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Regarding HKDF, the specification says a salt may be re-used.

However, I have set up my system to generate a new, long and cryptographically random salt that persists with the user. This salt is not a secret. (See the RFC, section 3.1)

In addition, I am taking our unique user identifier and using it as context info for the third parameter in HKDF.

So our HKDF looks like this:

DK = HKDF(MasterKey, PublicUserSpecificSalt, UUID)

My questions are:

  1. Are there any issues with creating a random, user-specific salt, and using this for the salt input in HKDF? It seems the spec expects the salt to be re-used, which I am not doing.
  2. If I do use a random, user-specific salt, what is the benefit of the UUID as context info? Should I just remove it, since it would add unnecessary complexity?
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  • $\begingroup$ @R1w Please don't make minimal edits until we don't have to perform an additional review, this is just not worth the cost. $\endgroup$
    – Maarten Bodewes ♦
    Sep 19, 2018 at 23:02

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By HKDF, I guess you mean HKDF-Expand, which takes as input a pseudorandom key, a label, and the output length. A public label is OK, but it is better unique rather than random due to HKDF-Expand's pseudorandom function security. I think using UUID only as the label is good enough.

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  • $\begingroup$ Hi! Thanks for your answer! Just to clarify -- by unique, do you mean a unique value that is shared across all users, or a value that is unique to that user? $\endgroup$
    – jrl
    Sep 20, 2018 at 17:58
  • $\begingroup$ @jrl If the server uses the same master key for all users, then the salt should be unique among all users. $\endgroup$
    – Shan Chen
    Sep 20, 2018 at 18:21
  • $\begingroup$ The HKDF function that takes a salt is HKDF-Extract, not HKDF-Expand. $\endgroup$
    – Luis Casillas
    Sep 21, 2018 at 0:05

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