Week 3 Flashcards
What is Public Key Encryption?
Two keys - one for encryption (public), one for decryption (private).
Can derive public from private, but not the other way around.
Explain m^-1 mod n?
Multiplicative inverse mod n.
Denotes number m’ such that m * m’ = 1 mod n.
m^-1 exists if m and n share no common factors.
How does Diffie-Hellman work?
Uses two public parameters: generator g (often 160 bits long) and prime p (often 1024 bits long).
Two random numbers picked and find:
tA = g^rA mod p, tB = g^rB mod p
Exchange these numbers and key = g^rArB = tB^rA = tA^rB mod p
What are the benefits and drawbacks of Diffie-Hellman?
Benefit: Cannot work out rA and rB from tA and tB.
No keys shared at the start.
Drawback: Additional authentication required.
What is Elgamal?
Diffie-Hellman as a public key scheme, with fixed g and p.
rA picked as private key, tA = g^rA mod p as public key.
Message encrypted by choosing rB and sending (g^rB mod p, M * tA^rB mod p).
What is RSA?
Public key system, which carefully generates e, d and n.
n is the product of two large primes, have (m^e)^d = m mod n for every m.
Public key is (e,n) and private key is (d,n).
How does encryption and decryption work in RSA?
Encrypt m: c = m^e mod n
Decrypt c: m = c^d mod n
Security depends on difficulty of finding prime factors of n.
How does Public Key Crypto work?
Ekp(KS), {M}ks
Make new symmetric key.
Encrypt that with the public key.
Encrypt message with the symmetric key.
Explain signatures?
Using RSA: Epub(Dpriv(M)) = M
Sign message with private key, can be verified with public key.
What is a hash?
Short string generated from a message.
Any small change makes hash different.
What are hashes used for?
Verification of message download.
Tying parts of a message together.
Signatures.
Protect passwords.
What is a Preimage attack?
Find a message with a given hash - very hard.
What is a Collision attack?
Finding two messages with the same hash.
What is a Prefix Collision attack?
Attacker picks a message prefix?
Name an attack on MAC?
Add data without knowing key.
