chapter 18: network security

Chapter 18:Network Security

Business Data Communications, 5e


2

Types of Security

• Information Security

• Computer Security

• Network Security


3

Security Requirements

• Confidentiality

• Integrity

• Availability

• Authenticity


4

Security Threats

• Passive attacks– Release of message contents– Traffic analysis– Difficult to detect because there is no data alteration– Emphasis on prevention through encryption

• Active attacks – Masquerae– Replay– Modification of messages– Denial of Service


5

Symmetric Encryption• The only form of encryption prior to late 1970s• Five components to the algorithm

– Plaintext– The original message or data– Encryption algorithm– Secret key– Ciphertext– Decryption algorithm

• Two requirements– Strong encryption algorithm– Secure exchange of keys


6

Conventional Encryption Operation


7

Symmetric Encryption Attacks

• Cryptanalysis– Exploits the algorithm to attempt to deduce a specific

plaintext or key being used– If successful, future and past messages encrypted with

that key are compromised

• Brute Force– Try every possible key on a piece of ciphertext until

an intelligible translation into plaintext is obtained– On average, half of all possible keys must be tried to

achieve success


8

Data Encryption Standard (DES)

• Dominant encryption algorithm after release in 1977

• 56-bit key made it too easy to crack by 1998• Life of DES extended by use of triple DES

(3DES– Repeats basic DES algorithm three times, using either

two or three unique keys– Key size of 112 or 168 bits– Drawbacks: Algorithm is sluggish in software, 64-bit

block size is inefficient


9

Advanced Encryption Standard


10

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– Encryption process carried out at two end systems – Encrypted data transmitted unaltered across network;

destination shares key with source to decrypt data– Packet headers cannot be secured


11

Symmetric EncryptionKey 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)


12

End-to-End Encryption

• Two types of keys– Session keys– Permanent keys

• Required components– Key distribution center– Security service module (SSM)


13

Automated Key Distribution


14

Traffic Padding

• A function that produces ciphertext output continuously, even in the absence of plaintext

• Continuous random data stream is generated. When plaintext is available, it is encrypted and transmitted. When input plaintext is not present, the random data are encrypted and transmitted

• Makes it impossible for an attacker to distinguish between true data flow and noise and therefore impossible to deduce the amount of traffic


15

Message Authentication

• Must verify that contents have not been altered and that source is authentic

• Approaches– Authentication using symmetric encryption– Authentication without message encryption– Message authentication code– One-way hash function


16

Hash Function (H) Requirements

• Can be applied to a block of data of any size.• Produces a fixed-length output.• H(x) is relatively easy to compute for any given x• For any given code h, it is computationally

infeasible to find x such that H(x) = h.• For any given block x, it is computationally

infeasible to find y ≠ x with H(y) = H(x).• It is computationally infeasible to find any pair

(x, y) such that H(x) = H(y).


17

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

– it is less cumbersome than conventional encryption


18

Public-Key Encryption Components

• Plaintext

• Encryption algorithm

• Public and private key

• Ciphertext

• Decryption algorithm


19

Public-Key Encryption Operation


20

Public-Key Signature Operation


21

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


22

Steps in Public Key Encryption

• Each user generates a pair of keys to be used for the encryption and decryption of messages.

• Each user places one of the two keys in a public register or other accessible file. This is the public key. The companion key is kept private.

• If Bob wishes to send a private message to Alice, Bob encrypts the message using Alice's public key.

• When Alice receives the message, she decrypts it using her private key. No other recipient can decrypt the message because only Alice knows Alice's private key.


23

Digital Signature Process


24

RSA Encryption Algorithm

• Developed in 1977, first published in 1978• Widely accepted and implemented approach to

public-key encryption • For plaintext block M and ciphertext block C

– C = Me mod n– M = Cd mod n = (Me)d mod n = Med mod n

• Both sender and receiver must know values of n and e; only receiver knows value of d

• Public key of KU = {e, n} • Private key of KR = {d, n}.


25

RSA Requirements

• It is possible to find values of e, d, n such that Med = M mod n for all M < n.

• It is relatively easy to calculate Me and Cd for all values of M < n.

• It is infeasible to determine d given e and n.– This requirement can be met with large values

of e and n


26

Approaches to Defeating RSA

• Brute force approach: try all possible private keys. – The larger the number of bits in e and d, the more

secure the algorithm. – However, the larger the size of the key, the slower the

system will run.

• Cryptanalysis: factoring n into its two prime factors– A hard problem, but not as hard as it used to be– Currently, a 1024-bit key size is considered strong

enough for virtually all applications


27

Key Management

• Symmetric encryption requires both parties to share a secret key

• Secure distribution of keys is the most difficult problem for symmetric encryption

• Public key encryption solves this problem, but adds the issue of authenticity

• Public key certiciates address this issue


28

Public Key Certificates


29

Public Key Certificate Process

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.


30

Virtual Private Networks (VPNs)

• Internet connectivity provides easier access for telecommuters and off-site employees

• Use of a public network exposes corporate traffic to eavesdropping and provides an entry point for unauthorized users

• A variety of encryption and authentication packages and products are available to secure and authenticate remote access

• Need for a standard that allows a variety of platforms to interconnect securely


31

Applications of IPSec

• Secures communications across a LAN, WANs, and/or the Internet

• Can encrypt and/or authenticate all traffic at the IP level

• 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


32

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


33

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


34

ESP Transport and Tunneling

• Transport Mode– provides protection

primarily for upper-layer protocols.

– Typically used for end-to-end communication between two hosts

– encrypts and optionally authenticates the IP payload but not the IP header

– useful for relatively small networks; for a full-blown VPN, tunnel mode is far more efficient

• Tunnel Mode– Provides protection to the

entire packet– Original packet is

encapsulated in ESP fields, protecting contents from examination

– Used when one or both ends is a security gateway

– Multiple hosts on networks behind firewalls may engage in secure communications without implementing IPSec


35

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


36

IPSec and VPNs

• Organizations need to isolate their networks and at the same time send and receive traffic over the Internet

• Authentication and privacy mechanisms of secure IP allow for security strategy

• IPSec can be implemented in routers or firewalls owned and operated by the organization, allowing the network manager complete control over security aspects of the VPN

chapter 18: network security

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