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