Notes - Computer Security MT24, Stream ciphers
Flashcards
What is the idea behind stream ciphers?
Attempt to simulate a perfectly secure one-time pad by using a cryptographic key to a seed pseudorandom stream of bits, which is then XORed with the plaintext for encryption.
Stream ciphers are generally less secure than block ciphers but are still used. Why?
They are useful in low latency applications, since you don’t need to wait for a complete block for encryption and transmission.
Formally @define a stream cipher?
- Definitions
- A hidden state from a set of possible states $s \in \mathcal S$
- An initialisation function $I: \mathcal K \to S$ which constructs an initial state from a key
- An update function $U : \mathcal S \to \mathcal S$ which updates the state
- An output function $f : \mathcal S \to \Sigma$ where $\Sigma$ is the alphabet for the stream cipher
- Operation
- Every time the output function is called to request a new symbol from the stream, the state is updated