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Public Key Infrastructure: Ensuring Secure Key Management

Learn the essentials of Public Key Infrastructure (PKI), including key distribution, establishment, and management. Explore the role of Certification Authorities and digital certificates in ensuring secure communication. Understand the principles of Diffie-Hellman key exchange and the management of group security in distributed systems.

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Public Key Infrastructure: Ensuring Secure Key Management

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  1. CS 194: Distributed SystemsSecurity Scott Shenker and Ion Stoica Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776

  2. Outline • Key Management • Group management • Authorization management • Example: Kerberos

  3. Security Management • Problem: how do you get keys in the first place? • Key distribution: securely associate an entity with a key • Example: Public Key Infrastructure (PKI) • Key establishment: establish session keys • Use public key cryptography (we already know ho to do it) • Diffie-Hellman key exchange

  4. Public Key Infrastructure (PKI) • System managing public key distribution on a wide-scale • Trust distribution mechanism • Allow arbitrary level of trust

  5. Components of a PKI

  6. Digital Certificate • Signed data structure that binds an entity (E) with its corresponding public key (KE+) • Signed by a recognized and trusted authority, i.e., Certification Authority (CA) • Provide assurance that a particular public key belongs to a specific entity • How? • CA generates KCA-(E, KE+) • Everyone can verify signature using KCA+

  7. Certification Authority (CA) • People, processes responsible for creation, delivery and management of digital certificates • Organized in an hierarchy (use delegation – see next) Root CA CA-1 CA-2

  8. Registration Authority • People, processes and/or tools that are responsible for • Authenticating the identity of new entities (users or computing devices) • Requiring certificates from CA’s.

  9. Certificate Repository • A database which is accessible to all users of a PKI, contains: • Digital certificates, • Certificate revocation information • Policy information

  10. private key public key Alice Alice private key public key Bob Bob Example • Alice generates her own key pair. • Bob generates his own key pair. • Both sent their public key to a CA and receive a digital certificate

  11. public key Bob private key Alice public key Alice private key Bob Example • Alice gets Bob’s public key from the CA • Bob gets Alice’s public key from the CA

  12. Certificate Revocation • Process of publicly announcing that a certificate has been revoked and should no longer be used. • Approaches: • Use certificates that automatically time out • Use certificate revocation list

  13. Key Establishment: Diffie-Hellman Key Exchange • Agree on two numbers n, g; both number can be made public! • Alice and Bob pick two secret numbers x and y • Similar to public-key cryptography • Example: For Alice, KA-=x, KA+ = gx mod n

  14. Outline • Key Management • Group management • Authorization management • Example: Kerberos

  15. Secure Group Management • Motivation: offer high availability for security services • How: replicate services • Problem: how to add a new replica to a group without compromising the integrity of the group?

  16. Securely admitting a new group member • CKG: secret key used for communication within group • KG+,KG-: public-private key pair to communicate with non-group members • KP,G: secret key • RP: reply pad • T: local time • Notation: [X]Y: X was signed by Y join request group admittance Process in group G New process

  17. Outline • Key Management • Group management • Authorization management • Example: Kerberos

  18. Outline • Key Management • Group management • Authorization management • Example: Kerberos

  19. Authorization Management • Granting authorization rights • Related with access control which verifies access rights (see book)

  20. Capabilities (1) • Capability: • Unforgeable data structure for a specific resource R • Specify access right the holder has with respect to R • Capability in Amoeba:

  21. Capabilities (2) • Generation of a restricted capability from an owner capability Owner

  22. Delegation • A wants to delegate an operation on a resource to B • Problem: how does A delegates its access rights to B? • Solutions: A signs (A, B, R) • If B wants to delegate operation to C, C needs to contact A • Avoid this problem using a proxy (Neuman scheme) • Proxy: a token allowing its owner to operate with the same or rstricted rights as the entity granting the token

  23. Delegation: Neuman Scheme • The general structure of a proxy as used for delegation:

  24. Delegation: Neuman Scheme • Using a proxy to delegate and prove ownership of access rights • In practice S+proxy, S-proxy can be a public-private key pair and N can be a nonce

  25. Kerberos • Based on Needham-Schroeder authentication scheme • Developed at MIT

  26. Example: Kerberos • Authentication in Kerberos • AS: Authentication server • TGS: Ticket Granting Service • T: timestamp used to avoid replay attacks of message 6

  27. Example: Kerberos • Setting up a secure channel in Kerberos:

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