Net-Centric Computing Division Department of Computer Science Bogor Agricultural University

1. Net-Centric Computing Division Department of Computer Science Bogor Agricultural University KOM 312 Komunikasi Data danJaringanKomputer Basic of Network Security Sri Wahjuni my_juni04(at)ipb.ac.id; my_juni04(at)yahoo.com

2. agenda • Network Security Threats • Security and Cryptography • Network Security Protocols swj/11

3. background • The combination of low-cost powerful computing and high-performance networks is a two-edged sword: • Many powerful new services and applications are enabled • But computer systems and networks become highly susceptible to a wide variety of security threats • Network security involves countermeasures to protect computer systems from intruders • Firewalls, security protocols, security practices swj/11

4. agenda • Network Security Threats • Security and Cryptography • Network Security Protocols swj/11

5. Threats, Security Requirements, and Countermeasures • Network Security Threats • Eavesdropping, man-in-the-middle, client and server imposters • Denial of Service attacks • Viruses, worms, and other malicious code • Network Security Requirements • Privacy, Integrity, Authentication, Non-Repudiation, Availability • Countermeasures • Communication channel security • Border security swj/11

6. Security Requirements Security threats motivate the following requirements: • Privacy/confidentially: information should be readable only by intended recipient • Integrity: recipient can confirm that a message has not been altered during transmission • Authentication: it is possible to verify that sender or receiver is who he claims to be • Non-repudiation: sender cannot deny having sent a given message. • Availability: of information and services swj/11

7. replay Request Server Client Response Threats : Eavesdropping • Information transmitted over network can be observed and recorded by eavesdroppers (using a packet sniffer) • Information can be replayed in attempts to access server • Requirements: privacy, authentication, non-repudiation swj/11

8. Server Client Imposter Threats : Client Imposter • Imposters attempt to gain unauthorized access to server • Ex. bank account or database of personal records • For example, in IP spoofing imposter sends packets with false source IP address • Requirements: privacy, authentication swj/11

9. Server Imposter Client Server Imposter • An imposter impersonates a legitimate server to gain sensitive information from a client • E.g. bank account number and associated user password • Requirements: privacy, authentication, non-repudiation swj/11

10. Man in the middle Client Server Threats : Man in the middle attack • An imposter manages to place itself as man in the middle • convincing the server that it is legitimate client • convincing legitimate client that it is legitimate server • gathering sensitive information and possibly hijacking session • Requirements: integrity, authentication swj/11

11. Server Attacker Threats : Denial of Service Attack • Attacker can flood a server with requests, overloading the server resources • Results in denial of service to legitimate clients • Distributed denial of service attack on a server involves coordinated attack from multiple (usually hijacked) computers • Requirement: availability swj/11

12. Server Imposter Client Threats : Malicious Code • A client becomes infected with malicious code • Opening attachments in email messages • Executing code from bulletin boards or other sources • Virus: code that, when executed, inserts itself in other programs • Worms: code that installs copies of itself in other machines attached to a network • Many variations of malicious code • Requirements: privacy, integrity, availability swj/11

13. Countermeasures Secure communication channels • Encryption • Cryptographic checksums and hashes • Authentication • Digital Signatures swj/11

14. Countermeasures (2) Secure borders • Firewalls • Virus checking • Intrusion detection • Authentication • Access Control swj/11

15. agenda • Network Security Threats • Security and Cryptography • Network Security Protocols swj/11

16. Cryptography • Encryption: transformation of plaintext message into encrypted (and unreadable) message called ciphertext • Decryption: recovery of plaintext from ciphertext • Cipher: algorithm for encryption & decryption • A secret key is required to perform encryption & decryption swj/11

17. Cryptography • Conventional : • Substitution Cipher • Transposition Cipher • Modern (Using mathematical methods): • Secret/symetric key • Public key • Cryptographic checksum and hash swj/11

18. Substitution Ciphers Substitution Cipher: Map each letter or numeral into another letter of numeral: a b c d e f g h i j k l m n o p q r s t u v w x y z z y x w v u t s r q p o n m l k j i h g f e d c b a • Example: • hvxfirgb security • Substitution ciphers are easy to break • Take histogram of frequency of occurrence of letters in a ciphertext message • Match to known frequencies of letters swj/11

19. Transposition Cipher Transposition Cipher: Rearrange order of letters/numerals in a message using a particular rearrangement: • interchange character k with character k+1 • Example: • security esuciryt • Transposition Ciphers are easy to break • Suppose plaintext and ciphertext are known • Matching of letters in plaintext and ciphertext will reveal transposition mapping swj/11

20. Encryption Decryption C=E (P) Ciphertext Plaintext P P K DK(.) EK(.) Key K Key K Secret/symetric Key Cryptography • Sender encrypts P by applying mapping EK which depends on secret key K: C = EK(P) • Receiver decrypts C by applying inverse mapping DK which also depends on K: DK(EK(P)) = P • Examples of symetric key methods : • Data Encryption Standard (DES) and Triple DES • Advanced Encryption Standard (AES) swj/11

21. Security using Secret Key Cryptography • Privacy: secret key renders messages confidential • Integrity: alteration of the cyphertext will be detected, because the decrypted message will be gibberish • When privacy is not required, encryption of the entire message is overkill because much processing involved • We will see that cryptographic checksums provide integrity and require less processing swj/11

22. Cryptographic Checksums and Hashes CrytoChk Message Message • Transmitter calculates a fixed number of bits (crypto checksum/hash) that depends on secret key K: HK(P) • Receiver recalculates hash from received message & compares to received hash • example : MD5, SHA-1 P P Crypto Checksum Calculator HK(P) K swj/11

23. Encryption Decryption DK2(.) Plaintext P EK1(.) Ciphertext P C = EK1(P) Private key K2 Public key K1 Public Key Cryptography • Public key cryptography provides privacy using two different keys: • Public key K1 available to all for encrypting messages to a certain user: C = EK1(P) • Private key K2 for user to decrypt messages: P = DK2(EK1(P)) • Example: RSA public key cryptography • Implementation: authentication, digital signature swj/11

24. Digital signatures using Public Key Cryptography • Digital signatures provide non repudiation • User “signs” a message that cannot be repudiated • Digital signature obtained as follows: • Transmitter obtains a hash of the message • Transmitter encrypts the hash using its private key; result is the digital signature • Transmitter sends message and signature • To check the signature: • Receiver obtains hash of message • Receiver decrypts signature using sender’s public key • Receiver compares hash computed from message and hash obtained from signature • Procedure also ensures message integrity swj/11

25. secret Key vs. Public Key • Public key systems have more capabilities • Secret key: privacy, integrity, authentication • Public key: all of above + digital signature • Public key algorithms are more complex • Require more processing and hence much slower than secret key • Practice: • Use public key method during session setup to establish a session key • Use secret key cryptography during session using the session key swj/11

26. Example: Pretty Good Privacy (PGP) • PGP developed by Phillip Zimmerman to provide secure email • http://www.philzimmermann.com/index.shtml • http://www.pgpi.org • Notorious for becoming publicly available for download over Internet in violation of US export restrictions • Uses public key cryptography to provide • Privacy, integrity, authentication, digital signature • De facto standard for email security • Also provides privacy and integrity for stored files swj/11

27. Key Distribution in Secret Key Systems • Kerberos: authentication service for users to access servers over network • Key distribution center(KDC) has secret key with every user • At login, user supplies ID and password • KDC authenticates user & generates session key • Session key & ticket-granting ticket (TGT) is sent to user encrypted using shared secret key • To access a particular server, user sends request to KDC with server name and TGT • KDC decrypts TGT to recover session key & then returns ticket to client for desired server swj/11

28. Key Distribution in Public Key Systems • In public key only one pair of keys per user • Key distribution problem: How to determine whether an advertised public key is not from an imposter? • Certification Authority (CA) • Issues digitally signed certificate that provides • User’s name & public key • Certificate serial #, expiration date • Certificates can be stored in publicly accessible directories • To communicate with B, a user contacts the CA to obtain the certificate for B • Users are configured to have the CA’s public key, which they use to verify the digital signature swj/11

29. agenda • Network Security Threats • Security and Cryptography • Network Security Protocols swj/11

30. Direct Connections to Internet • Computers A & B communicate across the Internet • Exposure to eavesdropping, imposters, DoS • Can encrypt some transmitted information • But IP headers need to be visible to routers & hence others • Eavesdropper can gather variety of usage information & deduce nature of interaction • Choice of which layer to apply security: IP, transport, or application layer swj/11

31. Gateway-to-Gateway • Computers A and B have gateways interposed between their internal network and Internet • Gateway can be a firewall • Controls external access to internal network • Packet filtering according to various header fields • IP addresses, port numbers, ICMP types, fields within payload • Secure tunnels can be established between gateways • All internal information including headers can be encrypted swj/11

32. Remote user to Gateway • Mobile host needs access to internal network • Gateway must provide user with access while barring intruders from accessing internal network • May also need to protect identity of mobile user • IP-address of mobile user changes swj/11

33. firewalls swj/11

34. Firewall Options • Firewalls can operate at different layers • IP-layer filtering cannot operate on payload contents • Circuit-Level Gateways • Direct client-to-server TCP connections not allowed • Relays TCP segments between actual client & actual server • Application-Level Gateways or Proxies • Interposed between actual client and actual server • Performs authentication and determines what features are available to client • Monitors, filters & relays messages swj/11

35. Virtual private networks (vpn) • Common design : implement firewall together with IPSec to create a secure tunnel • single authenticated • providing integrity, secrecy swj/11

36. Protocol Layer Options • Security Services can be provided at different layers of the protocol stack • Data Link Layer security • Point-to-point security between directly-connected devices, e.g. wireless LAN security • IP-Layer security • Security service between IP-layer & Transport layer • End-to-end security across an internet, e.g. IPsec • Transport Layer security • Security service between Transport & Application Layers • E.g. Secure Sockets Layer & Transport Layer Security swj/11

37. communication Security Services • Integrity Service: information received from network has not been altered during transmission • Authentication Service: the receiver can authenticate that information came from trusted sender • Privacy Service: information is readable only by intended recipient • In applications that require network security, integrity & authentication essential; privacy not always justified swj/11

38. Security Association • A Security Association (SA) is a logical simplex connection between two network-layer entities • Two SA’s required for bidirectional secure communication • SA is specified by • A unique identifier • Security services to be used • Cryptographic algorithms to be used • How shared keys will be established • Other attributes such as lifetime • SA negotiated before security service begins swj/11

39. 802.11 security : WEP • Wired Equivalent Privacy (WEP): • host requests authentication from access point • access point sends 128 bit nonce • host encrypts nonce using shared symmetric key (40 bits) • access point decrypts nonce, authenticates host swj/11

40. IP Security (IPsec) . • IPsec defined in RFCs 2401, 2402, 2406 • Provides authentication, integrity, confidentiality, and access control at the IP layer • Provides a key management protocol to provide automatic key distribution techniques (Internet key exchange/IKE). • Security service can be provided between a pair of communication nodes, where the node can be a host or a gateway (router or firewall). • Two protocols & two modes to provide traffic security: • Authentication Header and Encapsulating Security Payload • Transport mode or tunnel mode swj/11

41. IPv4 Header AH Upper Layer (e.g., TCP or UDP) IPsec Authentication Header • Authentication header (AH) placed after headers that are examined at every hop • Presence of AH indicated by protocol value = 51 in IPv4 header • Authentication performed over all fields including IP header, except fields that change at every hop

42. Secure Sockets Layer (SSL) • SSL developed by Netscape Communications • Operates on top of TCP • Security services: server authentication, data encryption, client authentication (optional) • Provides secure connections • HTTP, FTP, telnet, … • Electronic ordering & payment; e-mail • SSL 3.0 submitted to IETF for standardization • TLS standardized by IETF (RFC 2246) • Slight differences with SSL 3.0 swj/11

43. Handshake Protocol HTTP Protocol Alert Protocol Change cipher spec Protocol TLS Record Protocol TCP IP Transport Layer Security (TLS) • TLS protocols operate at two layers • TLS Record Protocol operates on top of TCP • Protocols on top of TLS Record Protocol • TLS Handshake Protocol • TLS Change Cipher Specification Protocol • TLS Alert Protocol swj/11

44. references • Garcia A.L., Widjaja A. 2004. Networks Communication : Fundamental Concepts and Key Architectures 2nd ed. – Chapter 11. McGraw-Hill Companies, Inc. swj/11