Understanding Cryptography by Christof Paar and Jan Pelzl - Chapter 2 Solutions - Ex2.3

- 1 min

Exercise 2.3

Assume an OTP-like encryption with a short key of 128 bit. This key is then being used periodically to encrypt large volumes of data. Describe how an attack works that breaks this scheme.


This solution is verified as correct by the official Solutions for Odd-Numbered Questions manual.

The main issue is that the key repeats every 128 bits. As such, the attacker may be able to exploit patterns in the underlying plaintext to recover the key. If a chosen plaintext attack is available then the attacker may be able to recover the entire key and then decrypt the rest of the message unhindered. In mathematical terms, the attack looks as follows (where is a keystream bit, is a chosen plaintext bit and is a ciphertext bit):

Thomas Busby

Thomas Busby

I write about computing stuff

comments powered by Disqus
rss facebook twitter github youtube mail spotify instagram linkedin google google-plus pinterest medium vimeo stackoverflow reddit quora