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  #`
                  #`````````` # # # # #`
                     # ` ` ` `# ##`
[download]

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

perl vigenere.pl | perl babbage.pl
[download]

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[$%++]}
[download]