MT is not suitable for crypto because after observing 637 values you can predict all further values. 637 is the size of MTs internal state vector and the output

MT is not suitable for crypto because after observing 637 values you can predict all further values. 637 is the size of MTs internal state vector and the output of MT are basically just values from that state vector run through a tempering function. That tempering function is invertible, thus by looking at 637 values you get the full state vector, at which point you can generate the next values as usual.