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Nifty! Out of curiosity, how much different is that from taking several partly-random streams and XORing them together? I always assumed what was going on was essentially a fancier version of that.

Oh, I guess you have to ensure the inputs aren’t correlated, or they’ll cancel out?

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The advantage of a secure XOF is that a malicious source of entropy needs to do a good deal more work than a simple XOR to generate controlled PRNG output (the attacker needs to do 2^n XOF operations to generate n bits of PRNG output, and that’s only if the attacker knows the output of all other sources of entropy—someone with that level of access can do far more effective attacks).

The sources of entropy can be correlated and won’t cancel out with a well designed secure XOF. SHAKE-256 is an example of a secure XOF.

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