root/fwknop/tags/fwknop-1.8.4-pre1/fwknop

#!/usr/bin/perl -w
#
#############################################################################
#
# File: fwknop
#
# Purpose: fwknop implements an authorization scheme known as Single Packet
#          Authorization (SPA) that requires only a single encrypted packet
#          to communicate various pieces of information including desired
#          access through an iptables policy and/or specific commands to
#          execute on the target system.  The main application of this
#          program is to protect services such as SSH with an additional
#          layer of security in order to make the exploitation of
#          vulnerabilities (both 0-day and unpatched code) much more
#          difficult.  For more information, see the fwknop(8) man page.
#
# Author: Michael Rash (mbr@cipherdyne.org)
#
# Version: 1.8.4-pre1
#
# Copyright (C) 2004-2007 Michael Rash (mbr@cipherdyne.org)
#
# License (GNU Public License):
#
#    This program is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License
#    along with this program; if not, write to the Free Software
#    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307
#    USA
#
#############################################################################
#
# $Id: fwknop 586 2006-11-04 20:45:49Z mbr $
#

use lib '/usr/lib/fwknop';
use Crypt::CBC;
use Net::IPv4Addr qw(ipv4_in_network);
use Net::Ping::External qw(ping);
use Digest::MD5 'md5_base64';
use IO::Socket;
use IO::Handle;
use MIME::Base64;
use Data::Dumper;
use POSIX;
use Term::ReadKey;
use Getopt::Long;
use strict;

my $version = '1.8.4-pre1';
my $revision_svn = '$Revision$';
my $rev_num = '1';
($rev_num) = $revision_svn =~ m|\$Rev.*:\s+(\S+)|;

my $print_version = 0;
my $print_help    = 0;
my $run_last_args = 0;
my $debug         = 0;
my $quiet         = 0;
my $verbose       = 0;
my $test_mode     = 0;
my $cmdl_homedir  = '';
my $knock_sleep   = 1;  ### default to 1 second difference between port knocks
my $knock_dst     = '';
my $homedir       = '';
my $spoof_src     = '';
my $server_mode   = 'pcap';
my $user_rc_file  = '';
my $server_proto  = '';
my $run_last_host = '';
my $show_last_host_cmd = '';
my $show_last_cmd = 0;
my $gpg_home_dir  = '';
my $gpg_recipient = '';
my $use_gpg_agent = 0;
my $gpg_agent_info = '';
my $max_msg_len   = 1500;
my $gpg_verbose   = 0;
my $gpg_default_key = 0;
my $err_wait_timer  = 30;  ### seconds
my $resolve_ip_url  = 'http://www.whatismyip.org/';
my $gpg_signing_key = '';
my $save_packet_mode = 0;
my $save_packet_file = '';
my $cmdline_pcap_cmd   = '';
my $no_save_last_args  = 0;
my $server_auth_method = '';
my $spa_established_tcp  = 0;
my $resolve_external_ip  = 0;
my $server_auth_crypt_pw = '';
my $pcap_sleep_interval  = 1;  ### seconds
my $knock_dst_pre_resolve = '';

### User agent for contacting http://www.whatismyip.com/, we don't
### advertise the fwknop client (can override with --User-agent)
my $ext_resolve_user_agent = "Firefox/1.0.5.4";

### mode numbers
my $command_mode = 0;
my $access_mode  = 1;

### default time values
my $knock_interval    = 60;
my $fw_access_timeout = 300;

### default to root (client must run as root in this mode)
my $spoof_username = '';
my $spoof_proto = 'udp';  ### default to udp

### encrypted port knock vars
my $cmdline_offset    = 0;
my $enc_port_offset   = 61000;  ### default offset
my $enc_key           = '';
my $enc_alg           = 'Rijndael';
my $enc_blocksize     = 32;

### there is a constant "RIJNDAEL_KEYSIZE" in the Crypt::Rijndael sources, but
### it is not used; a 16 byte key size is fine.
my $enc_keysize       = 16;

my $enc_shared_secret = '';
my $enc_allow_ip      = '';
my $enc_source_ip     = '';
my $enc_rotate_proto  = 0;
my $get_key_file      = '';  ### get key from file
my $enc_pcap_port     = 62201;  ### default pcap port
my $access_str        = '';

### packet counters
my $tcp_ctr  = 0;
my $udp_ctr  = 0;
my $icmp_ctr = 0;

### tcp option types
my $tcp_nop_type       = 1;
my $tcp_mss_type       = 2;
my $tcp_win_scale_type = 3;
my $tcp_sack_type      = 4;
my $tcp_timestamp_type = 8;

my %tcp_p0f_opt_types = (
    'N' => $tcp_nop_type,
    'M' => $tcp_mss_type,
    'W' => $tcp_win_scale_type,
    'S' => $tcp_sack_type,
    'T' => $tcp_timestamp_type
);

my $ip_re = qr|(?:[0-2]?\d{1,2}\.){3}[0-2]?\d{1,2}|;

my @args_cp = @ARGV;

### run GetOpt() to get comand line args
&handle_command_line();

&usage(0) if $print_help;

if ($print_version) {
    print "[+] fwknop v$version (file revision: $rev_num)\n",
        "      by Michael Rash <mbr\@cipherdyne.org>\n";
    exit 0;
}

die "[*] Must also specify a GnuPG signing key with --gpg-signing-key or\n",
    "    use --gpg-default-key to use a default key (specified in\n",
    "    ~/.gnupg/options with the default-key variable).\n"
    if ($gpg_recipient and (not $gpg_default_key and not $gpg_signing_key));

die "[*] Must specify a GnuPG recipient key (on the fwknopd side) with\n",
    "    --gpg-recipient"
    if (($gpg_default_key or $gpg_signing_key) and not $gpg_recipient);

die "[*] Cannot spoof source address for a real TCP socket."
    if ($spoof_src and $spa_established_tcp);

&get_homedir();

### save a copy
$knock_dst_pre_resolve = $knock_dst;

if ($run_last_args or $show_last_cmd) {

    ### run fwknop with same command line args as the previous
    ### execution
    &run_last_cmdline();

} elsif ($run_last_host or $show_last_host_cmd) {

    $run_last_host = $show_last_host_cmd if $show_last_host_cmd;

    ### run fwknop with the last args for this particular knock destination
    &run_last_host_cmdline();
}

die "[*] Must specify a destination server with -D <IP|Host>"
    unless $knock_dst;

my $print_mode = '';
if (lc($server_mode) eq 'pcap') {
    $print_mode = 'SPA';
} elsif (lc($server_mode) eq 'knock') {
    $print_mode = 'encrypted port knocking';
} elsif (lc($server_mode) eq 'shared') {
    $print_mode = 'shared sequence port knocking';
} else {
    die "[*] Unknown server mode: $server_mode ",
        qq|(must be "pcap", "knock", or "shared").|;
}

if ($debug) {
    print "[+] ***DEBUG*** Starting fwknop client ($print_mode mode)...\n";
} else {
    print "[+] Starting fwknop client ($print_mode mode)...\n"
        unless $quiet;
}

unless ($knock_dst =~ /$ip_re/) {
    print "[+] Resolving hostname: $knock_dst\n" unless $quiet;
    ### resolve to an IP
    my $iaddr = inet_aton($knock_dst)
        or die "[*] Could not resolve $knock_dst to an IP.";
    my $addr = inet_ntoa($iaddr)
        or die "[*] Could not resolve $knock_dst to an IP.";
    $knock_dst = $addr;
}

&validate_access_str() if $access_str;

if (lc($server_mode) eq 'pcap' or lc($server_mode) eq 'knock') {
    die "[*] Must also specify: -D <knock destination>\n"
        unless $knock_dst;

    unless ($enc_allow_ip
            or $enc_source_ip
            or $resolve_external_ip) {
        die "[*] Must either specify: --allow-IP <IP>, ",
            "--source-IP, or --Resolve-external-IP\n";
    }

    ### make fwknop server see "0.0.0.0" in the encrypted sequence.
    ### This will instruct the server to open the port for whatever
    ### source IP the sequence comes from.  This is useful for
    ### clients that are behind a NAT device.
    $enc_allow_ip = '0.0.0.0' if $enc_source_ip;

    ### resolve the extenal IP via http://www.whatismyip.com
    $enc_allow_ip = &resolve_external_ip() if $resolve_external_ip;

    if ($spoof_src) {
        $< == 0 && $> == 0 or
            die '[*] You must be root (or equivalent ',
                "UID 0 account) to spoof the source address.\n";
    }
    unless ($enc_allow_ip =~ /$ip_re/) {
        ### resolve to an IP
        my $iaddr = inet_aton($enc_allow_ip)
            or die "[*] Could not resolve $enc_allow_ip to IP.";
        my $addr = inet_ntoa($iaddr)
            or die "[*] Could not resolve $enc_allow_ip to IP.";
        $enc_allow_ip = $addr;
    }
    if ($cmdline_offset) {
        if (lc($server_mode) eq 'pcap') {
            die "[*] Port offset is meaningless in pcap mode ",
                "(only a single packet is sent).";
        }
        unless ($cmdline_offset < 65280 and $cmdline_offset > 0) {
            die "[*] Port offset must be 0 < port < 65280";
        }
        $enc_port_offset = $cmdline_offset;
    }
    if (lc($server_mode) eq 'pcap') {
        unless ($enc_pcap_port < 65535 and $enc_pcap_port > 0) {
            die "[*] Port offset must be 0 < port < 65535";
        }
    }
} else {
    if ($enc_rotate_proto) {
        die '[*] Can only specify --rotate-proto with ',
            'encrypted sequences.';
    }
}

if ($save_packet_mode) {
    ### save of copy of the packet
    unless ($save_packet_file) {
        $save_packet_file = "$homedir/fwknop_save_packet.$$";
    }
    unlink $save_packet_file if -e $save_packet_file;
}

### save our command line args (so -l can be used next time)
unless ($run_last_args or $run_last_host or $no_save_last_args
        or $show_last_cmd or $show_last_host_cmd) {
    &save_args();
}

if (lc($server_mode) eq 'pcap' or lc($server_mode) eq 'knock') {

    ### get the encryption key from the --get-key file
    ### or from STDIN if it's not in the file.
    &get_key();

    &handle_server_auth_method() if $server_auth_method;

    if (lc($server_mode) eq 'pcap') {

        ### construct and send the encrypted message to the server
        ### (sends a single packet).
        &pcap_send_encrypted_msg(&pcap_build_enc_msg());

    } else {
        ### we are running in port knocking mode, so get the
        ### encrypted port sequence (16 ports)
        &knock_ports(&encrypt_sequence());
    }
} else {
    ### we are running in non-encrypted port knocking mode, so get
    ### the port sequence
    &knock_ports(&import_shared_sequence());
}
exit 0;
#============================ end main ==============================

sub pcap_build_enc_msg() {

    my $msg         = '';
    my $msg_part1   = '';  ### contains everything before the md5 sum
    my $server_auth = '';  ### contains stuff after the md5 sum
    my $user        = '';
    my $random_num  = '';
    my $timestamp   = time();

    ### message format (all fields are separated by ":" characters
    #
    #  random number (16 bytes)
    #  username
    #  timestamp
    #  software version
    #  mode (command mode (0) or access mode (1))
    #  if command mode => command to execute
    #  if access mode  => IP,proto,port
    #  message md5 sum
    #  server_auth (post 0.9.2 release)

    ### some of the fields below might happen to contain ":" chars,
    ### so we base64 encode them

    print "\n[+] Building encrypted single-packet authorization (SPA) ",
        "message...\n" unless $quiet;

    if ($spoof_src) {
        if ($spoof_username) {
            $user = $spoof_username;
        } else {
            $user = 'root';
        }
    } else {
        ### getlogin() is better than using ENV{'USER'}, which is
        ### easily manipulated, so only use as a last resort.
        if ($spoof_username) {
            $user = $spoof_username;
        } else {
            $user = getlogin() || getpwuid($<) ||
                die "[*] Could not determine user; try using the ",
                    "--Spoof-user option";
        }
    }

    $random_num = int(rand(100000000000000));
    $random_num .= int(rand(10)) while (length($random_num) < 16);

    print "[+] Packet fields:\n\n",
        "        Random data: $random_num\n",
        "        Username:    $user\n",
        "        Timestamp:   $timestamp\n",
        "        Version:     $version\n"
        unless $quiet;

    my $msg_not_base64_encoded = "$random_num:$user:$timestamp:$version";

    ### append username and timestamp
    $msg_part1 = $random_num . ':' . encode_base64($user) .
        ':' . $timestamp . ':' . $version;

    if ($cmdline_pcap_cmd) {
        ### a specific command will be executed on the server.  Note we
        ### prepend the command string with the $enc_allow_ip so that the
        ### fwknopd server can apply the REQUIRE_SOURCE_ADDRESS criteria.
        $msg_part1 .= ":$command_mode:" .
            encode_base64("$enc_allow_ip,$cmdline_pcap_cmd");
        $msg_not_base64_encoded .= ":$command_mode:$enc_allow_ip" .
            ",$cmdline_pcap_cmd";

        print "        Action:      $command_mode (command mode)\n",
            "        Cmd:         $cmdline_pcap_cmd\n"
            unless $quiet;
    } else {
        ### access to port(s)/protocol(s) will be granted on the
        ### server
        $msg_part1 .= ":$access_mode:";
        $msg_not_base64_encoded .= ":$access_mode:$enc_allow_ip,$access_str";

        print "        Action:      $access_mode (access mode)\n"
            unless $quiet;
        if ($access_str) {
            $msg_part1 .= encode_base64("$enc_allow_ip,$access_str");
            print "        Access:      $enc_allow_ip,$access_str\n"
                unless $quiet;
        } else {
            $msg_part1 .= encode_base64("$enc_allow_ip,none,0");
            print "        Access:      $enc_allow_ip,none,0\n"
                unless $quiet;
        }
    }

    $msg_part1 =~ s/\n//g;

    if (lc($server_auth_method) eq 'crypt') {
        $server_auth = ':' . encode_base64("crypt,$server_auth_crypt_pw");
    }

    ### calculate md5 hash over entire message
    my $md5sum = md5_base64($msg_part1 . $server_auth);
    $msg = "$msg_part1:$md5sum" . $server_auth;
    $msg_not_base64_encoded = "$msg_not_base64_encoded:$md5sum"
            . $server_auth;

    print "        MD5 sum:     $md5sum\n" unless $quiet;

    if (lc($server_auth_method) eq 'crypt') {
        print "        Server auth: $server_auth_method,";
        for (my $i=0; $i<length($server_auth_crypt_pw); $i++) {
            print '*';
        }
        print "\n";
    }
    print "\n[+] Clear text message (not base64 encoded): ",
        "$msg_not_base64_encoded\n",
        "[+] Clear text message (fields base64 encoded): $msg\n"
        if $debug;

    if ($gpg_signing_key or $gpg_default_key) {
        return &pcap_GPG_encrypt_msg($msg);
    }
    return &pcap_Rijndael_encrypt_msg($msg);
}

sub pcap_GPG_encrypt_msg() {
    my $msg = shift;

    my $gnupg = GnuPG::Interface->new();

    $gpg_home_dir = "$homedir/.gnupg" unless $gpg_home_dir;

    if ($gpg_verbose) {
        $gnupg->options->hash_init(
            'homedir' => $gpg_home_dir);
    } else {
        $gnupg->options->hash_init(
            'batch' => 1,
            'homedir' => $gpg_home_dir);
    }

    ### if --gpg-default-key is given, then we trust that the user has
    ### set the default key with the default-key variable in ~/.gnupg/options
    $gnupg->options->default_key($gpg_signing_key) unless $gpg_default_key;

    $gnupg->options->push_recipients($gpg_recipient);

    my ($input, $output, $error, $pw, $status) =
        (IO::Handle->new(),
        IO::Handle->new(),
        IO::Handle->new(),
        IO::Handle->new(),
        IO::Handle->new());

    my $handles = GnuPG::Handles->new(
        stdin  => $input,
        stdout => $output,
        stderr => $error,
        passphrase => $pw,
        status => $status
    );

    my $pid;

    if ($use_gpg_agent or $gpg_agent_info) {
        if ($gpg_agent_info) {
            $ENV{'GPG_AGENT_INFO'} = $gpg_agent_info;
        }
        $pid = $gnupg->sign_and_encrypt('handles' => $handles,
            'command_args' => [ qw( --use-agent ) ]);
    } else {
        $pid = $gnupg->sign_and_encrypt('handles' => $handles);
    }

    print $pw $enc_key;
    close $pw;

    print $input $msg;
    close $input;

    my @ciphertext = <$output>;
    close $output;

    my @errors = <$error>;
    close $error;

    waitpid $pid, 0;

    my $ctext = '';
    if (@ciphertext) {
        $ctext = join '', @ciphertext;
    }

    unless ($ctext) {
        print "[*] GnuPG encrypt failed.\n";
        unless ($gpg_verbose) {
            print "    GnuPG errors:\n";
            print for @errors;
        }
        exit 1;
    }

    my $encoded_msg = encode_base64($ctext);

    $encoded_msg =~ s/=*$//;
    $encoded_msg =~ s/\n//g;

    print "[+] Encrypted message: $encoded_msg\n" if $debug;
    return $encoded_msg;
}

sub pcap_Rijndael_encrypt_msg() {
    my $msg = shift;

    my $cipher = Crypt::CBC->new(
        {
            'key'             => $enc_key,
            'cipher'          => $enc_alg
        }
    );
    my $encoded_msg = encode_base64($cipher->encrypt($msg));

    ### remove trailing "==" (the decrypt function will put
    ### them back before attempting to decrypt)... this is to
    ### make it more difficult for an IDS to detect fwknop
    ### traffic
    $encoded_msg =~ s/=*$//;
    $encoded_msg =~ s/\n//g;

    print "[+] Encrypted message: $encoded_msg\n" if $debug;
    return $encoded_msg;
}

sub pcap_send_encrypted_msg() {
    my $msg = shift;

    my $msg_len = length($msg);

    if ($msg_len > $max_msg_len) {
        die "[*] Message length is too long ($msg_len bytes), ",
            "must be less than $max_msg_len bytes";
    }

    if ($verbose) {
        print "\n[+] Packet data:\n\n", $msg, "\n\n" unless $quiet;
    }

    if ($save_packet_mode) {
        print "    Saving packet data to: $save_packet_file\n" unless $quiet;
        open F, "> $save_packet_file" or die "[*] Could not open ",
            "$save_packet_file: $!";
        print F $msg;
        close F;
    }

    if ($spoof_src) {
        unless ($spoof_src =~ /$ip_re/) {
            ### resolve to an IP
            my $iaddr = inet_aton($spoof_src)
                or die "[*] Could not resolve $spoof_src to IP.";
            my $addr = inet_ntoa($iaddr)
                or die "[*] Could not resolve $spoof_src to IP.";
            $spoof_src = $addr;
        }

        print
"\n[+] Sending $msg_len byte message to $knock_dst over $spoof_proto",
    "/$enc_pcap_port\n    (spoofed src ip: $spoof_src).\n" unless $quiet;
        my $rand_src_port = int(rand(65535));
        $rand_src_port = 65001 if $rand_src_port > 65535;
        $rand_src_port += 1024 if $rand_src_port < 1024;

        ### use Net::RawIP to spoof the packets
        require Net::RawIP;

        if ($spoof_proto eq 'udp') {
            my $rawpkt = new Net::RawIP({
                ip => {
                    saddr => $spoof_src,
                    daddr => $knock_dst
                },
                udp =>{}});
            $rawpkt->set({ ip => {
                    saddr => $spoof_src,
                    daddr => $knock_dst
                },
                udp => {
                    source => $rand_src_port,
                    dest   => $enc_pcap_port,
                    data   => $msg,
                }
            });
            $rawpkt->send();
        } elsif ($spoof_proto eq 'icmp') {
            my $rawpkt = new Net::RawIP({
                ip => {
                    saddr => $spoof_src,
                    daddr => $knock_dst
                },
                icmp =>{}});
            $rawpkt->set({ ip => {
                    saddr => $spoof_src,
                    daddr => $knock_dst
                },
                icmp => {
                    type => 0,
                    code => 0,
                    sequence => 0,
                    data => $msg
                }
            });
            $rawpkt->send();
        } elsif ($spoof_proto eq 'tcp') {
            my $rawpkt = new Net::RawIP({
                ip => {
                    saddr => $spoof_src,
                    daddr => $knock_dst
                },
                tcp =>{}});
            $rawpkt->set({ ip => {
                    saddr => $spoof_src,
                    daddr  => $knock_dst
                },
                tcp => {
                    ack => 1,
                    source => $rand_src_port,
                    dest   => $enc_pcap_port,
                    data => $msg
                }
            });
            if ($test_mode) {
                print "    --Test-mode enabled, SPA packet not sent.\n";
            } else {
                $rawpkt->send();
            }
        }
    } else {

        if ($spa_established_tcp) {  ### useful for Tor
            print "\n[+] Sending $msg_len byte message to $knock_dst ",
                "over established tcp/$enc_pcap_port socket...\n"
                unless $quiet;

            if ($test_mode) {
                print "    --Test-mode enabled, SPA packet not sent.\n";
            } else {
                my $socket = IO::Socket::INET->new(
                    PeerAddr => $knock_dst,
                    PeerPort => $enc_pcap_port,
                    Proto    => 'tcp',
                    Timeout  => 1
                ) or die "[*] Could not acquire TCP/$enc_pcap_port socket ",
                        "with $knock_dst: $!";

                $socket->send($msg);
                undef $socket;
            }
        } else {
            print "\n[+] Sending $msg_len byte message to $knock_dst ",
                "over udp/$enc_pcap_port...\n" unless $quiet;

            if ($test_mode) {
                print "    --Test-mode enabled, SPA packet not sent.\n";
            } else {
                my $socket = IO::Socket::INET->new(
                    PeerAddr => $knock_dst,
                    PeerPort => $enc_pcap_port,
                    Proto    => 'udp',
                    Timeout  => 1
                ) or die "[*] Could not acquire UDP socket: $!";

                $socket->send($msg);
                undef $socket;
            }
        }
    }
    return;
}

sub knock_ports() {
    my $ports_aref = shift;

    print "[+] Sending port knocking sequence to knock server: $knock_dst\n"
        unless $quiet;
    my $packet_ctr = 0;
    for my $href (@$ports_aref) {
        my $proto = $href->{'proto'};
        my $port  = $href->{'port'};
        ### note that we never care if the destination replies with a
        ### RST or icmp echo reply (or anything else).  In fact, hopefully
        ### the remote firewall is configued to not reply at all
        if ($proto eq 'icmp') {
            print "    icmp echo request -> $knock_dst\n";
            ping(hostname => "$knock_dst", count => 1, timeout => 1);
            sleep $knock_sleep;
        } else {
            print "    -> $knock_dst $proto/$port (packet: $packet_ctr)\n";
            my $socket = IO::Socket::INET->new(
                PeerAddr => $knock_dst,
                PeerPort => $port,
                Proto    => $proto,
                Timeout  => 1
            );  ### note there is no "or die" here since we just want to throw
                ### packets on the network
            if (defined $socket and $proto eq 'udp') {
                $socket->send('0');  ### have to actually send something for udp
                sleep $knock_sleep;
            }
            if ($proto eq 'tcp' and $knock_sleep >= 1) {
                sleep $knock_sleep;
            }
            undef $socket if defined $socket;
        }
        $packet_ctr++;
    }
    print "[+] Finished knock sequence.\n";
    return;
}

sub encrypt_sequence() {
    my $clear_txt = '';
    my $checksum = 0;
    my @encrypted_seq = ();

    my $cipher = Crypt::CBC->new({
        'key'    => $enc_key,
        'cipher' => $enc_alg,
    });

    my @octets = split /\./, $enc_allow_ip;

    $clear_txt .= chr($_) for @octets;
    $checksum += $_ for @octets;

    my $proto_num      = 0;
    my $enc_allow_port = 0;
    if ($access_str =~ /tcp/i) {
        $proto_num = 6;
        if ($access_str =~ /(\d+)/) {
            $enc_allow_port = $1;
        }
    } elsif ($access_str =~ /udp/i) {
        $proto_num = 17;
        if ($access_str =~ /(\d+)/) {
            $enc_allow_port = $1;
        }
    } elsif ($access_str =~ /icmp/i) {
        $proto_num = 1;
        $enc_allow_port = 0;
    }

    unless ($enc_allow_port) {
        die "[*] Must specify port to open."
            if $proto_num != 1;
    }
    my $port_upper_bits = $enc_allow_port;
    my $port_lower_bits = $enc_allow_port;

    if ($enc_allow_port == 0) {
        $port_upper_bits = 0;
        $port_lower_bits = 0;
    } else {
        $port_upper_bits = $port_upper_bits >> 8;
        $port_lower_bits = $port_lower_bits % 256;
    }

    $clear_txt .= chr($port_upper_bits);
    $clear_txt .= chr($port_lower_bits);

    $checksum += $port_upper_bits;
    $checksum += $port_lower_bits;

    $clear_txt .= chr($proto_num);
    $checksum += $proto_num;

    $checksum = $checksum % 256;

    $clear_txt .= chr($checksum);

    ### append username
    ### FIXME: either the checksum should be removed, or it should
    ### be applied to the username as well.
    my $username = getlogin() || getpwuid($<) || die "[*] Could not ",
        "get process username.";

    if ($username) {
        my @chars = split //, $username;
        for my $char (@chars) {
            if (length($clear_txt) < $enc_blocksize-1) {
                $clear_txt .= $char;
            }
        }
    }

    my @tmp_chars = split //, $clear_txt;
    print '[+] Clear-text sequence (' . length($clear_txt) . ' bytes): ';
    print ord($_) . ' ' for @tmp_chars;
    print "\n";

    my $cipher_txt = $cipher->encrypt($clear_txt);
    undef $cipher;

    @