Public Key Encryption

Question 1

What is RSA?

Answer 1

A public encryption algorithm, where the plaintext and ciphertext are integers between 0 and n-1

Question 2

n=pq. What are p and q?

Answer 2

Very large primes

Question 3

In RSA, what are: n, z, e, and d?

Answer 3

n = pq. z = (p-1)(q-1). e is smaller than z, and has no common factors with z. d is such that ed - 1 is divisible by z, or that (ed)mod(z) = 1.

Question 4

(n, e) or (n, d). Which is public and which is private?

Answer 4

(n, e) is public.

Question 5

How do you encrypt a message M using RSA?

Answer 5

C = M^e mod(n)

Question 6

How do you decrypt ciphertext c using RSA?

Answer 6

M = C^d mod(n)

Question 7

True or false? n must be kept secret in RSA, and so must d.

Answer 7

False. d must be kept private, but not n.

Question 8

True or false? For each pair p and q, there is only one pair e and d?

Answer 8

False.

Question 9

When using RSA keys for authentication, how is this done? Say for instance that Bob wants to authenticate a message which Alice has sent him.

Answer 9

If Alice just wants to authenticate her message, she can sign it with her private key. Bob can verify that Alice sent the message by checking the signature using Alice’s public key.

Question 10

How can RSA be beaten?

Answer 10

Brute force

Question 11

What is Diffie Hellman used for mainly?

Answer 11

Session key exchanges

Question 12

What principle is Diffie Hellman based on?

Answer 12

That is is very difficult to compute discrete logarithms

Question 13

If we send the message using Diffie Hellman: a^x mod(q), which part of this is secret information?

Answer 13

x is secret to the sender.