1. Handshake Protocols
COEN 150

2. Threat Model Passive Sniffing Spoofing Standard Attack Modes:
Malicious Mallory can read messages between Alice and Bob.
Spoofing
Malicious Mallory can create messages that seem to come from either Alice or Bob.
Standard Attack Modes:
Breaking Cryptography
Man-in-the-Middle
Replay Attacks
Reflection Attack (Open several connections)

3. Simple Password Protocol
Alice: Hi, I am Alice. My password is “fiddlesticks”.
Bob: Welcome, Alice.
Vulnerable to sniffing and replay attack:
Mallory sniffs the exchange.
Mallory: Hi, I am Alice. My password is “fiddlesticks”.

4. Shared Secret Alice and Bob share a secret key K. Alice: I am Alice.
Bob: Encrypt random number R.
Alice: EK(R)
Bob (calculates EK(R) as well.):
Welcome Alice.

5. Shared Secret Vulnerable to DOS attack. while(1) {
Mallory: I am Alice.
Bob: Encrypt R.
Mallory: X.
Bob (EK(R) != X): Access denied. }

6. Shared Secret
Vulnerable to sniffing and replay attack if R is not random or if R is repeated.

7. Shared Secret Use of Clock: Problems with clock accuracy:
Alice: I am Alice, EK(clock).
Bob calculates clock, compares with his value: Welcome Alice.
Problems with clock accuracy:
Demand tight accuracy: drifting clocks can be bad.
Otherwise: replay attack.

8. Shared secret, use of clock
Sniffing + replay attack:
Mallory to Bob: KILL, KILL, KILL, KILL.
(Bob cannot receive messages because his message buffer is filled up.)
Alice: Hi, I’m Alice. EK(clock).
Mallory to Alice: KILL, KILL, KILL, KILL.
(Alice cannot receive messages because her message buffer is filled up.)
Mallory to Bob: Hi, I’m Alice. EK(clock).
Bob: Hi, Alice.

9. Shared secret, use of clock
Mallory
KILL
KILL
Hi, I’m Alice, EK(clock)
Hi, I’m Alice, EK(clock)
Bob
Alice

10. Public Key Alice: “I’m Alice.” Bob: “R”. Alice: “EAlice(R)”.
Bob calculates “DAliceEAlice(R) == R”: Hi Alice.

11. Public Key Alice: “I’m Alice.”
Bob creates random challenge R: “EAlice(R)”.
Alice: “R”.
Bob checks “R == R”: Hi Alice.

12. Public Key: DOS attack Bob spends much more time computing than Trudy!
Trudy: “I’m Alice.”
Bob: “R”.
Trudy: “X”
Bob calculates “DAliceEAlice(X) != R: Access Denied.
Bob spends much more time computing than Trudy!

13. Mutual Authentication: Shared Secret
Alice: “I am Alice”
Bob: “RB”
Alice: EK(RB). RA.
Bob calculates EK(RB) himself: EK(RA). Hi Alice.
Alice calculates EK(RA) herself: Hi Bob.

14. Mutual Authentication with less messages?
Alice: I am Alice. RA
Bob: RB. EK(RA).
Alice: Hi Bob. EK(RB).
Bob: Hi Alice.

15. Mutual Authentication with less steps is vulnerable to the reflection attack
Session 1 Trudy: I am Alice. RA.
Session 1 Bob: RB. EK(RA).
Session 2 Trudy: I am Alice. RB.
Session 2 Bob: RB’. EK(RB).
Session 1 Trudy: Hi Bob. EK(RB).
Session 1 Bob: Hi Alice.

16. Warning Signals Requestor should authenticate herself first.
Don’t have requestor and requestee do exactly the same thing. (E.g. use different key pairs.)
If you provide encryption service, you set yourself up for a key guessing attack.

17. Public Key: Simple Mutual Authentication
Alice: “I am Alice. RA”
Bob: “EBob(RA). RB”
Alice DBobEBob (RA)=RA: Hello Bob. EAlice(RB).
Bob: DAliceEAlice(RB) = RB: Hello Alice.

18. Key Distribution Centers
Maintains a shared secret for each registered user.
To set-up a connection requires the KDC to set up a session key.

19. Key Distribution Center Original Algorithm
Alice to KDC: Alice wants Bob.
KDC to Alice: Here is your session key.
KDC to Bob: Here is your session key.
This needs to be modified.

20. Key Distribution Center: Needham Schroeder Protocol
Alice to KDC: N1, Alice wants Bob.
KDC to Alice: KA(N1,KS,Bob,Ticket), where Ticket=KB(KS,Alice).
Alice to Bob: Ticket, KS(N2).
Bob to Alice: KS(N2-1,N3).
Alice to Bob: K(N3-1).
N1, N2, N3 are nonces to prevent replay attacks.

21. Key Distribution Center: Needham Schroeder Protocol Variant
Alice to KDC: N1, Alice wants Bob.
KDC to Alice: KA(N1,KS,Bob,Ticket), where Ticket=KB(KS,Alice).
Alice to Bob: Ticket, KS(N2).
Bob to Alice: KS(N2-1),KS(N3).
Alice to Bob: K(N3-1).
N1, N2, N3 are nonces to prevent replay attacks.

22. Replay attack on modified NS
Alice to KDC: N1, Alice wants Bob.
KDC to Alice: KA(N1,KS,Bob,Ticket), where Ticket=KB(KS,Alice).
Alice to Bob: Ticket, KS(N2).
Bob to Alice: KS(N2-1),KS(N3).
Alice to Bob: KS(N3-1).
Trudy as Alice to Bob: Ticket, KS(N2)
Bob to Alice, but intercepted by Trudy: KS(N2-1), KS(N4)
Trudy as Alice to Bob: Ticket, KS(N4).
Bob to Alice, but intercepted by Trudy. KS(N4-1), KS(N5).
Trudy as Alice to Bob: KS(N4-1).

23. Key Distribution Center
Assume that Alice’s key has become compromised.
Trudy can now present herself as Alice to Bob with an old ticket.
Tickets need to have an expiration date!!!!!!!!!!!