use strict;

while(<DATA>){
    chomp;
    /(\d+)\s+([_a-z.]+)/ or last;
    my $plaintext = decipher($1, $2);
    print "$plaintext\n";
}

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

__DATA__
5 cs.
101 thqqxw.lui.qswer
3 b_ylxmhzjsys.virpbkr
0

##</code><code>##

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
}