For the lazy reader: The issue is that rd() returns at most a 64-bit int, whilst mt19937 has 19937 bits of state. This means you are very restricted. Note tha

For the lazy reader: The issue is that rd() returns at most a 64-bit int, whilst mt19937 has 19937 bits of state. This means you are very restricted. Note that, for example linux's /dev/random uses 4069 bits of entropy.