/* $OpenBSD: auth-rsa.c,v 1.72 2006/11/06 21:25:27 markus 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 <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 "xmalloc.h"

#include "rsa.h"

#include "packet.h"

#include "ssh1.h"

#include "uidswap.h"

#include "match.h"

#include "buffer.h"

#include "auth-options.h"

#include "pathnames.h"

#include "log.h"

#include "servconf.h"

#include "key.h"

#include "hostfile.h"

#include "auth.h"

#ifdef GSSAPI

#include "ssh-gss.h"

#endif

#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. */

if (BN_rand(challenge, 256, 0, 0) == 0)

fatal("auth_rsa_generate_challenge: BN_rand failed");

if ((ctx = BN_CTX_new()) == NULL)

fatal("auth_rsa_generate_challenge: BN_CTX_new failed");

if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)

fatal("auth_rsa_generate_challenge: BN_mod failed");

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;

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);

f = auth_openkeyfile(file, pw, options.strict_modes);

if (!f) {

xfree(file);

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);

}