This dialog window appears after clicking one of the two Secret buttons from within the Diffie-Hellman Demonstration.
The dialog prompts you to choose the secret numbers of Alice or Bob.
Remember to choose the secret numbers for Alice and Bob separately! That means you have to call up this dialog twice.
This points out that in practice nobody - neither Alice nor Bob - is allowed to reveal their secrets in order to keep the key exchange safe: Not even the co-operating person may gain knowledge of the secret because the communication is performed over a non-secure channel.
The secret of a party is an arbitrary natural number, preferably smaller than the prime module which was defined as part of the public parameters. If you choose one of the following values, then the computed session key is possibly not safe!
The button Generate Secret creates an appropriate secret number fulfilling the necessary conditions.