[Guest]

How do you manage a fair coin flip over the phone?

This is not a trick question. There are only two people involved, they can't see each other, they can't go anywhere...

This is a fairly well known problem and many of you may have already encountered it in college. I wasn't able to find it so I'm posting it. My apologies if this has already been posted.

I expect some of you will know the answer right away having seen it before. Please hide your answers so those who haven't seen it can enjoy.

Thanks!

[Guest]

ok, I'm a new user here and I give up. How do I find the correct answer (from the author's point of view)?

[Guest]

Lets say that A is the one tossing and B is the person guessing. Both A and B will create an encryption algorithm for heads and tails. For example 'A' may number the letters and add 10 to each letter. Then, A can toss and B can guess (without saying). then almost instantaneous, both can tell their encrypted choice. Then they can explain what it actually meant and can explain the encryption mehanism.

[Guest]

ok, I'm a new user here and I give up. How do I find the correct answer (from the author's point of view)?

I made a post earlier with comments on the answer. Here is a wikipedia article that describes it pretty good.

http://en.wikipedia.org/wiki/Commitment_scheme

[Guest]

Lets say that A is the one tossing and B is the person guessing. Both A and B will create an encryption algorithm for heads and tails. For example 'A' may number the letters and add 10 to each letter. Then, A can toss and B can guess (without saying). then almost instantaneous, both can tell their encrypted choice. Then they can explain what it actually meant and can explain the encryption mehanism.

The problem with having late revelation of the algorithm is that I could come up with two algorithms that give the same answer with different meaning and select the algorithm that makes me win.

[Guest]

Person A (the one with the coin) flips the coin. Person B (the one calling) counts 1-2-3, and on 4 person A says the outcome of the coin toss and person B calls. If there is any delay between their answers they agree to discount it and repeat the toss until they both speak at exactly the same time. Good old straightforward paper-scissor-stone rules.

LOL they should just forget the whole coin flip and just play paper-scissor-stone.