Preparation for implementing some cryptographic primitives in Python

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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:


Next install the PETLib package:

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.
   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.

Let me know how it goes.