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openssh-portable/auth-rsa.c

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/*
*
* auth-rsa.c
*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
*
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
* All rights reserved
*
* Created: Mon Mar 27 01:46:52 1995 ylo
*
* 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.
*
*/
#include "includes.h"
RCSID("$Id: auth-rsa.c,v 1.19 2000/04/30 00:00:53 damien 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 <openssl/rsa.h>
#include <openssl/md5.h>
/* Flags that may be set in authorized_keys options. */
extern int no_port_forwarding_flag;
extern int no_agent_forwarding_flag;
extern int no_x11_forwarding_flag;
extern int no_pty_flag;
extern char *forced_command;
extern struct envstring *custom_environment;
/*
* 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)
{
extern ServerOptions options;
char line[8192], file[1024];
int authenticated;
unsigned int bits;
FILE *f;
unsigned long linenum = 0;
struct stat st;
RSA *pk;
/* 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(buf);
packet_send_debug(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;
/* 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.
*/
authenticated = 1;
/* RSA part of authentication was accepted. Now process the options. */
if (options) {
while (*options && *options != ' ' && *options != '\t') {
cp = "no-port-forwarding";
if (strncmp(options, cp, strlen(cp)) == 0) {
packet_send_debug("Port forwarding disabled.");
no_port_forwarding_flag = 1;
options += strlen(cp);
goto next_option;
}
cp = "no-agent-forwarding";
if (strncmp(options, cp, strlen(cp)) == 0) {
packet_send_debug("Agent forwarding disabled.");
no_agent_forwarding_flag = 1;
options += strlen(cp);
goto next_option;
}
cp = "no-X11-forwarding";
if (strncmp(options, cp, strlen(cp)) == 0) {
packet_send_debug("X11 forwarding disabled.");
no_x11_forwarding_flag = 1;
options += strlen(cp);
goto next_option;
}
cp = "no-pty";
if (strncmp(options, cp, strlen(cp)) == 0) {
packet_send_debug("Pty allocation disabled.");
no_pty_flag = 1;
options += strlen(cp);
goto next_option;
}
cp = "command=\"";
if (strncmp(options, cp, strlen(cp)) == 0) {
int i;
options += strlen(cp);
forced_command = xmalloc(strlen(options) + 1);
i = 0;
while (*options) {
if (*options == '"')
break;
if (*options == '\\' && options[1] == '"') {
options += 2;
forced_command[i++] = '"';
continue;
}
forced_command[i++] = *options++;
}
if (!*options) {
debug("%.100s, line %lu: missing end quote",
SSH_USER_PERMITTED_KEYS, linenum);
packet_send_debug("%.100s, line %lu: missing end quote",
SSH_USER_PERMITTED_KEYS, linenum);
continue;
}
forced_command[i] = 0;
packet_send_debug("Forced command: %.900s", forced_command);
options++;
goto next_option;
}
cp = "environment=\"";
if (strncmp(options, cp, strlen(cp)) == 0) {
int i;
char *s;
struct envstring *new_envstring;
options += strlen(cp);
s = xmalloc(strlen(options) + 1);
i = 0;
while (*options) {
if (*options == '"')
break;
if (*options == '\\' && options[1] == '"') {
options += 2;
s[i++] = '"';
continue;
}
s[i++] = *options++;
}
if (!*options) {
debug("%.100s, line %lu: missing end quote",
SSH_USER_PERMITTED_KEYS, linenum);
packet_send_debug("%.100s, line %lu: missing end quote",
SSH_USER_PERMITTED_KEYS, linenum);
continue;
}
s[i] = 0;
packet_send_debug("Adding to environment: %.900s", s);
debug("Adding to environment: %.900s", s);
options++;
new_envstring = xmalloc(sizeof(struct envstring));
new_envstring->s = s;
new_envstring->next = custom_environment;
custom_environment = new_envstring;
goto next_option;
}
cp = "from=\"";
if (strncmp(options, cp, strlen(cp)) == 0) {
char *patterns = xmalloc(strlen(options) + 1);
int i;
options += strlen(cp);
i = 0;
while (*options) {
if (*options == '"')
break;
if (*options == '\\' && options[1] == '"') {
options += 2;
patterns[i++] = '"';
continue;
}
patterns[i++] = *options++;
}
if (!*options) {
debug("%.100s, line %lu: missing end quote",
SSH_USER_PERMITTED_KEYS, linenum);
packet_send_debug("%.100s, line %lu: missing end quote",
SSH_USER_PERMITTED_KEYS, linenum);
continue;
}
patterns[i] = 0;
options++;
if (!match_hostname(get_canonical_hostname(), patterns,
strlen(patterns)) &&
!match_hostname(get_remote_ipaddr(), patterns,
strlen(patterns))) {
log("RSA authentication tried for %.100s with correct key but not from a permitted host (host=%.200s, ip=%.200s).",
pw->pw_name, get_canonical_hostname(),
get_remote_ipaddr());
packet_send_debug("Your host '%.200s' is not permitted to use this key for login.",
get_canonical_hostname());
xfree(patterns);
/* key invalid for this host, reset flags */
authenticated = 0;
no_agent_forwarding_flag = 0;
no_port_forwarding_flag = 0;
no_pty_flag = 0;
no_x11_forwarding_flag = 0;
while (custom_environment) {
struct envstring *ce = custom_environment;
custom_environment = ce->next;
xfree(ce->s);
xfree(ce);
}
if (forced_command) {
xfree(forced_command);
forced_command = NULL;
}
break;
}
xfree(patterns);
/* Host name matches. */
goto next_option;
}
bad_option:
log("Bad options in %.100s file, line %lu: %.50s",
SSH_USER_PERMITTED_KEYS, linenum, options);
packet_send_debug("Bad options in %.100s file, line %lu: %.50s",
SSH_USER_PERMITTED_KEYS, linenum, options);
authenticated = 0;
break;
next_option:
/*
* Skip the comma, and move to the next option
* (or break out if there are no more).
*/
if (!*options)
fatal("Bugs in auth-rsa.c option processing.");
if (*options == ' ' || *options == '\t')
break; /* End of options. */
if (*options != ',')
goto bad_option;
options++;
/* Process the next option. */
continue;
}
}
/*
* Break out of the loop if authentication was successful;
* otherwise continue searching.
*/
if (authenticated)
break;
}
/* Restore the privileged uid. */
restore_uid();
/* Close the file. */
fclose(f);
RSA_free(pk);
if (authenticated)
packet_send_debug("RSA authentication accepted.");
/* Return authentication result. */
return authenticated;
}
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