CSCE 715: Network Systems Security

1. CSCE 715:Network Systems Security Chin-Tser Huang huangct@cse.sc.edu University of South Carolina

2. Security in Network Layer • Implementing security in application layer provides flexibility in security policy and key management • Problem is need to implement security mechanism in every application individually • To reduce the overhead, implement security in network layer to provide security for all applications between selected pair of computers

3. IPSec • Current security standard for IP layer • Provide general security services for IP • Authentication • Confidentiality • Anti-replay • Key management • Applicable to use over LANs, across public and private WANs, and for the Internet

4. Scenario of IPSec Uses

5. Benefits of IPSec • Provide strong security to all traffic crossing the perimeter if installed in a firewall/router • Resistant to bypass • IPSec is below transport layer, hence transparent to applications • Can be transparent to end users • Can provide security for individual users if desired

6. IP Security Architecture • Specification is quite complex • Defined in numerous RFC’s • RFC 2401/2402/2406/2408 • many others, grouped by category • Two protocols • Authentication Header (AH) • Encapsulating Security Payload (ESP) • Mandatory in IPv6, optional in IPv4

7. Security Association (SA) • A unidirectional relationship between sender and receiver that affords security for traffic flow • Each IPSec computer maintains a database of SA’s • Defined by 3 parameters • Security Parameters Index (SPI) • IP Destination Address • Security Protocol Identifier

8. SA Parameters • Sequence Number Counter • Sequence Number Overflow • Anti-Replay Window • AH and ESP information • Lifetime • IPSec Protocol Mode • Path MTU

9. Authentication Header (AH) • Provide support for data integrity and authentication of IP packets • end system/router can authenticate user/app • prevent address spoofing attacks • guard against replay attacks by tracking sequence numbers • Based on use of a MAC • HMAC-MD5-96 or HMAC-SHA-1-96 • MAC is calculated over IP header fields that are either immutable or predictable, AH header other than authentication data, and entire upper-level protocol data • Parties must share a secret key

10. Authentication Header

11. Transport vs Tunnel Mode AH • Transport mode is used to authenticate IP payload and selected portion of IP header • good for host to host traffic • Tunnel mode authenticates entire IP packet and selected portion of outer IP header • good for VPNs, gateway to gateway security

12. End-to-End vs End-to-Intermediate Authentication

13. Scope of AH Authentication

14. Encapsulating Security Payload (ESP) • Provide message content confidentiality and limited traffic flow confidentiality • Can optionally provide the same authentication services as AH • Support range of ciphers, modes, padding • DES, Triple-DES, RC5, IDEA, CAST etc • CBC most common • pad to meet blocksize, for traffic flow

15. Encapsulating Security Payload

16. Padding • Serve several purposes • expand the plaintext to required length • make Pad Length and Next Header fields aligned to 32-bit word boundary • conceal actual length of payload

17. Transport vs Tunnel Mode ESP • Transport mode is used to encrypt and optionally authenticate IP data • data protected but header left in clear • can suffer from traffic analysis but is efficient • good for ESP host to host traffic • Tunnel mode encrypts entire IP packet • add new header for next hop • can counter traffic analysis • good for VPNs, gateway to gateway security

18. Transport vs Tunnel Mode ESP

19. Scope of ESP Encryption and Authentication

20. Combining Security Associations • SAs can implement either AH or ESP, but each SA can implement only one • Some traffic flows may require services of both AH and ESP, while some other flows may require both transport and tunnel modes • To address these concerns, need to combine SAs to form a security association bundle

21. Authentication plus Confidentiality • Which one first? Three approaches to consider • ESP with Authentication Option • Transport mode or tunnel mode • Authentication after encryption • Transport Adjacency • A bundle of two transport SAs, with the inner being an ESP SA and the outer being an AH SA • Authentication after encryption • Transport-Tunnel Bundle • A bundle consisting of an inner AH transport SA and an outer ESP tunnel SA • Authentication before encryption

22. Combining Security Associations

23. Key Management • Handle key generation and distribution • Typically need 2 pairs of keys • 2 per direction for AH & ESP • Manual key management • sysadmin manually configures every system • Automated key management • automated system for on demand creation of keys for SA’s in large systems • Oakley and ISAKMP

24. OAKLEY • A key exchange protocol • Based on Diffie-Hellman key exchange • Add features to address weaknesses of Diffie-Hellman • cookies to counter clogging attacks • nonces to counter replay attacks • key exchange authentication to counter man-in-the-middle attacks • Can use arithmetic in prime fields or elliptic curve fields

25. Usage of Cookies • Three basic requirements • Must depend on specific parties • Impossible for anyone other than issuing entity to generate cookies that will be accepted by issuing entity • Cookie generation and verification must be fast • To create a cookie, perform a fast hash over src and dst IP addresses, src and dst ports, and a locally generated secret value

26. ISAKMP • Internet Security Association and Key Management Protocol • Provide framework for key management • Define procedures and packet formats to establish, negotiate, modify, and delete SAs • Independent of key exchange protocol, encryption algorithm, and authentication method

27. ISAKMP Header

28. ISAKMP Payload

29. ISAKMP Exchange

30. ISAKMP Exchange

31. Next Class • Denial-of-Service (DoS) attack • Hop Integrity