Stream Ciphers

Stream Ciphers

1. What Is a Stream Cipher? (beginner — bit-by-bit!)

Stream cipher = encrypts ONE bit/byte at a time with a pseudorandom keystream ($c = m$ XOR $k$!). Opposite of block ciphers (64-bit chunks!). Fast in hardware, no buffering — perfect for calls/video! Ideal = one-time pad (random key = message length!); real = short key stretched to long keystream!

2. Synchronous vs Self-Synchronizing (exam table!)

	Synchronous	Self-synchronizing
Keystream from	key + state only (NOT data!)	key + past CIPHERTEXT!
Sync loss	fatal (must resync markers!)	auto-heals after $t$ digits!
Error spread	none (1 bad bit = 1 bad bit!)	$t$ bits garbled, then OK!
Example	RC4, ChaCha20, OFB mode!	CFB mode!

3. LFSR Engine (exam hardware!)

Linear-Feedback Shift Register: bits shift right, new bit = XOR of taps!
4-bit example taps (3,0): 1000 → 0100 → 0010 → ... period up to 2⁴-1 = 15!
Pure LFSR = LINEAR (Berlekamp-Massey cracks from 2L bits!) → harden:
combine LFSRs (Geffe!), nonlinear filter, irregular clocking (A5/1!), or ditch LFSR (ChaCha ARX!)

4. Golden Rules (traps!)

• NEVER reuse key+nonce (two-time pad: $c₁$ XOR $c₂$ = $m₁$ XOR $m₂$ — reads both!).
• Stream = privacy ONLY (no integrity! flip bit → flips plaintext bit! add MAC!).
• Need sync markers/nonces per message!

5. Exam Points

• Stream vs block + sync table — 5-7 marks, very frequent.
• LFSR idea + why harden — 3-5 marks, frequent.
• Never-reuse rule — 2-3 marks, evergreen.