I'm trying to apply the hash function combination method described in https://eprint.iacr.org/2013/210.pdf to a HKDF PRNG construction. Thing is it needs 4 uniformly random parameters/keys chosen by the signer during the combination step, but that's what I was already using the PRNG for so it feels like I'm running into some chicken-egg problem.

What would be the recommended way to generate these keys? Is it at all possible to strengthen PRNGs with this trick?

  • $\begingroup$ In general collision resistance is not something that a PRF should be protected against, if I'm not mistaken. $\endgroup$ – Maarten Bodewes Mar 5 '18 at 22:31
  • $\begingroup$ It's about improving pre-image resistance to the strongest of the selected hash functions. The keys should be uncorrelated and are known to any attacker, so it does require pre-image resistance if I understand correctly... $\endgroup$ – user56697 Mar 5 '18 at 22:37
  • $\begingroup$ Simply XORing the results is also not an option, even if collision resistance is not important, the functions may be correlated. $\endgroup$ – user56697 Mar 5 '18 at 23:02

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