[openssh.git] / auth-rsa.c

/* $OpenBSD: auth-rsa.c,v 1.67 2006/03/25 18:29:35 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 "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
febd6f21	1	/* $OpenBSD: auth-rsa.c,v 1.67 2006/03/25 18:29:35 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
	25	#include "rsa.h"
	26	#include "packet.h"
	27	#include "xmalloc.h"
42f11eb2	28	#include "ssh1.h"
8efc0c15	29	#include "uidswap.h"
4fe2af09	30	#include "match.h"
38c295d6	31	#include "auth-options.h"
42f11eb2	32	#include "pathnames.h"
	33	#include "log.h"
	34	#include "servconf.h"
	35	#include "auth.h"
46df736f	36	#include "hostfile.h"
1853d1ef	37	#include "monitor_wrap.h"
82b00264	38	#include "ssh.h"
41feb690	39	#include "misc.h"
94ec8c6b	40
	41	/* import */
	42	extern ServerOptions options;
	43
aa3378df	44	/*
	45	* Session identifier that is used to bind key exchange and authentication
	46	* responses to a particular session.
	47	*/
1e3b8b07	48	extern u_char session_id[16];
8efc0c15	49
aa3378df	50	/*
	51	* The .ssh/authorized_keys file contains public keys, one per line, in the
	52	* following format:
	53	* options bits e n comment
	54	* where bits, e and n are decimal numbers,
	55	* and comment is any string of characters up to newline. The maximum
ea067773	56	* length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
aa3378df	57	* description of the options.
aa3378df	58	*/
8efc0c15	59
1853d1ef	60	BIGNUM *
46f1eece	61	auth_rsa_generate_challenge(Key *key)
	62	{
	63	BIGNUM *challenge;
	64	BN_CTX *ctx;
	65
	66	if ((challenge = BN_new()) == NULL)
	67	fatal("auth_rsa_generate_challenge: BN_new() failed");
	68	/* Generate a random challenge. */
	69	BN_rand(challenge, 256, 0, 0);
	70	if ((ctx = BN_CTX_new()) == NULL)
	71	fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	72	BN_mod(challenge, challenge, key->rsa->n, ctx);
	73	BN_CTX_free(ctx);
	74
	75	return challenge;
	76	}
	77
1853d1ef	78	int
46f1eece	79	auth_rsa_verify_response(Key key, BIGNUM challenge, u_char response[16])
	80	{
	81	u_char buf[32], mdbuf[16];
	82	MD5_CTX md;
	83	int len;
	84
52103b10	85	/* don't allow short keys */
82b00264	86	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
b7ced7d1	87	error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
b7ced7d1	88	BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
52103b10	89	return (0);
	90	}
	91
46f1eece	92	/* The response is MD5 of decrypted challenge plus session id. */
	93	len = BN_num_bytes(challenge);
	94	if (len <= 0 \|\| len > 32)
	95	fatal("auth_rsa_verify_response: bad challenge length %d", len);
	96	memset(buf, 0, 32);
	97	BN_bn2bin(challenge, buf + 32 - len);
	98	MD5_Init(&md);
	99	MD5_Update(&md, buf, 32);
	100	MD5_Update(&md, session_id, 16);
	101	MD5_Final(mdbuf, &md);
	102
	103	/* Verify that the response is the original challenge. */
	104	if (memcmp(response, mdbuf, 16) != 0) {
	105	/* Wrong answer. */
	106	return (0);
	107	}
	108	/* Correct answer. */
	109	return (1);
	110	}
	111
aa3378df	112	/*
	113	* Performs the RSA authentication challenge-response dialog with the client,
	114	* and returns true (non-zero) if the client gave the correct answer to
	115	* our challenge; returns zero if the client gives a wrong answer.
	116	*/
8efc0c15	117
8efc0c15	118	int
46f1eece	119	auth_rsa_challenge_dialog(Key *key)
8efc0c15	120	{
c8d54615	121	BIGNUM challenge, encrypted_challenge;
46f1eece	122	u_char response[16];
46f1eece	123	int i, success;
5260325f	124
b775c6f2	125	if ((encrypted_challenge = BN_new()) == NULL)
b775c6f2	126	fatal("auth_rsa_challenge_dialog: BN_new() failed");
5260325f	127
1853d1ef	128	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
5260325f	129
5260325f	130	/* Encrypt the challenge with the public key. */
46f1eece	131	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
5260325f	132
	133	/* Send the encrypted challenge to the client. */
	134	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	135	packet_put_bignum(encrypted_challenge);
	136	packet_send();
c8d54615	137	BN_clear_free(encrypted_challenge);
5260325f	138	packet_write_wait();
5260325f	139
c8d54615	140	/* Wait for a response. */
54a5250f	141	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
c8d54615	142	for (i = 0; i < 16; i++)
febd6f21	143	response[i] = (u_char)packet_get_char();
95500969	144	packet_check_eom();
c8d54615	145
1853d1ef	146	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
5260325f	147	BN_clear_free(challenge);
46f1eece	148	return (success);
8efc0c15	149	}
8efc0c15	150
aa3378df	151	/*
46f1eece	152	* check if there's user key matching client_n,
46f1eece	153	* return key if login is allowed, NULL otherwise
aa3378df	154	*/
8efc0c15	155
1853d1ef	156	int
46f1eece	157	auth_rsa_key_allowed(struct passwd pw, BIGNUM client_n, Key **rkey)
8efc0c15	158	{
ea067773	159	char line[SSH_MAX_PUBKEY_BYTES], *file;
1853d1ef	160	int allowed = 0;
1e3b8b07	161	u_int bits;
5260325f	162	FILE *f;
1e3b8b07	163	u_long linenum = 0;
5260325f	164	struct stat st;
46df736f	165	Key *key;
94ec8c6b	166
5260325f	167	/* Temporarily use the user's uid. */
63bd8c36	168	temporarily_use_uid(pw);
5260325f	169
5260325f	170	/* The authorized keys. */
c8445989	171	file = authorized_keys_file(pw);
c8445989	172	debug("trying public RSA key file %s", file);
5260325f	173
	174	/* Fail quietly if file does not exist */
	175	if (stat(file, &st) < 0) {
	176	/* Restore the privileged uid. */
	177	restore_uid();
c8445989	178	xfree(file);
159897f3	179	return (0);
8efc0c15	180	}
5260325f	181	/* Open the file containing the authorized keys. */
	182	f = fopen(file, "r");
	183	if (!f) {
	184	/* Restore the privileged uid. */
	185	restore_uid();
c8445989	186	xfree(file);
159897f3	187	return (0);
8efc0c15	188	}
c8445989	189	if (options.strict_modes &&
6978866a	190	secure_filename(f, file, pw, line, sizeof(line)) != 0) {
c8445989	191	xfree(file);
c8445989	192	fclose(f);
bbe88b6d	193	logit("Authentication refused: %s", line);
c8445989	194	restore_uid();
159897f3	195	return (0);
5260325f	196	}
46f1eece	197
	198	/* Flag indicating whether the key is allowed. */
	199	allowed = 0;
5260325f	200
46df736f	201	key = key_new(KEY_RSA1);
5260325f	202
aa3378df	203	/*
	204	* Go though the accepted keys, looking for the current key. If
	205	* found, perform a challenge-response dialog to verify that the
	206	* user really has the corresponding private key.
	207	*/
ea067773	208	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
5260325f	209	char *cp;
ca75d7de	210	char *key_options;
2ceb8101	211	int keybits;
5260325f	212
aa3378df	213	/* Skip leading whitespace, empty and comment lines. */
	214	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	215	;
5260325f	216	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	217	continue;
	218
aa3378df	219	/*
	220	* Check if there are options for this key, and if so,
	221	* save their starting address and skip the option part
	222	* for now. If there are no options, set the starting
	223	* address to NULL.
	224	*/
5260325f	225	if (cp < '0' \|\| cp > '9') {
5260325f	226	int quoted = 0;
ca75d7de	227	key_options = cp;
5260325f	228	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	229	if (*cp == '\\' && cp[1] == '"')
	230	cp++; /* Skip both */
	231	else if (*cp == '"')
	232	quoted = !quoted;
8efc0c15	233	}
5260325f	234	} else
ca75d7de	235	key_options = NULL;
5260325f	236
5260325f	237	/* Parse the key from the line. */
46df736f	238	if (hostfile_read_key(&cp, &bits, key) == 0) {
96a7b0cc	239	debug("%.100s, line %lu: non ssh1 key syntax",
42f11eb2	240	file, linenum);
5260325f	241	continue;
8efc0c15	242	}
5260325f	243	/* cp now points to the comment part. */
5260325f	244
5260325f	245	/* Check if the we have found the desired key (identified by its modulus). */
46df736f	246	if (BN_cmp(key->rsa->n, client_n) != 0)
aa3378df	247	continue;
5260325f	248
57112b5a	249	/* check the real bits */
2ceb8101	250	keybits = BN_num_bits(key->rsa->n);
2ceb8101	251	if (keybits < 0 \|\| bits != (u_int)keybits)
bbe88b6d	252	logit("Warning: %s, line %lu: keysize mismatch: "
57112b5a	253	"actual %d vs. announced %d.",
46df736f	254	file, linenum, BN_num_bits(key->rsa->n), bits);
57112b5a	255
5260325f	256	/* We have found the desired key. */
d343d900	257	/*
	258	* If our options do not allow this key to be used,
	259	* do not send challenge.
	260	*/
ca75d7de	261	if (!auth_parse_options(pw, key_options, file, linenum))
d343d900	262	continue;
5260325f	263
46f1eece	264	/* break out, this key is allowed */
46f1eece	265	allowed = 1;
33de75a3	266	break;
8efc0c15	267	}
8efc0c15	268
5260325f	269	/* Restore the privileged uid. */
5260325f	270	restore_uid();
8efc0c15	271
5260325f	272	/* Close the file. */
c8445989	273	xfree(file);
5260325f	274	fclose(f);
8efc0c15	275
46f1eece	276	/* return key if allowed */
	277	if (allowed && rkey != NULL)
	278	*rkey = key;
94ec8c6b	279	else
46f1eece	280	key_free(key);
	281	return (allowed);
	282	}
	283
	284	/*
	285	* Performs the RSA authentication dialog with the client. This returns
	286	* 0 if the client could not be authenticated, and 1 if authentication was
	287	* successful. This may exit if there is a serious protocol violation.
	288	*/
	289	int
fdaef11e	290	auth_rsa(Authctxt authctxt, BIGNUM client_n)
46f1eece	291	{
	292	Key *key;
	293	char *fp;
fdaef11e	294	struct passwd *pw = authctxt->pw;
46f1eece	295
46f1eece	296	/* no user given */
fdaef11e	297	if (!authctxt->valid)
46f1eece	298	return 0;
46f1eece	299
1853d1ef	300	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
94ec8c6b	301	auth_clear_options();
46f1eece	302	return (0);
	303	}
	304
	305	/* Perform the challenge-response dialog for this key. */
	306	if (!auth_rsa_challenge_dialog(key)) {
	307	/* Wrong response. */
	308	verbose("Wrong response to RSA authentication challenge.");
	309	packet_send_debug("Wrong response to RSA authentication challenge.");
	310	/*
	311	* Break out of the loop. Otherwise we might send
	312	* another challenge and break the protocol.
	313	*/
	314	key_free(key);
	315	return (0);
	316	}
	317	/*
	318	* Correct response. The client has been successfully
	319	* authenticated. Note that we have not yet processed the
	320	* options; this will be reset if the options cause the
	321	* authentication to be rejected.
	322	*/
	323	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	324	verbose("Found matching %s key: %s",
	325	key_type(key), fp);
	326	xfree(fp);
	327	key_free(key);
8efc0c15	328
46f1eece	329	packet_send_debug("RSA authentication accepted.");
46f1eece	330	return (1);
8efc0c15	331	}