IP Security and VPN

1. IP Securityand VPN Most of the slides are derived from the slides (Chapter-8) by the authors of «Computer Networking: A Top Down Approach», and from the slides of Prof. Yan Chen from Northwestern University.

2. IP Security • Have a range of application specific security mechanisms • eg. PGP, SSL/HTTPS • However there are security concerns that cut across protocol layers • Would like security implemented by the network for all applications

3. What is network-layer confidentiality ? between two network entities: • sending entity encrypts datagram payload, payload could be: • TCP or UDP segment, ICMP message, OSPF message …. • all data sent from one entity to other would be hidden: • web pages, e-mail, P2P file transfers, TCP SYN packets … • “blanket coverage”

4. Virtual Private Networks (VPNs) motivation: • institutions often want private networks for security. • costly: separate routers, links, DNS infrastructure. • VPN: institution’s inter-office traffic is sent over public Internet instead • encrypted before entering public Internet • logically separate from other traffic

5. IP header IP header IP header IPsec header IPsec header IPsec header Secure payload Secure payload Secure payload IP header IP header payload payload Virtual Private Networks (VPNs) laptop w/ IPsec publicInternet salespersonin hotel router w/ IPv4 and IPsec router w/ IPv4 and IPsec branch office headquarters

6. Benefits of IPSec • In a firewall/router provides strong security to all traffic crossing the perimeter • In a firewall/router is resistant to bypass • Is below transport layer, hence transparent to applications • Can be transparent to end users • Can provide security for individual users • Secures routing architecture

7. IPsec services • data integrity • origin authentication • replay attack prevention • confidentiality • two protocols providing different service models: • AH • ESP

8. IPsec transport mode • IPsec datagram emitted and received by end-system • protects upper level protocols IPsec IPsec

9. IPsec – tunneling mode • edge routers IPsec-aware IPsec IPsec IPsec IPsec • hosts IPsec-aware

10. Two IPsec protocols • Authentication Header (AH) protocol • provides source authentication & data integrity but not confidentiality • Encapsulation Security Protocol (ESP) • provides source authentication, data integrity, and confidentiality • more widely used than AH

11. Four combinations are possible! most common andmost important

12. Transport Mode • ESP protects higher layer payload only • AH can protect IP headers as well as higher layer payload IP header IP options IPSec header Higher layer protocol ESP Real IP destination AH

13. Tunnel Mode • ESP applies only to the tunneled packet • AH can be applied to portions of the outer header Outer IP header IPSec header Inner IP header Higher layer protocol ESP Real IP destination Destination IPSec entity AH

14. Security associations (SAs) • before sending data, “security association (SA)”established from sending to receiving entity • SAs are simplex: for only one direction • ending, receiving entitles maintain state information about SA • recall: TCP endpoints also maintain state info • IP is connectionless; IPsec is connection-oriented! • how many SAs in VPN w/ headquarters, branch office, and n traveling salespeople?

15. Security Association (SA) • Defined by 3 parameters: • Security Parameters Index (SPI) • IP Destination Address • Security Protocol Identifier • Have a database of Security Associations • Determine IPSec processing for senders • Determine IPSec decoding for destination • SAs are not fixed! Generated and customized per traffic flows

16. Example SA from R1 to R2 Internet headquarters branch office R1 stores for SA: • 32-bit SA identifier: Security Parameter Index (SPI) • origin SA interface (200.168.1.100) • destination SA interface (193.68.2.23) • type of encryption used (e.g., 3DES with CBC) • encryption key • type of integrity check used (e.g., HMAC with MD5) • authentication key 200.168.1.100 193.68.2.23 security association R1 R2 172.16.1/24 172.16.2/24

17. Security Association Database (SAD) • endpoint holds SA state in security association database (SAD), where it can locate them during processing. • with n salespersons, 2 + 2n SAs in R1’s SAD • when sending IPsec datagram, R1 accesses SAD to determine how to process datagram. • when IPsec datagram arrives to R2, R2 examines SPI in IPsec datagram, indexes SAD with SPI, and processes datagram accordingly.

18. Authenticated Header (AH) • Data integrity • Entire packet has not been tampered with • Authentication • Can “trust” IP address source • Use MAC to authenticate • Symmetric encryption, e.g, DES • One-way hash functions, e.g, HMAC-MD5-96 or HMAC-SHA-1-96 • Anti-replay feature • Integrity check value

19. IPSec Authenticated Header SAD … Length of the authentication header Payload Length Next Header (TCP/UDP) Reserved SPI Sequence Number ICV

20. Integrity Check Value - ICV • Keyed Message authentication code (MAC) calculated over • IP header field that do not change or are predictable • Source IP address, destination IP, header length, etc. • Prevent spoofing • Mutable fields excluded: e.g., time-to-live (TTL), IP header checksum, etc. • IPSec protocol header except the ICV value field • Upper-level data • Code may be truncated to first 96 bits

21. AH: Tunnel and Transport Mode • Original • Transport Mode • Cover most of the original packet • Tunnel Mode • Cover entire original packet

22. Encapsulating Security Payload (ESP) • Provide message content confidentiality • Provide limited traffic flow confidentiality • Can optionally provide the same authentication services as AH • Supports range of ciphers, modes, padding • Incl. DES, Triple-DES, RC5, IDEA, CAST etc • A variant of DES most common • Pad to meet blocksize, for traffic flow

23. ESP: Tunnel and Transport Mode • Original • Transport Mode • Good for host to host traffic • Tunnel Mode • Good for VPNs, gateway to gateway security

24. ESP Transport Example Original IP Header SPI Sequence Number Authentication coverage Payload (TCP Header and Data) Variable Length Encrypted Padding (0-255 bytes) Pad Length Next Header Integrity Check Value

25. “enchilada” authenticated encrypted new IPheader ESP hdr originalIP hdr Original IP datagram payload ESP trl ESP auth SPI Seq # padding padlength nextheader IPsec datagram focus for now on tunnel mode with ESP

26. “enchilada” authenticated encrypted new IPheader ESP hdr originalIP hdr Original IP datagram payload ESP trl ESP auth SPI Seq # padding padlength nextheader What happens? Internet headquarters branch office 200.168.1.100 193.68.2.23 security association R1 R2 172.16.1/24 172.16.2/24

27. R1: convert original datagram to IPsec datagram • appends to back of original datagram (which includes original header fields!) an “ESP trailer” field. • encrypts result using algorithm & key specified by SA. • appends to front of this encrypted quantity the “ESP header, creating “enchilada”. • creates authentication MAC over the whole enchilada, using algorithm and key specified in SA; • appends MAC to back of enchilada, forming payload; • creates brand new IP header, with all the classic IPv4 header fields, which it appends before payload.

28. “enchilada” authenticated encrypted new IPheader ESP hdr originalIP hdr Original IP datagram payload ESP trl ESP auth SPI Seq # padding padlength nextheader Inside the enchilada: • ESP trailer: Padding for block ciphers • ESP header: • SPI, so receiving entity knows what to do • Sequence number, to thwart replay attacks • MAC in ESP auth field is created with shared secret key

29. IPsec sequence numbers • for new SA, sender initializes seq. # to 0 • each time datagram is sent on SA: • sender increments seq # counter • places value in seq # field • goal: • prevent attacker from sniffing and replaying a packet • receipt of duplicate, authenticated IP packets may disrupt service • method: • destination checks for duplicates • doesn’t keep track of all received packets; instead uses a window

30. Security Policy Database (SPD) • policy: For a given datagram, sending entity needs to know if it should use IPsec • needs also to know which SA to use • may use: source and destination IP address; protocol number • info in SPD indicates “what” to do with arriving datagram • info in SAD indicates “how” to do it

31. SPD Entry Actions • Discard • Do not let in or out • Bypass • Outbound: do not apply IPSec • Inbound: do not expect IPSec • Protect – will point to an SA or SA bundle • Outbound: apply security • Inbound: check that security must have been applied

32. SPD Protect Action • If the SA does not exist… • Outbound processing: use IKE to generate SA dynamically • Inbound processing: drop packet

33. Outbound Processing Outbound packet (on A) A B IP Packet SPD(Policy) SA Database SPI & IPSec Packet … … Send to B Is it for IPSec?If so, which policy entry to select? IPSec processing Determine the SA and its SPI

34. Inbound Processing A B Inbound packet (on B) SPD(Policy) From A SPI & Packet SA Database … … Use SPI to index the SAD Was packet properly secured? Original IP Packet “un-process”

35. Key Management • Handles key generation & distribution • Typically need 2 pairs of keys • 2 per direction for AH & ESP • Manual key management • Sysadmin manually configures every system (IPSec SAs are manually established.) • Automated key management • manual keying is impractical for VPN with 100s of endpoints • instead use IPsec IKE (Internet Key Exchange) • IKE PKI: both sides start with public/private key pair, certificate • Similar to SSL handshake

36. Summary: IPsec services • suppose Trudy sits somewhere between R1 and R2. she doesn’t know the keys. • will Trudy be able to see original contents of datagram? How about source, dest IP address, transport protocol, application port? • flip bits without detection? • masquerade as R1 using R1’s IP address? • replay a datagram?

37. IPsec summary • Either AH or ESP protocol (or both) • AH provides integrity, source authentication • ESP protocol (with AH) additionally provides encryption • IPsec peers can be two end systems, two routers/firewalls, or a router/firewall and an end system • IKE message exchange for algorithms, secret keys, SPI numbers