An example little filter. Don't forget to specify the secret key when encoding. :-)

use List::Util q(shuffle);
use Getopt::Std;
getopts('ds:');

$opt_s and srand $opt_s;
@T{@$_} = shuffle @$_ for ([A..Z],[a..z]);
$opt_d and %T = reverse %T;

s{.}{$T{$&}//$&}eg, print for <>;
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Now, could someone please determine if a pair of keys exist that cancel each other (decode without -d, that is)?