Stream Cipher¶

links: Modern Cryptography MOC - Index

The output bits of a stream cipher are produced gradually and on demand, so that an application can request exactly as many pseudorandom bits as its needs.

Stream cipher with pseudorandom generator¶

A secure stream cipher without an IV is just a pseudorandom generator with a more flexible interface. - \(s\): seed/key - \(l\): length

\(G^l(s) := GetBits_1(Init(s), 1^l)\)

Stream cipher with pseudorandom function¶

Using different values for the \(IV\) should produce output streams that appear independently uniform. Practical stream ciphers typically do not support arbitrary values of \(n\) (which determines the length of the seed and the IV), but instead work only for some fixed values of \(n\).

\(F_s^l(IV) := GetBits_1(Init(s, IV), 1^l)\)

Example Output of a stream cipher

\(F_s(IV \parallel \langle 0 \rangle),F_s(IV \parallel \langle 1 \rangle), ...\)

Modes of Operation¶

Synchronized mode¶

Stateful encryption where the sender and receiver are required to maintain state between the encryption/decryption of different messages. Does not need to use an IV.

Unsynchronized mode¶

When a stream cipher does take an IV, it can be used to construct a stateless encryption scheme. The main advantage here is that the encryption scheme directly handles arbitrary-length messages.

links: Modern Cryptography MOC - Index