mxCrypto - Generic crypto wrapper type for Python

mxCrypto - Wrapping SSLeay's cryptographic algorithms

Interface : Ciphers : Hashes : Utils : Examples : Structure : Installation : Copyright : History : Home

Version 0.1.1

Introduction

A while ago I started getting interested in Andrew Kuchling's pycrypt package. This was mainly because I had some plans for implementing secure channels from a clients local server to the webserver. The connection going via HTTP through the firewalls to the server. Unfortunately, Andrew's package falls under the US ITAR export restrictions and he can only make the non-encrypting parts of the package available for export.

Since I want to use his package which I find very well structured, I had to find a way to fill in the blanks. Basically these three options were available:

Print the missing parts in the US, ship them to somewhere outside the US and scan them in again (this is how PGP gets exported). I tried that approach and it failed horribly.
Gather all the bits and pieces from different sources and reassemble them to work with his package structure. This would have caused serious support headaches.
Take an existing crypto lib and wrap it, e.g. SSLeay.

I decided to take the third approach. SSLeay is well supported, distributed world-wide and has a set of very fast implementations for most of the parts missing in Andrew's export version of pycrypt. So here goes: a wrapper for the ciphers and hash functions in Eric Young's fantastic SSLeay library.

Interface

The package is called CryptoWorld and includes these subpackages:

Ciphers,
Hashes and
Utils

It currently does not define an interface on its own.

Note that you can also access the ciphers and hash functions through Andrew's package if you have installed the package following the instructions given below. The documentation given here is merely provided to point out minor differences between pycrypt's interface and mxCrypto's.

Ciphers submodule

This subpackage wraps the cipher algorithms available in SSLeay in a way that is nearly 100% compatible with what Andrew has implemented in his package.

All ciphers are provided as objects with a common interface:

Cipher Constructors

These constructors are available in the Ciphers subpackage (DEFAULT_MODE and DEFAULT_IV are explained below):

RC2(key, mode=DEFAULT_MODE, IV=DEFAULT_IV)

Cipher Instance Methods

All cipher objects provide a similar interface. They define at least these methods:

encrypt(string): Returns the encrypted string. Block ciphers will generally expect the string to have a multiple of their blocksize as length while stream ciphers can handle arbitrary length strings. The internal state of the cipher object is updated according to the mode set at creation time.
decrypt(string): Returns the decrypted string. This is the inverse of the encrypt method.

The DES and DES3 objects also define this method:

isWeak(): Returns 1 iff the key given to the constructor falls into the class of weak (meaning more easily breakable) keys, 0 otherwise. The chances of picking a weak key are very low, so you might as well not check for them.

Cipher Instance Variables

Ciphers define these instance variables:

blocksize (readonly): Is 1 for stream ciphers and gives the blocksize for block ciphers (usually 8 bytes).
keysize (readonly): Is 0 for stream ciphers and gives the keysize for block ciphers (usually between 8 and 16 bytes).
mode (readonly): Mode the cipher operates in.
IV: Gives access to the initial vector used for the encryption. This may be written to while performing encryption to reset the cipher to a new initial state.

Constants

These constants are available:

ECB, CBC, CFB: Mode constants. See Andrew's documentation for more details.
DEFAULT_MODE = ECB: Default mode used when no mode is given on the constructors.
DEFAULT_IV: Default initial vector used when no IV is given on the constructors; it is set to '\0'*cipher.blocksize.

Hashes submodule

This subpackage wraps the hash algorithms available in SSLeay in a way that is 100% compatible with what Andrew has implemented in his package.

All hash algorithms are provided as objects with a common interface:

Hash Constructors

These constructors are available in the Hashes subpackage (DEFAULT_DATA is explained below):

MD2(string=DEFAULT_DATA)

Hash Instance Methods

All hash objects provide a similar interface. They define at least these methods:

update(string): Updates the internal state of the hash with the data given in string.
digest(): Returns a string of length digestsize (see below) that represents the current internal state of the hash algorithm. The state itself is not changed by calling this method.
hexdigest(): Same as digest() except that a 2-byte HEX version of the string is returned. See str2hex() for details on the format.
copy(): Returns a true copy of the object without changing the internal state.

Hash Instance Variables

Hashes define this instance variable:

digestsize (readonly): Gives the number of bytes you can expect when calling digest() on the hash object.

Constants

These constants are available:

DEFAULT_DATA = '': Default data string used when no initial data is given on the constructors.

Utils submodule

This subpackage provides some handy helper functions:

str2hex(string): Takes a string and converts each byte to a 2-byte lowercase HEX representation. The resulting string is twice as long as the original one.
hex2str(hexstring): Inverse of str2hex(). This function expects a HEX encoded string (2 HEX characters / byte). Case is not important. The result is returned as string.

Examples of Use

Here is a very simple one:

from CryptoWorld.Ciphers import RC4
from CryptoWorld.Utils import str2hex

c = RC4('MyKey123')
e = c.encrypt('Hello World!')
print 'Less readable:',str2hex(e)
c = RC4('MyKey123')
print 'More readable:',c.decrypt(e)

This should the following output:

Less readable: f166e053dd40552b97a9bc23
More readable: Hello World!

For more elaborate examples of how to use the ciphers and hash functions, have a look at the Examples/ subdirectory of the package.

It includes a script which let's you en/decrypt files of any size. (Use with care though: the script is not well tested yet.) To see all options run 'python enc.py -h'. The script shows how to deal with blocksizes, padding and writing cipher independant code.

Package Structure

[CryptoWorld]
       Doc/
       [mxCrypto]
              test.py
       Ciphers.py
       Hashes.py
       Utils.py

Entries enclosed in brackets are packages (i.e. they are directories that include a __init__.py file). Ones without brackets are just simple subdirectories that are not accessible via import.

The package imports all symbols from the extension module mxCrypto, so you only need to 'import CryptoWorld' to start working.

Installation

The package needs two other packages to be installed first:

Andrew Kuchling's export version of the pycrypt package version 1.1a2 or above and
SSLeay version 0.9.0 or above.

Next, download the archive (it's located on a German server), unzip it to a clean directory on your Python path and then follow these steps (assuming you have already installed Python):

Run make -f Makefile.pre.in boot in the mxCrypto directory
Edit Setup and fix the paths to SSLeay if necessary.
Run make
Unzip the included file pycrypt.zip in the Crypto installation directory of Andrew's export package and follow the instructions in Hash/README and possibly Cipher/README
Get Python 1.5 or above running, and execute test.py; it should not report any errors
Give me feedback :-)

Choosing a C++ compiler:

Since the wrapping code is written in C++ and uses exceptions you may run into trouble compiling it. Be sure to use the latest versions of your compiler (e.g. gcc 2.8.1 was reported to have no problems; the sparcworks CC fails to handle member constructors and gcc 2.7.2 doesn't handle exceptions). I'm using the latest egcs release without any difficulties. You can set the compiler by editing the Setup file.

Bug reports:

Please report any bugs or quirks, etc. directly to me.

What I'd like to hear from you...

Contributions of up-to-date compiled versions for WinXX are always welcome -- can't provide them myself, so I'm depending on some help from you...
Does the package compile out of the box for you ?

Copyrights, Disclaimer and Credits

Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee or royalty is hereby granted, provided that the above copyright notice appear in all copies and that both the copyright notice and this permission notice appear in supporting documentation or portions thereof, including modifications, that you make.

Note that your country's laws may restrict usage, copying and distribution of software that provides interfaces to data encryption algorithms.

THE AUTHOR MARC-ANDRE LEMBURG DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE !

To give you a quick overview of what the copyright conditions of the involved packages are I've copied the notices from latest versions I could find (they may be subject to change):

Andrew's pycrypt package (export version 1.1a2):

Distribute and use freely; there are no restrictions on further dissemination and usage except those imposed by the laws of your country of residence. This software is provided "as is" without warranty of fitness for use or suitability for any purpose, express or implied. Use at your own risk or not at all.

Note: The package also includes software written by third parties. See the included docs for credits and further copyright information. AFAIK, mxCrypto replaces most (if not all) code included in the export version that was not written or modified by Andrew Kuchling with versions provided by SSLeay.

Eric Young's SSLeay (version 0.9.0):

This package is an SSL implementation written by Eric Young (eay@cryptsoft.com). The implementation was written so as to conform with Netscapes SSL.

This library is free for commercial and non-commercial use as long as the following conditions are aheared to. The following conditions apply to all code found in this distribution, be it the RC4, RSA, lhash, DES, etc., code; not just the SSL code. The SSL documentation included with this distribution is covered by the same copyright terms except that the holder is Tim Hudson (tjh@cryptsoft.com).

Please note that MD2, MD5 and IDEA are publically available standards that contain sample implementations, I have re-coded them in my own way but there is nothing special about those implementations. The DES library is another matter :-).

Copyright remains Eric Young's, and as such any Copyright notices in the code are not to be removed. If this package is used in a product, Eric Young should be given attribution as the author of the parts of the library used. This can be in the form of a textual message at program startup or in documentation (online or textual) provided with the package.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
All advertising materials mentioning features or use of this software must display the following acknowledgement: "This product includes cryptographic software written by Eric Young (eay@cryptsoft.com)" The word 'cryptographic' can be left out if the rouines from the library being used are not cryptographic related :-).
If you include any Windows specific code (or a derivative thereof) from the apps directory (application code) you must include an acknowledgement: "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"

THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

The licence and distribution terms for any publically available version or derivative of this code cannot be changed. i.e. this code cannot simply be copied and put under another distribution licence [including the GNU Public Licence.]

The reason behind this being stated in this direct manner is past experience in code simply being copied and the attribution removed from it and then being distributed as part of other packages. This implementation was a non-trivial and unpaid effort.

History & Future

Things that still need to be done:

Wrap some more of SSLeay, e.g. RSA.
Add PGP mode to the ciphers.
Better integration with Andrew's work.
Fix bugs in the documentation...

Things that changed from 0.1.0 to 0.1.1:

Fixed some installation related bugs in the Setup.in file (this should make the module compile on Solaris).
Added fixed versions of the test scripts for Andrew's pycrypt.
Added an example of how to use the ciphers to encrypt files. The tool can be found in Examples/ and is called enc.py. Call it with '-h' parameter to see the options and read the source to find out how it works.

Version 0.1.0 was the intial release.