Data and Computer Communications Eighth Edition by William Stallings Lecture slides by Lawrie Brown Chapter 21 – Network Security

Network 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 Requirements  confidentiality - protect data content/access  integrity - protect data accuracy  availability - ensure timely service  authenticity - protect data origin

Passive Attacks  eavesdropping on transmissions  to obtain information release of possibly sensitive/confidential message contents release of possibly sensitive/confidential message contents traffic analysis which monitors frequency and length of messages to get info on senders traffic analysis which monitors frequency and length of messages to get info on senders  difficult to detect  can be prevented using encryption

Active Attacks  masquerade pretending to be a different entity pretending to be a different entity  replay  modification of messages  denial of service  easy to detect detection may lead to deterrent detection may lead to deterrent  hard to prevent focus on detection and recovery focus on detection and recovery

Requirements for Security  strong encryption algorithm even known, unable to decrypt without key even known, unable to decrypt without key even if many plaintexts & ciphertexts available even if many plaintexts & ciphertexts available  sender and receiver must obtain secret key securely  once key is known, all communication using this key is readable

Attacking Encryption  cryptanalysis relay on nature of algorithm plus some knowledge of general characteristics of plaintext relay on nature of algorithm plus some knowledge of general characteristics of plaintext attempt to deduce plaintext or key attempt to deduce plaintext or key  brute force try every possible key until plaintext is recovered try every possible key until plaintext is recovered rapidly becomes infeasible as key size increases rapidly becomes infeasible as key size increases 56-bit key is not secure 56-bit key is not secure

Block Ciphers  most common symmetric algorithms  process plain text in fixed block sizes producing block of cipher text of equal size  most important current block ciphers: Data Encryption Standard (DES) Data Encryption Standard (DES) Advanced Encryption Standard Advanced Encryption Standard

Data Encryption Standard  US standard  64 bit plain text blocks  56 bit key  broken in 1998 by Electronic Frontier Foundation special purpose US$250,000 machine special purpose US$250,000 machine with detailed published description with detailed published description less than three days less than three days DES now worthless DES now worthless

Location of Encryption Devices

Link Encryption  each communication link equipped at both ends  all traffic secure  high level of security  requires lots of encryption devices  message must be decrypted at each switch to read address (virtual circuit number)  security vulnerable at switches particularly on public switched network particularly on public switched network

End to End Encryption  encryption done at ends of system  data in encrypted form crosses network unaltered  destination shares key with source to decrypt  host can only encrypt user data otherwise switching nodes could not read header or route packet otherwise switching nodes could not read header or route packet  hence traffic pattern not secure  solution is to use both link and end to end

Message Authentication  protection against active attacks with falsification of data falsification of data falsification of source falsification of source  authentication allows receiver to verify that message is authentic has not been altered has not been altered is from claimed/authentic source is from claimed/authentic source timeliness timeliness

Authentication Using Symmetric Encryption  assume sender & receiver only know key  only sender could have encrypted message for other party  message must include one of: error detection code error detection code sequence number sequence number time stamp time stamp

Authentication Without Encryption  authentication tag generated and appended to each message  message not encrypted  useful when don’t want encryption because: messages broadcast to multiple destinations messages broadcast to multiple destinations have one destination responsible for authenticationhave one destination responsible for authentication one side heavily loaded one side heavily loaded encryption adds to workloadencryption adds to workload can authenticate random messagescan authenticate random messages programs authenticated without encryption can be executed without decoding programs authenticated without encryption can be executed without decoding

Message Authentication Code  generate authentication code based on shared key and message  common key shared between A and B  if only sender and receiver know key and code matches: receiver assured message has not altered receiver assured message has not altered receiver assured message is from alleged sender receiver assured message is from alleged sender if message has sequence number, receiver assured of proper sequence if message has sequence number, receiver assured of proper sequence  can use various algorithms, eg. DES

RSA Algorithm

RSA Example

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

Public Key Certificates

WiFi Protected Access  WiFi Protected Access (WPA) extensions to address security issues based on current i standard based on current i standard addresses authentication, key management, data transfer privacy addresses authentication, key management, data transfer privacy  uses authentication server and a more robust protocol  encryption with AES or 104-bit RC4

WiFi Protected Access

802.11i Access Control

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