A stream cipher is basically a cryptographically secure RNG. It takes a key (and sometimes an IV or nonce) and generates an endless stream of random bytes.
Counter (CTR) mode
One popular way to make stream ciphers recently is to take a regular block cipher and just encrypting a counter with it.
Let’s say you have an encryption function $E(k, x)$ that takes a key $k$ and the plaintext block $x$, and returns the encrypted block.
Sponge functions
An easy way to get a stream cipher out of a sponge function is as follows.
- Absorb the IV
- Absorb the key
- Squeeze as many bytes as you want to encrypt the entire plaintext
Tags: Cryptography
