Math Is Fun Forum

Simply use the methods of Public Key encryption...

Lets say our two people are A and B.

Both  A and B will set up a board.  The Chance and Community Chest cards will be ordered into a standard order (alphabetical based on their wording, for example), so that both parties have everything in the exact same order.

Both parties will also obtain a random number generator.  The generator will take a seed number and can be used to generate a stream of random numbers. There are numerous examples of such generators on the web - some simple, and some complex. It would be simpler if they used the same generator, but it is not necessary.

Finally, both parties will generate a Private and Public key pair using utilities easily found on the web.  They will initialize their random number generator with their private key.

Before play begins, each player will send a message to the other player with the following information:
- the algorithm for the random number generator (if they are not using the same generator)
- the public key

To determine who goes first, a function of the two public keys is made.  Say, count the number of instances of '7' in their sum.  If the result is odd, player A goes first.  If it is even, player B.

Now, a turn progresses as follows (assume it is player A's turn):
1. Player A will query their random number generator for a number.  Lets say the result is X.
2. Player A will then send a message to player B including:
  - The value of X
  - Any non-random moves being made (building houses, etc)
  - Their intent (i.e. roll the dice to move, pay to get out of jail, etc).
3. When Player B receives the message, they will advance their random number generator by X and then use the next 2 numbers to get numbers between 1 and 6.  This will be the dice roll of the player.
4. Player B will then make the move for Player A on their own copy of the board.  Additional numbers are generated as needed, if no further input is required from the other player.  For example:
  - If this roll puts them on Chest or Chance, another number is selected to determine which card was drawn.  If there are Y cards remaining in the pile, then the number is requested between 1 and Y and that card is then selected and played. 
  - If they rolled doubles and can do nothing further, then the next roll can be made
5. The results of the move and the dice rolls made are then sent back to Player A.
  - For efficiency, if there was nothing further that A might do, then B can include in the message the start of his turn (i.e. by including a new random number X in the message, and stating the intent of their move)

Play will proceed until the end of the game.

After the game, or at any point during the game, a player can request an "audit".  During an audit, the player will send their private key to the other player.  They can each create a new private+public key pair if play is to continue (i.e. if they are audited in the middle of the game).  Once in possession of the other player's private key, it can be validated that all of the random numbers generated by the opponent were fair by using their private key and algorithm to re-play all the moves so far. If any discrepancies are found, then one of the players was cheating.  By using the opponent's public key (provided at the beginning of the game), it can be verified that the given private key is in fact authentic - i.e. the one that was decided upon at the beginning of the game.

Using this method, one cannot force the outcome of any role.  An attempt to do so would be detectable by their opponent when they are audited.  For added security, they could basically perform an audit every turn.  Thus the message would include the private key used to generate the rolls, and a new public from the private/public key pair to be used for subsequent numbers.

Notes:
- The reason for sending 'X' to each other is to prevent the person from tailoring their private keys at the beginning of the game (or after an audit) to give random numbers that would provide you a disadvantage.  For example, it wouldn't be very difficult to generate a private key that when used as a seed, results in a roll of 8 right off the bat.  This would land the other player on the tax on their first move, putting them at a disadvantage.  By including X in the message, the other player is forced to pass the first X numbers, virtually eliminating the possibility of them tailoring the results.