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Lecture 7: Key Distribution

Lecture 7: Key Distribution. secret keys public keys. Secret Keys Distribution Problem. in the system where there are n users and potentially any user can communicate with any other? one scheme: each user knows the keys of all the others needs n 2 keys Key Distribution Center

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Lecture 7: Key Distribution

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  1. Lecture 7: Key Distribution secret keys public keys

  2. Secret Keys Distribution Problem • in the system where there are n users and potentially any user can communicate with any other? • one scheme: each user knows the keys of all the others • needs n2 keys • Key Distribution Center • each user has a key • The KDC has all keys • The KDC assigns a (new) key to any pair who need to talk

  3. Using KDC, variant 1 KDC Bob Alice A wants to talk to B Randomly choose Kab {“B”, Kab}Ka {“A”, Kab}Kb {Message}Kab

  4. A Common Variant KDC Bob Alice A wants to talk to B Randomly choose Kab {“B”, Kab}Ka ,{“A”, Kab}Kb {“A”, Kab}Kb ,{Message}Kab

  5. Interorganizational KDC Lotus KDC SUN KDC MIT KDC F G D E A B C KDC Realms • KDC hierarchy – a way to provide scalability • KDC realm: a KDC and the users of that KDC • issues with KDC • how would you talk to someone in another realm? • how would you know what realm? • how would you know a path to follow? • what can bad KDCs do?

  6. Public Key Distribution • Certification Authority (CA) signs “Certificates” • Certificate: a signed message saying “I, the CA, vouch that 489024729 is Mikhail’s public key” • If everyone has a certificate, a private key, and the CA’s public key, they can authenticate • CA vs. KDC • what if KDC database is stolen? CA private key? • what needs to be done if CA compromised? If KDC compromised? • What if KDC or CA down temporarily?

  7. Strategies for CA Hierarchies • strategies • One universally trusted organization • Top-Down, starting from a universally trusted organization’s well-known key • No rules. Anyone signs anything. End users decide who to trust • Many independent CA’s. Configure which ones to trust • more on that later when we cover PKI

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