Transcript Chapter 1 Security Problems in Computing

Chapter 9
Networking &
Distributed Security
(Part C)
Outline

Overview of Networking

Threats
Wiretapping, impersonation, message
interruption/modification, DoS
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Controls
Encryption, authentication, distributed
authentication, traffic control, integrity control
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Email privacy: PEM, PGP
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Firewalls
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Electronic Mails
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Security Goals vs Threats
Goals
Threats

confidentiality
1.
interception
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integrity
2.
interception and subsequent replay
content modification
content forgery by outsider
content forgery by recipient
origin forgery by recipient
3.
4.
5.
6.

authenticity
7.
8.
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nonrepudiation


reliable delivery

origin modification
origin forgery by outsider
Threats 2 through 8 above
interception (blocked delivery)
 denial of message transmission
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Privacy-enhanced E-Mails (PEM)

Internet standards
1987: RFC989 (PEM version 1)
1989: RFC1113 (version 2)
1993: RFC1421, 1422, 1423, 1424 (Part I, II, III, IV), version 3

Protection of privacy-enhanced emails occurs in the body of the
message. The header of the message is not changed to
ensure compatibility with the then existing email systems.

Overview: Fig. 9-27, 9-28 (p.424)
1) The message header and body is encrypted under a
symmetric key, K  E (message, K)
2) K is encrypted by the recipient’s public key  Rpub (K)
3) A duplicate header is prepended to the message, which
contains both Rpub(K) and E(message, K).

Q: In step 2, can symmetric key, instead of the recipient’s public
key, be used to encrypt the message key?
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Privacy-enhanced E-Mails (PEM)
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The answer: YES. See p.425.
Q: What would be the requirements if symmetric key is used?
Proc-Type field: processing type
DEK-Info field: data exchange key field
Key-Info: key exchange
Message encryption: DES
Key exchange: DES or RSA
In principle, any encryption algorithms can be used.
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Privacy-enhanced E-Mails (PEM)
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Security features:
Confidentiality – message encryption
Authenticity - ?
Nonrepudiability - ?
Integrity - ?
Answers: p.425
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Privacy-enhanced E-Mails (PEM)
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Advantages:
The user may choose to use PEM or not in sending an email.
PEM provide strong end-to-end security for emails.
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Problems?
1.
Key management
2.
The end points may not be secure.
Yet another privacy enhanced email protocol:
PGP: p.426
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Firewalls

Q: Which is more important, protection of emails or
protection of network-connected resources?
(see argument on p.427)
A firewall works in a way similar to a filter, which lets through

only desirable interactions while keeping all others out of the
protected network.
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Analogy: a gate keeper, a security gateway
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A firewall is a device or a process that filters all traffic
between a protected (inside) network and a less trustworthy
(outside) network.
Scenarios:

o
Internal users sending company secrets outside
o
Outside people breaking into systems inside
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Firewalls
Alternative security policies:

•
To block all incoming traffic, but allow outgoing traffic to
pass.
•
To allow accesses only from certain places
•
To allow accesses only from certain users
•
To allow accesses for certain activities (such as specific port
numbers)
o Port 79: finger; Port 23: telnet; Port 513: rlogin;
o Port 21: ftp; Port 177: X Windows
o ICMP messages: the PROTOCOL field of IP header = 1
o
Each of these mechanisms is a potential back door into
the system.
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Types of Firewalls
Screening Routers

•
The simplest, but may be the most effective type of
firewalls.
•
A router plays the role of a ‘gateway’ between two
networks. (Fig. 9-31, p.429)
•
A screening router takes advantage of a router’s ability of
“screening” passing-through packets and forwards only
packets that are desirable.
•
Example: Fig. 9-32.
•
A router has a unique advantage because it sits between
an outside and the inside network. (Fig. 9-33)
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Types of Firewalls
Proxy Gateways

•
“proxy”: authority or power to act for another
•
A firewall that simulates the effects of an application by
running “pseudo-applications”.
•
To the inside it implements part of the application protocol
to make itself look as if it is the outside connection.
•
To the outside it implements part of the application
protocol to act just like the inside process would.
•
It examines the content, not just the header, of a packet.
•
Examples of using proxy firewalls: pp.431-432
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Types of Firewalls
Guards
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•
A “sophisticated” proxy firewall
•
A guard firewall examines and interprets the content of a
packet.
•
A guard usually implements and enforces certain
business policies.
•
Example: enforcing an email “quota” (p.433)
•
Other examples
•
Trade-offs?
•
Table 9-3 (p.434) Comparing the types of firewalls
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Firewalls
Examples of Firewall Configurations
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Screening router only: Fig. 9-35
•
Proxy firewall only: Fig. 9-36
•
A combined approach: Fig. 9-37
Q: Does it make sense to reverse the position of the
screening router and the proxy firewall in Fig. 9-37?
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DMZ (Demilitarized zone)

The segment in a network bounded by two firewalls.
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Considerations about Firewalls
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Firewalls provide perimeter protection of a network, if the
network’s perimeter is clearly defined and can be controlled
by the firewall.
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A firewall is a prime target to attack.
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A firewall does not solve all security problems. Why not?
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A firewall may have a negative effect on software portability.
(See VM: Ch. 16 – Through the firewall)
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Summary

Network security is a rich area, in terms of complexity
of the problem and research opportunities.
•
•
•
•
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Intrusion detection
Honeypots
Security versus performance
…
Next:
– Buffer overflow (VM: Ch 7)
– Applying cryptography (VM: Ch 11)
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