The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


Crypt::SKey - Perl S/Key calculator


  # In perl script:
  use Crypt::SKey qw(compute);
  $output = compute($sequence_num, $seed, $password);
  @output = compute($sequence_num, $seed, $password, $count);
  # Command line:
  perl -MCrypt::SKey -e key 500 fo099804
  perl -MCrypt::SKey -e key 500 fo099804 100
  perl -MCrypt::SKey=key_md4 -e key_md4 500 fo099804
  # The following shell alias may be useful:
  alias key 'perl -MCrypt::SKey -e key'
  # This allows you to simply type:
  key 500 fo099804


This module contains a simple S/Key calculator (as described in RFC 1760) implemented in Perl. It exports the function key by default, and may optionally export the function compute.

compute_md4, compute_md5, key_md4, and key_md5 are provided as convenience functions for selecting either MD4 or MD5 hashes. The default is MD4; this may be changed with with the $Crypt::SKey::HASH variable, assigning it the value of MD4 or MD5. You can access any of these functions by exporting them in the same manner as compute in the above example.

Most S/Key systems use MD4 hashing, but a few (notably OPIE) use MD5.


Follow the usual steps for installing any Perl module:

  perl Makefile.PL
  make test
  make install


compute($sequence_num, $seed, $password [, $count])

compute_md4($sequence_num, $seed, $password [, $count])

compute_md5($sequence_num, $seed, $password [, $count])

Given three arguments, computes the hash value and returns it as a string containing six words separated by spaces (or as a string of 16 hex digits if $Crypt::SKey::HEX is set to a true value). If $count is specified and greater than one, returns a list of several such strings. The meanings of the arguments is as follows:

  • sequence_number

    Which output in the sequence of calculated S/Key responses to generate. This is called N in RFC 1760. It will usually be the first number shown in an S/Key challenge.

  • seed

    This is a random seed. It is usually the second number/string shown in an S/Key challenge.

  • password

    This is your secret password.

  • count

    This argument is optional and defaults to 1. It specifies the number of S/Key responses to generate. This may be useful if you want to pre-generate a bunch of responses and print them on a piece of paper so that you don't need to have an S/Key calculator around later.




Acts just like the 'key' executable program that comes with the standard distribution of s/key. Reads several arguments from the command line (@ARGV), prompts for the user's password, and prints one or more calculated s/key responses to STDOUT. The command line arguments are, in order:

  • sequence_number

  • seed

  • count (optional)

Their meanings are exactly the same as with the compute function above.


If you care about security, you'd probably be better off using SSH than S/Key, because SSH encrypts your entire session whereas S/Key only encrypts your password. I wrote this module because nobody else seemed to have done it yet, and because sometimes I'm on systems with neither SSH nor the key program, but I want to telnet to a system that offers S/Key password transmission.

The original key program takes the count parameter using the -n flag, but this version takes it as an optional final argument. Unless I hear from someone that needs the behavior changed, I'm not likely to add the -n flag.

I currently have no plans to write any code that checks the validity of S/Key responses at login, i.e. the code that the server has to run when authenticating users. It shouldn't be hard, though, and if someone wants to send me a patch implementing this functionality I'll be happy to add it.


Ken Williams,

Thanks to Chris Nandor and Allen Chen for testing MD5 functionality.


Copyright 2000-2002 Ken Williams. All rights reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


perl(1). RFC 1760. Digest::MD4(1). Digest::MD5(1). Term::ReadKey(1).