329 lines
8.3 KiB
C
329 lines
8.3 KiB
C
/* $NetBSD: auth-rsa.c,v 1.1.1.10 2002/04/22 07:35:43 itojun Exp $ */
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/*
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* Author: Tatu Ylonen <ylo@cs.hut.fi>
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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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* 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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* As far as I am concerned, the code I have written for this software
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* can be used freely for any purpose. Any derived versions of this
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* software must be clearly marked as such, and if the derived work is
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* incompatible with the protocol description in the RFC file, it must be
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* called by a name other than "ssh" or "Secure Shell".
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*/
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#include "includes.h"
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RCSID("$OpenBSD: auth-rsa.c,v 1.55 2002/03/29 19:18:33 stevesk Exp $");
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#include <openssl/rsa.h>
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#include <openssl/md5.h>
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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 "ssh1.h"
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#include "mpaux.h"
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#include "uidswap.h"
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#include "match.h"
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#include "auth-options.h"
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#include "pathnames.h"
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#include "log.h"
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#include "servconf.h"
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#include "auth.h"
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#include "hostfile.h"
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#include "monitor_wrap.h"
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#include "ssh.h"
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/* import */
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extern ServerOptions options;
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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 u_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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BIGNUM *
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auth_rsa_generate_challenge(Key *key)
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{
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BIGNUM *challenge;
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BN_CTX *ctx;
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if ((challenge = BN_new()) == NULL)
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fatal("auth_rsa_generate_challenge: BN_new() failed");
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/* Generate a random challenge. */
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BN_rand(challenge, 256, 0, 0);
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if ((ctx = BN_CTX_new()) == NULL)
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fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
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BN_mod(challenge, challenge, key->rsa->n, ctx);
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BN_CTX_free(ctx);
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return challenge;
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}
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int
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auth_rsa_verify_response(Key *key, BIGNUM *challenge, u_char response[16])
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{
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u_char buf[32], mdbuf[16];
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MD5_CTX md;
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int len;
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/* don't allow short keys */
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if (BN_num_bits(key->rsa->n) < SSH_RSA_MINIMUM_MODULUS_SIZE) {
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error("auth_rsa_verify_response: n too small: %d bits",
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BN_num_bits(key->rsa->n));
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return (0);
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}
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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_verify_response: 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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/* 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 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(Key *key)
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{
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BIGNUM *challenge, *encrypted_challenge;
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u_char response[16];
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int i, success;
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if ((encrypted_challenge = BN_new()) == NULL)
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fatal("auth_rsa_challenge_dialog: BN_new() failed");
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challenge = PRIVSEP(auth_rsa_generate_challenge(key));
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/* Encrypt the challenge with the public key. */
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rsa_public_encrypt(encrypted_challenge, challenge, key->rsa);
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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(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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packet_check_eom();
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success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
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BN_clear_free(challenge);
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return (success);
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}
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/*
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* check if there's user key matching client_n,
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* return key if login is allowed, NULL otherwise
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*/
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int
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auth_rsa_key_allowed(struct passwd *pw, BIGNUM *client_n, Key **rkey)
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{
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char line[8192], *file;
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int allowed = 0;
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u_int bits;
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FILE *f;
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u_long linenum = 0;
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struct stat st;
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Key *key;
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/* Temporarily use the user's uid. */
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temporarily_use_uid(pw);
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/* The authorized keys. */
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file = authorized_keys_file(pw);
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debug("trying public RSA key file %s", file);
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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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xfree(file);
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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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xfree(file);
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return (0);
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}
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if (options.strict_modes &&
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secure_filename(f, file, pw, line, sizeof(line)) != 0) {
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xfree(file);
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fclose(f);
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log("Authentication refused: %s", line);
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restore_uid();
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return (0);
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}
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/* Flag indicating whether the key is allowed. */
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allowed = 0;
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key = key_new(KEY_RSA1);
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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 (hostfile_read_key(&cp, &bits, key) == 0) {
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debug("%.100s, line %lu: non ssh1 key syntax",
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file, 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(key->rsa->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(key->rsa->n))
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log("Warning: %s, line %lu: keysize mismatch: "
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"actual %d vs. announced %d.",
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file, linenum, BN_num_bits(key->rsa->n), bits);
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/* We have found the desired key. */
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/*
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* If our options do not allow this key to be used,
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* do not send challenge.
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*/
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if (!auth_parse_options(pw, options, file, linenum))
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continue;
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/* break out, this key is allowed */
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allowed = 1;
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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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xfree(file);
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fclose(f);
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/* return key if allowed */
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if (allowed && rkey != NULL)
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*rkey = key;
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else
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key_free(key);
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return (allowed);
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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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Key *key;
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char *fp;
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/* no user given */
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if (pw == NULL)
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return 0;
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if (!PRIVSEP(auth_rsa_key_allowed(pw, client_n, &key))) {
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auth_clear_options();
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return (0);
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}
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/* Perform the challenge-response dialog for this key. */
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if (!auth_rsa_challenge_dialog(key)) {
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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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/*
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* Break out of the loop. Otherwise we might send
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* another challenge and break the protocol.
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*/
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key_free(key);
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return (0);
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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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fp = key_fingerprint(key, SSH_FP_MD5, SSH_FP_HEX);
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verbose("Found matching %s key: %s",
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key_type(key), fp);
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xfree(fp);
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key_free(key);
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packet_send_debug("RSA authentication accepted.");
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return (1);
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}
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