This project shows how to extend PyCrypto with your own ciphers.

In PyCrypto the various ciphering modes are added to a block cipher in two steps. First the basic ciphering modes: CBC, CFB, OFB, CTR, these are added to a cipher in a C file: block_template.c. This only works for the ciphers bundled with PyCrypto.

The more complicated ciphering modes ( CCM, EAX, SIV, OPENPGP, GCM ) are added to a cipher by building upon the simple modes, in Crypto/Cipher/blockalgo.py

So if you want to use blockalgo to add CCM to your own cipher, you will have to make sure it supports the simple ciphering modes first.

You can do this by deriving from the _BlockCipher class offered here. It expects the user to implement only the encrypt_block and decrypt_block methods.

In order to add the more complex modes you can pass the above cipher as the factory class to a BlockAlgo derived object.

Three examples are provided:

• AESCipher : should behave exactly the same as Crypto.Cipher.AES - for testing.
• TandemCipher : A Cipher as used in GSMK cryptophone, XORring AES and Twofish.
• TwofishCipher: wraps the twofish module, adding PyCrypto ciphering modes.
• Modes.py: For the purpose of testing the CipherAdapter, several wrappers which add a ciphering mode to a cipher, using only it's ECB mode.

Note that in AESCipher and TandemCipher the new factory method is in the top level class, while in the TwofishCipher module the new method is a module level function. These are implemented differently showing two different ways of implementing a cipher.

• For CFB mode currently only segment_size=128 is supported.

Author: Willem Hengeveld [email protected]