Re: [secdir] SECDIR review of draft-ietf-dnsop-dns-zone-digest-09

[Donald Eastlake <[e-mail]>]

Hi Duane,

Thanks for accepting most of my suggestions.

See my responses below at <de> on the items we are still discussing.

On Tue, Sep 15, 2020 at 11:38 AM Wessels, Duane <[e-mail]>
wrote:

> Thanks Don for the extensive review!  tl;dr, we did accept most/many of
> your suggestions already, except these items may warrant further discussion:
>
> - minimum digest length
> - more hash algorithms than SHA-384
> - local policy when verifying multiple digests
> - implementation status section
>
> More responses inline below.
>
> DW
>
> > On Sep 12, 2020, at 6:53 PM, Donald Eastlake <[e-mail]> wrote:
> >
>
...

> >
> > 2) The minimum size of the Digest Field, 1 octet, seems too small.
> > Typical minimum size for such a field in IETF protocols seems to be 96
> > bits or 12 octets, so that, at least with a strong hash function, you
> > have a reasonable resistance against brute force attacks. Also, while
> > it is fairly obvious, you might want to mention how a receiver
> > determines the length of the Digest Field.
> >
> > OLD (Section 2.2.4)
> > The Digest field must not be empty.
> >
> > NEW
> > The Digest Field MUST NOT be shorter than 12 octets. If it is, the
> > ZONEMD RR containing that short digest cannot be used to verify a
> > zone.  The length of the digest field is determined by deducting the
> > fixed size of the Serial, Scheme, and Hash Algorithm fields from the
> > RDATA size in the ZONEMD RR header.
>
> I'm not necessarily opposed to a minimum digest size.  Do you envision
> any complications with private use algorithms though?  It seems a little
> strange to allow private use algorithms whose digest value could be
> anything,
> but requiring a minimum size (which could then simply be padded I know).
>

<de> Well, it looked to me like the draft already had a minimum Digest
field, namely one octet, since it said the Digest Field couldn't be empty.
As I recall, the digest didn't say what to do if the Digest field was zero
length but I presume it would mean that the ZONEMD RR was malformed and
thus could not be used to validate a zone. Seems like the minimum length
would apply to private use algorithms also. See Section 5.2.2.1 of
https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-rfc2845bis-09

> 3) SHA384 / algorithm agility
> > -- here are some thoughts/comments on SHA384 and algorithm agility:
>
...

> >
> > 3c) SHA2-384 is just SHA2-512 with some different initialization
> > values and with the 512 bit output truncated to 384. Thus the
> > incremental effort of providing SHA2-512 in tiny if you have SHA2-384
> > (and vice versa). So, if SHA2-384 is mandatory, then you might as well
> > make SHA2-512 mandatory also since you get it for probably <1%
> > incremental effort. Having two mandatory algorithms with some people
> > using one and some the other would provide some real confidence that
> > implementations were actually looking at the Hash Algorithm Field,
> > could handle Digest Fields of different length, etc., and the
> > algorithm agility mechanisms might actually work. It currently looks a
> > bit like only lip service is being paid to algorithm agility.
>
> In an earlier version of the draft, four algorithms were defined, although
> none "past" SHA-384.  There was a (Oct 2018) discussion to make SHA-384
> the mandatory algorithm and drop all the weaker ones.
>
> I'm not necessarily opposed to adding more mandatory-to-implement
> algorithms
> if there is working group consensus about that, but it feels like a pretty
> significant and late change?
>

<de> I agree it is a significant change in the draft. But, as I say, if you
have SHA-384 you have 99+% of the crypto code you need for SHA-384 and
SHA-512. Of course, SHA-512 has an even longer value, 64 bytes, but with
just one or at most a few ZONEMD RRs at the zone apex, that does not seem
to be a problem. Of my security comments, I rate adding a 2nd hash
algorithm as the most important. It could be that the 2nd algorithm would
be a SHOULD rather than a MUST. And, if a second algorithm is added, I
think SHA-512 would be the easiest for implementers.


> > 3d) On algorithm agility generally: There is a reasonable provision in
> > the ZONEMD RR for a hash algorithm identifier.  But, given that this
> > is Standards Track I would have expected there to be hash algorithms
> > specified of different types with one a MUST implement and one a
> > SHOULD implement -- possible more but at least that. Blake or SHA3
> > would be example of a different algorithm type from SHA2. But I admit
> > that SHA2-384/512 is very strong and a break in it significant enough
> > to make much difference seems very unlikely.
>
> Same as above.
>

<de> Response above.

> 4) There is no mention of local policy. If at some point there are
> > multiple hash algorithms and/or schemes specified (which could include
> > private use algorithms and schemes), a ZONEMD zone validator would
> > likely need a local policy as to which algorithm/scheme digest it will
> > pay attention to.  See, for example, the first paragraph of Section 4
> > which assumes any good digest should be good enough to validate the
> > zone for any verifier.
>
> The draft-ietf-dnsop-dns-zone-digest-00 version did have the following
> text that spoke to this:
>
> >   It is RECOMMENDED that implementations maintain a (possibly
> >   configurable) list of supported Digest Type algorithms ranked from
> >   most to least preferred.  It is further RECOMMENDED that recipients
> >   use only their most preferred algorithm that is present in the zone
> >   for digest verification.
> >
> >   As a matter of local policy, the recipient MAY require that all
> >   supported and present Digest Type algorithms verify the zone.
>
> There was a thread on DNSOP where we had agreement to
> take those two paragraphs out:
>
> https://mailarchive.ietf.org/arch/msg/dnsop/RFCklH7Lx00bL-tOVRCAc0j5ftw/


<de> I went and read that thread. I agree that the text removed was kind of
complex. I was just worried that the text in Section 4 too strongly implied
that an implementation must accept a zone if a ZONEMD validates. True, it
does have text about "supporting" the Scheme and Hash Algorithm but
conceivable you could support a Scheme and a Hash Algorithm but have a
policy against accepting a ZONEMD RR that combined them and in any case I
suspect the draft is using "supported" in the sense of "implemented". I'd
be happy with adding just one sentence in Section 4 something like "The
verifier MAY ignore a ZONEMD if the Scheme and Hash Algorithm violates
local policy."
...

> > 6e. Actually, 240-254 seems like a lot of values for Private Use. Why
> > would you need 15 private hash algorithms within one administrative
> > area? If you think there are going to be lots of private/proprietary
> > hashes (national vanity crypto?) that get used moderately widely,
> > there should be a provision for "vendor specific" hash algorithms
> > where the "Digest Field" actually starts with a "vendor" identifier.
>
> It seemed a reasonable amount to us, but if others think it should be
> smaller that shouldn't be a problem.
>

 <de> OK. It isn't gigantic or anything, just a bit larger than the 3 or 4
or so values I typically see.

...
> > 12. Having an Implementation Status section (Section 10) is good while
> > a draft is going through the approval process, but I think it is, as
> > indicated in RFC 7942, inappropriate in a resulting standards track
> > RFC. Furthermore, all of the disclaimer text given in RFC 7942 is
> > missing here. So, I believe similar disclaimer text should be added
> > and the RFC Editor should be requested to drop this section before
> > publication.
>
> The authors feel it could be useful, even though it is expected to
> become out-of-date.   Looks like some other published RFCs have kept
> their Implementation Status sections, sometimes as an appendix?
>

<de> OK... I think that if retained it would be much better as an appendix.

 ...

> > ATTACHMENT
> >
> > Taking into account the above and to make other editorial suggestions,
> > I have done an editing pass over the draft the results of which are
> > attached.  I hope you find it helpful.
>
> Thank you, we did find it very helpful.
>

<de> You're welcome.

<de> Donald
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