| /* $OpenBSD: auth-rsa.c,v 1.66 2006/03/25 13:17:01 djm 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 "includes.h" |
| |
| #include <sys/types.h> |
| #include <sys/stat.h> |
| |
| #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); |
| } |