1. Authentication System

2. Bob can not “see” Alice, so Trudy simply declares
Introduction
Authentication is the process of reliably verifying the identity of someone (or something).
in a network,
Bob can not “see” Alice, so Trudy simply declares
herself to be Alice
“I am Alice”

3. Kinds of Authentication
Password-Based Authentication
Symmetric key Based Authentication
Public-Key Based Authentication

4. Password-Based Authentication
Attaining the benefits of cryptographic authentication with the user being able to remember passwords only
Problems of password protocols:
Eavesdropping
Password guessing attack
On-line password guessing
Off-line password guessing

6. Encrypted Key Exchange (EKE)
Key establishment as well as authentication
One of the W{.} may possibly be removed.
In that case, the non-encrypting side should not issue the first challenge. (Why not?)
Alice
Bob
“Alice”, W{EA}
W{EA{KAB}}
KAB{CA}
KAB{CA, CB}
KAB{CB}

7. EKE with Diffie-Hellman
Why are ga, gb encrypted?
(authentication)
Alice
Bob
“Alice”, W{ga mod p}
(KAB = gab mod p)
KAB{CA, CB}
KAB{CA}
W{gb mod p}, CB

8. Augmented EKE
EKE vulnerable to database disclosure (since server has to store W)
Augmented EKE: defense against this threat
Client has to know the password. Server stores a one-way derivation of it.

9. Augmented EKE with Diffie-Hellman
Server stores gW mod p
How does this protocol protect against database disclosure?
Why is this protocol not secure?
Alice
Bob
“ Alice”, ga mod p
gb mod p, H(gab mod p, gbW mod p)
H’(gab mod p, gbW mod p)

10. Secure Remote Password (SRP)
Secure Remote Password Protocol
Thomas Wu
Notation

11. Protocol
To establish a password P with Steve, Carol picks a random salt s, and computes .
Carol
Steve

12. Off- vs. On-Line Password Guessing
On-line password guessing attack:
Type passwords at the system that is going to verify the password.
The system can make it impossible to guess too many passwords in this manner. Ex: ATM.
The system can be designed to be slow, so as not to allow very many guesses per unit time.

13. Off-line password guessing attack:
Dictionary attack
An attacker guesses a password and verifies his guess off-line.
If his guess fails the attacker tries again with another password, until he finds the proper one.

14. Symmetric key based Authentication
Both Entity using a secret key k, |k|>64
Not need CA.
Compare with PASSWORD BASE
user need not to input password
More faster
Must store KEY
More security in theory
In 1994, Bellare, Rogaway present Entity Authencation and Key Distribution
The MAP1, and AKEP1 are more important.
MAP1 has Matural Authentcation
AKEP1 add the part of Key Exchange

15. MAP1 Suppose the entities Alice and Bob share a secret key . AliceRA
[B, A, RA, RB]
[A, RB] 

16. AKEP1

17. we assume the entities A and B share secret keys  and .
 is the session agreed after the protocol.
Alice
Bob RA
[B, A, RA, RB, {} ]
[A, RB] 