[Cryptography] Atomic or simultaneous exchange of secrets.

[jamesd at echeque.com]

Ann and Bob want to exchange secrets.

She does not want to give him her secret, unless he gives her his.

He does not want to give her his secret, unless she gives him hers.

Seems like they are stuck.

Each secret is an unpredictable integer modulo a very large prime, if
that makes a difference.

I seem to recall that there was a solution to this problem, but when I
read the solution, was not too happy with it.