#!/usr/bin/perl 
#
# A perl implementation of the "crypt" program.
#
# This program implements an rc4 encrytion algorythm 
# using a variable length key single piece key.
#
# It takes a file or data stream and encrypts it using the
# provided key.  Running the output of the program back
# through the program with the same key will decrypt it.
#
# USAGE: rc4 <keyval> [file name]
# If no file name is provided, the script will read from 
# STDIN.  
#
# Andy Welter
# www.the-welters.com
# May 2001
#

#
# Encrypt a buffer at a type.  Encryption is a stateful
# process, so we use the "@state" global variable to track
# the state.
sub rc4 {
my ($buf) = @_;
my ($ebuf, $char);
for(unpack('C*',$buf)) {
	$x++;
	$y=($state[$x%=256]+$y)%256;
	@state[$x,$y]=@state[$y,$x];
	#&swap;
	$char= pack (C, 
		$_^=$state[ ($state[$x] + $state[$y]) %256 ]);
	$ebuf= $ebuf . $char;
	};
return $ebuf;
};

sub prepkey {
#
# Prepare the encryption key
#
my @key=@_;
my @hexkey=unpack('C*',pack('H*',shift @key));
my ($x, $y);
my @t;
my @state;
#
# prepare key
for(@t=@state=0..255){
	$y=($hexkey[$_%@hexkey]+$state[$x=$_]+$y)%256;
	@state[$x,$y]=@state[$y,$x];
	#&swap;
}
return @state;
};

local @state=prepkey("$ARGV[0]");
my $x=0;
my $y=0;
my $file;
if ( $ARGV[1] eq "" ) {
	# If no file name was specified, 
	# use standard-in as the file name
	$file="<&=0";
} else {
	$file=$ARGV[1];
};

open (IN,"$file") || die "error: can not open $file";
while (read (IN,$buf, 1024)) {
	print rc4 ($buf);
};