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Dining cryptographers protocol/Rewrite

This article is a rewrite of the existing article dining cryptographers protocol. Since this page is bound to be incomplete, unedited, or otherwise messy, please do not merge changes from here into the main article just yet.

Problem Statement

Dining Cryptographers

A group of cryptographers is enjoying dinner at a local restaurant. Upon requesting their bill, the cryptographers are surprised to learn from their host that payment for the dinner has already been anonymously arranged and that the group owes nothing. They speculate that the payer might be one of the cryptographers in the party, but then they realize that the dinner may have been paid for by the National Security Agency, their employer. Though everybody at the table respects each other's right to make an anonymous payment, they still wish to know whether their meal was in fact funded by the NSA.

Problem: If it turns out that one of the cryptographers at the table is the payer, how can he anonymously signal this fact to his peers?

Solution: Each cryptographer flips a coin privately with any other member to his left and right. Then they all stand up and announce true if the two coins he can see were different (head and tails) or false if the two coins were the same (head and head). If one of the cyptographers is the payer, he states the opposite. If there is an odd number of trues and the number of cryptographers are odd or there is an even number of trues and the number of cryptographers are even, then the NSA paid. Elsewhere, the check was paid by a member of the group. Who actually paid is not revealed.

Aging Cryptographers

Alice and Bob are attending a prestigious awards ceremony. At this event, it is custom for attendees to sit next to the Master of Ceremonies in order of age. In such a manner the MC wishes to have the oldest and most seasoned recipients sitting closest to him; the youngest members sit at the far end of the table. Alice and Bob wish to determine which of the two is older and thus should be seated closer to the head of the table. However, having never met before, they do not know each other's age. Neither wants to seem rude at a formal function, so they quickly discount the idea of asking each other for his or her age.

Problem: How can Alice and Bob determine which is older without telling each other their ages?

Voting Cryptographers

The CEO of a company that produces cryptographic software is retiring, so the company's board of directors chooses two candidates to replace him. The board convenes, and during this meeting they discuss the merits of each candidate and the likely benefits that each would bring to the company. Since this is a fair and respected organization, company policy states that at the end of the meeting a secret ballot will be used to elect a new CEO. The election process is a democratic one: each board member may cast one vote, and all votes are given equal weight. The candidate who received the greatest number of votes is promoted to CEO.

Problem: Given the requirements of the voting process, how can the board members elect a new CEO?

General problem statement

  • How to calculate F(x_1, x_2, ldots, x_n) without revealing any individual's x_i
  • Describe need for new protocol to solve problem

History

  • (Does this protocol even have a well-defined history?)
    • CCG '88, about CDG '87: "This solution was the first one to raise the hope that such protocols could be implemented in an unconditionally secure way." (p. 12)
  • What were the design goals for this protocol?

The Dining Cryptographers Protocol

The dining cryptographers protocol allows for any member of a group to multicast data to every other member of the group. Though the broadcast is public, the protocol guarantees that its sender remains anonymous. This protocol allows only for one member of the group to transmit data during any given round.

Transmitting one bit

Consider that there are n cryptographers sitting around a circular table, so for convenience they shall be numbered P_1, P_2, ..., P_{n-1}, P_n. The cryptographers are arranged such that P_i has as his neighbors P_{i-1} and P_{i+1}. (P_1 sits between P_n and P_2; P_n sits between P_{n-1} and P_1.) Additionally, there are n pairs of adjacent cryptographers. Each pair is written as N_{(h,i)}, where P_h and P_i are the cryptographers in the pair. It is obvious then that each cryptographer P_i is a member of exactly two pairs N_{(h,i)} and N_{(i,j)}. (Note that h and j are not necessarily distinct.)

Each pair N_{(h,i)} secretly chooses one bit at random; this bit b_{(h,i)} is known only to P_h and P_i. In this manner a total of n random bits are chosen among all adjacent pairs of cryptographers. Then each cryptographer P_i should know exactly two bits of information: b_{(h,i)} and b_{(i,j)}.

Each cryptographer P_i now computes a value v_i = b_{(h,i)} oplus b_{(i,j)} oplus s_i, where the b values are the secret bits known by P_i and s_i is the signal that he wishes to send anonymously. This value v_i is made public to all persons sitting at the table. When all v values have been made public, the existence of a signal s can be detected by calculating the bitwise XOR of every v_i. This XOR operation yields the following:

s = v_1 oplus cdots oplus v_n

s = (b_{(n, 1)} oplus b_{(1, 2)} oplus s_1) oplus cdots oplus (b_{(n-1, n)} oplus b_{(n, 1)} oplus s_n)

s = (b_{(n, 1)} oplus b_{(n, 1)} oplus s_1) oplus cdots oplus (b_{(n-1, n)} oplus b_{(n-1, n)} oplus s_n)

s = s_1 oplus cdots oplus s_n

Assuming that at most one person is attempting to send a signal over the channel, at most one value s_i on the right-hand side of the last equation should be 1, yielding s = 1. If nobody tried sending a signal over the channel, then it is evident that this equation yields s = 0. Hence all cryptographers can detect the existence of a signal if one is sent.

This is trivially anonymous as determining the sender requires knowing the secrets. As s = s_1 oplus cdots oplus s_n, and saying node i was the sender, without knowing all secrets except for the sender (s_1,ldots,s_{i-1},s_{i+1},ldots,s_n) any of the nodes could have transmitted the message, and each therefore appears equally likely to any attacker as long as the number of attackers is less than n - 2.

Example

  • One-bit communication using n coins as entropy source
  • Pictures are nice :)

Transmitting multiple bits

  • Explain protocol for multi-bit signal
  • Quick overview of proof (should be same as one-bit)

The Dining Cryptographers in the Disco

  • Add short description of this protocol

Security considerations

  • Advantages
    • Anonymous sender
    • Anonymous recipient (if key used)
      • Allow key to be sent in main channel rather than key channel
  • Disadvantages
    • Slow Isn't really slow, as calculation are just some ADD's or XOR's. But much data overhead. 2n bytes transfer for one byte in case of peer to peer communication (n is number of participants)
    • Malicious party can inject random bits to mess up data
      • Chaum present trap massagesmessages to prevent malicious data
    • Collusion to detect who sent the signal
      • Only if described as above with only one key exchange to the "right" participant. Chaum used this as introductional example only and quickly came to exchange keys between all participants. This prevents collusion completely.

The Ageing Cryptographers Protocol

The ageing cryptographers protocol allows for every member of a group to contribute inputs to a function that can be calculated by all members of the group. The protocol guarantees both that an input to the function cannot be traced back to any particular participant and that each participant calculates the same result. (In other words, a correct implementation of the protocol guarantees that the participant calculates the correct result.) All members of the group may transmit data simultaneously during any given round.

Protocol

  • Description of protocol
  • Proof of correctness
  • Proof of anonymitity (of age)

Example

  • Alice and Bob, as described earlier

Security considerations

  • (?)

The Voting Cryptographers Protocol

The voting cryptographers protocol is similar to the ageing cryptographers protocol. It guarantees both that any particular input cannot be traced back to its source and that all participants correctly implementing the protocol agree on the final result. Additionally, this protocol is immune to attack from a participant trying to change another's vote or otherwise causing disruption. All members of the group may transmit data simultaneously during any given round.

Two candidates

  • Description of protocol
  • Proof of correctness
  • Proof of anonymitity (of vote)

Example

More than two candidates

Security considerations

  • Collusion (?)

Possible References

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