This is a compliment to my recent obfuscated post, Vigenére cipher. In honor of Charles Babbage (pictured here so you don't have to squint too hard), who is credited with breaking Vigenére's cipher ca. 1854, but his method was not published until several years later, and as a result credit for the development was instead given to Friedrich Kasiski, who made the same discovery some years after Babbage.

What is now commonly referred to as the Kasiski examination allows a cryptanalyst to deduce the length of the keyword used in the polyalphabetic substitution cipher. Once the length of the keyword is discovered, the cryptanalyst lines up the ciphertext in N columns, where N is the length of the keyword. Then, each column can be treated as the ciphertext of a monoalphabetic substitution cipher. As such, each column can be attacked with frequency analysis

This homage to Charles Babbage uses this approach.

$_='`$t` `.=lc for<>;u($_` )for` 3..``6;%t=qw ` `(a 82 b 15 c 28 d 43 e ` `127 f 22 ` g 20 h 61 i 70 ` `j 2 k 8 ```````l 40 m 24 n 67 ` o 75` ` p 19 q 1` ` r ` 60 s 63 t ` 91 ` u 28 v 1 `0 w ` 24 x 2 y ` `20 ` ` z 1);$k= k() ` ``;$d+=$t{$` `_}f o``r keys%t;$l =$d` /in` ``t(`length($t)/ $k)/100 ` ;map{%n=f(t($_));@g=b(1,\` `%n);$y.= i(\@g)}0..$k-1;@_=(a..z); map{@$_= @_;if($;++){for$"(2..$;){ pu ` sh` `` @$_,shift@$_} `` `` `}` }@_;map{$p=i` n` d`ex `((join\'\',` ` ` `@`{(sp `lit//,$y)[$c ` ]}),$_);` `$o```.=$p>=0?$`a` `` [ $p]: $_;$c+=$c<$k-1?1 ````: `-$` ``k+1}split//,$t;s ``ub \'b{my($e,$s `,@g)=@_;p ` ``ush@ `g`,[$_,(s pli` `` ``t//,\'#\' ``x in` `` `t($$s{`$_}*$e )`)]for ` `+sort+keys%$s;retur ```n@g}s` ub\'c{my$x=shift;$x=g($x,shift ```)while@_; return$x}sub\'f{my%d;$d{$_}++f` or grep/[a-z]/ ,split//,shift;$d{$_}||=0for a..z;return%d}su b\'g{my($x,$y)=@_;($x,$y)=($y,$x%$y)while$y;r eturn$x}sub\'i{my($g,@c)=@_;map{push@c,o(v($g),` `` ` $$g[0][0]);w($g)}0..25;return(map{$_->[1]}sort{$` b-`` >[0]<=>$a->[0]}@c)[0]} sub\'k{my@g;for(sort{$s{` `$b}`` <=>$s{$a}}keys%s){last ``if$s{$_}<3;next unless y `/a-``` z//>2;my@f ;push@f,(pos `($t)-3)while$t=~/$_/g;m` ````````y$g=c(n(@f) );`$g```` >2&&push@g,$g}return c(@` g)}sub\'n{my$o= shift;return map{$_-$o}@_ }sub\'o{my($g,$w) =@_;my$c=0;map{map{/\+/&&` $c++;/\-/&&$c--}@ $_}@$g;return[$c,$w]}sub\' `t{my($o)=@_;my$c= 0;my$r;map{$r.=$_ unless( `$k-$o+$c)%$k;$c++} split//,$t;$r=~s/[^a-z]/ /g;return$r}sub\'u{ my$l=$_[0];$s{substr($t` ,$_,$l)}++for 0..(le ngth($t)-$l)}sub\'v{my($ `m)=@_;my@g=b($l,\%t );$s=\@g;$z=0;map{$x=0;ma `p{$$s[$z][$x]=$$m` [$z][$x]eq\'#\'&&$$s[$z][ `$x]eq\'#\'?\'+` \':\'-\';$x++}@$_;$z++}@$m `;return$s}sub \'w{$R=shift;push@$R,shif` `t@$R}print" Key: $y\nPlaintext:\n$o\`` `n";';s-\s \s+--gmx;s&`&&gm;eval#;` #etur#`` `#my($x($v());$y=$z#`#` ##```` ``# charles #`` #`````` ````# babbage #` #`````````` # # # # #` # ` ` ` `# ##`

This reads in ciphertext via STDIN. If saved as, and Vigenére cipher is saved as, you can see it in action via:

perl | perl

I have a more fully featured (unobfuscated, commented, and POD'd) version which even produces HTML output detailing the results of the Kasiski examination, and optionally the letter frequency analysis charts, which I will post to the code contributions section shortly. But not too quickly, as it's basically a giant spoiler for this :-)

I'd like to give a special thanks to liverpole, as without his Latent Image Obfuscation Generator, I wouldn't have been so easily able to generate an ascii 'portrait' of Charles Babbage that I could then turn into code.

$,=42;for(34,0,-3,9,-11,11,-17,7,-5){$*.=pack'c'=>$,+=$_}for(reverse s +plit//=>$* ){$%++?$ %%2?push@C,$_,$":push@c,$_,$":(push@C,$_,$")&&push@c,$"}$C[$# +C]=$/;($#C >$#c)?($ c=\@C)&&($ C=\@c):($ c=\@c)&&($C=\@C);$%=$|;for(@$c){print$_^ +$$C[$%++]}

In reply to Breaking the indecipherable cipher, courtesy Charles Babbage. by chargrill

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.