Here's a cheap way to get a non-repeating xor pattern of the proper length. It works by seeding rand with the key and relying on repeatability. The key is a single perl integer. The symmetric property^* of xor encryption is handy.

# Usage: cypher KEY, LIST;
sub cypher {
    srand shift;
    my $string = join '', @_;
    my $here = 0;
    while ( 2*$here < length( $string)) {
        vec( $string, $here, 16) ^= int rand 1<<16;
        $here++;
    }
    $string;
}

my $encoded = cypher 12345, <<'EOT';
     Listen, my children, and you shall hear
     Of the midnight ride of Paul Revere.
EOT

print map {sprintf '%x ', $_ } unpack 'C*', $encoded;
print $/, cypher 12345, $encoded;
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This is better than rot13, but probably is still weak crypto. The keyspace is small. Decryption depends on having the same particular implementation of rand. Regularities of rand can be exploited. But it is certainly enough to discourage casual readers.

^* the same procedure is used for both encryption and decryption.

The conventional nomenclatura for such modules, if one exists in XS, is to add "PurePerl" or "PP" to the name So, looking for crypt PP, I found these likely candidates:

• Crypt::Twofish_PP
• Crypt::Rijndael_PP
• Crypt::GOST_PP
• Crypt::Blowfish_PP
• Crypt::CAST5_PP
• Crypt::DES_PP

If it's not particularly important data, that's cool, but be careful never to use the rational, "No one will figure it out." Someone will always be curious enough to see if they can do it, or just are melicious enough to do it for kicks.

The data is not sensitive, I can do away with any encryption. I just wanted to have something like a plastice cover on a notebook. Thanks for the simple suggestion.

we should emulate the Jedi masters by learning how to build strong cryptography programs all by ourselves.

Let's try that cpan link again... Crypt::CipherSaber

#!/usr/bin/perl -w
use strict;

my $i = "abcdefghijkl m nopqrst uvxwyz";
print "$i\n";
my $j = mess_it($i);
print "$j\n";
print fix_it($j),"\n";

sub fix_it{
my $s = shift;
$s =~ s#(.)#chr(ord($1)/2)#ge; 
return $s;
}

sub mess_it{
my $s = shift;
$s =~ s/(.)/chr(ord($1)*2)/ge;
return $s;
}
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