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Am I correct in assuming that HMAC_DRBG would return different results if I, e.g. first request 4 bytes followed by a request of another 4 bytes instead of asking for 8 bytes at once?

It seems that in NIST SP800-90A chapter 10.1.2.5: Generating Pseudorandom Bits Using HMAC_DRBG first the K and V values are changed, then the requested number of bits is generated (changing just V), and then the K and V values are changed again. Besides that, leftover bits get chucked away as well.

Isn't it a strange property for a DRNG to return different value if DRNG(4) || DRNG(4) != DRNG(8) if the DRNG is not reseeded in between? Do other DRNG's have the same kind of property?

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  • $\begingroup$ Or maybe I'm just too used to the properties of KBKDFs :) $\endgroup$ – Maarten Bodewes Oct 16 '14 at 22:40
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Yes, you are reading this right. The requests for random value from NIST 800-90 drbgs perturbed the state. If this is a problem you can add a layer that optionally buffers values and always makes constant size requests.

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  • $\begingroup$ Whoops, waited a bit too long for other possible answers, sorry about that :) $\endgroup$ – Maarten Bodewes Oct 23 '14 at 22:32

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