Copyright (c) Nick Payne 1996-1997

Cryptext v2.31

**IMPORTANT** This version of Cryptext is _not_ compatible with any version
of Cryptext prior to v2.30. If you have files you have encrypted with an earlier
version of Cryptext, you must decrypt them before installing the new version.
If you do not, you will not be able to decrypt these files without 
un-installing the new version and re-installing the previous version.

Cryptext is a freeware Windows 95 / NT4 shell extension that performs strong 
file encryption. It uses a combination of SHA-1 and RC4 to encrypt files using 
a 160-bit key. The current version of Cryptext can be found on my home page at 
http://www.pcug.org.au/~njpayne.

Cryptext may be used in any way, for any purpose, at no cost. It may be 
distributed by any means, provided that the original files as supplied by the 
author remain intact and that no charge is made other than for reasonable 
distribution costs. Note that Cryptext contains strong cryptographic routines 
upon which some countries place distribution and/or use restrictions. Verify 
that you are allowed to use or distribute Cryptext before doing so.

DISCLAIMER OF WARRANTY
THIS SOFTWARE AND DOCUMENTATION ARE PROVIDED "AS IS" AND WITHOUT WARRANTIES 
AS TO PERFORMANCE OF MERCHANTABILITY OR ANY OTHER WARRANTIES WHETHER EXPRESSED 
OR IMPLIED. BECAUSE OF THE VARIOUS HARDWARE AND SOFTWARE ENVIRONMENTS INTO 
WHICH THIS PROGRAM MAY BE PUT, NO WARRANTY OF FITNESS FOR PARTICULAR PURPOSE 
IS OFFERED. GOOD DATA PROCESSING PROCEDURE DICTATES THAT ANY PROGRAM BE 
THOROUGHLY TESTED WITH NON-CRITICAL DATA BEFORE RELYING ON IT. THE USER MUST 
ASSUME THE ENTIRE RISK OF USING THE PROGRAM.

Any product or brand names mentioned in this document are trademarks or 
registered trademarks of their respective owners.

To install Cryptext
===================
1. If you have a previous version of Cryptext installed, decrypt any 
   encrypted files.

2. Unzip the contents of CRYPTEXT.ZIP into a temporary directory

3. In Explorer, right-click on CRYPTEXT.INF and select Install from the popup 
   context menu. 

To install this shell extension on Windows NT you must be logged in as 
administrator or a user who is a member of the Administrators group. This is 
because installing a shell extension on NT requires update rights to a part of 
the registry to which a normal user cannot write.

To uninstall Cryptext
=====================
1. From the Start menu, select Settings, then Control Panel, then Add/Remove 
   Programs. 

2. Select the "Cryptext (Remove only)" entry.

3. Click on the Add/Remove button.

How does it work?
=================
1. When you install Cryptext it adds "Encrypt" and "Decrypt" items to the 
   context menu you get when right-clicking on files or directories in 
   Explorer.

2. When you encrypt a file, Cryptext takes your passphrase and uses the SHA-1 
   one-way hash function to generate a 160-bit key. 

3. For each file selected, it then concatenates the key from step 2 with a 
   salt value generated from (a) the number of 100-nanosecond intervals since 
   January 1, 1601, and (b) a 32-bit random number, and hashes this concatenation 
   with SHA-1 to produce the key which is used for the encryption. This step is 
   taken to ensure that no two files are encrypted with the same keystream.

4. The salt values from step 3 are stored in plaintext with the encrypted file, 
   so that the file can be successfully decrypted when the correct passphrase is 
   supplied. There is no requirement that these values be kept secret, only that 
   they be unique for each file.

5. In order to verify your passphrase on second and subsequent executions, 
   Cryptext takes the key generated in step 2, adds it to the end of your 
   passphrase, and applies the SHA-1 function to the concatenation of the 
   passphrase and key. The resultant hash is saved so that subsequent passphrases 
   can be checked for validity by being put through the same two-step hash and 
   compared with the stored value.

For decryption, Cryptext first reads the values saved in step 4 from the encrypted 
file and concatenates them with your hashed passphrase to obtain the decryption 
key.

Verification
============
To verify that Cryptext actually does what I claim, I have provided the source code 
and a Visual C++ make file to build a Win32 console application that uses SHA-1 and 
RC4 to decrypt files which have been encrypted with Cryptext. To verify:
1. Check the source code
2. Use the make file to build DECRYPT.EXE
3. Encrypt a file with Cryptext
4. Run DECRYPT.EXE in a DOS window to decrypt the file. You need to supply two command
   line parameters. The first is the passphrase; the second is the filename. e.g.
      DECRYPT " my pass phrase" c:\temp\test.txt.$#!
   Don't forget that a file encrypted with Cryptext will have the added extension "$#!"
   which is not shown in Explorer.

Notes
=====
1. If you are running on Windows NT 4 then you have the option to retain the 
   encryption password in memory for the duration of your NT session. If you 
   are running on Windows 95 you must enter the password for each encryption 
   or decryption.

2. After the initial execution of Cryptext, you cannot change your encryption 
   password unless you know the existing password. If you forget the password, 
   you have to uninstall and reinstall Cryptext. If you have files you have 
   encrypted and you have forgotten the password then those files are not 
   recoverable. 

3. Cryptext assumes that the file system on which it is running supports long 
   filenames. When it encrypts a file it adds the extension ".$#!" to the 
   filename. This name change will fail if the existing filename already 
   exceeds 252 characters or if the file system does not support long 
   filenames (such as a NetWare server volume which does not have long 
   namespace support loaded). The resulting file is still encrypted but it 
   does not have the extension which Cryptext recognises, and when you decrypt 
   it you will be queried whether the file is actually encrypted.

4. Password selection. Cryptext allows a password to be up to 255 characters 
   long. As a file encryption password is generally in use for much longer 
   than a login password, you should use more care selecting it. For more 
   information on good password selection, use one of the www search engines 
   to search for web documents containing "password", "selection", and "good".

5. My thanks to Gary Mulhern for showing how to add the registry entries to 
   get a meaningful icon and file type to display for encrypted files in 
   Explorer.

If you are interested in finding out more about encryption and cryptography:
a) RSA's web site at www.rsa.com has a good cryptography FAQ available both 
   online and as a downloadable PDF file
b) There is a lot of cryptographic source code at the ftp site
   idea.sec.dsi.unimi.it/pub/security/crypt/code
c) Bruce Schneier's book "Applied Cryptography" has a comprehensive coverage 
   of both protocols and algorithms
d) You can find an encryption library which provides a consistent interface
   to many encryption algorithms, as both source code and 16- and 32-bit DLLs,
   at http://www.cs.auckland.ac.nz/~pgut001/cryptlib.html.

Nick Payne
njpayne@pcug.org.au