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Computer and Network Security - Message Digests , Kerberos, PKI –

Computer and Network Security - Message Digests , Kerberos, PKI –. Course textbook. References: Kaufman, C., Perlman, R. and Speciner, M., Network Security (Private Communication in a Public World) , 2 nd edition, Prentice Hall 2002.

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Computer and Network Security - Message Digests , Kerberos, PKI –

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  1. Computer and Network Security- Message Digests, Kerberos, PKI –

  2. Course textbook References: Kaufman, C., Perlman, R. and Speciner, M., Network Security (Private Communication in a Public World), 2nd edition, Prentice Hall 2002. William Stalling, Cryptography and Network Security, 2nd edition, Prentice Hall 1999. Class notes: Computer Security, Pennsylvania State University http://www.cse.psu.edu/~cg543/

  3. Computationally expensive to public-key-encrypt long messages Goal: fixed-length,easy to compute digital signature, “fingerprint” apply hash function H to m, get fixed size message digest, H(m). Hash function properties: Many-to-1 Produces fixed-size msg digest (fingerprint) Given message digest x, computationally infeasible to find m such that x = H(m) computationally infeasible to find any two messages m and m’ such that H(m) = H(m’). Message Digests

  4. Bob sends digitally signed message: Alice verifies signature and integrity of digitally signed message: Digital signature = Signed message digest

  5. Internet checksum would make a poor message digest. Too easy to find two messages with same checksum. MD5 hash function widely used. Computes 128-bit message digest in 4-step process. arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x. SHA-1 is also used. US standard 160-bit message digest Hash Function Algorithms

  6. Problem: How do two entities establish shared secret key over network? Solution: trusted key distribution center (KDC) acting as intermediary between entities Problem: When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: trusted certification authority (CA) Trusted Intermediaries

  7. Trusted Key Distribution Center (KDC) • Keberos • Key Distribution Center • Ticket Granting Server

  8. Keberos • Kerberos authenticates users. • Authentication: Assessing identity of users • Most widely used centralized passwordsystem • Now: part of Windows 2K, XP network authentication

  9. The setup of Kerberos • The players – Principal - person being authenticated – Service - entity performing authentication – Key Distribution Center (KDC) • Trusted third party for key distribution • Each principal and service has a Kerberos password known toKDC • Ticket granting server • Server granting transient authentication • The objectives – Authenticate Alice (Principal) to Bob (Service) – Negotiate a symmetric (secret) session key kAB

  10. Alice,Bob need shared symmetric key. KDC: server shares different secret key with each registered user. Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC. Key Distribution Center (KDC) • Alice communicates with KDC, gets session key R1, and KB-KDC(A,R1) • Alice sends Bob KB-KDC(A,R1), Bob extracts R1 • Alice, Bob now share the symmetric key R1.

  11. Kerberos Ticket • Bob’s Ticket KA-KDC(R1, KB-KDC(A,R1))contains –Alice’s identity – The session key (R1) • Only Alice and KDC can open KA-KDC(R1, KB-KDC(A,R1)) • Only Bob and KDC can open KB-KDC(A,R1).

  12. Trusted Certificate Authorities (CA) • Public Key Infrastructure • Public Key Cryptography • CA (Certificate Authorities)

  13. What is Certificate? • A certificate … – … makes an association between a user • identity/job/attribute and a private key – … contains public key information {e,n} – … has a validity period – … is signed by some certificate authority (CA) • Issued by CA for some purpose – Verisign is in the business of issuing certificates – People trust Verisign to vet identity

  14. Certification authority (CA) binds public key to particular entity. Entity (person, router, etc.) can register its public key with CA. Entity provides “proof of identity” to CA. CA creates certificate binding entity to public key. Certificate digitally signed by CA. When Alice wants Bob’s public key: gets Bob’s certificate (Bob or elsewhere). Apply CA’s public key to Bob’s certificate, get Bob’s public key Certification Authorities

  15. Diffie-Hellman Key Exchange • Given • p: Prime number • g: g < p, prime root of p {g1 mod p, g2 mod p, g3 mod p,…, gp-1 mod p} = {1, 2, 3, 4, …, p-1}

  16. Diffie-Hellman Alice Bob S1.1 choose random RA choose random RB --------------------------------------------------------------------------------------------------------------------- S1.2 compute compute ----------------------------------------------------------------------------------------------------------------- S1.3 A  B : QA B  A : QB --------------------------------------------------------------------------------------------------------------------- S1.4 compute compute

  17. So far, we talked about Basic Techniques of Security… • Those are used in many different security scenarios • secure email • secure transport (SSL) • IPsec

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