auth-rsa.c

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/*
 * 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 1.63 2005/06/17 02:44:32 djm Exp $");

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

#include "rsa.h"
#include "packet.h"
#include "xmalloc.h"
#include "ssh1.h"
#include "uidswap.h"
#include "match.h"
#include "auth-options.h"
#include "pathnames.h"
#include "log.h"
#include "servconf.h"
#include "auth.h"
#include "hostfile.h"
#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. */
        BN_rand(challenge, 256, 0, 0);
        if ((ctx = BN_CTX_new()) == NULL)
                fatal("auth_rsa_generate_challenge: BN_CTX_new() failed");
        BN_mod(challenge, challenge, key->rsa->n, ctx);
        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] = 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;
        struct stat st;
        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);

        /* Fail quietly if file does not exist */
        if (stat(file, &st) < 0) {
                /* Restore the privileged uid. */
                restore_uid();
                xfree(file);
                return (0);
        }
        /* Open the file containing the authorized keys. */
        f = fopen(file, "r");
        if (!f) {
                /* Restore the privileged uid. */
                restore_uid();
                xfree(file);
                return (0);
        }
        if (options.strict_modes &&
            secure_filename(f, file, pw, line, sizeof(line)) != 0) {
                xfree(file);
                fclose(f);
                logit("Authentication refused: %s", line);
                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);
}

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