[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 1.61 2004/12/06 11:41:03 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"

/* 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"
ea067773	17	RCSID("$OpenBSD: auth-rsa.c,v 1.61 2004/12/06 11:41:03 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"
94ec8c6b	36
	37	/* import */
	38	extern ServerOptions options;
	39
aa3378df	40	/*
	41	* Session identifier that is used to bind key exchange and authentication
	42	* responses to a particular session.
	43	*/
1e3b8b07	44	extern u_char session_id[16];
8efc0c15	45
aa3378df	46	/*
	47	* The .ssh/authorized_keys file contains public keys, one per line, in the
	48	* following format:
	49	* options bits e n comment
	50	* where bits, e and n are decimal numbers,
	51	* and comment is any string of characters up to newline. The maximum
ea067773	52	* length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
aa3378df	53	* description of the options.
aa3378df	54	*/
8efc0c15	55
1853d1ef	56	BIGNUM *
46f1eece	57	auth_rsa_generate_challenge(Key *key)
	58	{
	59	BIGNUM *challenge;
	60	BN_CTX *ctx;
	61
	62	if ((challenge = BN_new()) == NULL)
	63	fatal("auth_rsa_generate_challenge: BN_new() failed");
	64	/* Generate a random challenge. */
	65	BN_rand(challenge, 256, 0, 0);
	66	if ((ctx = BN_CTX_new()) == NULL)
	67	fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	68	BN_mod(challenge, challenge, key->rsa->n, ctx);
	69	BN_CTX_free(ctx);
	70
	71	return challenge;
	72	}
	73
1853d1ef	74	int
46f1eece	75	auth_rsa_verify_response(Key key, BIGNUM challenge, u_char response[16])
	76	{
	77	u_char buf[32], mdbuf[16];
	78	MD5_CTX md;
	79	int len;
	80
52103b10	81	/* don't allow short keys */
82b00264	82	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
b7ced7d1	83	error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
b7ced7d1	84	BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
52103b10	85	return (0);
	86	}
	87
46f1eece	88	/* The response is MD5 of decrypted challenge plus session id. */
	89	len = BN_num_bytes(challenge);
	90	if (len <= 0 \|\| len > 32)
	91	fatal("auth_rsa_verify_response: bad challenge length %d", len);
	92	memset(buf, 0, 32);
	93	BN_bn2bin(challenge, buf + 32 - len);
	94	MD5_Init(&md);
	95	MD5_Update(&md, buf, 32);
	96	MD5_Update(&md, session_id, 16);
	97	MD5_Final(mdbuf, &md);
	98
	99	/* Verify that the response is the original challenge. */
	100	if (memcmp(response, mdbuf, 16) != 0) {
	101	/* Wrong answer. */
	102	return (0);
	103	}
	104	/* Correct answer. */
	105	return (1);
	106	}
	107
aa3378df	108	/*
	109	* Performs the RSA authentication challenge-response dialog with the client,
	110	* and returns true (non-zero) if the client gave the correct answer to
	111	* our challenge; returns zero if the client gives a wrong answer.
	112	*/
8efc0c15	113
8efc0c15	114	int
46f1eece	115	auth_rsa_challenge_dialog(Key *key)
8efc0c15	116	{
c8d54615	117	BIGNUM challenge, encrypted_challenge;
46f1eece	118	u_char response[16];
46f1eece	119	int i, success;
5260325f	120
b775c6f2	121	if ((encrypted_challenge = BN_new()) == NULL)
b775c6f2	122	fatal("auth_rsa_challenge_dialog: BN_new() failed");
5260325f	123
1853d1ef	124	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
5260325f	125
5260325f	126	/* Encrypt the challenge with the public key. */
46f1eece	127	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
5260325f	128
	129	/* Send the encrypted challenge to the client. */
	130	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	131	packet_put_bignum(encrypted_challenge);
	132	packet_send();
c8d54615	133	BN_clear_free(encrypted_challenge);
5260325f	134	packet_write_wait();
5260325f	135
c8d54615	136	/* Wait for a response. */
54a5250f	137	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
c8d54615	138	for (i = 0; i < 16; i++)
c8d54615	139	response[i] = packet_get_char();
95500969	140	packet_check_eom();
c8d54615	141
1853d1ef	142	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
5260325f	143	BN_clear_free(challenge);
46f1eece	144	return (success);
8efc0c15	145	}
8efc0c15	146
aa3378df	147	/*
46f1eece	148	* check if there's user key matching client_n,
46f1eece	149	* return key if login is allowed, NULL otherwise
aa3378df	150	*/
8efc0c15	151
1853d1ef	152	int
46f1eece	153	auth_rsa_key_allowed(struct passwd pw, BIGNUM client_n, Key **rkey)
8efc0c15	154	{
ea067773	155	char line[SSH_MAX_PUBKEY_BYTES], *file;
1853d1ef	156	int allowed = 0;
1e3b8b07	157	u_int bits;
5260325f	158	FILE *f;
1e3b8b07	159	u_long linenum = 0;
5260325f	160	struct stat st;
46df736f	161	Key *key;
94ec8c6b	162
5260325f	163	/* Temporarily use the user's uid. */
63bd8c36	164	temporarily_use_uid(pw);
5260325f	165
5260325f	166	/* The authorized keys. */
c8445989	167	file = authorized_keys_file(pw);
c8445989	168	debug("trying public RSA key file %s", file);
5260325f	169
	170	/* Fail quietly if file does not exist */
	171	if (stat(file, &st) < 0) {
	172	/* Restore the privileged uid. */
	173	restore_uid();
c8445989	174	xfree(file);
159897f3	175	return (0);
8efc0c15	176	}
5260325f	177	/* Open the file containing the authorized keys. */
	178	f = fopen(file, "r");
	179	if (!f) {
	180	/* Restore the privileged uid. */
	181	restore_uid();
c8445989	182	xfree(file);
159897f3	183	return (0);
8efc0c15	184	}
c8445989	185	if (options.strict_modes &&
6978866a	186	secure_filename(f, file, pw, line, sizeof(line)) != 0) {
c8445989	187	xfree(file);
c8445989	188	fclose(f);
bbe88b6d	189	logit("Authentication refused: %s", line);
c8445989	190	restore_uid();
159897f3	191	return (0);
5260325f	192	}
46f1eece	193
	194	/* Flag indicating whether the key is allowed. */
	195	allowed = 0;
5260325f	196
46df736f	197	key = key_new(KEY_RSA1);
5260325f	198
aa3378df	199	/*
	200	* Go though the accepted keys, looking for the current key. If
	201	* found, perform a challenge-response dialog to verify that the
	202	* user really has the corresponding private key.
	203	*/
ea067773	204	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
5260325f	205	char *cp;
ca75d7de	206	char *key_options;
5260325f	207
aa3378df	208	/* Skip leading whitespace, empty and comment lines. */
	209	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	210	;
5260325f	211	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	212	continue;
	213
aa3378df	214	/*
	215	* Check if there are options for this key, and if so,
	216	* save their starting address and skip the option part
	217	* for now. If there are no options, set the starting
	218	* address to NULL.
	219	*/
5260325f	220	if (cp < '0' \|\| cp > '9') {
5260325f	221	int quoted = 0;
ca75d7de	222	key_options = cp;
5260325f	223	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	224	if (*cp == '\\' && cp[1] == '"')
	225	cp++; /* Skip both */
	226	else if (*cp == '"')
	227	quoted = !quoted;
8efc0c15	228	}
5260325f	229	} else
ca75d7de	230	key_options = NULL;
5260325f	231
5260325f	232	/* Parse the key from the line. */
46df736f	233	if (hostfile_read_key(&cp, &bits, key) == 0) {
96a7b0cc	234	debug("%.100s, line %lu: non ssh1 key syntax",
42f11eb2	235	file, linenum);
5260325f	236	continue;
8efc0c15	237	}
5260325f	238	/* cp now points to the comment part. */
5260325f	239
5260325f	240	/* Check if the we have found the desired key (identified by its modulus). */
46df736f	241	if (BN_cmp(key->rsa->n, client_n) != 0)
aa3378df	242	continue;
5260325f	243
57112b5a	244	/* check the real bits */
46df736f	245	if (bits != BN_num_bits(key->rsa->n))
bbe88b6d	246	logit("Warning: %s, line %lu: keysize mismatch: "
57112b5a	247	"actual %d vs. announced %d.",
46df736f	248	file, linenum, BN_num_bits(key->rsa->n), bits);
57112b5a	249
5260325f	250	/* We have found the desired key. */
d343d900	251	/*
	252	* If our options do not allow this key to be used,
	253	* do not send challenge.
	254	*/
ca75d7de	255	if (!auth_parse_options(pw, key_options, file, linenum))
d343d900	256	continue;
5260325f	257
46f1eece	258	/* break out, this key is allowed */
46f1eece	259	allowed = 1;
33de75a3	260	break;
8efc0c15	261	}
8efc0c15	262
5260325f	263	/* Restore the privileged uid. */
5260325f	264	restore_uid();
8efc0c15	265
5260325f	266	/* Close the file. */
c8445989	267	xfree(file);
5260325f	268	fclose(f);
8efc0c15	269
46f1eece	270	/* return key if allowed */
	271	if (allowed && rkey != NULL)
	272	*rkey = key;
94ec8c6b	273	else
46f1eece	274	key_free(key);
	275	return (allowed);
	276	}
	277
	278	/*
	279	* Performs the RSA authentication dialog with the client. This returns
	280	* 0 if the client could not be authenticated, and 1 if authentication was
	281	* successful. This may exit if there is a serious protocol violation.
	282	*/
	283	int
fdaef11e	284	auth_rsa(Authctxt authctxt, BIGNUM client_n)
46f1eece	285	{
	286	Key *key;
	287	char *fp;
fdaef11e	288	struct passwd *pw = authctxt->pw;
46f1eece	289
46f1eece	290	/* no user given */
fdaef11e	291	if (!authctxt->valid)
46f1eece	292	return 0;
46f1eece	293
1853d1ef	294	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
94ec8c6b	295	auth_clear_options();
46f1eece	296	return (0);
	297	}
	298
	299	/* Perform the challenge-response dialog for this key. */
	300	if (!auth_rsa_challenge_dialog(key)) {
	301	/* Wrong response. */
	302	verbose("Wrong response to RSA authentication challenge.");
	303	packet_send_debug("Wrong response to RSA authentication challenge.");
	304	/*
	305	* Break out of the loop. Otherwise we might send
	306	* another challenge and break the protocol.
	307	*/
	308	key_free(key);
	309	return (0);
	310	}
	311	/*
	312	* Correct response. The client has been successfully
	313	* authenticated. Note that we have not yet processed the
	314	* options; this will be reset if the options cause the
	315	* authentication to be rejected.
	316	*/
	317	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	318	verbose("Found matching %s key: %s",
	319	key_type(key), fp);
	320	xfree(fp);
	321	key_free(key);
8efc0c15	322
46f1eece	323	packet_send_debug("RSA authentication accepted.");
46f1eece	324	return (1);
8efc0c15	325	}