Network Security

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42.444 Telecommunications: A
Management Perspective

Network Security

Lecture 15

(Chapters 20)


Dr Gerald Grant

Copyright © 1997
-
2001

Dr Gerald Grant

2

Security Threats


Passive attacks


Eavesdropping on, or monitoring,
transmissions


Electronic mail, file transfers, and client/server
exchanges are examples of transmissions that
can be monitored


Active attacks


Modification of transmitted data


Attempts to gain unauthorized access to
computer systems

Copyright © 1997
-
2001

Dr Gerald Grant

3

Encryption Methods


The essential technology underlying
virtually all automated network and
computer security applications is
cryptography


Two fundamental approaches are in use:


conventional encryption, also known as
symmetric encryption


public
-
key encryption, also known as
asymmetric encryption

Copyright © 1997
-
2001

Dr Gerald Grant

4

Conventional Encryption


The only form of encryption prior to late 1970s


Five components to the algorithm


Plaintext: The original message or data


Encryption algorithm: Performs various substitutions and
transformations on the plaintext.


Secret key: Input to the encryption algorithm.
Substitutions and transformations performed depend on
this key


Ciphertext: Scrambled message produced as output.
depends on the plaintext and the secret key


Decryption algorithm: Encryption algorithm run in reverse.
Uses ciphertext and the secret key to produce the original
plaintext.

Copyright © 1997
-
2001

Dr Gerald Grant

5

Conventional Encryption
Operation

Copyright © 1997
-
2001

Dr Gerald Grant

6

Conventional Encryption
Requirements & Weaknesses


Requirements


A strong encryption algorithm


Secure process for sender & receiver to
obtain secret keys


Methods of Attack


Cryptanalysis


Brute force

Copyright © 1997
-
2001

Dr Gerald Grant

7

Data Encryption Standard (DES)


Adopted in 1977, reaffirmed for 5 years in
1994, by NBS/NIST


Plaintext is 64 bits (or blocks of 64 bits),
key is 56 bits


Plaintext goes through 16 iterations, each
producing an intermediate 64
-
bit value
that is used in the next iteration.


DES is now too easy to crack to be a
useful encryption method

Copyright © 1997
-
2001

Dr Gerald Grant

8

Triple DEA


Alternative to DES, uses multiple
encryption with DES and multiple keys


With three distinct keys, TDEA has an
effective key length of 168 bits, so is
essentially immune to brute force attacks


Principal drawback of TDEA is that the
algorithm is relatively sluggish in software

Copyright © 1997
-
2001

Dr Gerald Grant

9

Public
-
Key Encryption


Based on mathematical functions rather
than on simple operations on bit patterns


Asymmetric, involving the use of two
separate keys


Misconceptions about public key
encryption


it is more secure from cryptanalysis


it is a general
-
purpose technique that has
made conventional encryption obsolete

Copyright © 1997
-
2001

Dr Gerald Grant

10

Public
-
Key Encryption
Components


Plaintext


Encryption algorithm


Public key


Private key


Ciphertext


Decryption algorithm

Copyright © 1997
-
2001

Dr Gerald Grant

11

Public
-
Key Encryption Operation

Copyright © 1997
-
2001

Dr Gerald Grant

12

Public
-
Key Signature Operation

Copyright © 1997
-
2001

Dr Gerald Grant

13

Characteristics of Public
-
Key


Computationally infeasible to determine the
decryption key given knowledge of the
cryptographic algorithm and the encryption key.


Either of the two related keys can be used for
encryption, with the other used for decryption.


Slow, but provides tremendous flexibility to
perform a number of security
-
related functions


Most widely used algorithm is RSA

Copyright © 1997
-
2001

Dr Gerald Grant

14

Location of Encryption Devices


Link encryption


Each vulnerable communications link is equipped on
both ends with an encryption device.


All traffic over all communications links is secured.


Vulnerable at each switch


End
-
to
-
end encryption


the encryption process is carried out at the two end
systems.


Encrypted data are transmitted unaltered across the
network to the destination, which shares a key with the
source to decrypt the data


Packet headers cannot be secured

Copyright © 1997
-
2001

Dr Gerald Grant

15

Conventional Encryption

Key Distribution


Both parties must have the secret key


Key is changed frequently


Requires either manual delivery of keys,
or a third
-
party encrypted channel


Most effective method is a Key
Distribution Center (e.g. Kerberos)

Copyright © 1997
-
2001

Dr Gerald Grant

16

Public
-
Key Encryption

Key Distribution


Parties create a pair of keys; public key is broadly
distributed, private key is not


To reduce computational overhead, the following
process is then used:

1. Prepare a message.

2. Encrypt that message using conventional encryption with
a one
-
time conventional session key.

3. Encrypt the session key using public
-
key encryption with
recipient’s public key.

4. Attach the encrypted session key to the message and send
it.

Copyright © 1997
-
2001

Dr Gerald Grant

17

Digital Signature Process

Copyright © 1997
-
2001

Dr Gerald Grant

18

Public Key Certificates

1. A public key is generated by the user and
submitted to Agency X for certification.

2. X determines by some procedure, such as a face
-
to
-
face meeting, that this is authentically the
user’s public key.

3. X appends a timestamp to the public key,
generates the hash code of the result, and
encrypts that result with X’s private key forming
the signature.

4. The signature is attached to the public key.

Copyright © 1997
-
2001

Dr Gerald Grant

19

Web Vulnerabilities


Unauthorized alteration of data at the Web
site


Unauthorized access to the underlying
operating system at the Web server


Eavesdropping on messages passed
between a Web server and a Web browser


Impersonation

Copyright © 1997
-
2001

Dr Gerald Grant

20

Methods for Improving

Web Security


Securing the Web site itself


install all operating system security patches


install the Web server software with minimal
system privileges


use a more secure platform


Securing the Web application

Copyright © 1997
-
2001

Dr Gerald Grant

21

Web Application Security


Secure HyperText Transfer Protocol
(SHTTP)


Secure Sockets Layer (SSL)


Web server packages should incorporate
both of these protocols

Copyright © 1997
-
2001

Dr Gerald Grant

22

Virtual Private Networks (VPNs)


The use of encryption and authentication
in the lower protocol layers to provide a
secure connection through an otherwise
insecure network, typically the Internet.


Generally cheaper than real private
networks using private lines but rely on
having the same encryption and
authentication system at both ends.


The encryption may be performed by
firewall software or possibly by routers.

Copyright © 1997
-
2001

Dr Gerald Grant

23

IPSec


Can secure communications across a
LAN, WANs, and/or the Internet


Examples of use:


Secure branch office connectivity over the
Internet


Secure remote access over the Internet


Establishing extranet and intranet connectivity
with partners


Enhancing electronic commerce security

Copyright © 1997
-
2001

Dr Gerald Grant

24

Benefits of IPSec


When implemented in a firewall or router,
provides strong security for all traffic
crossing the perimeter


IPSec in a firewall is resistant to bypass


Runs below the transport layer (TCP, UDP)
and so is transparent to applications


Can be transparent to end users


Can provide security for individual users if
needed

Copyright © 1997
-
2001

Dr Gerald Grant

25

IPSec Functions


IPSec provides three main facilities


authentication
-
only function referred to as
Authentication Header (AH)


combined authentication/encryption function
called Encapsulating Security Payload (ESP)


a key exchange function


For VPNs, both authentication and
encryption are generally desired

Copyright © 1997
-
2001

Dr Gerald Grant

26

ESP Encryption & Authentication


Copyright © 1997
-
2001

Dr Gerald Grant

27

IPSec Key Management


Manual


System administrator manually configures each system
with its own keys and with the keys of other
communicating systems


Practical for small, relatively static environments


Automated


Enables the on
-
demand creation of keys for SAs and
facilitates the use of keys in a large distributed system


Most flexible but requires more effort to configure and
requires more software