CS338 Computer Security
Wednesday, 27 September 2023

+ Eve debrief part 1: big integers
- Try (65426 ** 119537) in python
- Try (65426 ** 119537) % 170171 in python
- ???
- Didn't need it, because python is smart, but here's one approach...

def mod_exponentiate(a, b, n):
    ''' returns (a ** b) mod n '''
    assert b >= 0
    if b == 0:
        result = 1
    elif b == 1:
        result = a % n
    elif b % 2 == 0:
        k = mod_exponentiate(a, b // 2, n)
        result = (k * k) % n
    else:
        k = mod_exponentiate(a, b // 2, n)
        result = (k * k * a) % n

    return result

+ Eve debrief part 2: integers, bytes, hexadecimal

+ Eve debrief part 3: shared secret
- Suppose Alice & Bob have performed a DH exchange and have secret K
    - Can they use K as an AES key?
    - Is Alice confident she's talking to Bob?
    - Is Bob confident he's talking to Alice?
    - If any of these answers is no, now what?

+ Questions

====

+ Digital signatures
- How I can prove I have the secret key S that goes
    with that public key P 
- How does that fit with hashes?

+ Next assignment: the pragmatics of keys
- Scenario: you want to SSH to mantis without a password
    - What does the Alice (your ssh command)
        to Bob (the ssh server on mantis)
        conversation need to include?
- How does the relevant stuff get stored?

+ Next: PKI and X.509 certificates