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

SYNOPSIS
      use Crypt::Skip32;

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

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 obscure them while keeping the encrypted output size small
    (also only 32 bits).

    One example where Crypt::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 Crypt::Skip32 to scramble ids and put the resulting 32-bit
    value in URLs (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" bytes (10).

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

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

        See the EXAMPLE below for hints on how to take a plain integer,
        encrypt it, and encode it for use in URLs and other non-binary
        formats.

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

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

    blocksize
          my $blocksize = $cipher->blocksize;
          my $blocksize = Crypt::Skip32->blocksize;

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

    keysize
          my $keysize = $cipher->keysize;
          my $keysize = Crypt::Skip32->keysize;

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

NOTES
    If Crypt::Skip32::XS is installed, this module will use it and the
    constructor will return an object of that type, though the interface is
    identical. You can stick with the pure Perl version by setting the
    CRYPT_SKIP32_PP environment variable before using this module.

    If reporting a bug, please try to determine (if possible) if it is this
    module or the XS one, and report it to the corresponding maintainer.

EXAMPLE
    This sample code demonstrates how Crypt::Skip32 can be used to encrypt
    unsigned integers and encode them for use in web URLs, form values, and
    other places where short encrypted text might be useful.

      use Crypt::Skip32;

      # Create a cipher. Change the long hex string to your secret key.
      my $key         = pack("H20", "112233445566778899AA");
      my $cipher      = new Crypt::Skip32 $key; # Always 10 bytes!

      # Encrypt an unsigned integer (under 2^32) into an 8-digit hex string.
      my $number      = 3493209676;
      my $plaintext   = pack("N", $number);
      my $ciphertext  = $cipher->encrypt($plaintext); # Always 4 bytes!
      my $cipherhex   = unpack("H8", $ciphertext);
      print "$number encrypted and converted to hex: $cipherhex\n";

      # Decrypt an encrypted, hexified unsigned integer.
      my $ciphertext2 = pack("H8", $cipherhex);
      my $plaintext2  = $cipher->decrypt($ciphertext2); # Always 4 bytes!
      my $number2     = unpack("N", $plaintext2);
      print "$cipherhex converted back and decrypted: $number2\n";

    The above code generates the output:

      3493209676 encrypted and converted to hex: 6da27100
      6da27100 converted back and decrypted: 3493209676

CAVEATS
    This initial alpha Perl implementation of Crypt::Skip32 has not been
    extentively reviewed by cryptographic experts, nor has it been tested
    extensively on many different platforms. It is recommended that this
    code not be used for applications which require a high level of
    security.

    Reviewers and testers welcomed.

    Though this module has been coded to follow a Crypt::CBC usable
    interface, it is not intended for use in encrypting long chunks of text.
    For those purposes, it is suggested you use another high quality, proven
    cipher with a longer block size.

INSTALLATION
    If your Linux distro does not have a prepared package for this module,
    then the preferred method for installation is directly from the CPAN
    using a command like:

        sudo cpan Crypt::Skip32

SOURCE
    The source for this module is being maintained on github:

        https://github.com/alestic/Crypt-Skip32

    Forks and patches will be reviewed, but please be aware that the
    targeted functionality of this particular module is very narrow.

    Feel free to build other abstractions on top of this module if you want
    to make it easier to use or to create a particular application for its
    use.

BUGS
    Problems and feature requests can be submitted through the github
    "issues" link:

        https://github.com/alestic/Crypt-Skip32/issues

    A gentle reminder sent directly to the author (below) may also help
    increase awareness and attention.

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

    Crypt::Skip32::XS

AUTHOR
    Perl code maintained by Eric Hammond <[email protected]>
    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.

CREDITS
    The following have contributed to the Perl version:

        John Laur
        Joe Edmonds
        gray

    And, of course, this is entirely based on the C version written by:

        Greg Rose

COPYRIGHT AND LICENSE
    Copyright (C) 2007-2019 Eric Hammond <[email protected]>

    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 original C version of SKIP32 by Greg Rose (see below) is explicitly
    "not copyright, no rights reserved". Even so, permission was requested
    and granted to make a Perl version available on the CPAN.

ORIGINAL C SOURCE
     /*
        SKIP32 -- 32 bit block cipher based on SKIPJACK.
        Written by Greg Rose, QUALCOMM Australia, 1999/04/27.

        In common: F-table, G-permutation, key schedule.
        Different: 24 round feistel structure.
        Based on:  Unoptimized test implementation of SKIPJACK algorithm
                   Panu Rissanen <[email protected]>

        SKIPJACK and KEA Algorithm Specifications
        Version 2.0
        29 May 1998

        Not copyright, no rights reserved.
     */

     typedef unsigned char   BYTE; /* 8 bits */
     typedef unsigned short  WORD; /* 16 bits */

     const BYTE ftable[256] = {
     0xa3,0xd7,0x09,0x83,0xf8,0x48,0xf6,0xf4,0xb3,0x21,0x15,0x78,0x99,0xb1,0xaf,0xf9,
     0xe7,0x2d,0x4d,0x8a,0xce,0x4c,0xca,0x2e,0x52,0x95,0xd9,0x1e,0x4e,0x38,0x44,0x28,
     0x0a,0xdf,0x02,0xa0,0x17,0xf1,0x60,0x68,0x12,0xb7,0x7a,0xc3,0xe9,0xfa,0x3d,0x53,
     0x96,0x84,0x6b,0xba,0xf2,0x63,0x9a,0x19,0x7c,0xae,0xe5,0xf5,0xf7,0x16,0x6a,0xa2,
     0x39,0xb6,0x7b,0x0f,0xc1,0x93,0x81,0x1b,0xee,0xb4,0x1a,0xea,0xd0,0x91,0x2f,0xb8,
     0x55,0xb9,0xda,0x85,0x3f,0x41,0xbf,0xe0,0x5a,0x58,0x80,0x5f,0x66,0x0b,0xd8,0x90,
     0x35,0xd5,0xc0,0xa7,0x33,0x06,0x65,0x69,0x45,0x00,0x94,0x56,0x6d,0x98,0x9b,0x76,
     0x97,0xfc,0xb2,0xc2,0xb0,0xfe,0xdb,0x20,0xe1,0xeb,0xd6,0xe4,0xdd,0x47,0x4a,0x1d,
     0x42,0xed,0x9e,0x6e,0x49,0x3c,0xcd,0x43,0x27,0xd2,0x07,0xd4,0xde,0xc7,0x67,0x18,
     0x89,0xcb,0x30,0x1f,0x8d,0xc6,0x8f,0xaa,0xc8,0x74,0xdc,0xc9,0x5d,0x5c,0x31,0xa4,
     0x70,0x88,0x61,0x2c,0x9f,0x0d,0x2b,0x87,0x50,0x82,0x54,0x64,0x26,0x7d,0x03,0x40,
     0x34,0x4b,0x1c,0x73,0xd1,0xc4,0xfd,0x3b,0xcc,0xfb,0x7f,0xab,0xe6,0x3e,0x5b,0xa5,
     0xad,0x04,0x23,0x9c,0x14,0x51,0x22,0xf0,0x29,0x79,0x71,0x7e,0xff,0x8c,0x0e,0xe2,
     0x0c,0xef,0xbc,0x72,0x75,0x6f,0x37,0xa1,0xec,0xd3,0x8e,0x62,0x8b,0x86,0x10,0xe8,
     0x08,0x77,0x11,0xbe,0x92,0x4f,0x24,0xc5,0x32,0x36,0x9d,0xcf,0xf3,0xa6,0xbb,0xac,
     0x5e,0x6c,0xa9,0x13,0x57,0x25,0xb5,0xe3,0xbd,0xa8,0x3a,0x01,0x05,0x59,0x2a,0x46
     };

     WORD
     g(BYTE *key, int k, WORD w)
     {
         BYTE g1, g2, g3, g4, g5, g6;

         g1 = (w>>8)&0xff;
         g2 = w&0xff;

         g3 = ftable[g2 ^ key[(4*k)%10]] ^ g1;
         g4 = ftable[g3 ^ key[(4*k+1)%10]] ^ g2;
         g5 = ftable[g4 ^ key[(4*k+2)%10]] ^ g3;
         g6 = ftable[g5 ^ key[(4*k+3)%10]] ^ g4;

         return ((g5<<8) + g6);
     }

     void
     skip32(BYTE key[10], BYTE buf[4], int encrypt)
     {
         int         k; /* round number */
         int         i; /* round counter */
         int         kstep;
         WORD        wl, wr;

         /* sort out direction */
         if (encrypt)
             kstep = 1, k = 0;
         else
             kstep = -1, k = 23;

         /* pack into words */
         wl = (buf[0] << 8) + buf[1];
         wr = (buf[2] << 8) + buf[3];

         /* 24 feistel rounds, doubled up */
         for (i = 0; i < 24/2; ++i) {
             wr ^= g(key, k, wl) ^ k;
             k += kstep;
             wl ^= g(key, k, wr) ^ k;
             k += kstep;
         }

         /* implicitly swap halves while unpacking */
         buf[0] = wr >> 8;   buf[1] = wr & 0xFF;
         buf[2] = wl >> 8;   buf[3] = wl & 0xFF;
     }

     #include <stdio.h>
     int main(int ac, char *av[])
     {
         BYTE        in[4] = { 0x33,0x22,0x11,0x00 };
         BYTE        key[10] = { 0x00,0x99,0x88,0x77,0x66,0x55,0x44,0x33,0x22,0x11 };
         int         i, encrypt;
         int         bt;

         if (ac == 1) {
             skip32(key, in, 1);
             printf("%02x%02x%02x%02x\n", in[0], in[1], in[2], in[3]);
             if (in[0] != 0x81 || in[1] != 0x9d || in[2] != 0x5f || in[3] != 0x1f) {
                 printf("819d5f1f is the answer! Didn't encrypt correctly!\n");
                 return 1;
             }
             skip32(key, in, 0);
             if (in[0] != 0x33 || in[1] != 0x22 || in[2] != 0x11 || in[3] != 0x00) {
                 printf("%02x%02x%02x%02x\n", in[0], in[1], in[2], in[3]);
                 printf("33221100 is the answer! Didn't decrypt correctly!\n");
                 return 1;
             }
         }
         else if (ac != 4) {
             fprintf(stderr, "usage: %s e/d kkkkkkkkkkkkkkkkkkkk dddddddd\n", av[0]);
             return 1;
         }
         else {
             encrypt = av[1][0] == 'e';
             for (i = 0; i < 10; ++i) {
                 sscanf(&av[2][i*2], "%02x", &bt);
                 key[i] = bt;
             }
             for (i = 0; i < 4; ++i) {
                 sscanf(&av[3][i*2], "%02x", &bt);
                 in[i] = bt;
             }
             skip32(key, in, encrypt);
             printf("%02x%02x%02x%02x\n", in[0], in[1], in[2], in[3]);
         }
         return 0;
     }