lecture_4

Modes of Operation
Block cipher: operates on fixed length b-bit input to produce b-bit ciphertext
What about encrypting plaintext longer than b bits?
Break plaintext into b-bit blocks (padding if necessary) and apply cipher on each block
Security issues arise: different modes of operation have been developed
Summary
Each block of 64 plaintext bits is encoded independently using same key
Typical applications: secure transmission of single values (e.g. encryption key)
Problem: with long message, repetition in plaintext may cause repetition in ciphertext
CBC Summary
Input to encryption algorithm is XOR of next 64-bits plaintext and preceding 64-bits ciphertext
Typical applications: General-purpose block-oriented transmission; authentication
Initialisation Vector (IV) must be known by sender/receiver, but secret from attacker
CFB Summary
Converts block cipher into stream cipher
No need to pad message to integral number of blocks
Operate in real-time: each character encrypted and transmitted immediately
Input processed s bits at a time
Preceding ciphertext used as input to cipher to produce pseudo-random output
XOR output with plaintext to produce ciphertext
Typical applications: General-purpose stream-oriented transmission; authentication