Vigenère cipher/Cryptanalysis: Difference between revisions

{{out}}

<pre>Key THECHESHIRECAT

Key length 14

Plaintext THISWASTHEPOEMTHATALICEREADJABBERWOCKYTWASBRILLIGANDTHESLITHYTOVESDIDGYREANDGIMB...

Key THECHESCIRECATTHECHESHIRECAT

Key length 28

Plaintext THISWASYHEPOEMTHATALICEREADJABBERWOHKYTWASBRILLIGANDTHESLITHYTOAESDIDGYREANDGIMB...

Key TJGGAHET

Key length 8

Plaintext TFGODXGHWMNKEYIVLMBJACIPPTXWTBBNFRADSITFHCOSMGOTFYPOXCASLGRDFQCJTABEDSNFPTOBYIQZ...

Key THECSAS

Key length 7

Plaintext THISLESHIRRYENTHATPPIQFEGKDKABBEGAOQLLVGATBRILAMGOOQVRETLITHNXOJFFFSDHYREACHGWNO...

Key THEC

Key length 4

Plaintext THISKXGYWOPOLYIMLODNHCIGPVZAABBEFTCHZITWHEQWTGOKFARSECAJLITHMQCATCDIKSNWPVQFFIQQ...</pre>

=={{header|Perl 6}}==

{{trans|Perl}}

<lang perl6># from Wikipedia

constant %English-letter-freq = (

E => 12.70, L => 4.03, Y => 1.97, P => 1.93, T => 9.06, A => 8.17, O => 7.51, I => 6.97, N => 6.75,

S => 6.33, H => 6.09, R => 5.99, D => 4.25, C => 2.78, U => 2.76, M => 2.41, W => 2.36, F => 2.23,

G => 2.02, B => 1.29, V => 0.98, K => 0.77, J => 0.15, X => 0.15, Q => 0.10, Z => 0.07

);

constant @alphabet = %English-letter-freq.keys.sort;

constant max_key_lengths = 5; # number of keylengths to try

sub myguess ($text) {

my ($seqtext, @spacing, @factors, $pos, %freq, %Keys);

# Kasiski examination

$seqtext = $text;

while ($seqtext ~~ /$<sequence>=[...].*$<sequence>/) {

$seqtext = substr($seqtext, 1+index($seqtext, $<sequence>));

push @spacing, 1 + index($seqtext, $<sequence>);

}

for @spacing -> $j {

%freq{$_}++ for grep { $j %% $_ }, 2..$j;

}

# discard very short keys, and test only the most likely remaining key lengths

(%freq.keys.grep(* > 3).sort({%freq{$_}}).tail(max_key_lengths)).race(:1batch).map: -> $keylen {

my $key-guess = '';

loop (my $i = 0; $i < $keylen; $i++) {

my ($mykey, %chi-square, $best-guess);

loop (my $j = 0; $j < $text.chars; $j += $keylen) {

$mykey ~= substr($text, ($j+$i) % $text.chars, 1);

}

for @alphabet -> $subkey {

my $decrypted = mycrypt($mykey, $subkey);

my $length = $decrypted.chars;

for @alphabet -> $char {

my $expected = %English-letter-freq{$char} * $length / 100;

my $observed = $decrypted.comb.grep(* eq $char).elems;

%chi-square{$subkey} += ($observed - $expected)² / $expected if $observed;

}

}

%Keys{$keylen}{'score'} = %chi-square{@alphabet[0]};

for %chi-square.keys.sort -> $sk {

if (%chi-square{$sk} <= %Keys{$keylen}{'score'}) {

$best-guess = $sk;

%Keys{$keylen}{'score'} = %chi-square{$sk};

}

}

$key-guess ~= $best-guess;

}

%Keys{$keylen}{'key'} = $key-guess;

}

%Keys.keys.sort({ %Keys{$_}{'score'} }).map:{ %Keys{$_}{'key'} };

}

sub mycrypt ($text, $key) {

constant %values-numbers = @alphabet Z=> ^@alphabet;

constant %values-letters = %values-numbers.invert;

my ($new-text);

my $keylen = $key.chars;

loop (my $i = 0; $i < $text.chars; $i++) {

my $val = -1 * %values-numbers{substr( $key, $i%$keylen, 1)} # negative shift for decode

+ %values-numbers{substr($text, $i, 1)};

$new-text ~= %values-letters{ $val % @alphabet };

}

return $new-text;

}

my $cipher-text = q:to/EOD/;

MOMUD EKAPV TQEFM OEVHP AJMII CDCTI FGYAG JSPXY ALUYM NSMYH

VUXJE LEPXJ FXGCM JHKDZ RYICU HYPUS PGIGM OIYHF WHTCQ KMLRD

ITLXZ LJFVQ GHOLW CUHLO MDSOE KTALU VYLNZ RFGBX PHVGA LWQIS

FGRPH JOOFW GUBYI LAPLA LCAFA AMKLG CETDW VOELJ IKGJB XPHVG

ALWQC SNWBU BYHCU HKOCE XJEYK BQKVY KIIEH GRLGH XEOLW AWFOJ

ILOVV RHPKD WIHKN ATUHN VRYAQ DIVHX FHRZV QWMWV LGSHN NLVZS

JLAKI FHXUF XJLXM TBLQV RXXHR FZXGV LRAJI EXPRV OSMNP KEPDT

LPRWM JAZPK LQUZA ALGZX GVLKL GJTUI ITDSU REZXJ ERXZS HMPST

MTEOE PAPJH SMFNB YVQUZ AALGA YDNMP AQOWT UHDBV TSMUE UIMVH

QGVRW AEFSP EMPVE PKXZY WLKJA GWALT VYYOB YIXOK IHPDS EVLEV

RVSGB JOGYW FHKBL GLXYA MVKIS KIEHY IMAPX UOISK PVAGN MZHPW

TTZPV XFCCD TUHJH WLAPF YULTB UXJLN SIJVV YOVDJ SOLXG TGRVO

SFRII CTMKO JFCQF KTINQ BWVHG TENLH HOGCS PSFPV GJOKM SIFPR

ZPAAS ATPTZ FTPPD PORRF TAXZP KALQA WMIUD BWNCT LEFKO ZQDLX

BUXJL ASIMR PNMBF ZCYLV WAPVF QRHZV ZGZEF KBYIO OFXYE VOWGB

BXVCB XBAWG LQKCM ICRRX MACUO IKHQU AJEGL OIJHH XPVZW JEWBA

FWAML ZZRXJ EKAHV FASMU LVVUT TGK

EOD

my $text = $cipher-text.uc.subst("rx/<-[{join '',@alphabet}]>/".EVAL, '', :g);

for myguess($text) -> $key {

say "Key $key\n" ~

"Key length {$key.chars}\n" ~

"Plaintext {substr(mycrypt($text, $key), 0, 80)}...\n";

}</lang>