[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 1.32 2000/10/14 12:19:45 markus Exp $");

#include "rsa.h"
#include "packet.h"
#include "xmalloc.h"
#include "ssh.h"
#include "mpaux.h"
#include "uidswap.h"
#include "match.h"
#include "servconf.h"
#include "auth-options.h"

#include <openssl/rsa.h>
#include <openssl/md5.h>


/* import */
extern ServerOptions options;

/*
 * Session identifier that is used to bind key exchange and authentication
 * responses to a particular session.
 */
extern unsigned 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 8000 characters.  See the documentation for a
 * description of the options.
 */

/*
 * 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(RSA *pk)
{
	BIGNUM *challenge, *encrypted_challenge;
	BN_CTX *ctx;
	unsigned char buf[32], mdbuf[16], response[16];
	MD5_CTX md;
	unsigned int i;
	int plen, len;

	encrypted_challenge = BN_new();
	challenge = BN_new();

	/* Generate a random challenge. */
	BN_rand(challenge, 256, 0, 0);
	ctx = BN_CTX_new();
	BN_mod(challenge, challenge, pk->n, ctx);
	BN_CTX_free(ctx);

	/* Encrypt the challenge with the public key. */
	rsa_public_encrypt(encrypted_challenge, challenge, pk);

	/* 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(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
	packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
	for (i = 0; i < 16; i++)
		response[i] = packet_get_char();

	/* The response is MD5 of decrypted challenge plus session id. */
	len = BN_num_bytes(challenge);
	if (len <= 0 || len > 32)
		fatal("auth_rsa_challenge_dialog: 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);
	BN_clear_free(challenge);

	/* 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 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(struct passwd *pw, BIGNUM *client_n)
{
	char line[8192], file[1024];
	int authenticated;
	unsigned int bits;
	FILE *f;
	unsigned long linenum = 0;
	struct stat st;
	RSA *pk;

	/* no user given */
	if (pw == NULL)
		return 0;

	/* Temporarily use the user's uid. */
	temporarily_use_uid(pw->pw_uid);

	/* The authorized keys. */
	snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
		 SSH_USER_PERMITTED_KEYS);

	/* Fail quietly if file does not exist */
	if (stat(file, &st) < 0) {
		/* Restore the privileged uid. */
		restore_uid();
		return 0;
	}
	/* Open the file containing the authorized keys. */
	f = fopen(file, "r");
	if (!f) {
		/* Restore the privileged uid. */
		restore_uid();
		packet_send_debug("Could not open %.900s for reading.", file);
		packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
		return 0;
	}
	if (options.strict_modes) {
		int fail = 0;
		char buf[1024];
		/* Check open file in order to avoid open/stat races */
		if (fstat(fileno(f), &st) < 0 ||
		    (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
		    (st.st_mode & 022) != 0) {
			snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
				 "bad ownership or modes for '%s'.", pw->pw_name, file);
			fail = 1;
		} else {
			/* Check path to SSH_USER_PERMITTED_KEYS */
			int i;
			static const char *check[] = {
				"", SSH_USER_DIR, NULL
			};
			for (i = 0; check[i]; i++) {
				snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
				if (stat(line, &st) < 0 ||
				    (st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
				    (st.st_mode & 022) != 0) {
					snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
						 "bad ownership or modes for '%s'.", pw->pw_name, line);
					fail = 1;
					break;
				}
			}
		}
		if (fail) {
			fclose(f);
			log("%s",buf);
			packet_send_debug("%s",buf);
			restore_uid();
			return 0;
		}
	}
	/* Flag indicating whether authentication has succeeded. */
	authenticated = 0;

	pk = RSA_new();
	pk->e = BN_new();
	pk->n = BN_new();

	/*
	 * 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 (fgets(line, sizeof(line), f)) {
		char *cp;
		char *options;

		linenum++;

		/* 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;
			options = cp;
			for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
				if (*cp == '\\' && cp[1] == '"')
					cp++;	/* Skip both */
				else if (*cp == '"')
					quoted = !quoted;
			}
		} else
			options = NULL;
		/*
		 * If our options do not allow this key to be used,
		 * do not send challenge.
		 */
		if (!auth_parse_options(pw, options, linenum))
			continue;

		/* Parse the key from the line. */
		if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) {
			debug("%.100s, line %lu: bad key syntax",
			      SSH_USER_PERMITTED_KEYS, linenum);
			packet_send_debug("%.100s, line %lu: bad key syntax",
					  SSH_USER_PERMITTED_KEYS, 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(pk->n, client_n) != 0)
			continue;

		/* check the real bits  */
		if (bits != BN_num_bits(pk->n))
			log("Warning: %s, line %ld: keysize mismatch: "
			    "actual %d vs. announced %d.",
			    file, linenum, BN_num_bits(pk->n), bits);

		/* We have found the desired key. */

		/* Perform the challenge-response dialog for this key. */
		if (!auth_rsa_challenge_dialog(pk)) {
			/* Wrong response. */
			verbose("Wrong response to RSA authentication challenge.");
			packet_send_debug("Wrong response to RSA authentication challenge.");
			continue;
		}
		/*
		 * 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.
		 * Break out of the loop if authentication was successful;
		 * otherwise continue searching.
		 */
		authenticated = 1;
		break;
	}

	/* Restore the privileged uid. */
	restore_uid();

	/* Close the file. */
	fclose(f);

	RSA_free(pk);

	if (authenticated)
		packet_send_debug("RSA authentication accepted.");
	else
		auth_clear_options();

	/* Return authentication result. */
	return authenticated;
}
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"
33de75a3	17	RCSID("$OpenBSD: auth-rsa.c,v 1.32 2000/10/14 12:19:45 markus Exp $");
8efc0c15	18
	19	#include "rsa.h"
	20	#include "packet.h"
	21	#include "xmalloc.h"
	22	#include "ssh.h"
	23	#include "mpaux.h"
	24	#include "uidswap.h"
4fe2af09	25	#include "match.h"
6fa724bc	26	#include "servconf.h"
38c295d6	27	#include "auth-options.h"
8efc0c15	28
	29	#include <openssl/rsa.h>
	30	#include <openssl/md5.h>
	31
94ec8c6b	32
	33	/* import */
	34	extern ServerOptions options;
	35
aa3378df	36	/*
	37	* Session identifier that is used to bind key exchange and authentication
	38	* responses to a particular session.
	39	*/
8efc0c15	40	extern unsigned char session_id[16];
8efc0c15	41
aa3378df	42	/*
	43	* The .ssh/authorized_keys file contains public keys, one per line, in the
	44	* following format:
	45	* options bits e n comment
	46	* where bits, e and n are decimal numbers,
	47	* and comment is any string of characters up to newline. The maximum
	48	* length of a line is 8000 characters. See the documentation for a
	49	* description of the options.
	50	*/
8efc0c15	51
aa3378df	52	/*
	53	* Performs the RSA authentication challenge-response dialog with the client,
	54	* and returns true (non-zero) if the client gave the correct answer to
	55	* our challenge; returns zero if the client gives a wrong answer.
	56	*/
8efc0c15	57
8efc0c15	58	int
4fe2af09	59	auth_rsa_challenge_dialog(RSA *pk)
8efc0c15	60	{
c8d54615	61	BIGNUM challenge, encrypted_challenge;
c8d54615	62	BN_CTX *ctx;
5260325f	63	unsigned char buf[32], mdbuf[16], response[16];
	64	MD5_CTX md;
	65	unsigned int i;
	66	int plen, len;
	67
	68	encrypted_challenge = BN_new();
	69	challenge = BN_new();
5260325f	70
	71	/* Generate a random challenge. */
	72	BN_rand(challenge, 256, 0, 0);
c8d54615	73	ctx = BN_CTX_new();
4fe2af09	74	BN_mod(challenge, challenge, pk->n, ctx);
c8d54615	75	BN_CTX_free(ctx);
5260325f	76
5260325f	77	/* Encrypt the challenge with the public key. */
5260325f	78	rsa_public_encrypt(encrypted_challenge, challenge, pk);
5260325f	79
	80	/* Send the encrypted challenge to the client. */
	81	packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
	82	packet_put_bignum(encrypted_challenge);
	83	packet_send();
c8d54615	84	BN_clear_free(encrypted_challenge);
5260325f	85	packet_write_wait();
5260325f	86
c8d54615	87	/* Wait for a response. */
	88	packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
	89	packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
	90	for (i = 0; i < 16; i++)
	91	response[i] = packet_get_char();
	92
5260325f	93	/* The response is MD5 of decrypted challenge plus session id. */
	94	len = BN_num_bytes(challenge);
	95	if (len <= 0 \|\| len > 32)
	96	fatal("auth_rsa_challenge_dialog: bad challenge length %d", len);
	97	memset(buf, 0, 32);
	98	BN_bn2bin(challenge, buf + 32 - len);
	99	MD5_Init(&md);
	100	MD5_Update(&md, buf, 32);
	101	MD5_Update(&md, session_id, 16);
	102	MD5_Final(mdbuf, &md);
5260325f	103	BN_clear_free(challenge);
5260325f	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;
8efc0c15	112	}
8efc0c15	113
aa3378df	114	/*
	115	* Performs the RSA authentication dialog with the client. This returns
	116	* 0 if the client could not be authenticated, and 1 if authentication was
	117	* successful. This may exit if there is a serious protocol violation.
	118	*/
8efc0c15	119
8efc0c15	120	int
6fa724bc	121	auth_rsa(struct passwd pw, BIGNUM client_n)
8efc0c15	122	{
5260325f	123	char line[8192], file[1024];
	124	int authenticated;
	125	unsigned int bits;
	126	FILE *f;
	127	unsigned long linenum = 0;
	128	struct stat st;
4fe2af09	129	RSA *pk;
5260325f	130
94ec8c6b	131	/* no user given */
	132	if (pw == NULL)
	133	return 0;
	134
5260325f	135	/* Temporarily use the user's uid. */
	136	temporarily_use_uid(pw->pw_uid);
	137
	138	/* The authorized keys. */
	139	snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
	140	SSH_USER_PERMITTED_KEYS);
	141
	142	/* Fail quietly if file does not exist */
	143	if (stat(file, &st) < 0) {
	144	/* Restore the privileged uid. */
	145	restore_uid();
	146	return 0;
8efc0c15	147	}
5260325f	148	/* Open the file containing the authorized keys. */
	149	f = fopen(file, "r");
	150	if (!f) {
	151	/* Restore the privileged uid. */
	152	restore_uid();
	153	packet_send_debug("Could not open %.900s for reading.", file);
	154	packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
	155	return 0;
8efc0c15	156	}
5260325f	157	if (options.strict_modes) {
	158	int fail = 0;
	159	char buf[1024];
	160	/* Check open file in order to avoid open/stat races */
	161	if (fstat(fileno(f), &st) < 0 \|\|
	162	(st.st_uid != 0 && st.st_uid != pw->pw_uid) \|\|
	163	(st.st_mode & 022) != 0) {
	164	snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
	165	"bad ownership or modes for '%s'.", pw->pw_name, file);
	166	fail = 1;
	167	} else {
	168	/* Check path to SSH_USER_PERMITTED_KEYS */
	169	int i;
	170	static const char *check[] = {
	171	"", SSH_USER_DIR, NULL
	172	};
	173	for (i = 0; check[i]; i++) {
	174	snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
	175	if (stat(line, &st) < 0 \|\|
	176	(st.st_uid != 0 && st.st_uid != pw->pw_uid) \|\|
	177	(st.st_mode & 022) != 0) {
	178	snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
	179	"bad ownership or modes for '%s'.", pw->pw_name, line);
	180	fail = 1;
	181	break;
	182	}
	183	}
8efc0c15	184	}
5260325f	185	if (fail) {
a306f2dd	186	fclose(f);
089fbbd2	187	log("%s",buf);
089fbbd2	188	packet_send_debug("%s",buf);
5260325f	189	restore_uid();
5260325f	190	return 0;
8efc0c15	191	}
5260325f	192	}
	193	/* Flag indicating whether authentication has succeeded. */
	194	authenticated = 0;
	195
4fe2af09	196	pk = RSA_new();
	197	pk->e = BN_new();
	198	pk->n = BN_new();
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	*/
5260325f	205	while (fgets(line, sizeof(line), f)) {
	206	char *cp;
	207	char *options;
	208
	209	linenum++;
	210
aa3378df	211	/* Skip leading whitespace, empty and comment lines. */
	212	for (cp = line; cp == ' ' \|\| cp == '\t'; cp++)
	213	;
5260325f	214	if (!cp \|\| cp == '\n' \|\| *cp == '#')
	215	continue;
	216
aa3378df	217	/*
	218	* Check if there are options for this key, and if so,
	219	* save their starting address and skip the option part
	220	* for now. If there are no options, set the starting
	221	* address to NULL.
	222	*/
5260325f	223	if (cp < '0' \|\| cp > '9') {
	224	int quoted = 0;
	225	options = cp;
	226	for (; cp && (quoted \|\| (cp != ' ' && *cp != '\t')); cp++) {
	227	if (*cp == '\\' && cp[1] == '"')
	228	cp++; /* Skip both */
	229	else if (*cp == '"')
	230	quoted = !quoted;
8efc0c15	231	}
5260325f	232	} else
5260325f	233	options = NULL;
33de75a3	234	/*
	235	* If our options do not allow this key to be used,
	236	* do not send challenge.
	237	*/
	238	if (!auth_parse_options(pw, options, linenum))
	239	continue;
5260325f	240
5260325f	241	/* Parse the key from the line. */
4fe2af09	242	if (!auth_rsa_read_key(&cp, &bits, pk->e, pk->n)) {
5260325f	243	debug("%.100s, line %lu: bad key syntax",
	244	SSH_USER_PERMITTED_KEYS, linenum);
	245	packet_send_debug("%.100s, line %lu: bad key syntax",
6ae2364d	246	SSH_USER_PERMITTED_KEYS, linenum);
5260325f	247	continue;
8efc0c15	248	}
5260325f	249	/* cp now points to the comment part. */
5260325f	250
5260325f	251	/* Check if the we have found the desired key (identified by its modulus). */
4fe2af09	252	if (BN_cmp(pk->n, client_n) != 0)
aa3378df	253	continue;
5260325f	254
57112b5a	255	/* check the real bits */
4fe2af09	256	if (bits != BN_num_bits(pk->n))
57112b5a	257	log("Warning: %s, line %ld: keysize mismatch: "
57112b5a	258	"actual %d vs. announced %d.",
4fe2af09	259	file, linenum, BN_num_bits(pk->n), bits);
57112b5a	260
5260325f	261	/* We have found the desired key. */
	262
	263	/* Perform the challenge-response dialog for this key. */
4fe2af09	264	if (!auth_rsa_challenge_dialog(pk)) {
5260325f	265	/* Wrong response. */
	266	verbose("Wrong response to RSA authentication challenge.");
	267	packet_send_debug("Wrong response to RSA authentication challenge.");
	268	continue;
8efc0c15	269	}
aa3378df	270	/*
	271	* Correct response. The client has been successfully
	272	* authenticated. Note that we have not yet processed the
	273	* options; this will be reset if the options cause the
	274	* authentication to be rejected.
aa3378df	275	* Break out of the loop if authentication was successful;
	276	* otherwise continue searching.
	277	*/
33de75a3	278	authenticated = 1;
33de75a3	279	break;
8efc0c15	280	}
8efc0c15	281
5260325f	282	/* Restore the privileged uid. */
5260325f	283	restore_uid();
8efc0c15	284
5260325f	285	/* Close the file. */
5260325f	286	fclose(f);
8efc0c15	287
4fe2af09	288	RSA_free(pk);
8efc0c15	289
5260325f	290	if (authenticated)
5260325f	291	packet_send_debug("RSA authentication accepted.");
94ec8c6b	292	else
94ec8c6b	293	auth_clear_options();
8efc0c15	294
5260325f	295	/* Return authentication result. */
5260325f	296	return authenticated;
8efc0c15	297	}