478 lines
13 KiB
C
478 lines
13 KiB
C
/*
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*
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* auth-rsa.c
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*
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* Author: Tatu Ylonen <ylo@cs.hut.fi>
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*
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* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
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* All rights reserved
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*
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* Created: Mon Mar 27 01:46:52 1995 ylo
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*
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* RSA-based authentication. This code determines whether to admit a login
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* based on RSA authentication. This file also contains functions to check
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* validity of the host key.
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*
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*/
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#include "includes.h"
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RCSID("$Id: auth-rsa.c,v 1.18 2000/02/11 10:59:11 markus Exp $");
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#include "rsa.h"
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#include "packet.h"
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#include "xmalloc.h"
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#include "ssh.h"
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#include "mpaux.h"
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#include "uidswap.h"
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#include "servconf.h"
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#include <ssl/rsa.h>
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#include <ssl/md5.h>
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/* Flags that may be set in authorized_keys options. */
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extern int no_port_forwarding_flag;
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extern int no_agent_forwarding_flag;
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extern int no_x11_forwarding_flag;
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extern int no_pty_flag;
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extern char *forced_command;
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extern struct envstring *custom_environment;
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/*
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* Session identifier that is used to bind key exchange and authentication
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* responses to a particular session.
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*/
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extern unsigned char session_id[16];
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/*
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* The .ssh/authorized_keys file contains public keys, one per line, in the
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* following format:
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* options bits e n comment
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* where bits, e and n are decimal numbers,
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* and comment is any string of characters up to newline. The maximum
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* length of a line is 8000 characters. See the documentation for a
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* description of the options.
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*/
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/*
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* Performs the RSA authentication challenge-response dialog with the client,
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* and returns true (non-zero) if the client gave the correct answer to
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* our challenge; returns zero if the client gives a wrong answer.
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*/
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int
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auth_rsa_challenge_dialog(BIGNUM *e, BIGNUM *n)
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{
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BIGNUM *challenge, *encrypted_challenge;
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RSA *pk;
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BN_CTX *ctx;
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unsigned char buf[32], mdbuf[16], response[16];
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MD5_CTX md;
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unsigned int i;
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int plen, len;
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encrypted_challenge = BN_new();
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challenge = BN_new();
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/* Generate a random challenge. */
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BN_rand(challenge, 256, 0, 0);
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ctx = BN_CTX_new();
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BN_mod(challenge, challenge, n, ctx);
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BN_CTX_free(ctx);
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/* Create the public key data structure. */
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pk = RSA_new();
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pk->e = BN_new();
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BN_copy(pk->e, e);
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pk->n = BN_new();
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BN_copy(pk->n, n);
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/* Encrypt the challenge with the public key. */
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rsa_public_encrypt(encrypted_challenge, challenge, pk);
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RSA_free(pk);
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/* Send the encrypted challenge to the client. */
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packet_start(SSH_SMSG_AUTH_RSA_CHALLENGE);
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packet_put_bignum(encrypted_challenge);
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packet_send();
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BN_clear_free(encrypted_challenge);
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packet_write_wait();
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/* Wait for a response. */
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packet_read_expect(&plen, SSH_CMSG_AUTH_RSA_RESPONSE);
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packet_integrity_check(plen, 16, SSH_CMSG_AUTH_RSA_RESPONSE);
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for (i = 0; i < 16; i++)
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response[i] = packet_get_char();
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/* The response is MD5 of decrypted challenge plus session id. */
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len = BN_num_bytes(challenge);
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if (len <= 0 || len > 32)
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fatal("auth_rsa_challenge_dialog: bad challenge length %d", len);
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memset(buf, 0, 32);
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BN_bn2bin(challenge, buf + 32 - len);
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MD5_Init(&md);
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MD5_Update(&md, buf, 32);
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MD5_Update(&md, session_id, 16);
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MD5_Final(mdbuf, &md);
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BN_clear_free(challenge);
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/* Verify that the response is the original challenge. */
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if (memcmp(response, mdbuf, 16) != 0) {
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/* Wrong answer. */
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return 0;
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}
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/* Correct answer. */
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return 1;
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}
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/*
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* Performs the RSA authentication dialog with the client. This returns
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* 0 if the client could not be authenticated, and 1 if authentication was
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* successful. This may exit if there is a serious protocol violation.
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*/
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int
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auth_rsa(struct passwd *pw, BIGNUM *client_n)
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{
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extern ServerOptions options;
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char line[8192], file[1024];
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int authenticated;
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unsigned int bits;
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FILE *f;
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unsigned long linenum = 0;
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struct stat st;
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BIGNUM *e, *n;
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/* Temporarily use the user's uid. */
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temporarily_use_uid(pw->pw_uid);
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/* The authorized keys. */
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snprintf(file, sizeof file, "%.500s/%.100s", pw->pw_dir,
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SSH_USER_PERMITTED_KEYS);
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/* Fail quietly if file does not exist */
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if (stat(file, &st) < 0) {
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/* Restore the privileged uid. */
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restore_uid();
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return 0;
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}
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/* Open the file containing the authorized keys. */
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f = fopen(file, "r");
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if (!f) {
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/* Restore the privileged uid. */
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restore_uid();
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packet_send_debug("Could not open %.900s for reading.", file);
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packet_send_debug("If your home is on an NFS volume, it may need to be world-readable.");
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return 0;
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}
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if (options.strict_modes) {
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int fail = 0;
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char buf[1024];
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/* Check open file in order to avoid open/stat races */
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if (fstat(fileno(f), &st) < 0 ||
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(st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
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(st.st_mode & 022) != 0) {
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snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
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"bad ownership or modes for '%s'.", pw->pw_name, file);
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fail = 1;
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} else {
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/* Check path to SSH_USER_PERMITTED_KEYS */
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int i;
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static const char *check[] = {
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"", SSH_USER_DIR, NULL
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};
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for (i = 0; check[i]; i++) {
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snprintf(line, sizeof line, "%.500s/%.100s", pw->pw_dir, check[i]);
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if (stat(line, &st) < 0 ||
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(st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
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(st.st_mode & 022) != 0) {
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snprintf(buf, sizeof buf, "RSA authentication refused for %.100s: "
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"bad ownership or modes for '%s'.", pw->pw_name, line);
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fail = 1;
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break;
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}
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}
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}
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if (fail) {
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log(buf);
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packet_send_debug(buf);
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restore_uid();
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return 0;
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}
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}
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/* Flag indicating whether authentication has succeeded. */
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authenticated = 0;
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e = BN_new();
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n = BN_new();
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/*
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* Go though the accepted keys, looking for the current key. If
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* found, perform a challenge-response dialog to verify that the
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* user really has the corresponding private key.
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*/
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while (fgets(line, sizeof(line), f)) {
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char *cp;
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char *options;
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linenum++;
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/* Skip leading whitespace, empty and comment lines. */
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for (cp = line; *cp == ' ' || *cp == '\t'; cp++)
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;
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if (!*cp || *cp == '\n' || *cp == '#')
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continue;
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/*
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* Check if there are options for this key, and if so,
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* save their starting address and skip the option part
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* for now. If there are no options, set the starting
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* address to NULL.
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*/
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if (*cp < '0' || *cp > '9') {
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int quoted = 0;
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options = cp;
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for (; *cp && (quoted || (*cp != ' ' && *cp != '\t')); cp++) {
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if (*cp == '\\' && cp[1] == '"')
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cp++; /* Skip both */
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else if (*cp == '"')
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quoted = !quoted;
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}
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} else
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options = NULL;
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/* Parse the key from the line. */
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if (!auth_rsa_read_key(&cp, &bits, e, n)) {
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debug("%.100s, line %lu: bad key syntax",
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SSH_USER_PERMITTED_KEYS, linenum);
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packet_send_debug("%.100s, line %lu: bad key syntax",
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SSH_USER_PERMITTED_KEYS, linenum);
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continue;
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}
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/* cp now points to the comment part. */
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/* Check if the we have found the desired key (identified by its modulus). */
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if (BN_cmp(n, client_n) != 0)
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continue;
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/* check the real bits */
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if (bits != BN_num_bits(n))
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log("Warning: %s, line %ld: keysize mismatch: "
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"actual %d vs. announced %d.",
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file, linenum, BN_num_bits(n), bits);
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/* We have found the desired key. */
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/* Perform the challenge-response dialog for this key. */
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if (!auth_rsa_challenge_dialog(e, n)) {
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/* Wrong response. */
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verbose("Wrong response to RSA authentication challenge.");
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packet_send_debug("Wrong response to RSA authentication challenge.");
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continue;
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}
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/*
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* Correct response. The client has been successfully
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* authenticated. Note that we have not yet processed the
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* options; this will be reset if the options cause the
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* authentication to be rejected.
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*/
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authenticated = 1;
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/* RSA part of authentication was accepted. Now process the options. */
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if (options) {
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while (*options && *options != ' ' && *options != '\t') {
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cp = "no-port-forwarding";
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if (strncmp(options, cp, strlen(cp)) == 0) {
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packet_send_debug("Port forwarding disabled.");
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no_port_forwarding_flag = 1;
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options += strlen(cp);
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goto next_option;
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}
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cp = "no-agent-forwarding";
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if (strncmp(options, cp, strlen(cp)) == 0) {
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packet_send_debug("Agent forwarding disabled.");
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no_agent_forwarding_flag = 1;
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options += strlen(cp);
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goto next_option;
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}
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cp = "no-X11-forwarding";
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if (strncmp(options, cp, strlen(cp)) == 0) {
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packet_send_debug("X11 forwarding disabled.");
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no_x11_forwarding_flag = 1;
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options += strlen(cp);
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goto next_option;
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}
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cp = "no-pty";
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if (strncmp(options, cp, strlen(cp)) == 0) {
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packet_send_debug("Pty allocation disabled.");
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no_pty_flag = 1;
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options += strlen(cp);
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goto next_option;
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}
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cp = "command=\"";
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if (strncmp(options, cp, strlen(cp)) == 0) {
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int i;
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options += strlen(cp);
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forced_command = xmalloc(strlen(options) + 1);
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i = 0;
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while (*options) {
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if (*options == '"')
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break;
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if (*options == '\\' && options[1] == '"') {
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options += 2;
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forced_command[i++] = '"';
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continue;
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}
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forced_command[i++] = *options++;
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}
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if (!*options) {
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debug("%.100s, line %lu: missing end quote",
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SSH_USER_PERMITTED_KEYS, linenum);
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packet_send_debug("%.100s, line %lu: missing end quote",
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SSH_USER_PERMITTED_KEYS, linenum);
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continue;
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}
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forced_command[i] = 0;
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packet_send_debug("Forced command: %.900s", forced_command);
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options++;
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goto next_option;
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}
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cp = "environment=\"";
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if (strncmp(options, cp, strlen(cp)) == 0) {
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int i;
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char *s;
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struct envstring *new_envstring;
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options += strlen(cp);
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s = xmalloc(strlen(options) + 1);
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i = 0;
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while (*options) {
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if (*options == '"')
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break;
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if (*options == '\\' && options[1] == '"') {
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options += 2;
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s[i++] = '"';
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continue;
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}
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s[i++] = *options++;
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}
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if (!*options) {
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debug("%.100s, line %lu: missing end quote",
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SSH_USER_PERMITTED_KEYS, linenum);
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packet_send_debug("%.100s, line %lu: missing end quote",
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SSH_USER_PERMITTED_KEYS, linenum);
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continue;
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}
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s[i] = 0;
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packet_send_debug("Adding to environment: %.900s", s);
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debug("Adding to environment: %.900s", s);
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options++;
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new_envstring = xmalloc(sizeof(struct envstring));
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new_envstring->s = s;
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new_envstring->next = custom_environment;
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custom_environment = new_envstring;
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goto next_option;
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}
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cp = "from=\"";
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if (strncmp(options, cp, strlen(cp)) == 0) {
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char *patterns = xmalloc(strlen(options) + 1);
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int i;
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options += strlen(cp);
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i = 0;
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while (*options) {
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if (*options == '"')
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break;
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if (*options == '\\' && options[1] == '"') {
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options += 2;
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patterns[i++] = '"';
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continue;
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}
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patterns[i++] = *options++;
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}
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if (!*options) {
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debug("%.100s, line %lu: missing end quote",
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SSH_USER_PERMITTED_KEYS, linenum);
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packet_send_debug("%.100s, line %lu: missing end quote",
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SSH_USER_PERMITTED_KEYS, linenum);
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continue;
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}
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patterns[i] = 0;
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options++;
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if (!match_hostname(get_canonical_hostname(), patterns,
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strlen(patterns)) &&
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!match_hostname(get_remote_ipaddr(), patterns,
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strlen(patterns))) {
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log("RSA authentication tried for %.100s with correct key but not from a permitted host (host=%.200s, ip=%.200s).",
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pw->pw_name, get_canonical_hostname(),
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get_remote_ipaddr());
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packet_send_debug("Your host '%.200s' is not permitted to use this key for login.",
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get_canonical_hostname());
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xfree(patterns);
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/* key invalid for this host, reset flags */
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authenticated = 0;
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no_agent_forwarding_flag = 0;
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no_port_forwarding_flag = 0;
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no_pty_flag = 0;
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no_x11_forwarding_flag = 0;
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while (custom_environment) {
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struct envstring *ce = custom_environment;
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custom_environment = ce->next;
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xfree(ce->s);
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xfree(ce);
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}
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if (forced_command) {
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xfree(forced_command);
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forced_command = NULL;
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}
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break;
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}
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xfree(patterns);
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/* Host name matches. */
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goto next_option;
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}
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bad_option:
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log("Bad options in %.100s file, line %lu: %.50s",
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SSH_USER_PERMITTED_KEYS, linenum, options);
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packet_send_debug("Bad options in %.100s file, line %lu: %.50s",
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SSH_USER_PERMITTED_KEYS, linenum, options);
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authenticated = 0;
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break;
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next_option:
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/*
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* Skip the comma, and move to the next option
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* (or break out if there are no more).
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*/
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if (!*options)
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fatal("Bugs in auth-rsa.c option processing.");
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if (*options == ' ' || *options == '\t')
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break; /* End of options. */
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if (*options != ',')
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goto bad_option;
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options++;
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/* Process the next option. */
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continue;
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}
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}
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/*
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* Break out of the loop if authentication was successful;
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* otherwise continue searching.
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*/
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if (authenticated)
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break;
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}
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/* Restore the privileged uid. */
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restore_uid();
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/* Close the file. */
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fclose(f);
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BN_clear_free(n);
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BN_clear_free(e);
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if (authenticated)
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packet_send_debug("RSA authentication accepted.");
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/* Return authentication result. */
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return authenticated;
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}
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