NAME

Tea_JS.pm - The Tiny Encryption Algorithm in Perl and JavaScript

SYNOPSIS

Usage:

 use Crypt::Tea_JS;
 $key = 'PUFgob$*LKDF D)(F IDD&P?/';
 $ascii_cyphertext = &encrypt ($plaintext, $key);
 ...
 $plaintext_again = &decrypt ($ascii_cyphertext, $key);
 ...
 $signature = &asciidigest ($text);

In CGI scripts:

 use Crypt::Tea_JS;
 print &tea_in_javascript;
 # now the browser can encrypt and decrypt ! In JS:
 var ascii_ciphertext = encrypt (plaintext, key);
 var plaintext_again  = decrypt (ascii_ciphertext, key);
 var signature = asciidigest (text);

DESCRIPTION

This module implements TEA, the Tiny Encryption Algorithm, and some Modes of Use, in Perl and JavaScript.

The $key is a sufficiently longish string; at least 17 random 8-bit bytes for single encryption.

Version 2.14

(c) Peter J Billam 1998

SUBROUTINES

encrypt( $plaintext, $key );

Encrypts with CBC (Cipher Block Chaining)

decrypt( $cyphertext, $key );

Decrypts with CBC (Cipher Block Chaining)

asciidigest( $a_string );

Returns an asciified binary signature of the argument.

tea_in_javascript();

Returns a compatible implementation of TEA in JavaScript, for use in CGI scripts to communicate with browsers.

EXPORT_OK SUBROUTINES

The following routines are not exported by default, but are exported under the ALL tag, so if you need them you should:

 import Crypt::Tea_JS qw(:ALL);

binary2ascii( $a_binary_string );

Provides an ascii text encoding of the binary argument. If Tea_JS.pm is not being invoked from a GCI script, the ascii is split into lines of 72 characters.

ascii2binary( $an_ascii_string );

Provides the binary original of an ascii text encoding.

JAVASCRIPT

At the browser end, the following functions offer the same functionality as their perl equivalents above:

encrypt ( str, keystr )

decrypt ( ascii, keystr )

asciidigest ( str );

Of course the same Key must be used by the Perl on the server and by the JavaScript in the browser, and of course you don't want to transmit the Key in cleartext between them. Let's assume you've already asked the user to fill in a form asking for their Username, and that this username can be transmitted back and forth in cleartext as an ordinary form variable.

On the server, typically you will retrieve the Key from a database of some sort, for example:

 $plaintext = "<P>Hello World !</P>\n";
 dbmopen %keys, "/home/wherever/passwords";
 $key = $keys{$username};
 dbmclose %keys;
 $cyphertext = &encrypt ($plaintext, $key);

At the browser end there are various ways of doing it. Easiest is to ask the user for their password every time they view an encrypted page, or submit a form.

 print &tea_in_javascript(), <<EOT;
 <SCRIPT LANGUAGE="JavaScript"> <!--
 var key = prompt("Password ?","");
 document.write(decrypt("$cyphertext", key));
 // -->
 </SCRIPT>
 EOT

To submit an encrypted FORM, the neatest way is to contruct two FORMs; one overt one which the user fills in but which never actually gets submitted, and one covert one which will hold the cyphertext.

 print <<'EOT';
 <SCRIPT LANGUAGE="JavaScript"> <!--
 function submitter (form) { // NB: a javascript: url passes the frame
   var plaintext = "";
   for (var i=0; i<form.length; i++) { // construct ff-separated list
     var e = form.elements[i];
     plaintext += (e.name+"\f"+ e.value+"\f"); 
   }
   document.covert.cyphertext.value = encrypt (plaintext, key);
   document.covert.submit();   // submit the covert form
   return false;  // don't actually submit the overt form
 }
 // -->
 </SCRIPT>
 EOT

 print <<EOT;
 <FORM NAME="covert" ACTION="$ENV{SCRIPT_NAME}" METHOD="post">
 <INPUT TYPE="hidden" NAME="username" VALUE="$username">
 <INPUT TYPE="hidden" NAME="cyphertext" VALUE="">
 </FORM>

 <FORM NAME="overt" onSubmit="return submitter(this)">
 <TABLE>
 <TR><TH>Contact</TH><TD><INPUT TYPE="text" NAME="contact"></TD></TR>
 <TR><TH>Date   </TH><TD><INPUT TYPE="text" NAME="date">   </TD></TR>
 <TR><TH>Report </TH><TD><INPUT TYPE="text" NAME="report"> </TD></TR>
 </TABLE>
 <INPUT TYPE="submit">
 </FORM>
 EOT

See the cgi script examples/tea_demo.cgi in the distribution directory.

If you want the browser to remember its Key from page to page, to form a session, then things get harder. If you store the Key in a Cookie, it is vulnerable to any imposter server who imitates your IP address, and also to anyone who sits down at the user's computer. However, this remains the most practical option.

The alternative is to store the Key in a JavaScript variable, but unfortunately all JavaScript variables get anihilated when you load a new page into the same target frame. Therefore you have to store the Key in a JavaScript variable in a frameset, open up a subframe covering almost all the screen, and load the subsequent pages into that subframe; they can then use parent.key to encrypt and decrypt. This can become intractable. See CGI::Htauth.pm for attempts to use this kind of technique.

ROADMAP

Crypt::Tea conflicted with a similarly-named Crypt::TEA by Abhijit Menon-Sen. Unfortunately, Microsoft operating systems confused the two names and are unable to install both. Version 2.09 of Crypt::Tea is mature, and apart perhaps from minor bug fixes will probably remain the final version. Further development will take place under the name Crypt::Tea_JS. The calling interface is identical.

I've taken advantage of the new name to make two important changes. Firstly, Crypt::Tea_JS uses the New Improved version of the Tea algorithm, which provides even stronger encryption, though it does surrender backward-compatibility for files encrypted by the old Crypt::Tea. Secondly, some of the core routines are now implemented in C, for improved performance (at the server end, if you're using it in a CGI context).

AUTHOR

Peter J Billam ( http://www.pjb.com.au/comp/contact.html ).

CREDITS

Based on TEA, as described in http://www.cl.cam.ac.uk/ftp/papers/djw-rmn/djw-rmn-tea.html , and on some help from Applied Cryptography by Bruce Schneier as regards the modes of use. Thanks also to Neil Watkiss for the MakeMaker packaging, to Scott Harrison for suggesting workarounds for MacOS 10.2 browsers, to Morgan Burke for pointing out the problem with URL query strings, and to Slaven Razic for portability advice in spite of "use bytes".

SEE ALSO

examples/tea_demo.cgi, perldoc Encode, http://www.pjb.com.au/comp, CGI::Htauth.pm, tea(1), perl(1).