3- Public-key encryption

Question 1

Requirements for public-key encryption

Answer 1

1- easy generation of a pair of keys
2- easy to encrypt
3- easy to decrypt
4- computationally infeasible to decrypt knowing public key and ctext

Question 2

Public key process

Answer 2

1- users generate public and private keys
2- alice encrypts her message with bob’s public key
3- bob decrypts using bob’s private key

Question 3

RSA Enc.

Answer 3

C = P^e mod n

Question 4

RSA Dec.

Answer 4

P = C^d mod n

Question 5

Selecting RSA keys

Answer 5

1- n: choose prime p&q, n=pq
2- e: relatively prime to
(p-1)(q-1)
3- d: solve equation [ed=1 mod n]

Question 6

RSA security

Answer 6

1- impossible to decrypt w/o private key
2- impossible to derive private key from public info

Question 7

RSA analogues

Answer 7

elliptic curve, el gamal

Question 8

Problems for public-key Enc.

Answer 8

1- high computational cost
2- security issues for long ptext: Enc. of blocs with the same key = ECB

Question 9

Hybrid encryption

Answer 9

1- use public key to establish symmetric keys
2- use symmetric encryption