20. Protocols. Definitions.Rules of communication.Types of protocols.Problems.

A protocol is a series of steps, involving two or more parties, designed to accomplish a task;Rules of communication.

Protocols have other characteristics as well:

— Everyone involved in the protocol must know the protocol and all of the steps to follow in

advance.

— Everyone involved in the protocol must agree to follow it.

— The protocol must be unambiguous; each step must be well defined and there must be no

chance of a misunderstanding.

— The protocol must be complete; there must be a specified action for every possible

situation.

A cryptographic protocol is a protocol that uses cryptography.

Types:

Basic Protocols;

Intermediate Protocols

Advanced Protocols

EsotericProtocols

Problems:

• Trust - will Alice and Bob really trust a faceless arbitrator atsome unknown location on the network?

• Cost – extra hardware/software/processing required

Delay – sending messages to an arbitrator will slow downthe transaction

• Bottleneck – every transaction will involve the arbitrator

• Vulnerability – the arbitrator is likely to be the target of anyattack, since if successful, the rewards will be substantial

21.Secure Protocols. Three types of Protocol

Additional property

• it should not be possible to do more or to learn more thanis specified in the protocol

• In designing secure protocols, we need to answer to questions:

What is the threat model, and is it realistic?

Does the protocol handle it correctly?

Three types of Protocol.

• Arbitrated Protocol:

  • Protocol requires the participation of a disinterested trusted third party

    • disinterested – no allegiance to either partyno vested interest in the outcome

• trusted – everybody accepts that what he says is true

• Adjudicated Protocol

  • Third party (again disinterested and trusted) only called in when disputes arise

• Self-enforcing Protocol

Protocol is designed to work between two mutually untrustworthy and unreliable parties

22.SecureElections. Simplistic Protocol #1,2

The ideal protocol has, at the very least, these

six requirements:

1. Only authorized voters can vote.

2. No one can vote more than once.

3. No one can determine for whom anyone else voted.

4. No one can duplicate anyone else’s vote. (This turns out to be the hardest requirement.)

5. No one can change anyone else’s vote without being discovered.

6. Every voter can make sure that his vote has been taken into account in the final tabulation.

Additionally, some voting schemes may have the following requirement:

7. Everyone knows who voted and who didn’t.

Simplistic Voting Protocol #1

(1) Each voter encrypts his vote with the public key of a Central Tabulating Facility (CTF).

(2) Each voter sends his vote in to the CTF.

(3) The CTF decrypts the votes, tabulates them, and makes the results public.

The CTF has no idea where the votes are from, so it doesn’t even

know if the votes are coming from eligible voters. It has no idea if eligible voters are voting more

than once.

Simplistic Voting Protocol #2

(1) Each voter signs his vote with his private key.

(2) Each voter encrypts his signed vote with the CTF’s public key.

(3) Each voter sends his vote to a CTF.

(4) The CTF decrypts the votes, checks the signatures, tabulates the votes, and makes the results public.

This protocol satisfies properties one and two: Only authorized voters can vote and no one can vote more than once—the CTF would record votes received in step (3). Each vote is signed with the voter’s private key, so the CTF knows who voted, who didn’t, and how often each voter voted.

The problem with this protocol is that the signature is attached to the vote; the CTF knows who voted for whom.