char buf[32], mdbuf[16];
	79	MD5_CTX md;
	80	int len;
	81
52103b10	82	/* don't allow short keys */
82b00264	83	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
b7ced7d1	84	error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
b7ced7d1	85	BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
52103b10	86	return (0);
	87	}
	88
46f1eece	89	/* The response is MD5 of decrypted challenge plus session id. */
	90	len = BN_num_bytes(challenge);
	91	if (len <= 0 \|\| len > 32)
	92	fatal("auth_rsa_verify_response: bad challenge length %d", len);
	93	memset(buf, 0, 32);
	94	BN_bn2bin(challenge, buf + 32 - len);
	95	MD5_Init(&md);
	96	MD5_Update(&md, buf, 32);
	97	MD5_Update(&md, session_id, 16);
	98	MD5_Final(mdbuf, &md);
	99
	100	/* Verify that the response is the original challenge. */
	101	if (memcmp(response, mdbuf, 16) != 0) {
	102	/* Wrong answer. */
	103	return (0);
	104	}
	105	/* Correct answer. */
	106	return (1);
	107	}
	108
aa3378df	109	/*
	110	* Performs the RSA authentication challenge-response dialog with the client,
	111	* and returns true (non-zero) if the client gave the correct answer to
	112	* our challenge; returns zero if the client gives a wrong answer.
	113	*/
8efc0c15	114
8efc0c15	115	int
46f1eece	116	auth_rsa_challenge_dialog(Key *key)
8efc0c15	117	{
c8d54615	118	BIGNUM challenge, encrypted_challenge;
46f1eece	119	u_char response[16];
46f1eece	120	int i, success;
5260325f	121
b775c6f2	122	if ((encrypted_challenge = BN_new()) == NULL)
b775c6f2	123	fatal("auth_rsa_challenge_dialog: BN_new() failed");
5260325f	124
1853d1ef	125	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
5260325f	126
5260325f	127	/* Encrypt the challenge with the public key. */
46f1eece	128	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
5260325f	129
	130	/* Send the encrypted challenge to the client. */
	131	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	132	packet_put_bignum(encrypted_challenge);
	133	packet_send();
c8d54615	134	BN_clear_free(encrypted_challenge);
5260325f	135	packet_write_wait();
5260325f	136
c8d54615	137	/* Wait for a response. */
54a5250f	138	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
c8d54615	139	for (i = 0; i < 16; i++)
c8d54615	140	response[i] = packet_get_char();
95500969	141	packet_check_eom();
c8d54615	142
1853d1ef	143	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
5260325f	144	BN_clear_free(challenge);
46f1eece	145	return (success);
8efc0c15	146	}
8efc0c15	147
aa3378df	148	/*
46f1eece	149	* check if there's user key matching client_n,
46f1eece	150	* return key if login is allowed, NULL otherwise
aa3378df	151	*/
8efc0c15	152
1853d1ef	153	int
46f1eece	154	auth_rsa_key_allowed(struct passwd pw, BIGNUM client_n, Key **rkey)
8efc0c15	155	{
ea067773	156	char line[SSH_MAX_PUBKEY_BYTES], *file;
1853d1ef	157	int allowed = 0;
1e3b8b07	158	u_int bits;
5260325f	159	FILE *f;
1e3b8b07	160	u_long linenum = 0;
5260325f	161	struct stat st;
46df736f	162	Key *key;
94ec8c6b	163
5260325f	164	/* Temporarily use the user's uid. */
63bd8c36	165	temporarily_use_uid(pw);
5260325f	166
5260325f	167	/* The authorized keys. */
c8445989	168	file = authorized_keys_file(pw);
c8445989	169	debug("trying public RSA key file %s", file);
5260325f	170
	171	/* Fail quietly if file does not exist */
	172	if (stat(file, &st) < 0) {
	173	/* Restore the privileged uid. */
	174	restore_uid();
c8445989	175	xfree(file);
159897f3	176	return (0);
8efc0c15	177	}
5260325f	178	/* Open the file containing the authorized keys. */
	179	f = fopen(file, "r");
	180	if (!f) {
	181	/* Restore the privileged uid. */
	182	restore_uid();
c8445989	183	xfree(file);
159897f3	184	return (0);
8efc0c15	185	}
c8445989	186	if (options.strict_modes &&
6978866a	187	secure_filename(f, file, pw, line, sizeof(line)) != 0) {
c8445989	188	xfree(file);
c8445989	189	fclose(f);
bbe88b6d	190	logit("Authentication refused: %s", line);
c8445989	191	restore_uid();
159897f3	192	return (0);
5260325f	193	}
46f1eece	194
	195	/* Flag indicating whether the key is allowed. */
	196	allowed = 0;
5260325f	197
46df736f	198	key = key_new(KEY_RSA1);
5260325f	199
aa3378df	200	/*
	201	* Go though the accepted keys, looking for the current key. If
	202	* found, perform a challenge-response dialog to verify that the
	203	* user really has the corresponding private key.
	204	*/
ea067773	205	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
5260325f	206	char *cp;
ca75d7de	207	char *key_options;
5260325f	208
aa3378df	209	/* Skip leading whitespace, empty and comment lines. */
	210	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	211	;
5260325f	212	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	213	continue;
	214
aa3378df	215	/*
	216	* Check if there are options for this key, and if so,
	217	* save their starting address and skip the option part
	218	* for now. If there are no options, set the starting
	219	* address to NULL.
	220	*/
5260325f	221	if (cp < '0' \|\| cp > '9') {
5260325f	222	int quoted = 0;
ca75d7de	223	key_options = cp;
5260325f	224	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	225	if (*cp == '\\' && cp[1] == '"')
	226	cp++; /* Skip both */
	227	else if (*cp == '"')
	228	quoted = !quoted;
8efc0c15	229	}
5260325f	230	} else
ca75d7de	231	key_options = NULL;
5260325f	232
5260325f	233	/* Parse the key from the line. */
46df736f	234	if (hostfile_read_key(&cp, &bits, key) == 0) {
96a7b0cc	235	debug("%.100s, line %lu: non ssh1 key syntax",
42f11eb2	236	file, linenum);
5260325f	237	continue;
8efc0c15	238	}
5260325f	239	/* cp now points to the comment part. */
5260325f	240
5260325f	241	/* Check if the we have found the desired key (identified by its modulus). */
46df736f	242	if (BN_cmp(key->rsa->n, client_n) != 0)
aa3378df	243	continue;
5260325f	244
57112b5a	245	/* check the real bits */
46df736f	246	if (bits != BN_num_bits(key->rsa->n))
bbe88b6d	247	logit("Warning: %s, line %lu: keysize mismatch: "
57112b5a	248	"actual %d vs. announced %d.",
46df736f	249	file, linenum, BN_num_bits(key->rsa->n), bits);
57112b5a	250
5260325f	251	/* We have found the desired key. */
d343d900	252	/*
	253	* If our options do not allow this key to be used,
	254	* do not send challenge.
	255	*/
ca75d7de	256	if (!auth_parse_options(pw, key_options, file, linenum))
d343d900	257	continue;
5260325f	258
46f1eece	259	/* break out, this key is allowed */
46f1eece	260	allowed = 1;
33de75a3	261	break;
8efc0c15	262	}
8efc0c15	263
5260325f	264	/* Restore the privileged uid. */
5260325f	265	restore_uid();
8efc0c15	266
5260325f	267	/* Close the file. */
c8445989	268	xfree(file);
5260325f	269	fclose(f);
8efc0c15	270
46f1eece	271	/* return key if allowed */
	272	if (allowed && rkey != NULL)
	273	*rkey = key;
94ec8c6b	274	else
46f1eece	275	key_free(key);
	276	return (allowed);
	277	}
	278
	279	/*
	280	* Performs the RSA authentication dialog with the client. This returns
	281	* 0 if the client could not be authenticated, and 1 if authentication was
	282	* successful. This may exit if there is a serious protocol violation.
	283	*/
	284	int
fdaef11e	285	auth_rsa(Authctxt authctxt, BIGNUM client_n)
46f1eece	286	{
	287	Key *key;
	288	char *fp;
fdaef11e	289	struct passwd *pw = authctxt->pw;
46f1eece	290
46f1eece	291	/* no user given */
fdaef11e	292	if (!authctxt->valid)
46f1eece	293	return 0;
46f1eece	294
1853d1ef	295	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
94ec8c6b	296	auth_clear_options();
46f1eece	297	return (0);
	298	}
	299
	300	/* Perform the challenge-response dialog for this key. */
	301	if (!auth_rsa_challenge_dialog(key)) {
	302	/* Wrong response. */
	303	verbose("Wrong response to RSA authentication challenge.");
	304	packet_send_debug("Wrong response to RSA authentication challenge.");
	305	/*
	306	* Break out of the loop. Otherwise we might send
	307	* another challenge and break the protocol.
	308	*/
	309	key_free(key);
	310	return (0);
	311	}
	312	/*
	313	* Correct response. The client has been successfully
	314	* authenticated. Note that we have not yet processed the
	315	* options; this will be reset if the options cause the
	316	* authentication to be rejected.
	317	*/
	318	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	319	verbose("Found matching %s key: %s",
	320	key_type(key), fp);
	321	xfree(fp);
	322	key_free(key);
8efc0c15	323
46f1eece	324	packet_send_debug("RSA authentication accepted.");
46f1eece	325	return (1);
8efc0c15	326	}