CSCE 715: Network Systems Security

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

2. Points of Vulnerability • Adversary can eavesdrop from a machine on the same LAN • Adversary can eavesdrop by dialing into communication server • Adversary can eavesdrop by gaining physical control of part of external links • twisted pair, coaxial cable, or optical fiber • radio or satellite links

3. Placement of Symmetric Encryption • Two major placement alternatives • Link encryption • encryption occurs independently on every link • implies must decrypt traffic between links • requires many devices, and a unique key between each pair of nodes sharing a link • End-to-end encryption • encryption occurs between original source and final destination • need devices at each end with shared keys

4. Characteristics ofLink and End-to-End Encryption

5. Placement of Encryption in Protocol Stack • Can place encryption function at various layers in OSI Reference Model • link encryption occurs at layers 1 or 2 • end-to-end can occur at layers 3, 4, 6, 7 • If move encryption toward higher layer • less information is encrypted but is more secure • application layer encryption is more complex, with more entities and need more keys

6. Scope of Encryption

7. Traffic Analysis • When using end-to-end encryption, must leave headers in clear so network can correctly route information • Hence although contents are protected, traffic patterns are not protected • Ideally both are desired • end-to-end protects data contents over entire path and provides authentication • link protects traffic flows from monitoring

8. Key Distribution • Symmetric schemes require both parties to share a common secret key • Need to securely distribute this key • If key is compromised during distribution, all communications between two parties are compromised

9. Key Distribution Schemes • Various key distribution schemes for two parties • A can select key and physically deliver to B • third party C can select and deliver key to A and B • if A and B have shared a key previously, can use previous key to encrypt a new key • if A and B have secure communications with third party C, C can relay key between A and B

10. A Key Distribution Scenario

11. Key Distribution Issues • Hierarchies of KDC’s are required for large networks, but must trust each other • Session key lifetimes should be limited for greater security • Use of automatic key distribution on behalf of users, but must trust system • Use of decentralized key distribution • Controlling purposes keys are used for

12. Automated Key Distribution

13. Summary of Symmetric Encryption • Traditional symmetric cryptography uses one key shared by both sender and receiver • If this key is disclosed, communications are compromised • Symmetric because parties are equal • Provide confidentiality, but does not provide non-repudiation

14. Insufficiencies with Symmetric Encryption • Symmetric encryption is not enough to address two key issues • key distribution – how to have secure communications in general without having to trust a KDC with your key? • digital signatures – how to verify that a received message really comes from the claimed sender?

15. Thoughts about KDC What good would it do after all to develop impenetrable cryptosystems, if their users were forced to share their keys with a KDC that could be compromised by either burglary or subpoena? - Whitfield Diffie, 1988

16. Advent of Asymmetric Encryption • Probably most significant advance in the 3000 year history of cryptography • Use two keys: a public key and a private key • Asymmetric since parties are not equal • Clever application of number theory concepts instead of merely substitution and permutation

17. How Asymmetric Encryption Works • Asymmetric encryption uses two keys that are related to each other • a public key, which may be known to anybody, is used to encrypt messages, and verify signatures • a private key, known only to the owner, is used to decrypt messages encrypted by the matching public key, and create signatures • the key used to encrypt messages or verify signatures cannot decrypt messages or create signatures

18. Asymmetric Encryptionfor Confidentiality

19. Asymmetric Encryptionfor Authentication

20. Public-Key Cryptosystems

21. Public-Key Characteristics • Public-Key algorithms rely on two keys where: • it is computationally infeasible to find decryption key knowing only algorithm & encryption key • it is computationally easy to en/decrypt messages when the relevant (en/decrypt) key is known • either of the two related keys can be used for encryption, with the other used for decryption (for some algorithms)

22. Applications for Asymmetric Encryption • Three categories • Encryption/decryption: sender encrypts a message with receiver’s public key • Digital signature: sender “signs” a message with its private key • Key exchange: two sides exchange a session key

23. Security of Asymmetric Encryption • Like symmetric schemes brute-force exhaustive search attack is always theoretically possible, but keys used are too large (>512bits) • Not more secure than symmetric encryption, dependent on size of key • Security relies on a large enough difference in difficulty between easy (en/decrypt) and hard (cryptanalyse) problems • Generally the hard problem is known, just that it is made too hard to do in practice • Require using very large numbers, so is slow compared to symmetric schemes

24. Next Class • RSA • Key management with asymmetric encryption • Diffie-Hellman key exchange • Read Chapter 10