use Digest::MD5;

# Given a message and key, returns a message authentication code
# with the following properties relevant to our example:
# - a 22-character string that may contain + / 0-9 a-z A-Z
# - any given message and key will always produce the same MAC
# - if you don't know the key, it's very hard to guess it
#   even if you have a message, its MAC, and this source code
# - if you have a message, its MAC, and even the key, it's 
#   very hard to find a different message with the same MAC
# - even a tiny change to a message, including adding on to
#   the end of it, will produce a very different MAC
sub compute_mac {
    my ($message, $key) = @_;
    Digest::MD5::md5_base64($key, Digest::MD5::md5($key, $message));
}

# Load a secret key string from somewhere safe
my $secret = 'skS>DrF1d:R-6<g8qmm7@Ml}?JQD1C';

# Ensures that an integer score is decorated with it's MAC
sub authenticated_score {
    my $score = shift;
    my $mac   = compute_mac(int($score), $secret);
    "$score/$mac";
}