Difference between revisions of "Preparation for implementing some cryptographic primitives in Python"

Revision as of 14:36, 1 September 2017

Later on in the semester we will be implementing some cryptographic primitives in Python. Want to see if you need to do some preparatory work? Here is a good practice task to use as a gauge.

First, set up a virtualenv or conda environment. For conda:

  https://conda.io/docs/user-guide/tasks/manage-environments.html

Next install the PETLib package:

  https://petlib.readthedocs.io/en/latest/index.html#quick-install

Once you have petlib installed and have passed the unit tests, a mini-example you can try is to implement symmetric encryption using AES-GCM. You don't really need knowledge of AES-GCM to do this; it's all about using PETLib effectively.

• Note: don't look at the official documentation for quick_gcm_enc until you're ready to check your work, as it contains the answer as an example. Here is the documentation for quick_gcm_enc that you will need, sans example:

 quick_gcm_enc(key, iv, msg, assoc=None, tagl=16)[source]
   One operation GCM encryption.
 Args:
   key (str): the AES symmetric key. Length depends on block cipher choice. iv (str): an Initialization Vector of up to the block size. (Can be shorter.) 
   msg (str): the message encrypt. 
   assoc (str): associated data that will be integrity protected, but not encrypted. 
   tagl (int): the length of the tag, up to the block length.

• Consider these imports:

   from os import urandom 
   from petlib.cipher import Cipher

• Note that urandom produces cryptographically strong bytes, which is handy for keys and ivs.

• Use the encoded plaintext rather than the input directly (you can encypt bytes not unicode strings, hence the need for encoding and decoding with UTF8 first).

• The documentation for petlib.cipher is available here.

• Once you've given this a try, you can check your work with the docs by looking at the quick_gcm_enc example

Let me know how it goes.