--- /dev/null
+/* $OpenBSD: auth-rsa.c,v 1.80 2011/05/23 03:30:07 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 <pwd.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <string.h>
+
+#include "xmalloc.h"
+#include "rsa.h"
+#include "packet.h"
+#include "ssh1.h"
+#include "uidswap.h"
+#include "match.h"
+#include "buffer.h"
+#include "pathnames.h"
+#include "log.h"
+#include "servconf.h"
+#include "key.h"
+#include "auth-options.h"
+#include "hostfile.h"
+#include "auth.h"
+#ifdef GSSAPI
+#include "ssh-gss.h"
+#endif
+#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. */
+ if (BN_rand(challenge, 256, 0, 0) == 0)
+ fatal("auth_rsa_generate_challenge: BN_rand failed");
+ if ((ctx = BN_CTX_new()) == NULL)
+ fatal("auth_rsa_generate_challenge: BN_CTX_new failed");
+ if (BN_mod(challenge, challenge, key->rsa->n, ctx) == 0)
+ fatal("auth_rsa_generate_challenge: BN_mod failed");
+ 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 (timingsafe_bcmp(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] = (u_char)packet_get_char();
+ packet_check_eom();
+
+ success = PRIVSEP(auth_rsa_verify_response(key, challenge, response));
+ BN_clear_free(challenge);
+ return (success);
+}
+
+static int
+rsa_key_allowed_in_file(struct passwd *pw, char *file,
+ const BIGNUM *client_n, Key **rkey)
+{
+ char line[SSH_MAX_PUBKEY_BYTES];
+ int allowed = 0;
+ u_int bits;
+ FILE *f;
+ u_long linenum = 0;
+ Key *key;
+
+ debug("trying public RSA key file %s", file);
+ if ((f = auth_openkeyfile(file, pw, options.strict_modes)) == NULL)
+ return 0;
+
+ /*
+ * 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.
+ */
+ key = key_new(KEY_RSA1);
+ 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);
+
+ /* Never accept a revoked key */
+ if (auth_key_is_revoked(key))
+ break;
+
+ /* 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;
+ if (key_is_cert_authority)
+ continue;
+ /* break out, this key is allowed */
+ allowed = 1;
+ break;
+ }
+
+ /* Close the file. */
+ fclose(f);
+
+ /* return key if allowed */
+ if (allowed && rkey != NULL)
+ *rkey = key;
+ else
+ key_free(key);
+
+ return allowed;
+}
+
+/*
+ * 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 *file;
+ u_int i, allowed = 0;
+
+ temporarily_use_uid(pw);
+
+ for (i = 0; !allowed && i < options.num_authkeys_files; i++) {
+ file = expand_authorized_keys(
+ options.authorized_keys_files[i], pw);
+ allowed = rsa_key_allowed_in_file(pw, file, client_n, rkey);
+ xfree(file);
+ }
+
+ restore_uid();
+
+ 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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