1. CHAPTER 7
SECURITY IN NETWORKS AND
DISTRIBUTED SYSTEM

2. INTRODUCTION
 Network is two devices connected across some
medium by hardware and software that complete the
communications (simple definition of network).
User (Client)
Host
Server
Communication medium
Simple View of Network

3. Introduction
 A network is normally not just single client to a
single server; typically many clients interact with
many servers.
User (Client) Host Server
User (Client)
User (Client)
User (Client)
Host Server
User (Client)
User (Client)
User (Client)
System A
System B

4. Network Security Issues
Network have security problems for the following reasons:
 Sharing – resources and workload sharing
 Complexity of system
 Unknown parameter – expandability of a network also implies
uncertainty about the network boundary
 Many points of attack – file may past through many host before
reaching the destination
 Anonymity – attacker can mount an attack with touching the
system
 Unknown path – there may be many path from one host to
another.

5. Possible Network Security Threats
 Wiretapping
 Impersonation
 Message confidence violations
 Message integrity violations
 Hacking
 Denial of Service (DoS)

6. Possible Network Security Threats
Wiretapping
 Wiretap means to intercept communications.
 Passive / Active Wiretapping
 Packet sniffer can retrieve all packets on the net.
 “Inductance” is a process where an intruder can tap a
wire without making physical contact with the cable.
 Microwave and satellite – higher possibility of
interception due to wider broadcasting.

7. Possible Network Security Threats
Wiretapping
 Optical fiber offers two significant security
advantages:
 The entire optical network must be tuned carefully each
time a new connection is made. Therefore, no one can tap
an optical system without detection.
 Optical fiber carries light energy, not electricity. Light
does not emanate a magnetic field as electricity does.
Therefore an inductive tap is impossible on an optical
fiber cable.

8. Possible Network Security Threats
Wiretapping
 However, optical fiber also has weaknesses
where wiretappers will try to tap at the
repeaters, splices and other equipments that
connects to the fiber optic and thus creates
vulnerabilities.

9. Possible Network Security Threats
Impersonation
 Pretend to be someone (personnel) or something
(process).
 In an impersonation, the attacker has several choices:
 Guess the identity and authentication details of the target
 Pick up the identity and authentication details of the target
from a previous communication
 Circumvent or disable the authentication mechanism at
the target computer
 Use a target that will not be authenticated
 Use a target whose authentication data is known

10. Possible Network Security Threats
Message Confidentiality Violations
 Misdelivery
 Exposure
 Traffic Flow Analysis

11. Possible Network Security Threats
Message Integrity Violations
 Falsification of Messages
 Change the content of a message
 Change any part of the content of a message
 Replace a message entirely
 Redirect a message
 Destroy or delete the message
 Noise – unintentional interference

12. Possible Network Security Threats
Hacking
 A source of threat to security in computer
communication.
 Hacker is considered as a separate threat because a
hacker can develop tools to search widely and
quickly for particular weaknesses and move swiftly
to exploit weaknesses.
 In this way, hacker has unlimited time to analyze,
plan, code, simulate and test for future attack.
 In reviewing the effects of this attack ; if it succeeds,
what additional capability would that give the hacker
for future attacks?

13. Possible Network Security Threats
Denial of Service
 Result of any action or series of actions that
prevents any part of a telecommunications
system from functioning.
 Connectivity
 Flooding
 Routing problems
 Disruption of Service

14. Network Security Control
 Encryption – link encryption, end-to-end encryption
 Link Encryption:
 Data is encrypted just before the system places it on the
physical communication links.
 Decryption occurs just as the communication enters the
receiving computer.

15. Application
Presentation
Session
Transport
Network
Data Link
Physical
Sender ReceiverMessage
Intermediate
Host
Message
(Plaintext)
Exposed
Message Encrypted Message in Plaintext: Exposed
Link Encryption

16. Network Security Control
 End-to-end encryption:
 Provides security from one end of a transmission
through the other.

17. Application
Presentation
Session
Transport
Network
Data Link
Physical
Sender Message
Intermediate
Host
Message Encrypted Message in Plaintext: Exposed
Receiver
End-to-End Encryption

18. Network Security Control
Link Encryption versus End-to-end Encryption:
Link Encryption End-to-end Encryption
Security Within Hosts
Message exposed in the sending host
Message expose in intermediate nodes
Security Within Hosts
Message encrypted in sending host
Message encrypted in intermediate nodes
Role of User
Applied by sending host
Invisible to user
Host maintains encryption
Can be done in hardware
All or no messages encrypted
Role of User
Applied by sending process
User applies encryption
User must find algorithm
Software implementation
User chooses to encrypt or not, for each
message

19. Authentication Issues in Distributed System
There are two main concern regarding authentication
issue in distributed system which are:
(1) How to ensure the authenticity of the communicating
hosts?
(2) How to ensure authenticity of users who are using the
hosts?

20. Authentication Issues in Distributed System
That is by using:
 Digital Distributed Authentication
 DCE (Distributed Computer Environment)
 Kerberos
 SESAME
 CORBA

21. Authentication Issues in Distributed System
Kerberos
 Is a system that supports authentication in distributed
systems.
 Was designed at Massachusetts Institute of
technology.
 The basis of kerberos is a central server that provides
authenticated tokens called tickets to requesting
applications.

22. Authentication Issues in Distributed System
KERBEROS
Initiating a Kerberos Session:

23. Authentication Issues in Distributed System
KERBEROS
Obtaining a Ticket to Access a File:

24. KERBEROS:
Access to Services and Servers in Kerberos

25. Authentication Issues in Distributed System
Kerberos was carefully designed to withstand attacks in
distributed environments:
 No password communicated on the network
 Cryptographic protection against spoofing
 Limited period of validity
 Time stamps to prevent replay attacks
 Mutual authentication

26. Authentication Issues in Distributed System
 Kerberos is not a perfect answer to security problems
in distributed systems because:
 Kerberos requires continuous availability of a trusted
ticket granting server.
 Authenticity of servers requires a trusted relationship
between the ticket granting server and every server
 Kerberos requires timely transactions
 A subverted workstation can save and later replay user
passwords

27. Authentication Issues in Distributed System
 Kerberos is not a perfect answer to security
problems in distributed systems because:
 Password guessing works
 Kerberos does not scale well
 Kerberos is not a complete solution

28. Privacy Enhanced Electronic Mail (PEM)
 The basis of PEM is encryption.
 In order to send a PEM message the sender
must have a certificate for the receiver.

29. Message header
+ Body
Message
Encryption
key
Receiver’s
public key
New header
Encrypted data
Encrypted key
Encrypted
Message
Header +
Body
Public key encryption
Symmetric key
encryption

31. Compose
message
PEM processing
requested ?
PEM
Send message
Receive message
Privacy
enhanced ?
PEM
View message
Yes
No
Yes
No
PEM processing in Message Transmission

32. Privacy Enhanced Electronic Mail (PEM)
 The major problem with PEM is key management.
 Therefore PGP was designed to overcome this
problem.

33. Pretty Good Privacy (PGP)
 Was designed by Phil Zimmerman to offer a reasonable
degree of privacy for email.
 It uses a message structuring scheme similar to PEM.
 The key management for PGP is ad hoc.
 Each user has a set of people he or she knows and trusts.
 The user exchanges public keys with those friends, exactly as
one might swap business card at meeting.
 Some people accept not just the friends’ public key but also
all public keys their friends have.

34. Pretty Good privacy (PGP)
 The assumption here is that any friend of yours is a
friend of mine.
 A PGP user builds a key ring which is the set of all
public keys that person possesses.
 In that way, when an encrypted messages arrives, the
person can decrypt it if the key is on that person’s
key ring.

35. Firewalls
 A firewall is a process that filters all traffic between
a protected or “inside” network and a less
trustworthy or “outside” network.
 There are three types of firewall:
 Screening Routers
 Proxy gateways
 Guards

36. Firewalls
Screening Router
 Is the simplest and in some situations the most effective type
of firewall.
 Hosts tend not to be connected directly to a wide area
network; more often hosts are connected to a router.

37. Firewalls
Router joining LAN to two WANs

39. Firewalls
Screening Router
 Router will only see the header of the message.
 Header will contain information on:
 The sender/receiver address
 Protocol
 Port
 Length of a packet
 It can also control the traffic based on application – by using
port numbers (eg: 21 for FTP and 25 for SMTP)
 It can also decide which application is acceptable and not
acceptable.
 It can also determine the authentication of an inside address.

41. Firewalls
Proxy Gateway
 Is also called a bastion host.
 Is a firewall that simulates the (proper) effects
of an application so that the application will
receive only requests to act properly.

42. Firewalls
Proxy Gateway
 To understand the real purpose of a proxy
gateway, we consider some examples:
 A company wants to set up an online lists so that
outsiders can see the products and prices offered.
It wants to be sure that no outsider can change the
prices or product list and that outsiders can access
only the price list not any of the more sensitive
files stored inside.

44. Firewalls
Guard
 A guard is a sophisticated proxy firewall.
 The guard decides what services to perform on the
user’s behalf based on its available knowledge such
as whether it can reliably know of the (outside)
user’s identity, previous interactions and so forth.

45. Firewalls
Guard
 Here are some more sophisticated examples of guard
activities:
 A university wants to allow its students to use email up to
a limit of so many messages or so many characters of
email in the last so many days. Although this result could
be achieved by modifying email handlers it is more easily
done by monitoring the common point through which all
email flows (the mail transfer protocol).
 A school wants its students to be able to access the WWW
but because of the slow speed of its connection to the
Web it will allow only so many characters per download
image.

46. Firewalls
Firewalls are not complete solutions to all
computer security problems.
 Firewalls can protect an environment only if the firewalls
control the entire perimeter.
 Firewall do not protect data outside the perimeter.
 Firewall are the most visible part of an installation to the
outside and therefore is the most attractive point of attack.
 Firewalls are targets of penetrators.
 Firewalls must be correctly configured.
 Firewalls exercise only minor control over the content
admitted to the inside – inaccurate data or malicious code
must be controlled inside the perimeter.