1. Strong Password Protocols, Firewalls
Network Security
Design Fundamentals
ET-IDA-082
Lecture-22
Network Defense
Strong Password Protocols, Firewalls
, v07
Prof. W. Adi

2. Outlines Firewalls Types and applications Strong Password Protocols
Lamport´s Hash
Strong Protocols
Firewalls
Types and applications

3. Password Schemes Strong Password Protocols
There are many different electronic devices for e-payment system. Different banks may be concerted in e-payment and the financial network is neccessary. E-payment flatform is built connecting the financial network and other open network, where the electronic devices can communicate with the flatform. PC is the most common device. Other devices include mobile devices, e.g. laptop, PDA, mobile telephone, ATM(Automatic Teller Machine), POS(Position of Sale), telephone and terminal. The electronic devices can connect the e-payment flatform using different open network.

4. Challenge-Response Authentication
User, system share a secret function F
(in practice, f is a known function with unknown parameters,
such as a cryptographic key Ks)
A random source generates r as a challenge
Secret key ks r F
F(r)
request to authenticate
system
user
user
random message r
(the challenge)
system
F(r)
(the response)
user
system

5. Pass Algorithms
Challenge-response with the function F itself as secret
Example:
Challenge is a random string of characters such as “abcdefg”, “ageksido”
Response is some function of that string such as “ bdf” , “gkio” (each second letter is selected)
Can alter algorithm based on alternative selections
Network connection is as above, dial-up might require “aceg”, “aesd”
Usually deployed in conjunction with fixed, reusable password

6. One-Time Passwords
One-Time Password: Password that can be used exactly once After use, it is immediately invalidated
Use strategies:
1- Any One-Time Password out of a shared list (Banking TAN)
Challenge: give a valid authenticated passwords from a securely shared list (PINS) p1 p2 p3 ….. pn
Response is any password for the list used only once
2- One-Time Password selected from a shred list (Selected Banking TAN)
Challenge: is a serial number i for a password from the
shared authenticated list p1 p2 p3 … pi .. pn
Response is password for a particular random selection pi
Problems
Synchronization of user, system
Generation of good random passwords
Password distribution problem

7. S/Key (TM Bellcore 1980) h(p2) = p1 h(pi) = pi-1
One-time password scheme based on idea of Lamport (1981)
h is a one-way hash function (MD5 or SHA-1, for example)
User chooses initial seed k0
System calculates:
h(k0) = k1
h(k1) = k2
h(k2) = k3 …
h(kn–2) = kn–1
h(kn–1) = kn pn
pn-1
pn-2 … p2 p1 h
ki+1 ki
Initialize with k0 at t=0
h(p2) = p1
h(pi) = pi-1
Passwords pi ‘s are recalled in the reversed order

8. S/Key Protocol
System stores maximum number of authentications n, number
of next authentication i, last correctly supplied password pi–1.
{ name }
{ i }
{ pi }
system
user
system
user
h(pi) = pi-1
user
system
System computes h(pi) = h(kn–i+1) = kn–i = pi-1.
If match with what is stored, system replaces pi–1 with pi and increments i.

9. Next used password in revesred order
Source Wikipedia

10. Compare password to Hn-1 password
Compare password to Hn-1 password. If equal, n authen-tication is successfull.
Store password for the n-1 future reference
Next
password
Source Wikipedia

11. Hardware Support Token-based Temporally-based
Used to compute response to challenge
May encipher or hash challenge
May require PIN from user
Temporally-based
Every minute (or so) different number shown
Computer knows what number to expect when
User enters number and fixed password

12. C-R and Dictionary Attacks
Same as for fixed passwords
Attacker knows challenge r and response f(r); if f encryption function, can try different keys
May only need to know form of response; attacker can tell if guess correct by looking to see if deciphered object is of right form
Example: Kerberos Version 4 used DES, but keys had 20 bits of randomness; Purdue attackers guessed keys quickly because deciphered tickets had a fixed set of bits in some locations

13. Encrypted Key Exchange
Defeats off-line dictionary attacks
Idea: random challenges enciphered, so attacker cannot verify correct decipherment of challenge
Assume Alice, Bob share secret password s
In what follows, Alice needs to generate a random public key p and a corresponding private key q
Also, k is a randomly generated session key, and RA and RB are random challenges

14. EKE: Encrypted Key Exchange Protocol
( Starting with W as a weak secret password between Alice and Bob ) and E is a cipher
Alice || Ew( ga mod p)
Alice
Bob
Bob || Ew( gb mod p)
Alice
Bob
Now Alice, Bob share a randomly generated
secret Diffie-Hellman session key k = gab mod p
Ek(RA)
Alice
Bob
Ek(RARB)
Alice
Bob
Ek(RB)
Alice
Bob

15. Biometrics
Automated measurement of biological, behavioral features that identify a person
Fingerprints: optical or electrical techniques
Maps fingerprint into a graph, then compares with database
Measurements not exact, so approximate matching algorithms used
Voices: speaker verification or recognition
Verification: uses statistical techniques to test hypothesis that speaker is who is claimed (speaker dependent)
Recognition: checks content of answers (speaker independent)

16. Other Characteristics
Can use several other characteristics
Eyes: patterns in irises unique
Measure patterns, determine if differences are random; or correlate images using statistical tests
Faces: image, or specific characteristics like distance from nose to chin
Lighting, view of face, other noise can hinder this
Keystroke dynamics: believed to be unique
Keystroke intervals, pressure, duration of stroke, where key is struck
Statistical tests used
Cautions: Known patters can be optically attacked by copying!

17. Location
If you know where user is, validate identity by seeing if person is where the user is
Requires special-purpose hardware to locate user
GPS (global positioning system) device gives location signature of entity
Host uses LSS (location signature sensor) to get signature for entity

18. Multiple Methods
Example: “where you are” also requires entity to have LSS (Location Signature Sensor) and/or GPS, so also “which means you have?”
Can assign different methods to different tasks
As users perform more and more sensitive tasks, must authenticate in a variety of ways
includes controls on access (time of day, etc.), resources, and requests to change passwords
Pluggable Authentication Modules (Physical Security)

19. Key Points Authentication is not cryptography Passwords are useful
You have to consider physical security of system components
Passwords are useful
They provide a basis for most forms of authentication
Protocols are important
Make attacks more difficult

20. Internal Defenses Firewalls etc. (Optional)

21. Perimeter and Internal Defenses
Commonly deployed defenses
Perimeter defenses – Firewall, IDS
Protect local area network and hosts
Keep external threats from internal network
Internal defenses – Virus scanning
Protect hosts from threats that get through the perimeter defenses
Extend the “perimeter” – VPN
Common practices, but could be improved
Internal threats are significant
Unhappy employees
Compromised hosts

22. Standard perimeter defense mechanisms
Firewall
Packet filter (stateless, stateful)
Application layer proxies
Traffic shaping
Intrusion detection
Anomaly and misuse detection
Methods applicable to network or host

23. Basic Firewall Concept
Separate local area net from internet
Firewall
Local area network
Internet
Router
All packets between LAN and internet routed through firewall

24. Firewall goals Prevent malicious attacks on hosts
Port sweeps, ICMP echo to broadcast addr, syn flooding, …
Worm propagation
Exploit buffer overflow in program listening on network
Prevent general disruption of internal network
External SMNP packets
Provide defense in depth
Programs contain bugs and are vulnerable to attack
Network protocols may contain;
Design weaknesses (SSH CRC)
Implementation flaws (SSL, NTP, FTP, SMTP...)
Control traffic between “zones of trusts”

25. Review: TCP Protocol Stack
Application protocol
Application
Application
TCP, UDP protocol
Transport
Transport
Network
IP protocol IP
IP protocol
Network
Link
Network Access
Link
Data Link
Data Link
Transport layer provides ports, logical channels identified by number

26. Types of Firewalls Three common types of Firewalls:
Packet-filtering routers
Application-level gateways
Circuit-level gateways

27. Types of Firewalls
Packet-filtering Router

28. Types of Firewalls Packet-filtering Router
Applies a set of rules to each incoming IP packet and then forwards or discards the packet
Filter packets going in both directions
The packet filter is typically set up as a list of rules based on matches to fields in the IP or TCP header
Two default policies (discard or forward)

29. Packet-filtering Router
Advantages:
Simplicity
Transparency to users
High speed
Disadvantages:
Difficulty of setting up packet filter rules
Lack of Authentication

30. Packet-filtering Router
Possible attacks and appropriate countermeasures
IP address spoofing
Source routing attacks
Tiny fragment attacks

31. Types of Firewalls
Application-level Gateway

32. Types of Firewalls Application-level Gateway Also called proxy server
Acts as a relay of application-level traffic

33. Application-level Gateway
Advantages:
Higher security than packet filters
Only need to scrutinize a few allowable applications
Easy to log and audit all incoming traffic
Disadvantages:
Additional processing overhead on each connection (gateway as splice point)

34. Types of Firewalls
Circuit-level Gateway

35. Circuit-level Gateway
Stand-alone system or
Specialized function performed by an Application-level Gateway
Sets up two TCP connections
The gateway typically relays TCP segments from one connection to the other without examining the contents

36. Circuit-level Gateway
The security function consists of determining which connections will be allowed
Typically use is a situation in which the system administrator trusts the internal users
An example is the SOCKS package

37. Firewall Configurations
In addition to the use of simple configuration of a single system (single packet filtering router or single gateway), more complex configurations are possible
Three common configurations

38. Firewall Configurations
Screened host firewall system (single-homed bastion host)

39. Firewall Configurations
Screened host firewall, single-homed bastion configuration
Firewall consists of two systems:
A packet-filtering router
A bastion host

40. Firewall Configurations
Configuration for the packet-filtering router:
Only packets from and to the bastion host are allowed to pass through the router
The bastion host performs authentication and proxy functions

41. Firewall Configurations
Greater security than single configurations because of two reasons:
This configuration implements both packet-level and application-level filtering (allowing for flexibility in defining security policy)
An intruder must generally penetrate two separate systems

42. Firewall Configurations
This configuration also affords flexibility in providing direct Internet access (public information server, e.g. Web server)

43. Firewall Configurations
Screened host firewall system (dual-homed bastion host)

44. Firewall Configurations
Screened host firewall, dual-homed bastion configuration
The packet-filtering router is not completely compromised
Traffic between the Internet and other hosts on the private network has to flow through the bastion host

45. Firewall Configurations
Screened-subnet firewall system

46. Firewall Configurations
Screened subnet firewall configuration
Most secure configuration of the three
Two packet-filtering routers are used
Creation of an isolated sub-network

47. Firewall Configurations
Advantages:
Three levels of defense to thwart intruders
The outside router advertises only the existence of the screened subnet to the Internet (internal network is invisible to the Internet)

48. Firewall Configurations
Advantages:
The inside router advertises only the existence of the screened subnet to the internal network (the systems on the inside network cannot construct direct routes to the Internet)

49. Trusted System Technology Trusted Computing
One way to enhance the ability of a system to defend against intruders and malicious programs is to implement trusted system technology