Cryptography · Interactive Exercises

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Core Concepts

5 questions

Cryptology splits into cryptography (defending information) and cryptanalysis (attacking it). Modern schemes are designed under Kerckhoffs's principle: assume attackers know every algorithm involved — only the short secret key stays hidden.

"Cryptology" is made up of two branches: cryptography (which defends) and ______ (which attacks).

1 mark

Cryptanalysis

Steganography

Digital forensics

Obfuscation

In the classic Alice / Bob / Eve / Mallory terminology, who represents an attacker who can only eavesdrop, not modify data?

1 mark

Alice

Bob

Eve

Mallory

Kerckhoffs's principle assumes attackers already know everything about a cryptosystem except:

1 mark

The ciphertext

The algorithm used

The secret key

The plaintext length

A classical substitution cipher maps 27 symbols (26 letters + a space character) to 27 symbols, giving 27! possible keys. Roughly how many bits are needed to represent one key (log₂(27!))?

1 mark

About 27 bits

About 94 bits

About 128 bits

About 256 bits

Why is exhaustive (brute-force) search infeasible against a 27!-key substitution cipher, per the lecture's back-of-envelope estimate?

1 mark

Computers cannot perform XOR operations

It would take roughly as long as the solar system has left to exist

The algorithm itself is a state secret

No computer can generate random numbers

Core Concepts score

0/5

🔤

Classical Ciphers & Attacks

3 questions

Substitution ciphers aren't secure against a determined attacker: a known plaintext reveals the key directly, and even a ciphertext-only attacker can exploit the fact that English letters (and letter pairs) occur with predictable frequency.

Classical substitution ciphers can be broken with only a ciphertext (no known plaintext) using:

1 mark

Frequency analysis of letters/letter-pairs

Quantum computing

SQL injection

The birthday paradox

According to the lecture, roughly how much ciphertext is "sufficiently long" for frequency analysis to reliably break a substitution cipher in English?

1 mark

5 characters

50 characters

500 characters

5000 characters

Given a known plaintext/ciphertext pair for a substitution cipher, an attacker can recover the key by:

1 mark

Running the AES key schedule backwards

Matching up corresponding plaintext and ciphertext letters directly

Guessing randomly until the checksum matches

Waiting for the algorithm to be leaked online

Classical Ciphers & Attacks score

0/3

🧊

Modern Ciphers & Modes

4 questions

AES is the workhorse modern block cipher — but a block cipher only encrypts one fixed-size block. Extending that to real (large) messages needs a mode of operation, and the choice of mode matters enormously for security.

Select all TRUE statements about AES (select all that apply):

2 marks

Block length is 128 bits

Key length can be 128, 192, or 256 bits

Proposed to NIST in 2000 through an open, worldwide selection process

Q10

Which block-cipher mode of operation is known for leaking plaintext patterns — e.g. an encrypted image's flat background stays visible — because identical plaintext blocks always produce identical ciphertext blocks?

1 mark

CTR

GCM

ECB

CBC

Q11

Among the AES modes mentioned in the lecture, which is described as the most secure, able to withstand attackers who can probe a "decryption oracle"?

1 mark

ECB

CTR

CBC

GCM

Q12

In CTR mode, what happens if the same key and the same IV (nonce) are mistakenly reused to encrypt two different messages, X and Y?

2 marks

The ciphertext becomes shorter

Nothing — CTR mode is immune to IV reuse

The AES algorithm simply refuses to run

XOR-ing the two ciphertexts cancels the shared keystream, leaking X⊕Y

Modern Ciphers & Modes score

0/6

⚠️

Pitfalls & Side Channels

5 questions

Modern ciphers like AES are believed mathematically secure — but real systems are broken through implementation mistakes, not by breaking the maths.

Q13

A key derived from an 8-character password (e.g. via SHA3), even when it feeds 128-bit AES, is realistically only as strong as:

1 mark

The much smaller password-guessing space

The full 2¹²⁸ AES keyspace

The SHA3 output size (256+ bits)

It cannot be attacked at all

Q14

"Security through obscurity" failed for RC4 and MIFARE Classic because:

1 mark

Their secret algorithms were eventually leaked or reverse-engineered, exposing real weaknesses

Both were open-source from day one

Both used quantum-resistant algorithms

Neither was ever deployed in production

Q15

A side-channel attack is best defined as one that:

1 mark

Exploits a mathematical weakness in the cipher's core algorithm

Requires directly breaking AES

Only works against quantum computers

Uses information (often physical — power, timing, sound) outside the cipher's abstract security model

Q16

Which of these are side-channel attack techniques mentioned in the lecture? (select all)

2 marks

Power analysis on a smartcard

Acoustic cryptanalysis (keystroke / 3D-printer sound)

SQL injection

Electromagnetic emissions from an LCD panel

Cache/page-fault timing on trusted hardware like Intel SGX

Q17

Which real CVE example from the lecture illustrates a "predictable key generation" pitfall?

1 mark

CVE-2020-7010: a Kubernetes cloud app generating passwords from deployment time

Heartbleed leaking OpenSSL memory

The BEAST attack on TLS 1.0

ZeroLogon's static AES-CFB8 IV

Pitfalls & Side Channels score

0/6

📜

History & Real-World Failures

4 questions

Cryptography has a long track record of ciphers that looked strong when introduced, then fell to advances in cryptanalysis or plain hardware progress.

Q18

DES, standardized in 1977 with a 56-bit key, was practically broken in 1998 by:

1 mark

Quantum computers

A side-channel power-analysis attack

A dedicated brute-force machine cracking a key in about 56 hours

Reverse-engineering the algorithm

Q19

RC4 became insecure in real deployments primarily because:

1 mark

It was mathematically broken from the very first day

WEP's use of RC4 with weak/reused IVs was exploited, and the industry moved to WPA2/AES

It required specialised hardware nobody owned

It was never actually implemented anywhere

Q20

During WWII, the Enigma machine's secret key was set physically using its:

1 mark

Keyboard layout alone

Power cable

Lampboard bulbs

Rotors and plugboard settings

Q21

Triple DES is still in limited use today with an effective key strength of about:

1 mark

56 bits

168 bits

112 bits

256 bits

History & Real-World Failures score

0/4

🗝️

Symmetric vs Asymmetric Keys

4 questions

Symmetric crypto uses one shared secret key to both encrypt and decrypt — fast, but both sides must somehow get that key safely first. Asymmetric (public-key) crypto uses a matched key pair: a public key anyone may have, which only locks (or verifies signatures), and a private key that never leaves its owner, which unlocks (or signs). It is much slower, so real protocols like TLS and SSH are hybrid: asymmetric crypto sets up a shared key, then symmetric crypto (AES, ChaCha20) protects the data.

Worked example: toy RSA with tiny numbers

Bob makes his keys. He picks two primes, 3 and 11, and multiplies them: n = 33. His public key is (e = 3, n = 33), which he gives to everyone. His private key is (d = 7, n = 33), which he keeps secret.
Alice encrypts the message 4 with Bob's public key: 4³ = 64, and 64 mod 33 = 31. She sends 31.
Bob decrypts with his private key: 31⁷ mod 33 = 4 ✓

mod means “the remainder after dividing”: 64 ÷ 33 = 1 remainder 31, so 64 mod 33 = 31. A clock works the same way — 15:00 is 3 o’clock because 15 mod 12 = 3. Eve sees 31 and knows the public key (3, 33). To find the private key 7 she must first factor 33 = 3 × 11 — trivial here, but real RSA uses an n about 600 digits long that nobody can factor in any practical time. That difficulty is what makes RSA secure. Try it in Python: pow(4, 3, 33) → 31, then pow(31, 7, 33) → 4.

Q22

In asymmetric (public-key) cryptography, a message encrypted with Bob's public key can be decrypted by:

1 mark

Anyone who has Bob's public key

Only Bob, using his matching private key

Anyone who knows which algorithm was used

Eve, by XOR-ing the ciphertext with the public key

Q23

Every cipher covered earlier in this session (the substitution cipher, Enigma, DES, Triple DES, RC4, AES) is:

1 mark

Asymmetric — each uses a public/private key pair

A hash function, not a cipher

A mix: the classical ones are symmetric, the modern ones asymmetric

Symmetric — the same secret key encrypts and decrypts

Q24

Why does HTTPS/TLS use both asymmetric and symmetric cryptography?

1 mark

Asymmetric crypto safely agrees on a shared session key; fast symmetric crypto (e.g. AES) then encrypts the actual data

Symmetric crypto exchanges the keys; asymmetric crypto encrypts the bulk data because it is faster

For redundancy — if one breaks, the other still protects the data

Browsers cannot run AES without a public key

Q25

When you log in to a server with an SSH key pair, which file must never leave your own machine?

1 mark

Your public key (id_ed25519.pub)

The server's authorized_keys file

Your private key (id_ed25519)

The known_hosts file

Symmetric vs Asymmetric Keys score

0/4

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