Re: [babel] Shepherd's review of draft-ietf-babel-hmac-02

Juliusz Chroboczek <jch@irif.fr> Wed, 26 December 2018 19:03 UTC

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Date: Wed, 26 Dec 2018 20:03:12 +0100
Message-ID: <87ftuk9klr.wl-jch@irif.fr>
From: Juliusz Chroboczek <jch@irif.fr>
To: Donald Eastlake <d3e3e3@gmail.com>
Cc: Babel at IETF <babel@ietf.org>, babel-chairs <babel-chairs@ietf.org>, draft-ietf-babel-hmac@ietf.org
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Subject: Re: [babel] Shepherd's review of draft-ietf-babel-hmac-02
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Thanks for your review, Donald.

> "robust random number generator" -> "robust random number [RFC4086]
> generator". Could also add the RFC 4086 reference at other occurrences
> of "random" but reference should be included at least once.

Done, thanks.

> Section 1.2, Top of page 4, third bullet item.
> This is a little confusing to read. I assume that the point about the
> packet being accepted by node A is that it's a valid packet (has a
> good HMAC TLV). But I think the average reading will be wondering
> "What is this node A and why isn't it mentioned again later?" Not sure
> if there is a reasonable wording change that improves this...

I too am unhappy with this property.  We've spent a lot of time thinking
about how to reformulate it in a more comprehensible manner, but we've
failed.  If anyone has any idea, I'm listening.

> Section 5.2:
> The diagram is odd. I would recommend sliding the Index field and
> preceding vertical bar character to the left so it starts aligned
> with the Type.

I've tried, but I didn't like the result.  I'll leave it as it is, if
that's okay with you.

> Also, the PC is fixed length, which is not shown in the figure - I think
> you really have to at least mention the length of the PC in this
> section.

Done.

> While it is reasonable, for implementation convenience, that there is
> a maximum size for Index, perhaps for cryptographic strength, there
> should be a minimum length, maybe 12 bytes?

I disagree.  The only property that the index needs to satisfy is that it
is never reused for a different PC sequence for a given (key, sender)
pair.  In particular, if a node never loses state (e.g. it has reliable
persistent storage), then it never needs to change indices -- and in that
case, a 0-octet index is suitable.

> Thinking some more about PC, seems like you either (1) need to say
> that circular arithmetic is used [rfc1982] (bad idea in my opinion),
> or (2) say what happens when PC hits all 1's (presumably adopt a new
> Index, probably a better idea). It might be useful to also state that
> PC is an "unsigned integer" or the like.

In Section 4.2, we say:

      if the PC overflows, a new index is generated;

In Section 5.2, we say:

      A new index MUST be generated whenever the PC overflows.

Please let me know if you think that's not explicit enough.

> Should say IANA is "requested" rather than "instructed".

Done.

> Heading should end with "-hmac-01", not "-hman-00".

Done.

Thanks again,

-- Juliusz