# The Vigenère Cipher

This is a solution to task #2 from the 15th Perl Weekly Challenge, written in Perl 6.

Write a script to implement Vigenère cipher. The script should be able encode and decode.

The Vigenère Cipher is a method of encryption that uses a series of Caesar ciphers chosen using the lettes of a repeating key.

Two solution approaches spring to mind:

• Use a Vigenère square as a lookup table
• Compute the rotations directly

I have chosen to compute the rotations directly. The encode and decode operations can be implemented with exactly the same algorithm, with the only difference being whether to add or subtract the key offset.

As an aside, here is a one-liner to construct a Vigenère square:

my @vigenère-square[26;26] = gather { -> $n { take ('A'..'Z').List.rotate($n) } for 0..25 };


Here is my solution in full:

#!/usr/bin/env perl6

enum Operation <encode decode>;

#| Apply the Vigenère Cipher to text from STDIN, using the provided key.
sub MAIN(
Operation $operation, #= encode or decode Str$key)             #= Key to use
{
my @key-values = $key.uc.comb(/<[A..Z]>/).map(*.ord - 'A'.ord); slurp.uc.comb(/<[A..Z]>/).map(*.ord - 'A'.ord) .map(->$n {
LEAVE @key-values .= rotate(1);
given $operation { when encode { ($n + @key-values[0]) % 26;
}
when decode {
(\$n + 26 - @key-values[0]) % 26;
}
}
})
.map(* + 'A'.ord).rotor(60, :partial).map(*.chrs.join.say);
}


The encode/decode transforms are performed in 0..25 number space. The input data is normalized to uppercase and only the letters A..Z are processed. The solution ignores all other characters both in the source text and the key.

I have also added POD comments to the MAIN sub and to its parameters. This produces some nice friendly command-line usage. It is worth noting that source layout matters when using ~POD~ comments to generate usage. The parameter comments need to be attached to the parameters so each parameter is on its own line.

% ./Vigenère.p6
Usage:
./Vigenère.p6 <operation> <key> -- Apply the Vigenère Cipher to text from STDIN, using the provided key.

<operation>    encode or decode
<key>          key string to use


% echo 'ATTACKATDAWN' | ./Vigenère.p6 encode 'LEMON'
LXFOPVEFRNHR
% echo 'Attack at dawn!' | ./Vigenère.p6 encode 'lemon'
LXFOPVEFRNHR
% echo 'LXFOPVEFRNHR' | ./Vigenère.p6 decode 'Lemon'
ATTACKATDAWN


Here is an example with a longer source text:

% fortune | ./Vigenère.p6 encode fortune

As an added bonus, the solution uses .rotor to wrap the output text to 60 columns, regardless of the input format.