1. @Yuan Xue (yuan.xue@vanderbilt.edu) CS 285 Network Security Key Distribution and Management Yuan Xue Fall 2012

2. @Yuan Xue (yuan.xue@vanderbilt.edu) Overview Secret Key Distribution Introduction Using key distribution center (KDC) Decentralized Using public-key Public Key Management Public-key Announcement Publicly Available Directory Public-key Certificate Web of Trust (GnuPG)

3. @Yuan Xue (yuan.xue@vanderbilt.edu) Secret Key Distribution Message Encryption Secret key encryption vs. public key encryption  Both encryption algorithms can provide confidentiality  Secret Key Encryption is more efficient and faster To use secret key encryption  Communicating peers must share the same key  The key must be protected from access by others Key Distribution

4. @Yuan Xue (yuan.xue@vanderbilt.edu) Key Hierarchy A secret key becomes insecure when used for a long time, since more ciphertext encrypted using this key is available to the attacker, making it easies to derive the key. Keys that are used to encrypt the data need to be renewed frequently Solution – Key Hierarchy Session key – encrypt data, renewed each session Master key – distribute session keys, renewed infrequently using non- cryptographic approach

5. @Yuan Xue (yuan.xue@vanderbilt.edu) Secret Key Distribution Now the questions are What are master keys?  secret key or public key? Who should share master keys?  who needs to be trusted a priori? How to get session keys from master keys?  key distribution protocol

6. @Yuan Xue (yuan.xue@vanderbilt.edu) Secret Key Distribution Approaches Three approaches Via key distribution center (KDC)  KDC needs to share a secret key with each of the communication parties Decentralized  The communication parties need to share a master key Via public key  The communication parties need to have the public keys of each other Using public key encryption Using Diffie-Hellman key exchange

7. @Yuan Xue (yuan.xue@vanderbilt.edu) Key Distribution based on KDC Initially A and B both trust KDC KU A -- shared secret key between A and KDC KU B -- shared secret key between B and KDC Goal A and B trust each other A and B share a secret key K S KDC AB KU A, KU B KU A KU B KDC AB KU A, KU B K S, KU A K S, KU B

8. @Yuan Xue (yuan.xue@vanderbilt.edu) Overview Secret Key Distribution Introduction Using key distribution center (KDC) Decentralized Using public-key Public Key Management Public-key Announcement Publicly Available Directory Public-key Certificate Web of Trust (GnuPG)

9. @Yuan Xue (yuan.xue@vanderbilt.edu) KDC-based Key Distribution Protocol Ticket to B Nonce  guarantee the reply (the secret key) from KDC is fresh Match the KDC reply with the request, in case A issued multiple requests to KDC The Needham–Schroeder Symmetric Key Protocol Any math func, e.g., N+1 Vulnerable to Replay attack

10. @Yuan Xue (yuan.xue@vanderbilt.edu) Decentralized Key Distribution Initially A and B trust each other A and B share a master secret key K m Goal A and B share a session secret key K S AB KmKm KmKm AB K S, K m

11. @Yuan Xue (yuan.xue@vanderbilt.edu) Decentralized Key Distribution

12. @Yuan Xue (yuan.xue@vanderbilt.edu) Secret Key Distribution Approaches Three approaches Via key distribution center (KDC)  KDC needs to share a secret key with each of the communication parties Decentralized  The communication parties need to share a master key Via public key  The communication parties need to have the public keys of each other Using public key encryption Using Diffie-Hellman key exchange

13. @Yuan Xue (yuan.xue@vanderbilt.edu) Secret Key Distribution Via Public Key Using public key encryption RSA Algorithm Using Diffie-Hellman key exchange

14. @Yuan Xue (yuan.xue@vanderbilt.edu) Simple Secret Key Distribution Problem: Man-in-the-middle-attack

15. @Yuan Xue (yuan.xue@vanderbilt.edu) Man-in-the-middle Attack A D B KU A ||ID A KU D ||ID A E[KU D, K s ] E[KU A, K s ] Key Issue: Binding between public key and the ID. Solution: public key management Provides authenticated association between the public key and the ID

16. @Yuan Xue (yuan.xue@vanderbilt.edu) Public Key Management Distribution of Public Key Public-key Announcement Publicly Available Directory Public-key Certificate (focus) Others  Fingerprint (GnuPG)  Web of Trust (covered in HW2 and the class on GnuPG)

17. @Yuan Xue (yuan.xue@vanderbilt.edu) Public Announcement No Authentication Key Issue: Binding ID Public key

18. @Yuan Xue (yuan.xue@vanderbilt.edu) Publicly Available Directory Directory [ID, public key] A securely registers its public key In person Secure communication The entire directory is published periodically B can access the directory via secure authenticated communication

19. @Yuan Xue (yuan.xue@vanderbilt.edu) Public-Key Certificate Certificate C A = E[KR auth, T||ID A ||KU A ]

20. @Yuan Xue (yuan.xue@vanderbilt.edu) Put into practice Let’s take a look at a real certificate How to generate OpenSSL http://www.madboa.com/ge ek/openssl/

21. @Yuan Xue (yuan.xue@vanderbilt.edu) AB I want to talk to you Certificate E(KU bob,S) Now – Use Public key to distribute secret key No assurance that the key is fresh

22. @Yuan Xue (yuan.xue@vanderbilt.edu) K = Hash (S, R Alice, R Bob ) Nonce Pre-master Secret K = Hash (S, R Alice, R Bob ) Secret Key AB I want to talk to you, R Alice Certificate, R Bob E(KU bob,S) Solution from SSL Nonce

23. @Yuan Xue (yuan.xue@vanderbilt.edu) a is a primitive root of prime number p then a mod p, a 2 mod p, …, a p-1 mod p are distinct and consist of the integers from 1 through p-1 For any b and a primitive root a of p, unique exponent I can be found such that b = a i mod p (0<=i <= p-1) Diffie-Hellman Key Exchange

24. @Yuan Xue (yuan.xue@vanderbilt.edu) More on D-H Key Exchange Basic Version -- Anonymous Diffie-Hellman: no authentication, vulnerable to man-in-the- middle attacks Fixed Diffie-Hellman: based on public parameter in server’s CA; fixed secret key Ephemeral Diffie-Hellman: one time secret key; most secure D-H options

25. @Yuan Xue (yuan.xue@vanderbilt.edu) In A Nutshell KDC-based Decentralized Secret Key Distribution Message Encryption Message Authentication Public-key-based RSA, Diffie-Hellman Public-key-based RSA, Diffie-Hellman Announcement Directory Certificate Public-key management Session Keys Public Keys Decentralized (Web of Trust) Decentralized (Web of Trust)