Re: [Cfrg] Review of draft-selander-ace-cose-ecdhe-12

Russ Housley <housley@vigilsec.com> Fri, 01 March 2019 21:07 UTC

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From: Russ Housley <housley@vigilsec.com>
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Subject: Re: [Cfrg] Review of draft-selander-ace-cose-ecdhe-12
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> On Mar 1, 2019, at 3:29 PM, Stanislav V. Smyshlyaev <smyshsv@gmail.com> wrote:
> 
> Minor concerns:
> 
> This doesn't seem to be crucial for the security, but I believe that it would be helpful to provide a comment in Section 3.3 about the reasons why PSK is used as a 'salt', not as a "secret" in HKDF.

I did not comment on it in my review, but I did not this when I read the document.  I actually thought it was kind of clever.

That said, RFC 5869 defines the salt as:

      salt     optional salt value (a non-secret random value);
               if not provided, it is set to a string of HashLen zeros.

So, while I have no concerns about the security of the construction, I do think the Security Considerations need to say that the PSK needs to be kept secret even though it is being input into the KDF as salt.

Russ