RFC: Crypt::Skip32 - 32-bit block cipher based on Skipjack

I'm planning to upload this new module for distribution on the CPAN.
Feedback welcomed.

NAME
Crypt::Skip32 - 32-bit block cipher based on Skipjack

SYNOPSIS
use Crypt::Skip32;

$cipher = new Crypt::Skip32 $key;
$ciphertext = $cipher->encrypt($plaintext);
$plaintext = $cipher->decrypt($ciphertext);

DESCRIPTION
Skip32 is a 80-bit key, 32-bit block cipher based on Skipjack. The
Perl
code for the algorithm is a direct translation from C to Perl of
skip32.c by Greg Rose found here:

http://www.qualcomm.com.au/PublicationsDocs/skip32.c

This cipher can be handy for scrambling small (32-bit) values when
you
would like to make it not obvious what they are while keeping the
encrypted output size small.

One example where Skip32 has been useful: You have numeric
database
record ids which increment sequentially. You would like to use
them in
URLs, but you don't want to make it obvious how many X's you have
in the
database by putting the ids directly in the URLs. You can use
Skip32 to
scramble the id and put the resulting 32-bit value in the URL
(perhaps
as 8 hex digits or some other shorter encoding). When a user
requests a
URL, you can unscramble the id to retrieve the object from the
database.

Warning: A 32-bit value can only go a little over 4 billion
(American).
Plan ahead if what you need to encrypt might eventually go over
this
limit.

FUNCTIONS
new
my $cipher = new Crypt::Skip32 $key;

Creates a new Crypt::Skip32 block cipher object, using $key,
where
$key is a key of "keysize()" (10) bytes.

encrypt
my $ciphertext = $cipher->encrypt($plaintext);

Encrypt $plaintext and return the $ciphertext where $plaintext
and
$ciphertext must be of "blocksize()" (4) bytes.

decrypt
my $plaintext = $cipher->decrypt($ciphertext);

Decrypt $ciphertext and return the $plaintext where $plaintext
and
$ciphertext must be of "blocksize()" (4) bytes.

blocksize
my $blocksize = $cipher->blocksize;

Returns the size (in bytes) of the block cipher. This is
always 4
bytes (for 32 bits).

keysize
my $keysize = $cipher->keysize;

Returns the size (in bytes) of the key. This is always 10
bytes.

EXAMPLE
use Crypt::Skip32;

my $key = pack("H20", "112233445566778899AA"); # Your
secret!
my $cipher = new Crypt::Skip32 $key; # Always 10 bytes!

my $plaintext1 = pack("N", 3493209676);
my $ciphertext = $cipher->encrypt($plaintext1); # Always 4
bytes!
print "scrambled 3493209676 -> 0x", unpack("H8", $ciphertext),
"\n";

my $plaintext2 = $cipher->decrypt($ciphertext);
die "something went horribly wrong" unless $plaintext2 eq
$plaintext1;

print "blocksize: ", $cipher->blocksize, " bytes \n";
print "keysize: ", $cipher->keysize, " bytes \n";

TODO
This version of Skip32 is implemented entirely in Perl. Since
there is a
free C implementation of the algorithm, this should probably be
made
available with XS or Inline::C so that it can run a bit faster.

SEE ALSO
The original Skip32 implementation in C by Greg Rose:

http://www.qualcomm.com.au/PublicationsDocs/skip32.c

The 80-bit key, 64-bit block Skipjack cipher created by the NSA
(Perl
code maintained by Julius C. Duque):

Crypt::Skipjack

AUTHOR
Perl code maintained by Eric Hammond com>
http://www.anvilon.com

Original Skip32 C code written 1999-04-27 by Greg Rose, based on
an
implementation of the Skipjack algorithm written by Panu Rissanen.

COPYRIGHT AND LICENSE
(C) Copyright 2007 Eric Hammond

This library is free software; you can redistribute it and/or
modify it
under the same terms as Perl itself, either Perl version 5.8.8 or,
at
your option, any later version of Perl 5 you may have available.

The C version of Skip32 by Greg Rose is explicitly "not copyright,
no
rights reserved" and he approved of making a Perl version
available
on the CPAN.

Re: RFC: Crypt::Skip32 - 32-bit block cipher based on Skipjack

It's been a couple decades since I've used Usenet so I decided to try
it through Google's web interface. Looks like they have a very narrow
word wrap and no way to preview, so here is another attempt to make it
more legible. Does anybody actually monitor these newsgroups with so
much spam?

NAME
Crypt::Skip32 - 32-bit block cipher based on Skipjack

SYNOPSIS
use Crypt::Skip32;

$cipher = new Crypt::Skip32 $key;
$ciphertext = $cipher->encrypt($plaintext);
$plaintext = $cipher->decrypt($ciphertext);

DESCRIPTION
Skip32 is a 80-bit key, 32-bit block cipher based on
Skipjack. The Perl code for the algorithm is a direct
translation from C to Perl of skip32.c by Greg Rose found
here:

http://www.qualcomm.com.au/PublicationsDocs/skip32.c

This cipher can be handy for scrambling small (32-bit) values
when you would like to make it not obvious what they are
while keeping the encrypted output size small.

One example where Skip32 has been useful: You have numeric
database record ids which increment sequentially. You would
like to use them in URLs, but you don't want to make it
obvious how many X's you have in the database by putting the
ids directly in the URLs. You can use Skip32 to scramble the
id and put the resulting 32-bit value in the URL (perhaps as
8 hex digits or some other shorter encoding). When a user
requests a URL, you can unscramble the id to retrieve the
object from the database.

Warning: A 32-bit value can only go a little over 4 billion
(American). Plan ahead if what you need to encrypt might
eventually go over this limit.

FUNCTIONS
new
my $cipher = new Crypt::Skip32 $key;

Creates a new Crypt::Skip32 block cipher object, using
$key, where $key is a key of "keysize()" (10) bytes.

encrypt
my $ciphertext = $cipher->encrypt($plaintext);

Encrypt $plaintext and return the $ciphertext where
$plaintext and $ciphertext must be of "blocksize()" (4)
bytes.

decrypt
my $plaintext = $cipher->decrypt($ciphertext);

Decrypt $ciphertext and return the $plaintext where
$plaintext and $ciphertext must be of "blocksize()" (4)
bytes.

blocksize
my $blocksize = $cipher->blocksize;

Returns the size (in bytes) of the block cipher. This is
always 4 bytes (for 32 bits).

keysize
my $keysize = $cipher->keysize;

Returns the size (in bytes) of the key. This is always 10
bytes.

EXAMPLE
use Crypt::Skip32;

my $key = pack("H20", "112233445566778899AA");
my $cipher = new Crypt::Skip32 $key;

my $plaintext1 = pack("N", 3493209676);
my $ciphertext = $cipher->encrypt($plaintext1);
print "scrambled 3493209676 -> 0x",
unpack("H8", $ciphertext), "\n";

my $plaintext2 = $cipher->decrypt($ciphertext);
die "something went horribly wrong"
unless $plaintext2 eq $plaintext1;

print "blocksize: ", $cipher->blocksize, " bytes \n";
print "keysize: ", $cipher->keysize, " bytes \n";

TODO
This version of Skip32 is implemented entirely in Perl. Since
there is a free C implementation of the algorithm, this
should probably be made available with XS or Inline::C so
that it can run a bit faster.

DOWNLOAD
Until this module is available on the CPAN, it may be
downloaded here:

http://www.anvilon.com/software/download/Crypt-Skip32-0.03.t ar.gz

SEE ALSO
The original Skip32 implementation in C by Greg Rose:

http://www.qualcomm.com.au/PublicationsDocs/skip32.c

The 80-bit key, 64-bit block Skipjack cipher created by the
NSA (Perl code maintained by Julius C. Duque):

Crypt::Skipjack

AUTHOR
Perl code maintained by Eric Hammond

http://www.anvilon.com

Original Skip32 C code written 1999-04-27 by Greg Rose, based
on an implementation of the Skipjack algorithm written by
Panu Rissanen.

COPYRIGHT AND LICENSE
Copyright (C) 2007 Eric Hammond


This library is free software; you can redistribute it and/or
modify it under the same terms as Perl itself, either Perl
version 5.8.8 or, at your option, any later version of Perl 5
you may have available.

The C version of Skip32 by Greg Rose is explicitly "not
copyright, no rights reserved" and he approved of making a Perl
version available on the CPAN.