#!/usr/bin/perl -w

# how to derive the key id for a given dnssec dnskey

use strict;
use warnings;
use MIME::Base64;

sub dnskey2id ($$$$)
{
  my ($flags, $protocol, $algorithm, $key) = @_;

  my $data = pack("nCC", $flags, $protocol, $algorithm);
  $data .= decode_base64($key);

  my $id = 0;
  map { $id += $_ } unpack("n*", $data);

  $id += ($id >> 16) & 0xffff;

  return ($id & 0xffff);
}

# e.g. for a RSASHA256 based ZSK
print dnskey2id("256", "3", "8", 'AwEAA...'), "\n";