How to design a circuit for genrating random numbers
There are a lot of circuits around advertised as "digital dice". The usual approach is to use a counter which runs at high speed, for a time determined by how long the user presses the button. Since a human can't press a button repeatably for the same number of microseconds, this produces acceptably random numbers.

The purist approach is to use a genuine noise source such as a silicon junction (diodes, especially Zeners), and amplify it. In a suitably shielded case this will produce "real" noise which you can use for cryptographic purposes.