kernel - Bring in dff23c692 from FreeBSD

[dragonfly.git] / crypto / openssh / sshconnect1.c

/* $OpenBSD: sshconnect1.c,v 1.76 2014/07/15 15:54:14 millert Exp $ */
/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * Code to connect to a remote host, and to perform the client side of the
 * login (authentication) dialog.
 *
 * 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/socket.h>

#include <openssl/bn.h>

#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <pwd.h>

#include "xmalloc.h"
#include "ssh.h"
#include "ssh1.h"
#include "rsa.h"
#include "buffer.h"
#include "packet.h"
#include "key.h"
#include "cipher.h"
#include "kex.h"
#include "uidswap.h"
#include "log.h"
#include "misc.h"
#include "readconf.h"
#include "authfd.h"
#include "sshconnect.h"
#include "authfile.h"
#include "canohost.h"
#include "hostfile.h"
#include "auth.h"
#include "digest.h"

/* Session id for the current session. */
u_char session_id[16];
u_int supported_authentications = 0;

extern Options options;
extern char *__progname;

/*
 * Checks if the user has an authentication agent, and if so, tries to
 * authenticate using the agent.
 */
static int
try_agent_authentication(void)
{
        int type;
        char *comment;
        AuthenticationConnection *auth;
        u_char response[16];
        u_int i;
        Key *key;
        BIGNUM *challenge;

        /* Get connection to the agent. */
        auth = ssh_get_authentication_connection();
        if (!auth)
                return 0;

        if ((challenge = BN_new()) == NULL)
                fatal("try_agent_authentication: BN_new failed");
        /* Loop through identities served by the agent. */
        for (key = ssh_get_first_identity(auth, &comment, 1);
            key != NULL;
            key = ssh_get_next_identity(auth, &comment, 1)) {

                /* Try this identity. */
                debug("Trying RSA authentication via agent with '%.100s'", comment);
                free(comment);

                /* Tell the server that we are willing to authenticate using this key. */
                packet_start(SSH_CMSG_AUTH_RSA);
                packet_put_bignum(key->rsa->n);
                packet_send();
                packet_write_wait();

                /* Wait for server's response. */
                type = packet_read();

                /* The server sends failure if it doesn't like our key or
                   does not support RSA authentication. */
                if (type == SSH_SMSG_FAILURE) {
                        debug("Server refused our key.");
                        key_free(key);
                        continue;
                }
                /* Otherwise it should have sent a challenge. */
                if (type != SSH_SMSG_AUTH_RSA_CHALLENGE)
                        packet_disconnect("Protocol error during RSA authentication: %d",
                                          type);

                packet_get_bignum(challenge);
                packet_check_eom();

                debug("Received RSA challenge from server.");

                /* Ask the agent to decrypt the challenge. */
                if (!ssh_decrypt_challenge(auth, key, challenge, session_id, 1, response)) {
                        /*
                         * The agent failed to authenticate this identifier
                         * although it advertised it supports this.  Just
                         * return a wrong value.
                         */
                        logit("Authentication agent failed to decrypt challenge.");
                        explicit_bzero(response, sizeof(response));
                }
                key_free(key);
                debug("Sending response to RSA challenge.");

                /* Send the decrypted challenge back to the server. */
                packet_start(SSH_CMSG_AUTH_RSA_RESPONSE);
                for (i = 0; i < 16; i++)
                        packet_put_char(response[i]);
                packet_send();
                packet_write_wait();

                /* Wait for response from the server. */
                type = packet_read();

                /* The server returns success if it accepted the authentication. */
                if (type == SSH_SMSG_SUCCESS) {
                        ssh_close_authentication_connection(auth);
                        BN_clear_free(challenge);
                        debug("RSA authentication accepted by server.");
                        return 1;
                }
                /* Otherwise it should return failure. */
                if (type != SSH_SMSG_FAILURE)
                        packet_disconnect("Protocol error waiting RSA auth response: %d",
                                          type);
        }
        ssh_close_authentication_connection(auth);
        BN_clear_free(challenge);
        debug("RSA authentication using agent refused.");
        return 0;
}

/*
 * Computes the proper response to a RSA challenge, and sends the response to
 * the server.
 */
static void
respond_to_rsa_challenge(BIGNUM * challenge, RSA * prv)
{
        u_char buf[32], response[16];
        struct ssh_digest_ctx *md;
        int i, len;

        /* Decrypt the challenge using the private key. */
        /* XXX think about Bleichenbacher, too */
        if (rsa_private_decrypt(challenge, challenge, prv) != 0)
                packet_disconnect(
                    "respond_to_rsa_challenge: rsa_private_decrypt failed");

        /* Compute the response. */
        /* The response is MD5 of decrypted challenge plus session id. */
        len = BN_num_bytes(challenge);
        if (len <= 0 || (u_int)len > sizeof(buf))
                packet_disconnect(
                    "respond_to_rsa_challenge: bad challenge length %d", len);

        memset(buf, 0, sizeof(buf));
        BN_bn2bin(challenge, buf + sizeof(buf) - len);
        if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
            ssh_digest_update(md, buf, 32) < 0 ||
            ssh_digest_update(md, session_id, 16) < 0 ||
            ssh_digest_final(md, response, sizeof(response)) < 0)
                fatal("%s: md5 failed", __func__);
        ssh_digest_free(md);

        debug("Sending response to host key RSA challenge.");

        /* Send the response back to the server. */
        packet_start(SSH_CMSG_AUTH_RSA_RESPONSE);
        for (i = 0; i < 16; i++)
                packet_put_char(response[i]);
        packet_send();
        packet_write_wait();

        explicit_bzero(buf, sizeof(buf));
        explicit_bzero(response, sizeof(response));
        explicit_bzero(&md, sizeof(md));
}

/*
 * Checks if the user has authentication file, and if so, tries to authenticate
 * the user using it.
 */
static int
try_rsa_authentication(int idx)
{
        BIGNUM *challenge;
        Key *public, *private;
        char buf[300], *passphrase, *comment, *authfile;
        int i, perm_ok = 1, type, quit;

        public = options.identity_keys[idx];
        authfile = options.identity_files[idx];
        comment = xstrdup(authfile);

        debug("Trying RSA authentication with key '%.100s'", comment);

        /* Tell the server that we are willing to authenticate using this key. */
        packet_start(SSH_CMSG_AUTH_RSA);
        packet_put_bignum(public->rsa->n);
        packet_send();
        packet_write_wait();

        /* Wait for server's response. */
        type = packet_read();

        /*
         * The server responds with failure if it doesn't like our key or
         * doesn't support RSA authentication.
         */
        if (type == SSH_SMSG_FAILURE) {
                debug("Server refused our key.");
                free(comment);
                return 0;
        }
        /* Otherwise, the server should respond with a challenge. */
        if (type != SSH_SMSG_AUTH_RSA_CHALLENGE)
                packet_disconnect("Protocol error during RSA authentication: %d", type);

        /* Get the challenge from the packet. */
        if ((challenge = BN_new()) == NULL)
                fatal("try_rsa_authentication: BN_new failed");
        packet_get_bignum(challenge);
        packet_check_eom();

        debug("Received RSA challenge from server.");

        /*
         * If the key is not stored in external hardware, we have to
         * load the private key.  Try first with empty passphrase; if it
         * fails, ask for a passphrase.
         */
        if (public->flags & SSHKEY_FLAG_EXT)
                private = public;
        else
                private = key_load_private_type(KEY_RSA1, authfile, "", NULL,
                    &perm_ok);
        if (private == NULL && !options.batch_mode && perm_ok) {
                snprintf(buf, sizeof(buf),
                    "Enter passphrase for RSA key '%.100s': ", comment);
                for (i = 0; i < options.number_of_password_prompts; i++) {
                        passphrase = read_passphrase(buf, 0);
                        if (strcmp(passphrase, "") != 0) {
                                private = key_load_private_type(KEY_RSA1,
                                    authfile, passphrase, NULL, NULL);
                                quit = 0;
                        } else {
                                debug2("no passphrase given, try next key");
                                quit = 1;
                        }
                        explicit_bzero(passphrase, strlen(passphrase));
                        free(passphrase);
                        if (private != NULL || quit)
                                break;
                        debug2("bad passphrase given, try again...");
                }
        }
        /* We no longer need the comment. */
        free(comment);

        if (private == NULL) {
                if (!options.batch_mode && perm_ok)
                        error("Bad passphrase.");

                /* Send a dummy response packet to avoid protocol error. */
                packet_start(SSH_CMSG_AUTH_RSA_RESPONSE);
                for (i = 0; i < 16; i++)
                        packet_put_char(0);
                packet_send();
                packet_write_wait();

                /* Expect the server to reject it... */
                packet_read_expect(SSH_SMSG_FAILURE);
                BN_clear_free(challenge);
                return 0;
        }

        /* Compute and send a response to the challenge. */
        respond_to_rsa_challenge(challenge, private->rsa);

        /* Destroy the private key unless it in external hardware. */
        if (!(private->flags & SSHKEY_FLAG_EXT))
                key_free(private);

        /* We no longer need the challenge. */
        BN_clear_free(challenge);

        /* Wait for response from the server. */
        type = packet_read();
        if (type == SSH_SMSG_SUCCESS) {
                debug("RSA authentication accepted by server.");
                return 1;
        }
        if (type != SSH_SMSG_FAILURE)
                packet_disconnect("Protocol error waiting RSA auth response: %d", type);
        debug("RSA authentication refused.");
        return 0;
}

/*
 * Tries to authenticate the user using combined rhosts or /etc/hosts.equiv
 * authentication and RSA host authentication.
 */
static int
try_rhosts_rsa_authentication(const char *local_user, Key * host_key)
{
        int type;
        BIGNUM *challenge;

        debug("Trying rhosts or /etc/hosts.equiv with RSA host authentication.");

        /* Tell the server that we are willing to authenticate using this key. */
        packet_start(SSH_CMSG_AUTH_RHOSTS_RSA);
        packet_put_cstring(local_user);
        packet_put_int(BN_num_bits(host_key->rsa->n));
        packet_put_bignum(host_key->rsa->e);
        packet_put_bignum(host_key->rsa->n);
        packet_send();
        packet_write_wait();

        /* Wait for server's response. */
        type = packet_read();

        /* The server responds with failure if it doesn't admit our
           .rhosts authentication or doesn't know our host key. */
        if (type == SSH_SMSG_FAILURE) {
                debug("Server refused our rhosts authentication or host key.");
                return 0;
        }
        /* Otherwise, the server should respond with a challenge. */
        if (type != SSH_SMSG_AUTH_RSA_CHALLENGE)
                packet_disconnect("Protocol error during RSA authentication: %d", type);

        /* Get the challenge from the packet. */
        if ((challenge = BN_new()) == NULL)
                fatal("try_rhosts_rsa_authentication: BN_new failed");
        packet_get_bignum(challenge);
        packet_check_eom();

        debug("Received RSA challenge for host key from server.");

        /* Compute a response to the challenge. */
        respond_to_rsa_challenge(challenge, host_key->rsa);

        /* We no longer need the challenge. */
        BN_clear_free(challenge);

        /* Wait for response from the server. */
        type = packet_read();
        if (type == SSH_SMSG_SUCCESS) {
                debug("Rhosts or /etc/hosts.equiv with RSA host authentication accepted by server.");
                return 1;
        }
        if (type != SSH_SMSG_FAILURE)
                packet_disconnect("Protocol error waiting RSA auth response: %d", type);
        debug("Rhosts or /etc/hosts.equiv with RSA host authentication refused.");
        return 0;
}

/*
 * Tries to authenticate with any string-based challenge/response system.
 * Note that the client code is not tied to s/key or TIS.
 */
static int
try_challenge_response_authentication(void)
{
        int type, i;
        u_int clen;
        char prompt[1024];
        char *challenge, *response;

        debug("Doing challenge response authentication.");

        for (i = 0; i < options.number_of_password_prompts; i++) {
                /* request a challenge */
                packet_start(SSH_CMSG_AUTH_TIS);
                packet_send();
                packet_write_wait();

                type = packet_read();
                if (type != SSH_SMSG_FAILURE &&
                    type != SSH_SMSG_AUTH_TIS_CHALLENGE) {
                        packet_disconnect("Protocol error: got %d in response "
                            "to SSH_CMSG_AUTH_TIS", type);
                }
                if (type != SSH_SMSG_AUTH_TIS_CHALLENGE) {
                        debug("No challenge.");
                        return 0;
                }
                challenge = packet_get_string(&clen);
                packet_check_eom();
                snprintf(prompt, sizeof prompt, "%s%s", challenge,
                    strchr(challenge, '\n') ? "" : "\nResponse: ");
                free(challenge);
                if (i != 0)
                        error("Permission denied, please try again.");
                if (options.cipher == SSH_CIPHER_NONE)
                        logit("WARNING: Encryption is disabled! "
                            "Response will be transmitted in clear text.");
                response = read_passphrase(prompt, 0);
                if (strcmp(response, "") == 0) {
                        free(response);
                        break;
                }
                packet_start(SSH_CMSG_AUTH_TIS_RESPONSE);
                ssh_put_password(response);
                explicit_bzero(response, strlen(response));
                free(response);
                packet_send();
                packet_write_wait();
                type = packet_read();
                if (type == SSH_SMSG_SUCCESS)
                        return 1;
                if (type != SSH_SMSG_FAILURE)
                        packet_disconnect("Protocol error: got %d in response "
                            "to SSH_CMSG_AUTH_TIS_RESPONSE", type);
        }
        /* failure */
        return 0;
}

/*
 * Tries to authenticate with plain passwd authentication.
 */
static int
try_password_authentication(char *prompt)
{
        int type, i;
        char *password;

        debug("Doing password authentication.");
        if (options.cipher == SSH_CIPHER_NONE)
                logit("WARNING: Encryption is disabled! Password will be transmitted in clear text.");
        for (i = 0; i < options.number_of_password_prompts; i++) {
                if (i != 0)
                        error("Permission denied, please try again.");
                password = read_passphrase(prompt, 0);
                packet_start(SSH_CMSG_AUTH_PASSWORD);
                ssh_put_password(password);
                explicit_bzero(password, strlen(password));
                free(password);
                packet_send();
                packet_write_wait();

                type = packet_read();
                if (type == SSH_SMSG_SUCCESS)
                        return 1;
                if (type != SSH_SMSG_FAILURE)
                        packet_disconnect("Protocol error: got %d in response to passwd auth", type);
        }
        /* failure */
        return 0;
}

/*
 * SSH1 key exchange
 */
void
ssh_kex(char *host, struct sockaddr *hostaddr)
{
        int i;
        BIGNUM *key;
        Key *host_key, *server_key;
        int bits, rbits;
        int ssh_cipher_default = SSH_CIPHER_3DES;
        u_char session_key[SSH_SESSION_KEY_LENGTH];
        u_char cookie[8];
        u_int supported_ciphers;
        u_int server_flags, client_flags;
        u_int32_t rnd = 0;

        debug("Waiting for server public key.");

        /* Wait for a public key packet from the server. */
        packet_read_expect(SSH_SMSG_PUBLIC_KEY);

        /* Get cookie from the packet. */
        for (i = 0; i < 8; i++)
                cookie[i] = packet_get_char();

        /* Get the public key. */
        server_key = key_new(KEY_RSA1);
        bits = packet_get_int();
        packet_get_bignum(server_key->rsa->e);
        packet_get_bignum(server_key->rsa->n);

        rbits = BN_num_bits(server_key->rsa->n);
        if (bits != rbits) {
                logit("Warning: Server lies about size of server public key: "
                    "actual size is %d bits vs. announced %d.", rbits, bits);
                logit("Warning: This may be due to an old implementation of ssh.");
        }
        /* Get the host key. */
        host_key = key_new(KEY_RSA1);
        bits = packet_get_int();
        packet_get_bignum(host_key->rsa->e);
        packet_get_bignum(host_key->rsa->n);

        rbits = BN_num_bits(host_key->rsa->n);
        if (bits != rbits) {
                logit("Warning: Server lies about size of server host key: "
                    "actual size is %d bits vs. announced %d.", rbits, bits);
                logit("Warning: This may be due to an old implementation of ssh.");
        }

        /* Get protocol flags. */
        server_flags = packet_get_int();
        packet_set_protocol_flags(server_flags);

        supported_ciphers = packet_get_int();
        supported_authentications = packet_get_int();
        packet_check_eom();

        debug("Received server public key (%d bits) and host key (%d bits).",
            BN_num_bits(server_key->rsa->n), BN_num_bits(host_key->rsa->n));

        if (verify_host_key(host, hostaddr, host_key) == -1)
                fatal("Host key verification failed.");

        client_flags = SSH_PROTOFLAG_SCREEN_NUMBER | SSH_PROTOFLAG_HOST_IN_FWD_OPEN;

        derive_ssh1_session_id(host_key->rsa->n, server_key->rsa->n, cookie, session_id);

        /*
         * Generate an encryption key for the session.   The key is a 256 bit
         * random number, interpreted as a 32-byte key, with the least
         * significant 8 bits being the first byte of the key.
         */
        for (i = 0; i < 32; i++) {
                if (i % 4 == 0)
                        rnd = arc4random();
                session_key[i] = rnd & 0xff;
                rnd >>= 8;
        }

        /*
         * According to the protocol spec, the first byte of the session key
         * is the highest byte of the integer.  The session key is xored with
         * the first 16 bytes of the session id.
         */
        if ((key = BN_new()) == NULL)
                fatal("ssh_kex: BN_new failed");
        if (BN_set_word(key, 0) == 0)
                fatal("ssh_kex: BN_set_word failed");
        for (i = 0; i < SSH_SESSION_KEY_LENGTH; i++) {
                if (BN_lshift(key, key, 8) == 0)
                        fatal("ssh_kex: BN_lshift failed");
                if (i < 16) {
                        if (BN_add_word(key, session_key[i] ^ session_id[i])
                            == 0)
                                fatal("ssh_kex: BN_add_word failed");
                } else {
                        if (BN_add_word(key, session_key[i]) == 0)
                                fatal("ssh_kex: BN_add_word failed");
                }
        }

        /*
         * Encrypt the integer using the public key and host key of the
         * server (key with smaller modulus first).
         */
        if (BN_cmp(server_key->rsa->n, host_key->rsa->n) < 0) {
                /* Public key has smaller modulus. */
                if (BN_num_bits(host_key->rsa->n) <
                    BN_num_bits(server_key->rsa->n) + SSH_KEY_BITS_RESERVED) {
                        fatal("respond_to_rsa_challenge: host_key %d < server_key %d + "
                            "SSH_KEY_BITS_RESERVED %d",
                            BN_num_bits(host_key->rsa->n),
                            BN_num_bits(server_key->rsa->n),
                            SSH_KEY_BITS_RESERVED);
                }
                if (rsa_public_encrypt(key, key, server_key->rsa) != 0 ||
                    rsa_public_encrypt(key, key, host_key->rsa) != 0)
                        fatal("%s: rsa_public_encrypt failed", __func__);
        } else {
                /* Host key has smaller modulus (or they are equal). */
                if (BN_num_bits(server_key->rsa->n) <
                    BN_num_bits(host_key->rsa->n) + SSH_KEY_BITS_RESERVED) {
                        fatal("respond_to_rsa_challenge: server_key %d < host_key %d + "
                            "SSH_KEY_BITS_RESERVED %d",
                            BN_num_bits(server_key->rsa->n),
                            BN_num_bits(host_key->rsa->n),
                            SSH_KEY_BITS_RESERVED);
                }
                if (rsa_public_encrypt(key, key, host_key->rsa) != 0 ||
                    rsa_public_encrypt(key, key, server_key->rsa) != 0)
                        fatal("%s: rsa_public_encrypt failed", __func__);
        }

        /* Destroy the public keys since we no longer need them. */
        key_free(server_key);
        key_free(host_key);

        if (options.cipher == SSH_CIPHER_NOT_SET) {
                if (cipher_mask_ssh1(1) & supported_ciphers & (1 << ssh_cipher_default))
                        options.cipher = ssh_cipher_default;
        } else if (options.cipher == SSH_CIPHER_INVALID ||
            !(cipher_mask_ssh1(1) & (1 << options.cipher))) {
                logit("No valid SSH1 cipher, using %.100s instead.",
                    cipher_name(ssh_cipher_default));
                options.cipher = ssh_cipher_default;
        }
        /* Check that the selected cipher is supported. */
        if (!(supported_ciphers & (1 << options.cipher)))
                fatal("Selected cipher type %.100s not supported by server.",
                    cipher_name(options.cipher));

        debug("Encryption type: %.100s", cipher_name(options.cipher));

        /* Send the encrypted session key to the server. */
        packet_start(SSH_CMSG_SESSION_KEY);
        packet_put_char(options.cipher);

        /* Send the cookie back to the server. */
        for (i = 0; i < 8; i++)
                packet_put_char(cookie[i]);

        /* Send and destroy the encrypted encryption key integer. */
        packet_put_bignum(key);
        BN_clear_free(key);

        /* Send protocol flags. */
        packet_put_int(client_flags);

        /* Send the packet now. */
        packet_send();
        packet_write_wait();

        debug("Sent encrypted session key.");

        /* Set the encryption key. */
        packet_set_encryption_key(session_key, SSH_SESSION_KEY_LENGTH, options.cipher);

        /*
         * We will no longer need the session key here.
         * Destroy any extra copies.
         */
        explicit_bzero(session_key, sizeof(session_key));

        /*
         * Expect a success message from the server.  Note that this message
         * will be received in encrypted form.
         */
        packet_read_expect(SSH_SMSG_SUCCESS);

        debug("Received encrypted confirmation.");
}

/*
 * Authenticate user
 */
void
ssh_userauth1(const char *local_user, const char *server_user, char *host,
    Sensitive *sensitive)
{
        int i, type;

        if (supported_authentications == 0)
                fatal("ssh_userauth1: server supports no auth methods");

        /* Send the name of the user to log in as on the server. */
        packet_start(SSH_CMSG_USER);
        packet_put_cstring(server_user);
        packet_send();
        packet_write_wait();

        /*
         * The server should respond with success if no authentication is
         * needed (the user has no password).  Otherwise the server responds
         * with failure.
         */
        type = packet_read();

        /* check whether the connection was accepted without authentication. */
        if (type == SSH_SMSG_SUCCESS)
                goto success;
        if (type != SSH_SMSG_FAILURE)
                packet_disconnect("Protocol error: got %d in response to SSH_CMSG_USER", type);

        /*
         * Try .rhosts or /etc/hosts.equiv authentication with RSA host
         * authentication.
         */
        if ((supported_authentications & (1 << SSH_AUTH_RHOSTS_RSA)) &&
            options.rhosts_rsa_authentication) {
                for (i = 0; i < sensitive->nkeys; i++) {
                        if (sensitive->keys[i] != NULL &&
                            sensitive->keys[i]->type == KEY_RSA1 &&
                            try_rhosts_rsa_authentication(local_user,
                            sensitive->keys[i]))
                                goto success;
                }
        }
        /* Try RSA authentication if the server supports it. */
        if ((supported_authentications & (1 << SSH_AUTH_RSA)) &&
            options.rsa_authentication) {
                /*
                 * Try RSA authentication using the authentication agent. The
                 * agent is tried first because no passphrase is needed for
                 * it, whereas identity files may require passphrases.
                 */
                if (try_agent_authentication())
                        goto success;

                /* Try RSA authentication for each identity. */
                for (i = 0; i < options.num_identity_files; i++)
                        if (options.identity_keys[i] != NULL &&
                            options.identity_keys[i]->type == KEY_RSA1 &&
                            try_rsa_authentication(i))
                                goto success;
        }
        /* Try challenge response authentication if the server supports it. */
        if ((supported_authentications & (1 << SSH_AUTH_TIS)) &&
            options.challenge_response_authentication && !options.batch_mode) {
                if (try_challenge_response_authentication())
                        goto success;
        }
        /* Try password authentication if the server supports it. */
        if ((supported_authentications & (1 << SSH_AUTH_PASSWORD)) &&
            options.password_authentication && !options.batch_mode) {
                char prompt[80];

                snprintf(prompt, sizeof(prompt), "%.30s@%.128s's password: ",
                    server_user, host);
                if (try_password_authentication(prompt))
                        goto success;
        }
        /* All authentication methods have failed.  Exit with an error message. */
        fatal("Permission denied.");
        /* NOTREACHED */

 success:
        return; /* need statement after label */
}