Re: [auth48] AUTH48: RFC-to-be 9496 <draft-irtf-cfrg-ristretto255-decaf448-08> for your review

[Jack Grigg <[e-mail]>]

Hi all,

Thank you RFC Editor/sg/ap for your work! I will perform my final review
next week, but in the interim I can respond to some of the questions.

For clarity, this email is NOT suggesting any immediate changes to the text
(I will make my own suggestions after my review).

On Sat, Oct 14, 2023 at 12:47 PM <[e-mail]> wrote:

> Authors,
>
> While reviewing this document during AUTH48, please resolve (as necessary)
> the following questions, which are also in the XML file.
>
> 1) <!-- [rfced] Please insert any keywords (beyond those that appear in
> the title) for use on https://www.rfc-editor.org/search. -->
>
>
> 2) <!-- [rfced] Please ensure that the guidelines listed in Section 2.1 of
> RFC 5743
> have been adhered to in this document.  See
> https://www.rfc-editor.org/rfc/rfc5743.html#section-2.1.
> -->
>
>
> 3) <!-- [rfced] Please consider including a reference for "Discrete Log
> Hardness".
> We do not see this phrase in RFCs and we did not find directly matching
> hits via
> our general searches.  Is this the same as the "hardness of the discrete
> logarithm
> problem"?
>
> Original:
>    This means the group has a cofactor
>    of 1, and all elements are equivalent from the perspective of
>    Discrete Log Hardness.
> -->
>
>
I think that "Discrete Log Hardness" and "hardness of the discrete
logarithm problem" could be used interchangeably here.


>
> 4) <!-- [rfced] We have updated the following for readability.  Please
> review to
> ensure we have not altered the meaning.
>
> Original:
>    Edwards curves provide a number of implementation benefits for
>    cryptography, such as complete addition formulas with no exceptional
>    points and formulas among the fastest known for curve operations.
>
> Current:
>    Edwards curves provide a number of implementation benefits for
>    cryptography, such as complete addition formulas with no exceptional
>    points and formulas known to be among the fastest for curve
>    operations.
> -->
>
>
> 5) <!-- [rfced] Is "hash_to_curve" considered an algorithm? RFC 9380
> refers to
> it as an encoding function.
>
> Original:
>    In some contexts this property would be a weakness, but it is
>    important in some contexts: in particular, it means that a
>    combination of a cryptographic hash function and the element
>    derivation function is suitable for use in algorithms such as
>    hash_to_curve [draft-irtf-cfrg-hash-to-curve-16].
> -->
>
>
A "hash-to-curve algorithm" is something we would have referred to, yes.
This specific reference should be adjusted however, because the
now-published RFC 9380 specifically constrains the definition of
hash_to_curve, and in Appendix B defines hash_to_ristretto255 with the same
security properties and interface (and similarly for decaf448 in Appendix
C). I'll think about a suggestion during my review.

Relatedly, I notice that RFC 9380 refers to an old version of this
RFC-to-be. Once RFC 9496 is published, I presume that RFC 9380's reference
will be fixed? Their reference link is the one that implementers will be
following more (our reference here is effectively an informational
backlink).


>
> 6) <!-- [rfced] To what does "its" refer in the last sentence?
>
> Original (the paragraph is provided for context):
>    Since ristretto255 is a prime-order group, every element except the
>    identity is a generator, but for interoperability a canonical
>    generator is selected, which can be internally represented by the
>    Curve25519 basepoint, enabling reuse of existing precomputation for
>    scalar multiplication.  This is its encoding as produced by the
>    function specified in Section 4.3.2:
> -->
>
>
"its" in the last sentence refers to "a canonical generator" in the
previous sentence.


>
> 7) <!-- [rfced] May we change "reflect" to "note" here?
>
> Original:
>    Implementations SHOULD reflect that: the
>    type representing an element of the group SHOULD be opaque to the
>    caller, meaning they do not expose the underlying curve point or
>    field elements.
>
> Suggested:
>    Implementations SHOULD note that the
>    type representing an element of the group SHOULD be opaque to the
>    caller, meaning they do not expose the underlying curve point or
>    field elements.
> -->
>
>
I don't think so, assuming that "note" is meant in the sense "to take
notice of". The SHOULD applies as-written to implementations, not
implementers. An implementation reflects what its implementer has noted.

I will consider during my review whether in context "Implementations SHOULD
reflect" or "Implementers SHOULD note" makes more sense.


>
> 8) <!--[rfced] May we clarify "allowed operations" as follows?
>
> Original:
>    The decoding
>    function always returns a valid internal representation, or an error,
>    and allowed operations on valid internal representations return valid
>    internal representations.
>
> Perhaps:
>    The decoding
>    function always returns a valid internal representation, or an error,
>    and operations that are allowed on valid internal representations
> return valid
>    internal representations.
> -->
>
>
> 9) <!-- [rfced] FYI, we have alphabetized the references. Please let us
> know
> of any objections.
> -->
>
>
> 10) <!-- [rfced] The sourcecode in Appendices A.1, A.2, and A.3 extend
> beyond the 69-character margin.  Please let us know how the lines may
> be broken.
> -->
>
>
As these are not actual source code with established parsing rules,
breaking the lines must be done carefully to ensure that adjacent test
vectors are correctly separated. I'll make a suggestion to this effect in
my review.


>
> 11) <!-- [rfced] Please review whether any of the notes in this document
> should be in the <aside> element. It is defined as "a container for
> content that is semantically less important or tangential to the
> content that surrounds it" (
> https://authors.ietf.org/en/rfcxml-vocabulary#aside).
> -->
>
>
> 12) <!-- [rfced] Please review the "Inclusive Language" portion of the
> online
> Style Guide <
> https://www.rfc-editor.org/styleguide/part2/#inclusive_language>
> and let us know if any changes are needed.
>
> For example, please consider whether "whitespace" should be updated.
> -->
>
>
"Whitespace" here means any character that represents horizontal or
vertical space in typography (
https://en.wikipedia.org/wiki/Whitespace_character). To my knowledge there
are no "positive or less harmful" connotations associated with this term
(unlike other historic terms of art in computer science), and I don't know
of another term that denotes the same set. But really all we need here is a
concise way to say "if you're copy-pasting these hex strings into your
code, any spaces, tabs, newlines, or whatever else the rendered version of
this RFC happens to put into those gaps can be ignored and must be removed,
and the position of them is irrelevant".

Cheers,
Jack


>
> Thank you.
>
> RFC Editor/sg/ap
>
>
> On Oct 13, 2023, at 4:46 PM, [e-mail] wrote:
>
> *****IMPORTANT*****
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> Updated 2023/10/13
>
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> --------------------------------------
> RFC9496 (draft-irtf-cfrg-ristretto255-decaf448-08)
>
> Title            : The ristretto255 and decaf448 Groups
> Author(s)        : H. Valence, J. Grigg, M. Hamburg, I. Lovecruft, G.
> Tankersley, F. Valsorda
> WG Chair(s)      :
> Area Director(s) :
>
>
>