Public Key Encryption

Question 1

what keys are required for public key encryption?

Answer 1

a public and private key

Question 2

Give two examples of one-way functions (2)

Answer 2

1. computing the product of two large primes
2. modular exponentiation with a large prime modulus

Question 3

What is a trapdoor one-way function?

Answer 3

a function that is very easy to compute for every input, but very difficult to “invert” for a given output without a secret trapdoor, however, easy to invert given a secret trapdoor

Question 4

What algorithms are used in public key encryption? (2)

Answer 4

RSA & El Gamal

Question 5

Which numbers form the RSA public key

Answer 5

n and e

Question 6

What algorithm is used to compute d?

Answer 6

Extended Euclidean algorithm

Question 7

what two properties of RSA make it secure?

Answer 7

1. computational impossibility of decrypting a ciphertext without knowledge of private key
2. computational impossibility of determining the private key directly from the public key (need p & q which secret or need to factor n into its prime factors p & q)

Question 8

deterministic encryption is susceptible to which type of attack?

Answer 8

an informed exhaustive plaintext search

Question 9

what is hybrid encryption?

Answer 9

a cryptographic technique in which the strengths of symmetric key encryption (efficient and simplicity) and public key encryption (ease of key sharing) are combined

Question 10

Why do long plaintexts cause security issues for public key encryption?

Answer 10

To encrypt a longer plaintext, we would have to split the plaintext into separate blocks and then encrypt them separately (public key equivalent of ECB mode) which is not desirable due to deterministic encryption

Question 11

Disadvantages of RSA (2)

Answer 11

1. Large computational costs due to expensive encryption and decryption computations
2. Not suitable for long plaintexts

Question 12

How does hybrid encryption work? (5)

Answer 12

1. The sender generates a symmetric key and encrypts plaintext using this key
2. The sender encrypts the symmetric key using the recipient’s public key
3. Sender sends both the encrypted plaintext and the encrypted symmetric key
4. Recipient uses their private key to decrypt the encrypted symmetric key
5. Recipient uses the decrypted symmetric key to decrypt the ciphertext containing the plaintext message
6. The symmetric key can be used for subsequent messages