Chapter 20:
Network Security
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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
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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
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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.
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Conventional Encryption
Operation
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Conventional Encryption
Requirements & Weaknesses
Requirements
A strong encryption algorithm
Secure process for sender & receiver to obtain secret keys
Methods of Attack
Cryptanalysis
Brute force
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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 value that is used in the next
iteration.
DES is now too easy to crack to be a useful
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encryption method
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
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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
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Public-Key Encryption
Components
Plaintext
Encryption algorithm
Public key
Private key
Ciphertext
Decryption algorithm
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Public-Key Encryption Operation
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Public-Key Signature Operation
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Characteristics of Public-Key
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
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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
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Conventional Encryption
Key Distribution
Both parties must have the secret key
Key is changed frequently
Requires either manual delivery of keys, or a thirdparty encrypted channel
Most effective method is a Key Distribution Center
(e.g. Kerberos)
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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.
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Digital Signature Process
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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.
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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
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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
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Web Application Security
Secure HyperText Transfer Protocol (SHTTP)
Secure Sockets Layer (SSL)
Web server packages should incorporate both of
these protocols
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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.
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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
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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
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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
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ESP Encryption & Authentication
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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
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