Re: [secdir] secdir review of draft-ietf-ntp-extension-field

Danny Mayer <> Wed, 16 September 2015 17:02 UTC

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Subject: Re: [secdir] secdir review of draft-ietf-ntp-extension-field
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Sorry for the delay in responding. I've been up to my ears in problems.
See my feedback below.


On 8/27/2015 9:08 AM, Sean Turner wrote:
> Fear not as this is just the secdir review!
> I have reviewed this document as part of the security directorate’s
> ongoing effort to review all IETF documents being processed by the
> IESG.  These comments were written with the intent of improving
> security requirements and considerations in IETF drafts. Comments not
> addressed in last call may be included in AD reviews during the IESG
> review.  Document editors and WG chairs should treat these comments
> just like any other last call comments.
> draft summary: This draft updates NTPv4 Protocol and Algorithm
> Specification (aka RFC 5905) s7.5, which is the section that
> describes extension fields, and to paraphrase the: clarify the
> relationship between extension fields and MACs and define the
> behavior of a host that receives an unknown extension field.  Note
> that when comparing the “OLD” section to RFC 5905, you’ll should note
> that the “OLD” text incorporates a verified errata
> (  The “NEW"
> text requires things like when defining an extension the definition
> must specify whether it must be MACed or not, the MTI MAC, the length
> of the MAC, etc.
> secdir status summary: I need to clarify something in my mind, which
> I hope fall into the “you missed that in this spec over here” or the
> “these are *NOT* the droids you’re looking for” bucket, before I can
> say "ready with nits":
> 0) says an extension can support multiple MACs, that the
> extension’s document defines the MTI algorithm & MAC length, and that
> if more than one algorithm is allowed the extension includes an
> indication of which one was actually used; all great.  But, I’m
> trying to figure out how that fits with RFC 5905:
> - In s7.3, I see "dgst (128)” in f8
> - In s9.2, I see "There is no specific requirement for
> authentication; however, if authentication is implemented, then the
> MD5-keyed hash algorithm described in [RFC1321] must be supported”
> Doesn’t s7.3 limit the MAC to HMAC-MD5 and the length to 128?  I mean
> if you’re going to allow an extension to override s9.2 that seems
> like something worth noting in say the abstract/intro.

Now that you bring this up, I should tell you that the reference
implementation implements MD5 and NOT HMAC_MD5 but it also implements
DES (not 3DES) and SHA-1! None of this is documented of course and the
packets are inspected for which algorithm to use based on the size of
the MAC field! Since there is no way to know from the packet whether
there is one or more extension fields or if there is a MAC present the
code ends up guessing which in turn limits the size that you can give an
extension field. This all leads to the strange wording in section and in the draft and is necessary to detect the presence
of a MAC.

We probably need to update the dgest field in RFC5905 to make it clear
that it can have multiple lengths depending on the algorithm used. On
the other hand I would prefer to get rid of the MAC and turn it into an
extension field, assuming that the NTS/CMS scheme is not used. The
advantages of that is obvious especially as no guessing would be
required and we could specify the algorithm to use and you could have
multiple MAC extension fields that would cover different parts of the

> Thinking there’s got to be a reason for this I went off and looked at
> the other NTP WG drafts … after finding the NTS & CMS-based specs,
> are the changes proposed in this draft to to allow an NTP packet blob
> that doesn’t use the MAC mechanism described in RFC 5905 but instead
> use the NTS/CMS “scheme”, i.e., an NTP extension that is a CMS
> object, with no MAC in the 5905 sense - the CMS object instead of the
> NTP MAC field gives you the authentication?
> 1) s7.5.1.2 seems to be saying if extension A specifies alg X, and
> extension B specifies alg X and Y, and extension C specifies alg Y
> then extension A and B can appear together as can extension B and C,
> but A and C can’t appear together?   Sounds great, but what if A and
> C do appear together what happens?

I think that the draft makes it clear that you cannot have that case
since it requires that the MAC use one algorithm. "multiple extension
fields that require a MAC they MUST all require use of the same
algorithm and MAC length"

> 2) Still on 7.5.12: "If there are multiple extension fields that
> require a MAC they MUST all require use of the same algorithm and MAC
> length.”
> So if I specify extension A with X as the MUST, and extension B with
> X as the SHOULD and Y as the MUST, then I can’t include both
> extension A and B?  Extension A requires X, but extension B requires
> Y.

That's right.

> 3) s7.5.1.3: What’s the 24-octet limitation based on?

The MAC guessing game. See the insanity above.

> Minor:
> 0) The new s7.5 says:
> The Field Type field is specific to the defined function and is not
> elaborated here.  If a ….
> 0.a) I think what you’re trying to say is that the Field Type field
> is defined in an IANA registry and it’s length and value are defined
> by the document referred to by the registry?


> 0.b) Neither RFC5905 nor this document specify how the padding is
> done.  Is padding determined by the document referred to by the field
> type?  I.e., can I do padding with all 1s and somebody else do it
> with all zeros?

It shouldn't matter. If the extension field specification needs it to be
specific it should state that in the specification.

> Maybe:
> The Field Type field defines the extension’s semantics as well as the
>  extension’s syntax, i.e., length, value, and padding.  This
> document defines no extensions.


> If a …
> Nits:
> 0) process wise: RFC 5905 has a lot of other errata; some marked
> verified some marked HFDU.  Since this document updates RFC 5905, did
> the WG consider including the other verified errata and resolving
> whether the errata marked HFDU were worth including?  I’ll also
> readily admit that this could have been found in the WG archives, but
> I didn’t search them.
We didn't consider this. We were only concentrating of the extension
field specification and MAC. This was never meant as a proposed REF5905bis.

> 1) s3: Might be worth noting that the “OLD” text includes the errata
> text.  I can’t remember whether you can normatively or informatively
> point to errata (sorry).
> 2) The new s7.5 includes:
> If a host receives an extension field with an unknown Field Type
> value …
> I was like hmm there’s a Field Type value, which is # that goes in
> Field Type and there’s the Value field for the Field Type.  Maybe
> dropping “value” from the quoted sentence makes it clearer that
> you’re really talking about the # that goes in Field Type and not the
> Value?

It's supposed to be the value of the field type.

> 2) s5: Might be worth a reference to the NTP Extension Field Type
> IANA registry.  Though obviously folks reading the base spec and
> extensions are going to find it though.


> spt
> PS - RFC 5905 KoD packets are my favorite packets now.