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Module 20 — Cryptography · Practice Questions

Original, concept-based questions — not exam dumps. Answers collapsed: decide first, then expand. Target ≥80%. Pairs with facts.md · flashcards.csv · README.md.

Q1. To send a message that only the recipient can read, you encrypt it with:

  • A. Your private key
  • B. Your public key
  • C. The recipient's public key
  • D. The recipient's private key
Answer **C. The recipient's public key.** Only their matching *private* key can decrypt it. (Signing, by contrast, uses **your private** key. Swapping these two is the most common crypto trap.)

Q2. A digital signature provides all of the following except:

  • A. Integrity
  • B. Authenticity
  • C. Non-repudiation
  • D. Confidentiality
Answer **D. Confidentiality.** A signature (hash encrypted with the signer's private key) proves who sent it and that it's unaltered, and prevents denial — but it does **not** hide the message. Encryption provides confidentiality.

Q3. Which algorithm performs key exchange only (no encryption or signing)?

  • A. RSA
  • B. Diffie-Hellman
  • C. AES
  • D. ECC
Answer **B. Diffie-Hellman.** DH securely agrees a shared secret over an untrusted channel but can't encrypt or sign. RSA does both; **DSA** signs only; AES is symmetric. Memorize the capability of each.

Q4. Which block-cipher mode should you never use because identical plaintext blocks produce identical ciphertext (the "penguin")?

  • A. GCM
  • B. CBC
  • C. CTR
  • D. ECB
Answer **D. ECB.** It leaks structure. Prefer **GCM** (authenticated: confidentiality + integrity). CBC is acceptable but is the padding-oracle target; CTR turns a block cipher into a stream.

Q5. What does adding a salt to a password hash defeat?

  • A. Brute-force attacks
  • B. Rainbow-table (precomputation) attacks
  • C. Side-channel attacks
  • D. Phishing
Answer **B. Rainbow-table attacks.** A unique per-password salt makes precomputed tables useless. Slowing brute force is the job of a **work factor** (bcrypt/scrypt/Argon2/PBKDF2), a separate property.

Q6. Which is the correct choice for storing user passwords?

  • A. MD5
  • B. SHA-256
  • C. Argon2 (or bcrypt/scrypt/PBKDF2)
  • D. Base64 encoding
Answer **C. Argon2 / bcrypt / scrypt / PBKDF2.** These are *slow, salted, tunable* KDFs. MD5 and SHA-256 are built to be **fast** — exactly wrong for passwords. Base64 is encoding, not hashing at all.

Q7. In the TLS handshake, asymmetric cryptography is used to ___ and symmetric to ___.

  • A. encrypt bulk data; exchange keys
  • B. exchange/agree a session key; encrypt bulk data
  • C. hash the message; sign the message
  • D. compress data; pad data
Answer **B.** TLS is **hybrid**: asymmetric (RSA/ECDHE) agrees the session key, then fast symmetric (AES-GCM) encrypts the bulk data. TLS 1.3 mandates ephemeral DH for **forward secrecy**.

Q8. The birthday attack is relevant to which primitive, and roughly what work does it require for an n-bit output?

  • A. Hash functions; ~2^(n/2)
  • B. Symmetric ciphers; 2^n
  • C. Asymmetric keys; 2^n
  • D. Passwords; n²
Answer **A. Hash functions; ~2^(n/2).** The birthday paradox means a collision is found in about the square root of the output space — why 128-bit hashes (MD5) are weak and 256-bit is preferred.

Q9. Which pair correctly matches the email-security technology to its trust model?

  • A. PGP → X.509 CA hierarchy; S/MIME → web of trust
  • B. PGP → web of trust; S/MIME → X.509 CA hierarchy
  • C. Both use a CA hierarchy
  • D. Both use a web of trust
Answer **B. PGP/GPG = web of trust; S/MIME = X.509 / CA (PKI).** A frequent matching question.

Q10. A padding-oracle attack specifically targets:

  • A. CBC mode
  • B. ECB mode
  • C. Hash functions
  • D. RSA key generation
Answer **A. CBC mode.** When a server reveals whether padding is valid, an attacker can decrypt ciphertext byte-by-byte. Authenticated modes (GCM) or encrypt-then-MAC prevent it.

Q11. DES is considered broken primarily because:

  • A. It uses a 56-bit key (too short for modern brute force)
  • B. It has no S-boxes
  • C. It is an asymmetric cipher
  • D. It cannot be implemented in hardware
Answer **A. 56-bit key.** DES's key space is exhaustible by modern hardware. 3DES extended it (legacy); **AES** (128/192/256-bit) is the standard replacement.

Q12. What is the difference between cryptography and steganography?

  • A. They are the same thing
  • B. Cryptography only works on images
  • C. Steganography is always stronger
  • D. Cryptography scrambles content; steganography hides the message's very existence
Answer **D.** Cryptography makes content unreadable (but visibly encrypted); steganography *conceals that a message exists* (e.g., LSB embedding in an image). Detecting hidden data is **steganalysis**; combine both for defense-in-depth.

Q13. Which algorithm is preferred for constrained devices (mobile/IoT) because it offers equivalent strength with smaller keys?

  • A. RSA
  • B. 3DES
  • C. ECC
  • D. DSA
Answer **C. ECC.** Elliptic-curve crypto achieves RSA-equivalent security with much smaller keys, reducing compute/power — ideal for mobile and IoT (ties to Modules 17 and 18).

Score yourself

  • 12–13: excellent — you have the pairings cold.
  • 9–11: re-drill the asymmetric-capability and public/private-key-direction facts.
  • < 9: re-read README.md and the facts.md master tables, then redo the lab-walkthrough.md.