[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!

[akaslickster]

If there is a mutual trust then that could be fair.

[Guest]

If there is a mutual trust then that could be fair.

Define mutual trust. There is no third party or service involved.

[Guest]

Mutual trust, it seems is that the two parties involved trust each other.

Define mutual trust. There is no third party or service involved.

[Guest]

Mutual trust, it seems is that the two parties involved trust each other.

The parties involved have no trust for each other.

[akaslickster]

they can honor eachothers word for the outcome of the flip. If I say it was tails than you could take my word for it. I won't cheat or lie.

Define mutual trust. There is no third party or service involved.

[akaslickster]

blind fold them and then>

sure way is put the phone on the top of a ladder then one of the guys flips the coin over it as the other one watches.

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!

Edited May 16, 2008 by akaslickster

[Guest]

never heard this one before, lets see how i did...

Have both people decide on who's heads and who's tales. Then have both people flip a coin and say what their coin landed on. when both coins are the same side, that person wins. to be sure do best 2 out of 3 and alternate who says first. That or say at the same time.

Edited May 16, 2008 by puzzlerf

[Guest]

sure way is put the phone on the top of a ladder then one of the guys flips the coin over it as the other one watches.

LOL

I like your answer but no... that's not it

[Guest]

There is no fair way to use a true coin flip to settle a dispute between two parties over distance — for example, two parties on the phone. The flipping party could easily lie about the outcome of the toss. In telecommunications and cryptography, the following algorithm can be used:

Party A chooses two large primes, either both congruent to 1, or both congruent to 3, mod 4, called p and q, and produces N = pq; then N is communicated to party B, but p and q are not. It follows N will be congruent to 1 mod 4. The primes should be chosen large enough that factoring of N is not computationally feasible. The exact size will depend on how much time party B is to be given to make the choice in the next step, and on party B's expected resources.

Party B calls either "1" or "3", a claim as to the mod 4 status of p and q. For example, if p and q are congruent to 1 mod 4, and B called "3", B loses the toss.

Party A produces the primes, making the outcome of the toss obvious; party B can easily multiply them to check that A is being truthful.

Edited May 16, 2008 by storm

[Guest]

never heard this one before, lets see how i did...

Have both people decide on who's heads and who's tales. Then have both people flip a coin and say what their coin landed on. when both coins are the same side, that person wins. to be sure do best 2 out of 3 and alternate who says first. That or say at the same time.

Good guess, but no... There is no trust at any time that the other is being truthful and speaking at the same time is not the answer

[Guest]

Good guess, but no... There is no trust at any time that the other is being truthful and speaking at the same time is not the answer

damn...

Done the way I stated, the only way to get an outcome is to either be honest, or give up and let the other person win. So you'd have to have trust other wise no outcome.

but you are the puzzle writer...

that or...

have a 3rd party on 3 way calling flip.

Edited May 16, 2008 by puzzlerf

[Guest]

There is no fair way to use a true coin flip to settle a dispute between two parties over distance — for example, two parties on the phone. The flipping party could easily lie about the outcome of the toss. In telecommunications and cryptography, the following algorithm can be used:

Party A chooses two large primes, either both congruent to 1, or both congruent to 3, mod 4, called p and q, and produces N = pq; then N is communicated to party B, but p and q are not. It follows N will be congruent to 1 mod 4. The primes should be chosen large enough that factoring of N is not computationally feasible. The exact size will depend on how much time party B is to be given to make the choice in the next step, and on party B's expected resources.

Party B calls either "1" or "3", a claim as to the mod 4 status of p and q. For example, if p and q are congruent to 1 mod 4, and B called "3", B loses the toss.

Party A produces the primes, making the outcome of the toss obvious; party B can easily multiply them to check that A is being truthful.

No fair using wikipedia

That is a correct answer

The basic idea is that you need to be able to commit to an answer without giving it away. So by giving the heads/tails call in an encrypted form you have commited to it but the coin flipper has no way of knowing what you have commited to. Coin flipper says how the coin landed then the caller gives the key for unlocking the call. Another way to encrypt would be to have N = pq for heads or N = pqr for tails. where p, q and r are large prime numbers. The coin flipper has no idea how many primes make up the factorization. Regardless of method, the concept is the answer. Now stop using wikipedia

Is it possible for the caller to be forced to commit to heads or tails without the flipper knowing what is being called?

[akaslickster]

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!

Question: Are the 2 lads in the same room as the phone and unable to see one another?

[Guest]

Question: Are the 2 lads in the same room as the phone and unable to see one another?

Different rooms, can only hear each other.

[akaslickster]

one guy takes the coin from his own pocket and remembers the date and letter of it then slides it under the door for the other guy to flip then he asks which date or letter. Then the same with the others. Best out of 2.

No need to stay up all night if your tired.

Edited May 16, 2008 by akaslickster

[Guest]

one guy takes the coin from his own pocket and remembers the date and letter of it then slides it under the door for the other guy to flip then he asks which date or letter. Then the same with the others. Best out of 2.

No need to stay up all night if your tired.

Very creative, but no... there is no way to pass anything but conversation back and forth.

[akaslickster]

each guy has a coin and both rooms are video taped simutaneously so there is no cheating. They continue to flip over the phone until one gets heads and the other gets tails.???

[akaslickster]

Ok I gave up and saw answer but there are many other ways I can think up that would be just as fair. Although it would not involve the same means. By the way thanks for sharing that , it kept me busy and I learned a new trick!

Edited May 16, 2008 by akaslickster

[Guest]

Ok I gave up and saw answer but there are many other ways I can think up that would be just as fair. Although it would not involve the same means.

I enjoyed your dialog

Here is a wikipedia reference to the concept behind the answer:

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

[Guest]

what if party A calculates an answer for both out comes? couldn't part A just say Wrong and give them the number that would show that?

[Guest]

Both of them will need one coin each. The idea is that they will both flip the coin and then simultaneously blurt out what it comes up, say on the count of 3.

If the coins match then person A wins. If they the coins are different then person B wins. They can decide before hand who's who.

If I'm thinking of the probability right it should be an even match up and regardless of if either of them lies about their coins result, a clear winner can be decided.

[Guest]

Trying to do this in a way that doesn't involve the use of a computer:

Use a piece of information which is not commonly known, but can be checked.

Say we use B's birthday. Is it on an even or odd numbered day?

A can toss a coin, and if it lands on heads A says "even", if it lands on tails A says "odd". More likely A will just take a guess, but it amounts to the same thing. If A guessed correctly, A wins, otherwise B wins.

Birthdays might not be the best choice since A might know and the odds aren't exactly 50/50, but you could use anything, like the nth digit of B's tax code, someone's phone number, whatever, which B knows and A doesnt. As long as it can be verified afterwards.

If you need to verify it quicker then you may be able to use something happening on TV (both parties watching). You need a 50/50 event coming up that one person can bet on (say odd/even on the nth digit of some unpredictable number which is coming up soon). Shopping and other interactive channels have various numbers appearing on screen, so you could agree on using one of those.

[Guest]

Sorry, I should be using spoilers.

The simultaneous blurting out of things is a bit uncertain - a quick reaction could allow someone to modify their answer so as to win. But if you made it practically impossible to do this...

Let's say you both think of the name of a baseball team. Then you both simultaneously blurt out the names.

Afterwards, you add up the number of letters in both teams. If they come to an even number A wins, if odd, B wins.

Edited May 16, 2008 by octopuppy

[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.

[Guest]

The guy flipping the coin (Person A) could take a video of the flipping with his camera phone, and send the video to his friend (Person B). To avoid the possibility of there being two videos (one showing heads, the other showing tails) and Person A deciding which to send, Person B would choose heads or tails immediately as he receives the video on his cell phone.