[RussellDussel]

This instantly reminded me of the (true) story where people were able to cheat an online poker game because the site released its source to "prove" its fairness/randomness, showing that the deck was shuffled from a fixed state using a standard RNG with the time of day (in ms) as the seed.

Of course this meant there were only 86,400,000 possible decks, people could generate all the possible decks ahead of time and as soon as cards would be revealed in the game they could easily match the hand being played to the limited set of possibilities. Essentially they had foresight of which cards were coming out. Furthermore, once a few decks were matched up you could vastly reduce the set of possibilities even further by working out the approximate time of day on the server.

[swang]

This is PlanetPoker[0]

The gist of it is their algorithm never chose the last card in the array to swap with, and the last card was never guaranteed to be the last card. They also committed the common mistake of implementing the "naive" shuffle, which is not equally distributed.

[0] http://www.cigital.com/papers/download/developer_gambling.ph...

[im3w1l]

By the birthday property, this means that it is very likely to experience the exact same deck being played out more than once.