Computer Networks · free preview

Key Predistribution

Every cipher in the kit assumes keys already in place, and getting them there is the hard half of the problem, public keys bind identities to keys through certificates, a certification authority's signed statement of the binding, verifiable by anyone who trusts the authority and chainable into hierarchies, with the decentralized web of trust as the grass-roots alternative, secret keys face a worse arithmetic, one key per pair of entities, so key distribution centers hold one master key per participant and mint session keys on request, and Diffie-Hellman key exchange lets two strangers derive a shared secret over a public channel from modular exponentiation and the hardness of discrete logarithms, though without authentication it answers only half the question.

1. Binding Names to Keys

Binding Names to Keys

2. One Key per Pair

One Key per Pair

3. Choosing Among the Three

Choosing Among the Three

4. A Secret From Public Exchange

A Secret From Public Exchange

5. The certificate's claim

What exactly does a public key certificate assert?

6. Walking the chain

A browser knows only a set of root keys. How does it come to trust a website's certificate?

7. The quadratic problem

Why is predistributing secret keys harder than predistributing public keys?

8. The ticket

In a KDC exchange, what does the ticket Alice carries to Bob contain?

9. The discrete log's protection

An eavesdropper sees $g^a \bmod p$ and $g^b \bmod p$ in a Diffie-Hellman exchange. What protects the shared secret?

10. The man in the middle

Why is raw Diffie-Hellman insufficient on its own against an active attacker?

11. The signed binding

What is the digitally signed statement binding a public key to an identity called?

12. The grass-roots alternative

What decentralized trust model has users sign the keys of people they have verified, with no authorities at all?

13. The trusted mint

What centralized service shares a master key with every participant and mints session keys on request?

14. The hard inverse

Diffie-Hellman's security rests on the difficulty of computing exponents from powers modulo a prime, known as what problem?

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