Lecture 3 & 4

Question 1

What’s the difference between secret key crypto and public key crypto?

Answer 1

Secret key: Symmetric key, same key used to encrypt and decrypt or to authenticate and verify.
Public key crypto: Asymmetric (two keys), One public to encrypt or verify and one private to decrypt or sign.

Question 2

What’s reduction to a known hard problem and what does it imply?

Answer 2

Proof that if an attacker can break the system, I can use that to solve the hard problem. It implies that if the problem remain hard, the system is secure

Question 3

What’s RSA?

Answer 3

A cryptosystem, it involves the use of a pair of keys: a public key for encryption and a private key for decryption. The security of RSA is based on the difficulty of factoring the product of two large prime numbers.

Question 4

What’s DH (Diffie Hellman)?

Answer 4

A key exchange algorithm used to securely share cryptographic keys over an untrusted communication channel. It enables two parties to independently generate a shared secret key without directly exchanging the key itself.

Question 5

Where’s RSA used?

Answer 5

TLS, SSH, GnuPG

Question 6

What’s DH secure against and not against?

Answer 6

Secure against passive eavesdropper
Not against active adversary

Question 7

Explain how DH works

Answer 7

1. Parties agree on publicly known parameters: a large prime number (p) and a primitive root modulo p (g).
   These parameters are shared openly and can be used by anyone.
2. Each party independently generates a private key (a for Party A and b for Party B). These keys are kept secret.
3. Compute public key:
   A = g^a mod p, B = g^b mod p
   Exchange private key
4. Ka= B^a = (g^b)^a=g^^ba=(g^a)^b=A^b=Kb

Question 8

What does PRNG and DRBG stand for?

Answer 8

PRNG: Pseudorandom number generator
DRBG: Deterministic random bit generator

Question 9

CSPRNG’s have many security notions, but one of them is significantly important. Name and describe the notion.

Answer 9

The next-bit test: If an adversary is given the “i” first bits of an output, he should not be able to guess the bit “i + 1” (next bit) with a higher chance than 50%.

Question 10

What is a signature a public-key version of?

Answer 10

A signature is a public key version of a MAC.

Question 11

What’s forward secrecy in DH?

Answer 11

That the compromise of a secret key does not compromise the secrecy of past communication sessions. In other words, even if a long-term private key is later exposed, it should not be possible to retroactively decrypt past communications that were secured using that key.

Question 12

What are signatures good for?

Answer 12

• Prove who (which signing key) signed a message (Authenticity)
• Ensures a message has not been tampered with (Integrity)
• The signer cannot deny having signed a message once signed (non-repudiation)

Question 13

What’s ephemeral Diffie Hellman and how does it relate to security?

Answer 13

DFE (Diffie Hellman Ephemeral) creates new temporary keys for a session. This helps with forwards secrecy, if a secret key is compromised it does not compromise the data of old conversations.