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Data and Computer Data 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
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
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
Passive AttacksPassive Attacks
eavesdropping on transmissionseavesdropping on transmissions to obtain informationto obtain information
release of possibly sensitive/confidential release of possibly sensitive/confidential message contentsmessage contents
traffic analysis which monitors frequency and traffic 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
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
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 secret sender and receiver must obtain secret key securelykey securely
once key is known, all communication once key is known, all communication using this key is readableusing this key is readable
Attacking EncryptionAttacking Encryption
cryptanalysiscryptanalysis relay on nature of algorithm plus some knowledge of relay 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
Block CiphersBlock Ciphers
most common symmetric algorithmsmost common symmetric algorithms process plain text in fixed block sizes process 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:
Data Data Encryption Standard Encryption Standard (DES)(DES) Advanced Encryption StandardAdvanced Encryption Standard
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 Frontier broken 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
Location of Encryption Location of Encryption DevicesDevices
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 to message 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
End to End EncryptionEnd to End Encryption
encryption done at ends of systemencryption done at ends of system data in encrypted form crosses network data 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 or otherwise 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
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 that authentication 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
Authentication Using Authentication Using Symmetric EncryptionSymmetric Encryption
assume sender & receiver only know keyassume sender & receiver only know key only sender could have encrypted only sender could have encrypted
message for other partymessage for other party message must include one of:message must include one of:
error detection code error detection code sequence numbersequence number time stamptime stamp
Authentication Without Authentication Without EncryptionEncryption
authentication tag generated and appended to authentication 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 be programs authenticated without encryption can be executed without decodingexecuted without decoding
Message Authentication CodeMessage Authentication Code
generate authentication code based on shared generate 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 code if 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 assured if message has sequence number, receiver assured
of proper sequenceof proper sequence
can use various algorithms, eg. DES can use various algorithms, eg. DES
RSA RSA AlgorithmAlgorithm
RSA ExampleRSA Example
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) currently key size of 1024-bits (300 digits) currently
secure for most appssecure for most apps
Public Key CertificatesPublic Key Certificates
WiFi Protected AccessWiFi Protected Access
WiFi Protected Access (WPA) extensions WiFi 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 more uses authentication server and a more
robust protocolrobust protocol encryption with AES or 104-bit RC4encryption with AES or 104-bit RC4
WiFi Protected AccessWiFi Protected Access
802.11i Access Control802.11i Access Control
802.11i Privacy & Integrity802.11i Privacy & Integrity
have Temporal Key Integrity Protocol have 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) or and 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