Cryptographic Protocols

1
Cryptographic Protocols

• Reference , Charles P., Security in
  Computing, 2nd Edition, Prentice Hall, 1996.

2
Definition of Protocols

• A protocol is an orderly sequence of steps two
  ore more parties take to accomplish some task.

3
Characteristics of a Good Protocol

• Established in Advance
• Completely designed before it is used.
• Mutually Subscribed
• All parties agree to follow protocol steps, in
  order.
• Unambiguous
• Steps cannot be misunderstood.
• Complete
• For every situation that can occur there is a
  prescribed action to be taken.

4
Design and Implementation

• A protocol sets the rules of behavior, as well
  as separates the process of accomplishing the
  task from the mechanism by which it is done.
• Design assures that the protocol is correct.
• Implementation simply requires a mechanism, which
  can change without affecting the design.

5
Arbitrated Protocols

• An arbiter is a disinterested third party trusted
  to complete a transaction between two distrusting
  parties.
• In a computer protocol, an arbiter is a
  trustworthy third party who ensures fairness.
• The arbiter could be a person, program, or a
  machine.

6
Problems with Arbitrated Protocols

• Trustworthy neutral third party is hard to find.
• Maintaining availability of arbiter is costly.
• Arbitration causes communications delay.
• Arbitration service is a network bottleneck.
• Secrecy is vulnerable, since arbiter has access
  to so much sensitive information.

7
Adjudicated Protocols

• An adjudicator is a third party who can judge
  whether a transaction was performed fairly--for
  example, a notary public.
• With adjudicated computer protocols, enough data
  is available for a third party to determine if
  two parties acted fairly.
• However, failure is detected only after it has
  occurred.

8
Self-Enforcing Protocols

• A self-enforcing protocol guarantees fairness.
• If either party cheats, it is obvious to the
  other party.

9
Comparing the Protocols

• The self-enforcing protocols are preferred, but
  not always possible.
• Adjudicated protocols are preferred over
  arbitrated protocols.

10
Cryptographic Protocols

• There are a number of problems that can be solved
  using secure protocols.
• Key Distribution
• Authentication of Keys
• Authentication of Transactions
• Contract Signing
• Certified Mail