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Cryptographic modules for Python.

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Python Cryptography Toolkit (pycrypto)

This is a collection of both secure hash functions (such as SHA256 and RIPEMD160), and various encryption algorithms (AES, DES, RSA, ElGamal, etc.). The package is structured to make adding new modules easy. This section is essentially complete, and the software interface will almost certainly not change in an incompatible way in the future; all that remains to be done is to fix any bugs that show up. If you encounter a bug, please report it in the Launchpad bug tracker at

An example usage of the SHA256 module is:

>>> from Crypto.Hash import SHA256
>>> hash =
>>> hash.update('message')
>>> hash.digest()

An example usage of an encryption algorithm (AES, in this case) is:

>>> from Crypto.Cipher import AES
>>> obj ='This is a key123', AES.MODE_CBC, 'This is an IV456')
>>> message = "The answer is no"
>>> ciphertext = obj.encrypt(message)
>>> ciphertext
>>> obj2 ='This is a key123', AES.MODE_CBC, 'This is an IV456')
>>> obj2.decrypt(ciphertext)
'The answer is no'

One possible application of the modules is writing secure administration tools. Another application is in writing daemons and servers. Clients and servers can encrypt the data being exchanged and mutually authenticate themselves; daemons can encrypt private data for added security. Python also provides a pleasant framework for prototyping and experimentation with cryptographic algorithms; thanks to its arbitrary-length integers, public key algorithms are easily implemented.

As of PyCrypto 2.1.0, PyCrypto provides an easy-to-use random number generator:

>>> from Crypto import Random
>>> rndfile =

A stronger version of Python’s standard “random” module is also provided:

>>> from Crypto.Random import random
>>> random.choice(['dogs', 'cats', 'bears'])

Caveat: For the random number generator to work correctly, you must call Random.atfork() in both the parent and child processes after using os.fork()


PyCrypto is written and tested using Python version 2.1 through 3.3. Python 1.5.2 is not supported.

The modules are packaged using the Distutils, so you can simply run “python build” to build the package, and “python install” to install it.

If the script crashes with a DistutilsPlatformError complaining that the file /usr/lib/python2.2/config/Makefile doesn’t exist, this means that the files needed for compiling new Python modules aren’t installed on your system. Red Hat users often run into this because they don’t have the python2-devel RPM installed. The fix is to simply install the requisite RPM. On Debian/Ubuntu, you need the python-dev package.

To verify that everything is in order, run “python test”. It will test all the cryptographic modules, skipping ones that aren’t available. If the test script reports an error on your machine, please report the bug using the bug tracker (URL given above). If possible, track down the bug and include a patch that fixes it, provided that you are able to meet the eligibility requirements at

It is possible to test a single sub-package or a single module only, for instance when you investigate why certain tests fail and don’t want to run the whole suite each time. Use “python test –module=name”, where ‘name’ is either a sub-package (Cipher, PublicKey, etc) or a module (Cipher.DES, PublicKey.RSA, etc). To further cut test coverage, pass also the option “–skip-slow-tests”.

To install the package under the site-packages directory of your Python installation, run “python install”.

If you have any comments, corrections, or improvements for this package, please report them to our mailing list, accessible via the PyCrypto website:

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