Score Card

What leaked, the attack that read it, and what the attacker had to hold, side by side for every cipher above.

CryptanalysisComparisondemo

Objectives

Match each classical cipher to the structure its attack exploited.
Rank the attacks by what the adversary needed, from one ciphertext to known plaintext.
Trace each classical failure to the modern design rule that answers it.

50%

Cipher	What leaked	Attack	Attacker needs
Caesar	tiny keyspace	try all 25 shifts	one ciphertext
Monoalphabetic	letter frequency	frequency, hill climbing	about 100 letters
Playfair	digraph frequency	digraph analysis	about 300 pairs
Hill (2×2)	linearity	K = C·P⁻¹ mod 26	two known blocks
Vigenère	a repeating key	find the period, then χ²	about 300 letters
Autokey	the key is plaintext	crib cascade	one right guess
One-time pad	only if reused	crib drag on P₁ ⊕ P₂	two ciphertexts
Transposition	counts unchanged	bigram score over orderings	one ciphertext
Product	counts still intact	search rails, climb letters	about 100 letters

Not one of these is a brute-force attack. Every row is structure the cipher left in its own output, and a longer key removes none of it.

Every break became a design rule

Classical failure	What modern ciphers do about it
A keyspace you can enumerate	keys of 128 bits and up
Statistics survive encryption	diffusion across the whole block
The cipher is linear	a non-linear round step: the S-box
Same input, same output	modes with an IV, and no raw ECB
A keystream used twice	nonce discipline and rekeying
Ciphertext modifiable in silence	a MAC, or an authenticated mode
The key shared in advance	public-key exchange
A secret algorithm	public standards, open competition

You have now seen why each row on the right exists. None of it was invented in the abstract: every line is a classical failure someone had already paid for.

step 1/2

CPE 645 / CS 685

Score Card

Steganalysis