# Notes - Computer Security MT24, Stream ciphers

> Source: https://ollybritton.com/notes/uni/part-b/mt24/computer-security/notes/stream-ciphers/ · Updated: 2025-01-10 · Tags: uni, notes

- [Course - Computer Security MT24](https://ollybritton.com/notes/uni/part-b/mt24/computer-security/)
	- [Notes - Computer Security MT24, Symmetric key ciphers](https://ollybritton.com/notes/uni/part-b/mt24/computer-security/notes/symmetric-key-ciphers/)
	- [Notes - Computer Security MT24, Block modes](https://ollybritton.com/notes/uni/part-b/mt24/computer-security/notes/block-modes/)
	- [Notes - Computer Security MT24, RC4](https://ollybritton.com/notes/uni/part-b/mt24/computer-security/notes/rc4/)

### Flashcards
What is the idea behind stream ciphers?::

Attempt to simulate a perfectly secure one-time pad by using a cryptographic key to seed a 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 $I: \mathcal K \to \mathcal S$ 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

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