#!/usr/bin/perl # This is a letter-frequency analyzer by Chris Staecker # Modified 4/28/08 use strict; use CGI; my $query = new CGI; sub maketop() { print "Chris Staecker: Frequency counter"; print ""; print "

Chris Staecker's frequency counter!

"; print "

This tool can help you crack a substitution or Vigenere cipher. Type your text below, and hit the button to compute the letter frequencies. You can also count frequencies in groups of letters offset by a particular number. (Choosing 5 offsets will count the frequencies in the groups of letters obtained by taking every 5th letter from the text.)

"; print "

"; if ($query->param("text")) { print $query->param("text"); } else { print "Type the cyphertext here"; } print "
"; print "Count frequency using this many offsets:
"; print "
"; } sub isletter { my ($c) = @_; return (ord($c) > 64 and ord($c)<91); } sub makeoffsetfreqs { my ($t, $skip, $o) = @_; my %freq; my $total; # all upper-case $t = uc($t); # strip out all non alphas my $s = ''; while ($t) { my $c = chop($t); if (isletter($c)) { $s = $c . $s; } } $t = $s; # start at the offset $t = substr($t, $o); # reverse it since chop takes off the end $t = reverse($t); while ($t) { my $c = chop($t); $freq{$c}++; # skip some letters foreach (1..$skip-1) { chop($t); } } # go through to compute the total count, and put in 0s for missed letters foreach (65..90) { if ($freq{chr($_)}) { $total += $freq{chr($_)}; } else { $freq{chr($_)} = 0; } } # print the frequencies, highest first while (keys %freq) { my $maxkey = (keys %freq)[0]; foreach (keys %freq) { if ($freq{$_} > $freq{$maxkey}) { $maxkey = $_; } } if ($freq{$maxkey}) { print "$maxkey: "; printf("%.3f", 100 * $freq{$maxkey} / $total); print "%
"; } else { print "$maxkey: 0
"; } delete $freq{$maxkey}; } } print $query->header(); maketop(); if ($query->param('text')) { my %freq; my $t = $query->param('text'); my $o = $query->param('offset'); print "

Results

"; print "

"; for (0..$o-1) { print ""; } print ""; for (0..$o-1) { print ""; } print "
Offset $_
"; makeoffsetfreqs($t, $o, $_); print "

"; } print "

You can see the source code for this CGI script

"; print "";