4 - Public Key Cryptography

Question 1

Who first introduced the idea of Public key crypto?

Answer 1

Diffe & Hellman 1976/77

RSA 1977 still used

Question 2

Public-key crypto relies on operations that are…

Answer 2

easy to do but hard to “undo”

Question 3

RSA is based on the difficulty of…

Answer 3

factoring (easy to multiply, hard to factor)

Question 4

Steps of public key

Answer 4

Each person has public/private
Alice -> Bob, encrypt with Bob public key and Bob decrypt with own private key

Question 5

Possible attack, specific to pub0key

Answer 5

When there are not many possible plaintext,

attacker can encrypt all possible plaintext using Bob pub key and see what they intercept then match.

Question 6

Setting up RSA Keys

Answer 6

p, q are large primes (150 digits plus)
n is p * q
r is (p-1)(q-1)

e is chosen between 2 and r-1 such that gcd(e,r) = 1

d such that ed mod r =1

Pub = (e,n)
Priv = (d,p,q)

Question 7

Out of e,n,d,p,q, which of these are allocated to public/private key?

Answer 7

pub = (e,n)
priv = (d,p,q)

Question 8

RSA - Plaintext and ciphertexts are integers between …

Answer 8

0 and n-1

Question 9

To send message m to Bob (RSA), it should be encrypted as…

Answer 9

c = m^e mod n

e and n are from RSA steps

Question 10

TO decrypt a message (RSA), Bob uses…

Answer 10

c^d mod n

d and n are from RSA steps

Question 11

RSA’s current main use

Answer 11

Digital signatures

Not encryption or key exchange