/* * Author: Tatu Ylonen * Copyright (c) 1995 Tatu Ylonen , 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 1.56 2002/06/10 16:53:06 stevesk Exp $"); #pragma ident "%Z%%M% %I% %E% SMI" #include #include #include "rsa.h" #include "packet.h" #include "xmalloc.h" #include "ssh1.h" #include "mpaux.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 "ssh.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 8000 characters. See the documentation 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 = 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 = 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[8192], *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); log("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 (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 (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 */ if (bits != BN_num_bits(key->rsa->n)) log("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, 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(struct passwd *pw, BIGNUM *client_n) { Key *key; char *fp; /* no user given */ if (pw == NULL) return 0; if (!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); }