@b{@b{0..27}=(_,a..z,'.')}=0..27;@s=pop=~/./g;$p[$_[0]*$_%+@s]=$b{($b{$s[$_]}+$_)%28}for 0..$#s;join'',@p

##</code><code>##

@e=split//,pop;
@e=pop=~/./g;

##</code><code>##

@;{@;{0..27}=(_,a..z,'.')}=0..27;$,=0;$;[$_[0]*$,++%@s]=$;{($;{$_}+$,)%28}for@s=pop=~/./g;join'',@;