Chip 1998 September/CHIP Eylül 1998.iso/freeware/Crypt01/crypt.exe/README.TXT

Text File | 1997-11-11 | 10KB | 201 lines

Copyright (c) Nick Payne 1996-1997

Cryptext v2.54

Overview
========
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 versions of Cryptext can always be found on my home
page at http://www.pcug.org.au/~njpayne, and are mirrored to the ftp site
ftp.funet.fi/pub/crypt/utilities/file.

This version of Cryptext is available in English, French, and German language
versions. The ZIP file for the English version is CRYPTEXT.ZIP, for the French
version CRYPTFRA.ZIP, and for the German version CRYPTDEU.ZIP. Only the Cryptext
program itself is language specific. The readme file and help file are still in
English for all versions.

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.

**IMPORTANT** This version of Cryptext implements a checksum to prevent
accidental decryption of a file with the wrong password, and is _not_
compatible with any version of Cryptext prior to v2.30. If you have files you
have encrypted with a version of Cryptext older than v2.30, you must decrypt
them before installing this 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.

Alternatively, you can use the console application DECRYPT.EXE, which is
included in the ZIP file, to decrypt any file encrypted with Cryptext version
2.0 or later. However, DECRYPT.EXE can only decrypt a single file at a time,
and makes no check that you are using the same password for decryption as you
used when encrypting the file.

If you are presently using Cryptext v1.x, you _must_ decrypt all encrypted
files before installing the new version of Cryptext, or you will have no way
of decrypting previously encrypted files.

New in this version
===================
The explanatory text in the initial password dialog was changed to give a better
explanation of what was happening with the user's encryption password.

Acknowledgements
================
Thanks to Juergen Schultze and Juergen Wulf for the German translations.

Thanks to Gary Mulhern for information on the registry entries needed to get
a meaningful icon and file type to display for encrypted files in Explorer.

To install Cryptext
===================
1. If you have a previous version of Cryptext older than v2.30 installed,
   decrypt any encrypted files.

2. Unzip the contents of CRYPTEXT.ZIP into a temporary directory. Due to a
   limitation of the Microsoft INF installation facility, you should not unzip
   the files into a directory with a long pathname. Unzip into a path that
   conforms to DOS 8.3 naming conventions (eg C:\TEMP).

3. In Explorer, right-click on CRYPTEXT.INF and select Install from the popup
   context menu. Depending on how your Explorer options are configured, you may
   just see the filename as Cryptext rather than CRYPTEXT.INF, but Explorer
   shows the file type as "Setup Information".

4. Once Cryptext is installed, you can delete the files you unzipped into the
   temporary directory, as they are no longer needed.

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 only an Administrator has update rights to the part of the registry
where shell extensions are registered.

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 file is then encrypted in 16Kb blocks. Each block is read, encrypted,
   and then written back to disk, so that when the encryption of the file is
   complete, the original file has been completely overwritten by the
   encrypted version.

5. 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.

6. 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 stored in the registry 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 5 from the
encrypted file and concatenates them with your hashed passphrase to obtain
the decryption key.

Verification
============
CRYPTEXT.ZIP contains the PGP signature of CRYPTEXT.DLL in the signature file
CRYPTEXT.SIG. If you have a copy of PGP version 5, you can use this together
with my public key to verify that CRYPTEXT.DLL has not been altered before it
reached you. My public key is available either from the keyserver
pgpkeys.mit.edu or from my web page at www.pcug.org.au/~njpayne/verify.html.

To verify that Cryptext actually does what I claim, CRYTPEXT.ZIP contains a
second zip file, DECRYPT.ZIP. This contains 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 Cryptext:
1. Extract the contents of DECRYPT.ZIP into a directory
2. Check the source code
3. Use the make file to build DECRYPT.EXE
4. Encrypt a file with Cryptext
5. 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 ".$#!" to the
   filename. This name change will fail if the existing filename already
   exceeds 252 characters or if the file system does not support long
   filenames (such as a NetWare server volume which does not have long
   namespace support loaded). The resulting file is still encrypted but it
   does not have the extension which Cryptext recognises, and you will not
   be able to decrypt it until you change the file extension to ".$#!".

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".

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
[e-mail]