For CSPRNGs, we tend to use hardness proofs (i.e. prove that an attacker cannot predict the CSPRNG output in polynomial time). To understand these, you'll need

For CSPRNGs, we tend to use hardness proofs (i.e. prove that an attacker cannot predict the CSPRNG output in polynomial time). To understand these, you'll need to figure out what the algorithm in use is. This isn't particularly relevant to TOTPs since they are built on unforgeable message authentication codes (TOTP usually uses HMAC) so the proof would usually be less about predicting a bit (though, if you could, that would be a valid security break) and more about proving an attacker cannot forge a signature on a message without violating the hardness assumptions. For regular ol PRNGs (i.e. non-cryptographically secure), there are various tools [1] which let you feed in enormous amounts of samples and then automatically apply a variety of known techniques to try to find correlations. But, generally, you wouldn't expect a PRNG to pass these tests. Instead, you should focus on getting a distribution and behavior that is helpful to your use case. [1] https://pracrand.sourceforge.net/PractRand.txt https://pracrand.sourceforge.net/PractRand.txt