Randomizing output of HKDF for the same input key material

Asked Feb 17 '21 at 18:30

Active Feb 17 '21 at 21:16

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Given HKDF and a master input material key $k$, I would like to derive ephemeral keys that cannot be linked together. If I understand correctly, the specification of $HKDF$ adding a unique random $salt$ while generating each new ephemeral key should randomize the output. Is it the case?

An example: I have a master input key k (unknown to the adversary). I generate 10 ephemeral keys as $x_0 = HKDF(k, salt_0), x_1 = HKDF(k, salt_1), \ldots , x_9 = HKDF(k, salt_9)$. And $x_0 \neq x_1 \neq \ldots x_9$.

edited Oct 07 '21 at 07:59

Community

asked Feb 17 '21 at 18:30

Ziva

1

See this Multiple AES Key Derivation from a master key and we can't guarantee the non-equality. Check it after generation. Also, you can extract only once, and use context info to generate multiple keys. – kelalaka Feb 17 '21 at 18:36
I mean, use extract once, and call expand 10 times with different infos – kelalaka Feb 17 '21 at 19:10
@kelalaka The link which you shared states for the HKDF extraction that "in general, different salts with same result in fundamentally different PRKs." so it seems it is what I'm looking for. Why did you say that you can't guarantee non-equality? – Ziva Feb 17 '21 at 19:29
1

I've added the probability calculation to the answer. Now, this may be the dupe of Multiple AES Key Derivation from a master key – kelalaka Feb 18 '21 at 14:02

Randomizing output of HKDF for the same input key material

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