Bob Bets With Alice For One Million, But They Don’t Trust Anyone To Toss The Coin

Bob Bets With Alice For One Million, But They Don’t Trust Anyone To Toss The Coin

Living in a world without trust

Bob bets one million on the toss of a coin with Alice. “But, who will we get to toss the coin?”, says Bob. “Well. I don’t trust you!”, says Alice, and Bob says that he doesn’t trust her. And so they draw-up a list of people who could toss the coin, but cannot agree on anyone. So let’s look at a method that allows Bob and Alice to toss a coin, and then they both compute the result of the toss, and without any of them gaining an unfair advantage.

First Alice creates a random number (a), and Bob does the same (b). Next Alice calculates a hash of the value and computes A. Bob does the same and computes B. Next they exchange values. Once they acknowledge that they have received them, they then send each other their random values. The result of the coin toss will be the least significant bit of each of the values, EX-OR’ed together:

In this case Bob and Alice can’t change their values, as they have sent the hash of the values. Alice will prove the b value against the hash that Bob provided, and Bob will check the value of the a value against the hash that Alice sent. The following is a sample run:

===Bob random and hash=====

Bob random= 369
Bob hash= ae2810e1e85383a19f8433a73bad300cecb7047023a47d5adcb607891417fae4:b0f1919bec594f7095d79b533682462b

===Alice random and hash=====

Alice random= 835
Alice hash= 1c47f2ee558f83bcaadde316b35fc9b17d8e87fb1e55705689865ab4758e8083:b2c22fc9ac404387a30b44c1909a6bee
Heads 
====Checking the flips ====

Alice checks value with salt: True
Bob checks value with salt: True

====10 random flips====

Tails Tails Heads Heads Tails Heads Tails Heads Tails Tails

Here is a sample run [link].

An outline of the code is here:

Conclusions

We increasingly live in a world where we do not trust anything, and so this method allows us to generate the result of a computation without requiring a trusted agent to perform the calculation. It is a world without the need for Trent. This is defined more formally as secure multi-party computation and can be used in secure elections to compute a result. It could also be used for a bidding system, where the winner of a bid could be revealed to those involved in the bid, but not to others.