Feistel Cipher Example

This is a feistel cipher implementation I wrote for a Computerphile video. While I started with this being a simple couple of functions, I realised that if I wanted proper file IO, I needed to code up a mode of operation. It now encrypts files and outputs ciphertext as files, theoretically you could write your own functions to call the cipher itself, which is found in feistel.py. Feel free to edit, change, reuse the code for whatever you wish.

If you'd like to learn more about Feistel ciphers, please watch my video. There is also a detailed article on wikipedia here

Some examples of famous feistel ciphers are DES and Twofish

Usage

Encryption

python encrypt.py -e -m ECB input_file output_file

Decryption

python encrypt.py -d -m ECB input_file output_file

At the moment only ECB, CBC and CTR modes of operation are permitted.

Cool things that could be added

This cipher is a bit bland, it would benefit from:

• More modes of operation. Contributors have now added CBC and CTR, but there are other less common modes.
• Better handling of keys. Reading input is slow and mistake-prone, and keys are never long enough. Keys could be optionally read from an additional file.
• I'm sure the key schedule could be improved, but i'm tempted to leave it as that was what was used in the video.

Reasons not to use this cipher for real!

This cipher is just a demonstration, and for fun. Please don't use it on anything you actually need to secure. Here are some issues off the top of my head:

• It's slow, may not be a problem based on your use case, but noone is attempting to write fast ciphers in python anyway!
• It doesn't conveniently use 256-bit keys. Because it reads strings as an input, these are likely to be much too short to be secure. This will also encrypt using no key - bad!
• I've paid absolutely no attention to safe memory use, cache timings etc. So it's conceivable that the cipher is vulnerable while it's running - possibly after too since it doesn't wipe the key from memory.
• It defaults to ECB mode - use of this mode is generally not encouraged.
• It provides no message integrity since it doesn't use HMAC, GCM etc.