[openssh.git] / auth-rsa.c

/* $OpenBSD: auth-rsa.c,v 1.70 2006/08/01 23:22:47 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 <stdio.h>
#include <string.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
cf851879	1	/* $OpenBSD: auth-rsa.c,v 1.70 2006/08/01 23:22:47 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>
cf851879	26	#include <stdio.h>
00146caa	27	#include <string.h>
b1842393	28
8efc0c15	29	#include "rsa.h"
	30	#include "packet.h"
	31	#include "xmalloc.h"
42f11eb2	32	#include "ssh1.h"
8efc0c15	33	#include "uidswap.h"
4fe2af09	34	#include "match.h"
38c295d6	35	#include "auth-options.h"
42f11eb2	36	#include "pathnames.h"
	37	#include "log.h"
	38	#include "servconf.h"
	39	#include "auth.h"
46df736f	40	#include "hostfile.h"
1853d1ef	41	#include "monitor_wrap.h"
82b00264	42	#include "ssh.h"
41feb690	43	#include "misc.h"
94ec8c6b	44
	45	/* import */
	46	extern ServerOptions options;
	47
aa3378df	48	/*
	49	* Session identifier that is used to bind key exchange and authentication
	50	* responses to a particular session.
	51	*/
1e3b8b07	52	extern u_char session_id[16];
8efc0c15	53
aa3378df	54	/*
	55	* The .ssh/authorized_keys file contains public keys, one per line, in the
	56	* following format:
	57	* options bits e n comment
	58	* where bits, e and n are decimal numbers,
	59	* and comment is any string of characters up to newline. The maximum
ea067773	60	* length of a line is SSH_MAX_PUBKEY_BYTES characters. See sshd(8) for a
aa3378df	61	* description of the options.
aa3378df	62	*/
8efc0c15	63
1853d1ef	64	BIGNUM *
46f1eece	65	auth_rsa_generate_challenge(Key *key)
	66	{
	67	BIGNUM *challenge;
	68	BN_CTX *ctx;
	69
	70	if ((challenge = BN_new()) == NULL)
	71	fatal("auth_rsa_generate_challenge: BN_new() failed");
	72	/* Generate a random challenge. */
	73	BN_rand(challenge, 256, 0, 0);
	74	if ((ctx = BN_CTX_new()) == NULL)
	75	fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
	76	BN_mod(challenge, challenge, key->rsa->n, ctx);
	77	BN_CTX_free(ctx);
	78
	79	return challenge;
	80	}
	81
1853d1ef	82	int
46f1eece	83	auth_rsa_verify_response(Key key, BIGNUM challenge, u_char response[16])
	84	{
	85	u_char buf[32], mdbuf[16];
	86	MD5_CTX md;
	87	int len;
	88
52103b10	89	/* don't allow short keys */
82b00264	90	if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
b7ced7d1	91	error("auth_rsa_verify_response: RSA modulus too small: %d < minimum %d bits",
b7ced7d1	92	BN_num_bits(key->rsa->n), SSH_RSA_MINIMUM_MODULUS_SIZE);
52103b10	93	return (0);
	94	}
	95
46f1eece	96	/* The response is MD5 of decrypted challenge plus session id. */
	97	len = BN_num_bytes(challenge);
	98	if (len <= 0 \|\| len > 32)
	99	fatal("auth_rsa_verify_response: bad challenge length %d", len);
	100	memset(buf, 0, 32);
	101	BN_bn2bin(challenge, buf + 32 - len);
	102	MD5_Init(&md);
	103	MD5_Update(&md, buf, 32);
	104	MD5_Update(&md, session_id, 16);
	105	MD5_Final(mdbuf, &md);
	106
	107	/* Verify that the response is the original challenge. */
	108	if (memcmp(response, mdbuf, 16) != 0) {
	109	/* Wrong answer. */
	110	return (0);
	111	}
	112	/* Correct answer. */
	113	return (1);
	114	}
	115
aa3378df	116	/*
	117	* Performs the RSA authentication challenge-response dialog with the client,
	118	* and returns true (non-zero) if the client gave the correct answer to
	119	* our challenge; returns zero if the client gives a wrong answer.
	120	*/
8efc0c15	121
8efc0c15	122	int
46f1eece	123	auth_rsa_challenge_dialog(Key *key)
8efc0c15	124	{
c8d54615	125	BIGNUM challenge, encrypted_challenge;
46f1eece	126	u_char response[16];
46f1eece	127	int i, success;
5260325f	128
b775c6f2	129	if ((encrypted_challenge = BN_new()) == NULL)
b775c6f2	130	fatal("auth_rsa_challenge_dialog: BN_new() failed");
5260325f	131
1853d1ef	132	challenge = PRIVSEP(auth_rsa_generate_challenge(key));
5260325f	133
5260325f	134	/* Encrypt the challenge with the public key. */
46f1eece	135	rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
5260325f	136
	137	/* Send the encrypted challenge to the client. */
	138	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	139	packet_put_bignum(encrypted_challenge);
	140	packet_send();
c8d54615	141	BN_clear_free(encrypted_challenge);
5260325f	142	packet_write_wait();
5260325f	143
c8d54615	144	/* Wait for a response. */
54a5250f	145	packet_read_expect(SSH_CMSG_AUTH_RSA_RESPONSE);
c8d54615	146	for (i = 0; i < 16; i++)
febd6f21	147	response[i] = (u_char)packet_get_char();
95500969	148	packet_check_eom();
c8d54615	149
1853d1ef	150	success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
5260325f	151	BN_clear_free(challenge);
46f1eece	152	return (success);
8efc0c15	153	}
8efc0c15	154
aa3378df	155	/*
46f1eece	156	* check if there's user key matching client_n,
46f1eece	157	* return key if login is allowed, NULL otherwise
aa3378df	158	*/
8efc0c15	159
1853d1ef	160	int
46f1eece	161	auth_rsa_key_allowed(struct passwd pw, BIGNUM client_n, Key **rkey)
8efc0c15	162	{
ea067773	163	char line[SSH_MAX_PUBKEY_BYTES], *file;
1853d1ef	164	int allowed = 0;
1e3b8b07	165	u_int bits;
5260325f	166	FILE *f;
1e3b8b07	167	u_long linenum = 0;
5260325f	168	struct stat st;
46df736f	169	Key *key;
94ec8c6b	170
5260325f	171	/* Temporarily use the user's uid. */
63bd8c36	172	temporarily_use_uid(pw);
5260325f	173
5260325f	174	/* The authorized keys. */
c8445989	175	file = authorized_keys_file(pw);
c8445989	176	debug("trying public RSA key file %s", file);
5260325f	177
	178	/* Fail quietly if file does not exist */
	179	if (stat(file, &st) < 0) {
	180	/* Restore the privileged uid. */
	181	restore_uid();
c8445989	182	xfree(file);
159897f3	183	return (0);
8efc0c15	184	}
5260325f	185	/* Open the file containing the authorized keys. */
	186	f = fopen(file, "r");
	187	if (!f) {
	188	/* Restore the privileged uid. */
	189	restore_uid();
c8445989	190	xfree(file);
159897f3	191	return (0);
8efc0c15	192	}
c8445989	193	if (options.strict_modes &&
6978866a	194	secure_filename(f, file, pw, line, sizeof(line)) != 0) {
c8445989	195	xfree(file);
c8445989	196	fclose(f);
bbe88b6d	197	logit("Authentication refused: %s", line);
c8445989	198	restore_uid();
159897f3	199	return (0);
5260325f	200	}
46f1eece	201
	202	/* Flag indicating whether the key is allowed. */
	203	allowed = 0;
5260325f	204
46df736f	205	key = key_new(KEY_RSA1);
5260325f	206
aa3378df	207	/*
	208	* Go though the accepted keys, looking for the current key. If
	209	* found, perform a challenge-response dialog to verify that the
	210	* user really has the corresponding private key.
	211	*/
ea067773	212	while (read_keyfile_line(f, file, line, sizeof(line), &linenum) != -1) {
5260325f	213	char *cp;
ca75d7de	214	char *key_options;
2ceb8101	215	int keybits;
5260325f	216
aa3378df	217	/* Skip leading whitespace, empty and comment lines. */
	218	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	219	;
5260325f	220	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	221	continue;
	222
aa3378df	223	/*
	224	* Check if there are options for this key, and if so,
	225	* save their starting address and skip the option part
	226	* for now. If there are no options, set the starting
	227	* address to NULL.
	228	*/
5260325f	229	if (cp < '0' \|\| cp > '9') {
5260325f	230	int quoted = 0;
ca75d7de	231	key_options = cp;
5260325f	232	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	233	if (*cp == '\\' && cp[1] == '"')
	234	cp++; /* Skip both */
	235	else if (*cp == '"')
	236	quoted = !quoted;
8efc0c15	237	}
5260325f	238	} else
ca75d7de	239	key_options = NULL;
5260325f	240
5260325f	241	/* Parse the key from the line. */
46df736f	242	if (hostfile_read_key(&cp, &bits, key) == 0) {
96a7b0cc	243	debug("%.100s, line %lu: non ssh1 key syntax",
42f11eb2	244	file, linenum);
5260325f	245	continue;
8efc0c15	246	}
5260325f	247	/* cp now points to the comment part. */
5260325f	248
5260325f	249	/* Check if the we have found the desired key (identified by its modulus). */
46df736f	250	if (BN_cmp(key->rsa->n, client_n) != 0)
aa3378df	251	continue;
5260325f	252
57112b5a	253	/* check the real bits */
2ceb8101	254	keybits = BN_num_bits(key->rsa->n);
2ceb8101	255	if (keybits < 0 \|\| bits != (u_int)keybits)
bbe88b6d	256	logit("Warning: %s, line %lu: keysize mismatch: "
57112b5a	257	"actual %d vs. announced %d.",
46df736f	258	file, linenum, BN_num_bits(key->rsa->n), bits);
57112b5a	259
5260325f	260	/* We have found the desired key. */
d343d900	261	/*
	262	* If our options do not allow this key to be used,
	263	* do not send challenge.
	264	*/
ca75d7de	265	if (!auth_parse_options(pw, key_options, file, linenum))
d343d900	266	continue;
5260325f	267
46f1eece	268	/* break out, this key is allowed */
46f1eece	269	allowed = 1;
33de75a3	270	break;
8efc0c15	271	}
8efc0c15	272
5260325f	273	/* Restore the privileged uid. */
5260325f	274	restore_uid();
8efc0c15	275
5260325f	276	/* Close the file. */
c8445989	277	xfree(file);
5260325f	278	fclose(f);
8efc0c15	279
46f1eece	280	/* return key if allowed */
	281	if (allowed && rkey != NULL)
	282	*rkey = key;
94ec8c6b	283	else
46f1eece	284	key_free(key);
	285	return (allowed);
	286	}
	287
	288	/*
	289	* Performs the RSA authentication dialog with the client. This returns
	290	* 0 if the client could not be authenticated, and 1 if authentication was
	291	* successful. This may exit if there is a serious protocol violation.
	292	*/
	293	int
fdaef11e	294	auth_rsa(Authctxt authctxt, BIGNUM client_n)
46f1eece	295	{
	296	Key *key;
	297	char *fp;
fdaef11e	298	struct passwd *pw = authctxt->pw;
46f1eece	299
46f1eece	300	/* no user given */
fdaef11e	301	if (!authctxt->valid)
46f1eece	302	return 0;
46f1eece	303
1853d1ef	304	if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
94ec8c6b	305	auth_clear_options();
46f1eece	306	return (0);
	307	}
	308
	309	/* Perform the challenge-response dialog for this key. */
	310	if (!auth_rsa_challenge_dialog(key)) {
	311	/* Wrong response. */
	312	verbose("Wrong response to RSA authentication challenge.");
	313	packet_send_debug("Wrong response to RSA authentication challenge.");
	314	/*
	315	* Break out of the loop. Otherwise we might send
	316	* another challenge and break the protocol.
	317	*/
	318	key_free(key);
	319	return (0);
	320	}
	321	/*
	322	* Correct response. The client has been successfully
	323	* authenticated. Note that we have not yet processed the
	324	* options; this will be reset if the options cause the
	325	* authentication to be rejected.
	326	*/
	327	fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
	328	verbose("Found matching %s key: %s",
	329	key_type(key), fp);
	330	xfree(fp);
	331	key_free(key);
8efc0c15	332
46f1eece	333	packet_send_debug("RSA authentication accepted.");
46f1eece	334	return (1);
8efc0c15	335	}