1. Single Password, Multiple Accounts
Mohamed G. Gouda, Alex X. Liu, Lok M. Leung, Mohamed A. Alam Department of Computer Sciences The University of Texas at Austin June, 2005

2. Multiple Accounts Most users have multiple accounts on Internet
Bank:
Travel:
Each account requires a password
Insecure common practice: same password for all accounts
To steal someone’s password, attackers can:
set up a malicious server, or
break into a low security server

3. Single Password Protocol (SPP)
Allow a server to authenticate a client
(without server knowing the client’s password at any time)
Can counter the following:
Malicious server attacks
Password file attacks
Message log attacks
Server spoofing attacks

4. SPP Version 1 Currently used in HTTP
Communication is encrypted using session key (SSL)
Vulnerable to malicious server attacks
C knows P
S stores MD(P)
C  S: C, P

5. SPP Version 2 Use challenge/response
Vulnerable to password file attacks
C knows P
S stores n, MD(n|P)
C  S: C
C  S: n
C  S: MD(n|P)

6. SPP Version 3 Vulnerable to message log file attacks C knows P
S stores n, MD2(n|P)
C  S: C
C  S: n
C  S: MD(n|P)

7. SPP Version 4 Vulnerable to server spoofing attacks C knows P
S stores ni, MD2(ni|P)
C  S: C
C  S: ni
C  S: MD(ni|P), ni+1, MD2(ni+1|P)

8. Server Spoofing Attacks
Malicious server S knows: ni, MD2(ni|P)
Benign server S’ knows: mi’, MD2(mi|P)
C S S’ C C mi mi
MD(mi|P), mi+1, MD2(mi+1|P)
MD(mi|P), mi+1, MD2(mi+1|P)

9. Final Version SPP Two techniques: C knows P S stores ni, MD2(ni|S|P)
Challenge/Response
One-time server-specific tickets
C knows P
S stores ni, MD2(ni|S|P)
C  S: C
C  S: ni
C  S: MD(ni|S|P), ni+1, MD2(ni+1|S|P)

10. Related Work: One-Time Password Protocols
Use different password for each authentication
Protocols: [Lamport 81] [Rubin 95]
Motivation: prevent eavesdropping
Invented before SSL

11. Related Work: Strong Password Protocols
Strong security properties
Protocols: [Bellovin 92-EKE] [Wu 98-SRP]…
Motivation
Establish a session key (SPP uses SSL)
Prevent dictionary attacks (SPP uses single strong password)
Computational intensive (Not suitable for web)
modular exponentiations,
asymmetric encryptions/decryptions

12. Related Work: Single Sign-on Protocols
Use one central server to authenticates clients for multiple servers. Thus one password/user.
Protocols: Microsoft Password Protocol
Disadvantages:
Single point of failure
Lacks of wide deployment
High incentive for attackers

13. Conclusions
Single Password Protocol (SPP) is
Simple
Efficient
Secure