This method works well over the phone
, or in another circumstance where two parties are physically separated (like over the Internet
Alice and Bob agree on a hash function H which compresses its input to an N-bit result. H should have a result long enough to prevent a mortal from finding two inputs which hash to the same result. It should also be free of shortcuts for doing so. In other words, it should be a cryptographic hash.
One of the two people picks a random number, hashes it with H(x), and tells y=H(x) to the other party. The recipient of y guesses whether the random input was even or odd. After this, the party who generated the random number reveals this secret number. The recipient can compute H(x) and verify that it equals y, thus, that x is almost certainly the original random number. After this it is obvious to both sides whether the guess was correct.