Classical Ciphers
Which cipher works by rearranging letter order instead of replacing letters?
Transposition
What's the most common letter in English text, and the usual starting point for frequency analysis?
E
What kind of cipher processes data continuously, one bit/byte at a time, instead of in fixed-size chunks?
Stream Cipher
What Python library did the class install to move from hand-built ciphers to real, industry-grade encryption?
Cryptography
Which hands-on activity used arithmetic across 3 rounds to let students build confusion and diffusion by hand?
Fiestel network
In a Caesar cipher with shift k = 4, what does 'T' become?
X
What does Kasiski examination help an attacker find?
The Vigenère key length
Which mode of operation is famous for leaking plaintext patterns?
ECB
What's it called when a system leaks a yes/no answer to a question an attacker shouldn't get to ask?
Oracle
Which activity used a "reverse + shift" toy rule to expose ECB-style determinism before AES was introduced?
Stamping Machine
What do we call a cipher that maps each plaintext letter to corresponsing fixed ciphertext letter throughout the message?
Monoalphabetic cipher
What do we call a guessed plaintext fragment used to help break a cipher?
Crib
What must a one-time pad key be, length-wise, to guarantee perfect secrecy?
At least as long as the plaintext
In the MetroPay case study, what actually went wrong?
ECB mode encrypted identical amounts identically
What's the name of the principle stating a cryptosystem should stay secure even if everything about it except the key is public?
Kerckhoffs's Principle
What do we call the original, unencrypted message before a cipher transforms it?
Plaintext
What do we call the exhaustive attack that tries every possible key, one by one, until the correct one is found?
Brute Force
In CTR mode, what one-time value gets combined with a counter before encryption, so the same plaintext block never produces the same ciphertext twice?
Nonce
What's the one-word term for the fatal mistake of using the same one-time pad key more than once?
Reuse
What's the one-word term for the property where a tiny change in input causes a large, unpredictable change in output?
Avalanche effect
A monoalphabetic substitution cipher has a keyspace of 26!, yet it's actually weak. Why?
Brute force is infeasible, but frequency analysis exploits the fact that English letters (E, T, A…) appear at predictable rates
A longer Vigenère keyword makes frequency analysis harder, but not impossible. Why not?
A longer key spreads each plaintext letter across more shift values, diluting single-letter frequency patterns. But Kasiski examination (or index of coincidence) can still recover the key length from repeated ciphertext fragments
What actually breaks if a nonce gets reused twice in CTR mode?
Both messages get XORed with the same keystream and XOR of plaintexts is exposed.
Why is it dangerous for a system to return a different error message for "wrong key" versus "invalid padding"?
That difference becomes a leaked oracle.
Why do we still teach century-old ciphers like Caesar and Vigenère when nobody protects real data with them today?
Understanding why classical ciphers fail is the fastest route to understanding what makes a cipher strong.