Re: [CFRG] I-D Action: draft-irtf-cfrg-hpke-08.txt

Christopher Wood <caw@heapingbits.net> Tue, 20 April 2021 22:03 UTC

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Date: Tue, 20 Apr 2021 15:03:06 -0700
From: "Christopher Wood" <caw@heapingbits.net>
To: cfrg@irtf.org
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Subject: Re: [CFRG] I-D Action: draft-irtf-cfrg-hpke-08.txt
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Thanks, Frank! We applied your suggestions in this PR:

   https://github.com/cfrg/draft-irtf-cfrg-hpke/pull/217

Since this was mostly editorial, we'll hold off publishing a new version for now. (Chairs, please let us know if you'd like to see a new version in order to move this document forward.)

Best,
Chris

On Mon, Apr 19, 2021, at 12:51 PM, Frank Denis wrote:
> General comments on draft-irtf-cfrg-hpke-08:
> 
> * The document is well-written and every function is clearly specified.
> I wrote implementations solely based on it, that were verified to be 
> interoperable with other implementations, and didn’t hit anything that 
> required additional information.
> 
> * The test vectors also appear to be correct.
> 
> Given this, the maturity of the scheme, and the fact that it is a 
> dependency for other protocols, I’d like to see this document move 
> forward.
> 
> 
> ---
> 
> 
> Additional comments:
> 
> 
> * Section 4:
> 
> “This function can raise an DecapError” -> “a DecapError”
> 
> 
> 
> * Section 4: "The Seal() and Open() functions can return a NonceOverflowError."
> 
> The fact that HPKE uses increasing nonces is an internal detail; 
> exposing this to applications looks like the wrong abstraction level.
> 
> From an application perspective, a more meaningful error would be 
> `TooManyMessages` or a more generic `Overflow` error that could also 
> encompass the case of the AEAD’s internal counter reaching a limit.
> 
> Using `NonceOverflowError` may catalyze inconsistencies between the 
> specification and its implementations.
> 
> 
> 
> * Section 4: LabeledExtract() and LabeledExpand() functions
> 
> The document doesn’t mention any size limits regarding the `suite_id` 
> and `label` parameters of these functions.
> 
> This can be an issue for implementations favoring static/pre-allocated 
> storage space over heap allocations.
> 
> Specifying reasonable limits (64 bytes?) may be useful to avoid 
> interoperability issues.
> These limits can also be documented in section 7.2.1.
> 
> 
> 
> 
> * Section 8.1.3
> 
> Splitting this into 2 or 3 paragraphs would make the section more readable.
> 
> 
> 
> * Section 8.4: “Further, because HPKE uses AEAD schemes that are not 
> key-committing”
> 
> This seems to suggest that non key-committing schemes are a requirement 
> for HPKE, which is not the case.
> Further revisions of the document can include key-committing schemes, 
> and exported keys can safely be used with such AEADs.
> 
> 
> 
> 
> 
> Cheers,
> -Frank.
> 
> 
> 
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