The tutorial "Random Number Generation" explains the different choices for PRNG methods used by RandomInteger and related functions. The documentation says that the default method is called ExtendedCA, which the tutorial describes as using a cellular automaton, presumeably of Wolfram class III, like ECA rule 30 in the manner described by Wolfram here: http://www.wolframscience.com/nksonline/page-317
The tutorial says
This generator uses a particular five-neighbor rule, so each new cell depends on five nonadjacent cells from the previous step.
It goes on to describe the effectiveness of the method. What it doesn't include, however, is exactly what cellular automaton rule is being used. Does anyone know the rule number of the CA being used? Or is that information kept secret?