Chapter 21 of William Stallings' book on network security discusses critical security requirements, such as confidentiality, integrity, availability, and authenticity, alongside different types of attacks, including passive and active attacks. It examines encryption methods like symmetric and public key encryption, emphasizing the importance of strong algorithms and key management. The chapter also highlights the role of protocols like SSL and TLS in providing secure communications over networks.

1. Data and ComputerData and Computer
CommunicationsCommunications
Eighth EditionEighth Edition
by William Stallingsby William Stallings
Lecture slides by Lawrie BrownLecture slides by Lawrie Brown
Chapter 21 – Network SecurityChapter 21 – Network Security

2. Network SecurityNetwork Security
To guard against the baneful influence exerted by
strangers is therefore an elementary dictate of savage
prudence. Hence before strangers are allowed to enter
a district, or at least before they are permitted to
mingle freely with the inhabitants, certain ceremonies
are often performed by the natives of the country for
the purpose of disarming the strangers of their magical
powers, or of disinfecting, so to speak, the tainted
atmosphere by which they are supposed to be
surrounded.
—The Golden Bough, Sir James George Frazer

3. Security RequirementsSecurity Requirements
 confidentiality - protect data content/accessconfidentiality - protect data content/access
 integrity - protect data accuracyintegrity - protect data accuracy
 availability - ensure timely serviceavailability - ensure timely service
 authenticity - protect data originauthenticity - protect data origin

4. Passive AttacksPassive Attacks
 eavesdropping on transmissionseavesdropping on transmissions
 to obtain informationto obtain information

release of possibly sensitive/confidentialrelease of possibly sensitive/confidential
message contentsmessage contents

traffic analysis which monitors frequency andtraffic analysis which monitors frequency and
length of messages to get info on senderslength of messages to get info on senders
 difficult to detectdifficult to detect
 can be prevented using encryptioncan be prevented using encryption

5. Active AttacksActive Attacks
 masquerademasquerade

pretending to be a different entitypretending to be a different entity
 replayreplay
 modification of messagesmodification of messages
 denial of servicedenial of service
 easy to detecteasy to detect

detection may lead to deterrentdetection may lead to deterrent
 hard to preventhard to prevent

focus on detection and recoveryfocus on detection and recovery

6. Symmetric EncryptionSymmetric Encryption

7. Requirements for SecurityRequirements for Security
 strong encryption algorithmstrong encryption algorithm

even known, unable to decrypt without keyeven known, unable to decrypt without key

even if many plaintexts & ciphertexts availableeven if many plaintexts & ciphertexts available
 sender and receiver must obtain secretsender and receiver must obtain secret
key securelykey securely
 once key is known, all communicationonce key is known, all communication
using this key is readableusing this key is readable

8. Attacking EncryptionAttacking Encryption
 cryptanalysiscryptanalysis

relay on nature of algorithm plus some knowledge ofrelay on nature of algorithm plus some knowledge of
general characteristics of plaintextgeneral characteristics of plaintext

attempt to deduce plaintext or keyattempt to deduce plaintext or key
 brute forcebrute force

try every possible key until plaintext is recoveredtry every possible key until plaintext is recovered

rapidly becomes infeasible as key size increasesrapidly becomes infeasible as key size increases

56-bit key is not secure56-bit key is not secure

9. Block CiphersBlock Ciphers
 most common symmetric algorithmsmost common symmetric algorithms
 process plain text in fixed block sizesprocess plain text in fixed block sizes
producing block of cipher text of equal sizeproducing block of cipher text of equal size
 most important current block ciphers:most important current block ciphers:

DataData Encryption StandardEncryption Standard (DES)(DES)

Advanced Encryption StandardAdvanced Encryption Standard

10. Data Encryption StandardData Encryption Standard
 US standardUS standard
 64 bit plain text blocks64 bit plain text blocks
 56 bit key56 bit key
 broken in 1998 by Electronic Frontierbroken in 1998 by Electronic Frontier
FoundationFoundation

special purpose US$250,000 machinespecial purpose US$250,000 machine

with detailed published descriptionwith detailed published description

less than three daysless than three days

DES now worthlessDES now worthless

11. Triple DEATriple DEA
 ANSI X9.17 (1985)ANSI X9.17 (1985)
 incorporated in DEA standard 1999incorporated in DEA standard 1999
 uses 2 or 3 keysuses 2 or 3 keys
 3 executions of DEA algorithm3 executions of DEA algorithm
 effective key lengtheffective key length 112 or112 or 168 bit168 bit
 slowslow
 block size (64 bit) now too smallblock size (64 bit) now too small

12. Advanced EncryptionAdvanced Encryption
StandardStandard
 NIST issued call for proposals for an AdvancedNIST issued call for proposals for an Advanced
Encryption Standard (AES) in 1997Encryption Standard (AES) in 1997

security strength equal to or better than 3DESsecurity strength equal to or better than 3DES

significantly improved efficiencysignificantly improved efficiency

symmetric block ciphersymmetric block cipher withwith block length 128 bitsblock length 128 bits

key lengths 128, 192, and 256 bitskey lengths 128, 192, and 256 bits

evaluation include security, computational efficiency,evaluation include security, computational efficiency,
memory requirements, hardware and softwarememory requirements, hardware and software
suitability, and flexibilitysuitability, and flexibility

AESAES issued as FIPS 197 in 2001issued as FIPS 197 in 2001

13. AES DescriptionAES Description
 assume key length 128 bitsassume key length 128 bits
 input a 128-bit blockinput a 128-bit block ((square matrix of bytes)square matrix of bytes)

copied into state arraycopied into state array, m, modified at each stageodified at each stage

after final stage, state copied to outputafter final stage, state copied to output
 128-bit key (square matrix of bytes)128-bit key (square matrix of bytes)

expanded into array of 44 32-bit key schedule wordsexpanded into array of 44 32-bit key schedule words
 byte orderingbyte ordering by columnby column

1st 4 bytes of1st 4 bytes of 128-bit input occupy 1st column128-bit input occupy 1st column

1st 4 bytes of expanded key1st 4 bytes of expanded key occupy 1st columnoccupy 1st column

14. AESAES
EncryptionEncryption
andand
DecryptionDecryption

15. AES Encryption RoundAES Encryption Round

16. Location of EncryptionLocation of Encryption
DevicesDevices

17. Link EncryptionLink Encryption
 each communication link equipped at both endseach communication link equipped at both ends
 all traffic secureall traffic secure
 high level of securityhigh level of security
 requires lots of encryption devicesrequires lots of encryption devices
 message must be decrypted at each switch tomessage must be decrypted at each switch to
read address (virtual circuit number)read address (virtual circuit number)
 security vulnerable at switchessecurity vulnerable at switches

particularly on public switched networkparticularly on public switched network

18. End to End EncryptionEnd to End Encryption
 encryption done at ends of systemencryption done at ends of system
 data in encrypted form crosses networkdata in encrypted form crosses network
unalteredunaltered
 destination shares key with source to decryptdestination shares key with source to decrypt
 host can only encrypt user datahost can only encrypt user data

otherwise switching nodes could not read header orotherwise switching nodes could not read header or
route packetroute packet
 hence traffic pattern not securehence traffic pattern not secure
 solution is to use both link and end to endsolution is to use both link and end to end

19. Key DistributionKey Distribution
 symmetric encryption needs key distributionsymmetric encryption needs key distribution

protected for access by othersprotected for access by others

changed frequentlychanged frequently
 possibilities for key distributionpossibilities for key distribution
1.1. key selected by A and delivered to Bkey selected by A and delivered to B
2.2. third party selects key and delivers to A and Bthird party selects key and delivers to A and B
3.3. use old key to encrypt & transmit new key from A to Buse old key to encrypt & transmit new key from A to B
4.4. use old key to transmit new key from third party to Ause old key to transmit new key from third party to A
and Band B

20. Automatic Key DistributionAutomatic Key Distribution

21. Traffic PaddingTraffic Padding
 addresses concern about traffic analysisaddresses concern about traffic analysis

though link encryption reduces opportunitythough link encryption reduces opportunity

attacker can still assess traffic volumeattacker can still assess traffic volume
 traffic padding produces ciphertexttraffic padding produces ciphertext
continuouslycontinuously
 if no plaintext, sends random dataif no plaintext, sends random data
 makes traffic analysis impossiblemakes traffic analysis impossible

22. Message AuthenticationMessage Authentication
 protection against active attacks withprotection against active attacks with

falsification of datafalsification of data

falsification of sourcefalsification of source
 authentication allows receiver to verify thatauthentication allows receiver to verify that
message is authenticmessage is authentic

has not been alteredhas not been altered

is from claimed/authentic sourceis from claimed/authentic source

timelinesstimeliness

23. Authentication UsingAuthentication Using
Symmetric EncryptionSymmetric Encryption
 assume sender & receiver only know keyassume sender & receiver only know key
 only sender could have encryptedonly sender could have encrypted
message for other partymessage for other party
 message must include one of:message must include one of:

error detection codeerror detection code

sequence numbersequence number

time stamptime stamp

24. Authentication WithoutAuthentication Without
EncryptionEncryption
 authentication tag generated and appended toauthentication tag generated and appended to
each messageeach message
 message not encryptedmessage not encrypted
 useful when don’t want encryption because:useful when don’t want encryption because:

messages broadcast to multiple destinationsmessages broadcast to multiple destinations
• have one destination responsible for authenticationhave one destination responsible for authentication

one side heavily loadedone side heavily loaded
• encryption adds to workloadencryption adds to workload
• can authenticate random messagescan authenticate random messages

programs authenticated without encryption can beprograms authenticated without encryption can be
executed without decodingexecuted without decoding

25. Message Authentication CodeMessage Authentication Code
 generate authentication code based on sharedgenerate authentication code based on shared
key and messagekey and message
 common key shared between A and Bcommon key shared between A and B
 if only sender and receiver know key and codeif only sender and receiver know key and code
matches:matches:

receiver assured message has not alteredreceiver assured message has not altered

receiver assured message is from alleged senderreceiver assured message is from alleged sender

if message has sequence number, receiver assuredif message has sequence number, receiver assured
of proper sequenceof proper sequence
 can use various algorithms, eg. DEScan use various algorithms, eg. DES

26. Message Authentication CodeMessage Authentication Code

27. One Way Hash FunctionOne Way Hash Function
 accepts variable size message and producesaccepts variable size message and produces
fixed size tag (message digest)fixed size tag (message digest)

but without use of a secret keybut without use of a secret key
 send digest with messagesend digest with message
 in manner that validates authenticityin manner that validates authenticity
 advantages of authentication without encryptionadvantages of authentication without encryption

encryption is slowencryption is slow

encryption hardware expensiveencryption hardware expensive

encryption hardware optimized for large data setsencryption hardware optimized for large data sets

algorithms covered by patentsalgorithms covered by patents

algorithms subject to export controls (from USA)algorithms subject to export controls (from USA)

28. UsingUsing
OneOne
WayWay
HashHash
FunctionsFunctions

29. Secure Hash FunctionsSecure Hash Functions
 produce a “fingerprint” of message/fileproduce a “fingerprint” of message/file
 must have the following properties:must have the following properties:

can be applied to any size data blockcan be applied to any size data block

produce fixed length outputproduce fixed length output

easy to computeeasy to compute

not feasible to reversenot feasible to reverse

not feasible to find two messages with thenot feasible to find two messages with the
same hashsame hash
 giving “weak” & “strong” hash functionsgiving “weak” & “strong” hash functions
 also used for data integrityalso used for data integrity

30. Secure Hash AlgorithmSecure Hash Algorithm
 Secure Hash Algorithm (SHA)Secure Hash Algorithm (SHA)

SHA defined in FIPS 180 (1993), 160-bit hashSHA defined in FIPS 180 (1993), 160-bit hash

SHA-1 defined in FIPS 180-1 (1995)SHA-1 defined in FIPS 180-1 (1995)

SHA-256, SHA-384, SHA-512 defined in FIPSSHA-256, SHA-384, SHA-512 defined in FIPS
180-2 (2002), 256/384/512-bit hashes180-2 (2002), 256/384/512-bit hashes
 SHA-1 being phased out, attack knownSHA-1 being phased out, attack known
 SHA-512 processes input messageSHA-512 processes input message

with total size less than 2with total size less than 2128128
bitsbits

in 1024 bit blocksin 1024 bit blocks

to produce a 512-bit digestto produce a 512-bit digest

31. SHA-512 Hash FunctionSHA-512 Hash Function

32. Public KeyPublic Key EncryptionEncryption

33. Public Key Encryption -Public Key Encryption -
OperationOperation
 public key is used for encryptionpublic key is used for encryption
 private key is used for decryptionprivate key is used for decryption
 infeasible to determine decryption key giveninfeasible to determine decryption key given
encryption key and algorithmencryption key and algorithm
 steps:steps:

user generates pair of keysuser generates pair of keys

user places one key in public domainuser places one key in public domain

to send a message to user, encrypt using public keyto send a message to user, encrypt using public key

user decrypts using private keyuser decrypts using private key

34. Digital SignaturesDigital Signatures

35. Digital SignaturesDigital Signatures
 sender encrypts message with private keysender encrypts message with private key
 receiver decrypts with senders public keyreceiver decrypts with senders public key
 authenticates senderauthenticates sender
 does not give privacy of datadoes not give privacy of data

must send both original and encrypted copiesmust send both original and encrypted copies
 more efficient to sign authenticatormore efficient to sign authenticator

a secure hash of messagea secure hash of message

send signed hash with messagesend signed hash with message

36. RSARSA
AlgorithmAlgorithm

37. RSA ExampleRSA Example

38. RSA SecurityRSA Security
 brute force search of all keysbrute force search of all keys

given size of parameters is infeasiblegiven size of parameters is infeasible

but larger keys do slow calculationsbut larger keys do slow calculations
 factor n to recover p & qfactor n to recover p & q

a hard problema hard problem

well known 129 digit challenge broken in 1994well known 129 digit challenge broken in 1994

key size of 1024-bits (300 digits) currentlykey size of 1024-bits (300 digits) currently
secure for most appssecure for most apps

39. Public Key CertificatesPublic Key Certificates

40. Secure Sockets Layer /Secure Sockets Layer /
Transport Layer SecurityTransport Layer Security
 Secure Sockets Layer (SSL) is a widely used setSecure Sockets Layer (SSL) is a widely used set
of general purpose security protocolsof general purpose security protocols

use TCPuse TCP to provideto provide reliable end-to-end servicereliable end-to-end service
 Transport Layer Security (TLS) inTransport Layer Security (TLS) in RFC 2246RFC 2246
 two implementationtwo implementation optionsoptions

incorporated in underlying protocol suiteincorporated in underlying protocol suite

embedded in specific packagesembedded in specific packages
 minor differences betweenminor differences between SSLv3SSLv3 andand TLSTLS

41. SSL ArchitectureSSL Architecture

42. SSL Connection and SessionSSL Connection and Session
 SSL ConnectionSSL Connection

a transport connection providing suitable servicea transport connection providing suitable service

are peer-to-peerare peer-to-peer, t, transientransient

associated with one sessionassociated with one session

multiple secure connectionsmultiple secure connections between parties possiblebetween parties possible
 SSL sessionSSL session

an association between client and serveran association between client and server

createdcreated byby Handshake ProtocolHandshake Protocol

define set of cryptographic security parametersdefine set of cryptographic security parameters

to avoid negotiation of new security parameters forto avoid negotiation of new security parameters for
each connectioneach connection

multiple simultaneous sessions between partiesmultiple simultaneous sessions between parties
possible but not used in practicepossible but not used in practice

43. SSL Record ProtocolSSL Record Protocol
 provides confidentiality serviceprovides confidentiality service

used to encrypt SSL payload dataused to encrypt SSL payload data
 provides message integrity serviceprovides message integrity service

usedused to form message authentication codeto form message authentication code
(MAC)(MAC)
 Handshake Protocol defines shared secretHandshake Protocol defines shared secret
keys for each of above serviceskeys for each of above services

44. SSL Record ProtocolSSL Record Protocol
OperationOperation

45. Record Protocol HeaderRecord Protocol Header
 content type (8 bits)content type (8 bits)

change_cipher_spec, alert, handshake, andchange_cipher_spec, alert, handshake, and
application_dataapplication_data

no distinctionno distinction betweenbetween applications (eg. HTTP)applications (eg. HTTP)

content of application datacontent of application data opaque to SSLopaque to SSL
 major version (8 bits)major version (8 bits) – SSL v3 is 3– SSL v3 is 3
 minor version (8 bits)minor version (8 bits) -- SSLv3 value is 0SSLv3 value is 0
 compressed length (16 bits)compressed length (16 bits)

maximum 2maximum 21414
+ 2048+ 2048

46. Change Cipher Spec ProtocolChange Cipher Spec Protocol
 usesuses Record ProtocolRecord Protocol
 single messagesingle message

single byte value 1single byte value 1
 causecause pending state to be copied intopending state to be copied into
current statecurrent state

updates cipher suite to be used on thisupdates cipher suite to be used on this
connectionconnection

47. Alert ProtocolAlert Protocol
 convey SSL-related alerts to peer entityconvey SSL-related alerts to peer entity
 alertalert messages compressed and encryptedmessages compressed and encrypted
 two bytestwo bytes

first byte warning(1) or fatal(2)first byte warning(1) or fatal(2)
• if fatal, SSL immediately terminates connectionif fatal, SSL immediately terminates connection
• other connections on session may continueother connections on session may continue
• nono new connections on sessionnew connections on session

second byte indicates specific alertsecond byte indicates specific alert
• eg.eg. fatal alert is an incorrect MACfatal alert is an incorrect MAC
• eg.eg. nonfatal alert is close_notify messagenonfatal alert is close_notify message

48. Handshake ProtocolHandshake Protocol
 most complex protocolmost complex protocol
 allows parties to authenticate each otherallows parties to authenticate each other
 and negotiate encryption and MACand negotiate encryption and MAC
algorithmalgorithm and cryptographic keysand cryptographic keys
 series of messages with four phases:series of messages with four phases:

phase 1 Initiate Connectionphase 1 Initiate Connection

phase 2 Certificate/Key Exchangephase 2 Certificate/Key Exchange

phase 3 Client Verifies Certificate, Parametersphase 3 Client Verifies Certificate, Parameters

phase 4 Complete Secure Connection Setupphase 4 Complete Secure Connection Setup

49. SSLSSL
HandshakeHandshake
ProtocolProtocol

50. SSL Handshake ProtocolSSL Handshake Protocol
ParametersParameters
 versionversion
 randomrandom
 session IDsession ID
 ciphersuiteciphersuite
 compression methodcompression method

51. IPv4 and IPv6 SecurityIPv4 and IPv6 Security
 IP Security extensions (IPSec) for IPv4/v6IP Security extensions (IPSec) for IPv4/v6
 developed in response to observed weaknessesdeveloped in response to observed weaknesses
 to stop unauthorized traffic monitoring, secureto stop unauthorized traffic monitoring, secure
user traffic with authentication & encryptionuser traffic with authentication & encryption
 example uses:example uses:

secure branch office connectivity over Internetsecure branch office connectivity over Internet

secure remote access over Internetsecure remote access over Internet

extranet and intranet connectivityextranet and intranet connectivity

enhanced electronic commerce securityenhanced electronic commerce security
 can encrypt / authenticate all traffic at IP levelcan encrypt / authenticate all traffic at IP level

52. IPSec FacilitiesIPSec Facilities
 Authentication Header (AH)Authentication Header (AH)

authentication only serviceauthentication only service
 Encapsulated Security Payload (ESP)Encapsulated Security Payload (ESP)

combined authentication & encryption servicecombined authentication & encryption service

generally used for virtual private networksgenerally used for virtual private networks
 key exchangekey exchange

both manual and automatedboth manual and automated
 in RFC’s 2401,2402,2406,2408 (1998)in RFC’s 2401,2402,2406,2408 (1998)

53. Security Association (SA)Security Association (SA)
 one-way sender-receiver relationshipone-way sender-receiver relationship
 for two-way, need two security associationsfor two-way, need two security associations
 three SA identification parametersthree SA identification parameters

security parameter index (in AH/ESP header)security parameter index (in AH/ESP header)

IP destination address (unicast only)IP destination address (unicast only)

security protocol identifier (AH or ESP)security protocol identifier (AH or ESP)
 SA uniquely identified by dest address inSA uniquely identified by dest address in
IPv4/6 header and SPI in AH/ESP headerIPv4/6 header and SPI in AH/ESP header

54. SA ParametersSA Parameters
 sequence number countersequence number counter
 sequence counter overflowsequence counter overflow
 anti-reply windowsanti-reply windows
 AH informationAH information
 ESP informationESP information
 lifetime of this associationlifetime of this association
 IPSec protocol modeIPSec protocol mode
 path MTUpath MTU

55. Authentication HeaderAuthentication Header

56. Encapsulating SecurityEncapsulating Security
PayloadPayload

57. WiFi Protected AccessWiFi Protected Access
 WiFi Protected Access (WPA) extensionsWiFi Protected Access (WPA) extensions
to address 802.11 security issuesto address 802.11 security issues

based on current 802.11i standardbased on current 802.11i standard

addresses authentication, key management,addresses authentication, key management,
data transfer privacydata transfer privacy
 uses authentication server and a moreuses authentication server and a more
robust protocolrobust protocol
 encryption with AES or 104-bit RC4encryption with AES or 104-bit RC4

58. WiFi Protected AccessWiFi Protected Access

59. 802.11i Access Control802.11i Access Control

60. 802.11i Privacy & Integrity802.11i Privacy & Integrity
 have Temporal Key Integrity Protocolhave Temporal Key Integrity Protocol
(TKIP) or WPA-1(TKIP) or WPA-1

s/w only changes to existing equipments/w only changes to existing equipment

using same RC4 algorithm as older WEPusing same RC4 algorithm as older WEP
 and Counter Mode CBC MAC (CCMP) orand Counter Mode CBC MAC (CCMP) or
WPA-2 using AES encryptionWPA-2 using AES encryption
 both add message integrity code (MIC)both add message integrity code (MIC)

generated using Michael algorithmgenerated using Michael algorithm

61. SummarySummary
 security requirements and attackssecurity requirements and attacks
 confidentiality using symmetric encryptionconfidentiality using symmetric encryption
 message authentication & hash functionsmessage authentication & hash functions
 public-key encryption & digital signaturespublic-key encryption & digital signatures
 secure socket layer (SSL)secure socket layer (SSL)
 IPSecIPSec
 WiFi Protected AccessWiFi Protected Access