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Network Security in Practice: Application, Transport, Network, Data Link, Physical Layers

This lecture discusses network security in different layers - application, transport, network, data link, and physical. It explores examples of protocols in each layer and the factors that determine security implementations. The lecture also highlights the differences between PGP, SSL, and IPsec.

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Network Security in Practice: Application, Transport, Network, Data Link, Physical Layers

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  1. CMSC 414Computer and Network SecurityLecture 22 Jonathan Katz

  2. Network security in practice

  3. Network layers • Application • Transport • Network • Data link • Physical

  4. Roughly… • Application layer: the communicating processes themselves and the actual messages transmitted • Transport layer: handles transmissions on an “end-to-end” basis • Network layer: handles transmissions on a “hop-by-hop” basis

  5. Examples • Application layer: PGP, SSH • Transport layer: SSL/TLS • Network layer: IPsec • Security not usually provided at the data link layer, except possible within closed networks (e.g., military) • Security at the physical layer? (Shielded wires…)

  6. Security in what layer? • Depends on the purpose… • What information needs to be protected? • What is the attack model? • Who shares keys in advance? • Should the user be involved? • E.g., a network-layer protocol cannot authenticate two end-users to each other • An application-layer protocol cannot protect IP header information • Also affects efficiency, ease of deployment, etc.

  7. Example: PGP vs. SSL vs. IPsec • PGP is an application-level protocol for “secure email” • Can provide security on “insecure” systems • Users choose when to use PGP; user must be involved • Alice’s signature on an email proves that Alice actually generated the message, and it was received unaltered; also non-repudiation • In contrast, SSL would secure “the connection” from Alice’s computer

  8. Example: PGP vs. SSL vs. IPsec • SSL sits “on top of” the transport layer • End-to-end security, best for connection-oriented sessions • User does not need to be involved • The OS does not have to be changed • Easy to modify applications to use SSL • If SSL rejects packet accepted by TCP, then TCP rejects “correct” packet when it arrives! • SSL must then close the connection…

  9. Example: PGP vs. SSL vs. IPsec • IPsec sits “on top of” the network layer • End-to-end or hop-by-hop security • Best for connectionless channels • Need to modify OS • All applications are “protected” by default, without requiring any change to applications or actions on behalf of users • Can only authenticate hosts, not users • User completely unaware that IPsec is running

  10. Take home message… • Best solution may involve changes at both the OS and applications layers • The “best” solution is not to run SSL and IPsec! • Would have been better to design system with security in mind from the beginning… • (Keep in mind for future systems…)

  11. IPsec: AH and ESP

  12. Overview • IPsec consists of two components • AH/ESP • Used once a key is established (either using IKE or out-of-band) • IKE • Can be used to establish a key

  13. Security associations (SAs) • An SA is a crypto-protected connection • One SA in each direction… • At each end, the SA contains a key, the identity of the other party, the sequence number, and crypto parameters • IPsec header indicates which SA to use • Chosen by destination • Won’t go into more detail…

  14. More on SAs… • Parties will maintain a database of SAs for currently-open connections • Used both to send and receive packets

  15. AH vs. ESP • Authentication header (AH) • Provides integrity only • Encapsulating security payload (ESP) • Provides encryption and/or integrity • Both provide cryptographic protection of everything beyond the IP headers • AH additionally provides integrity protection of some fields of the IP header

  16. Transport vs. tunnel mode • Transport mode: add IPsec information between IP header and rest of packet • IP header | IPsec | packet • Most logical when IPsec used end-to-end

  17. Transport vs. tunnel mode • Tunnel mode: keep original IP packet intact; add new header information • New IP header | IPSec | old header | packet • Can be used when IPSec is applied at intermediate point along path (e.g., for firewall-to-firewall traffic) • E.g., change source/destination info… • Results in slightly longer packet • Note that data may be encrypted multiple times

  18. Firewalls… • Problem if data used for decision-making (like higher-layer information) is encrypted end-to-end • Arguments pro and con as to whether this data should be encrypted or not

  19. More on AH • AH provides integrity protection on header • But some fields change en route! • Only immutable fields are included in the integrity check • Mutable but predictable fields are also included in the integrity check • E.g., payload length • The final value of the field is used

  20. More on AH vs. ESP • Recall that ESP provides encryption and/or authentication • So why do we need AH? • AH also protects the IP header • Export restrictions • Firewalls need some high-level data to be unencrypted • None of these are compelling…

  21. The future of IPsec? • In the long run, it seems that AH will become obsolete • Better to encrypt everything anyway • No real need for AH • Certain performance disadvantages • AH is complex… • Etc. • IPsec is still evolving

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