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$cipher->encrypt($data) 

Encrypt data. The size of $data must be a multiple of blocksize 
(16 bytes), otherwise this function will croak. Apart from that, 
it can be of (almost) any length.

# this requires $username < 16 bytes and prefix packs with spaces
sprintf '%16s', $username;
...
    my $data = shift;
  return "\0" x ( 16 - length($data)%16 ) . $data;
}