Message Authentication Code for the Network Time Protocol
draft-ietf-ntp-mac-06

[Ballot comment]
§2: "is not a secure MAC and therefore MUST be
deprecated."

That "MUST" seems like a statement of fact rather than a normative requirement.

[Ballot comment]
Thanks to everyone who worked on this document.

I agree with Mirja's concerns about the backwards compatibility aspects,
especially from an operational perspective -- in particular, is there some way
to roll this change out incrementally, or does it require a flag day? It seems
to me that this document really needs an "operational considerations" section
that either explains how this change might be deployed or cites some already
existing NTP document that does so.

I also have a handful of minor and editorial comments that you may want to
consider addressing.

---------------------------------------------------------------------------

Please consider addressing these id-nits issues:

  ** The abstract seems to contain references ([RFC5905], [RFC4493]), which
    it shouldn't.  Please replace those with straight textual mentions of the
    documents in question.

  -- The draft header indicates that this document updates RFC5905, but the
    abstract doesn't seem to directly say this.

---------------------------------------------------------------------------

§1.1:

>  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
>  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
>  document are to be interpreted as described in RFC 2119 [RFC2119].

Please update to use the boilerplate in RFC 8174.

---------------------------------------------------------------------------

§3:

>  A symmetric key is a triplet of ID,
>  type (e.g.  MD5, AES-CMAC) and the key itself.

I don't follow this. A naïve reading of this sentence would lead me to believe
that the key contains three fields, one of which is itself.  I suspect you're
using "key" here to refer to two somewhat different constructs.  Consider
giving them more unique names.

[Ballot comment]
Thanks to everyone who worked on this document.

I agree with Mirja's concerns about the backwards compatibility aspects,
especially from an operational perspective -- in particular, is there some way
to roll this change out incrementally, or does it require a flag day? It seems
to me that this document really needs an "operational considerations" section
that either explains how this change might be deployed or cites some already
existing NTP document that does so.

I also have a handful of minor and editorial comments that you may want to
consider addressing.

---------------------------------------------------------------------------

Please consider addressing these id-nits issues:

  ** The abstract seems to contain references ([RFC5905], [RFC4493]), which
    it shouldn't.  Please replace those with straight textual mentions of the
    documents in question.

  -- The draft header indicates that this document updates RFC5905, but the
    abstract doesn't seem to directly say this.

---------------------------------------------------------------------------

§1.1:

>  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
>  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
>  document are to be interpreted as described in RFC 2119 [RFC2119].

Please update to use the boilerplate in RFC 8174.

---------------------------------------------------------------------------

§3:

>  A symmetric key is a triplet of ID,
>  type (e.g.  MD5, AES-CMAC) and the key itself.

I don't follow this. The key is composed of the ID, the type, and the key. Where
the ID is a key identifier, the type is the MAC algorithm, and the key is...
composed of the ID, the type, and the key, I guess? This seems infinitely
recursive.

I suspect you're using "key" here to refer to two somewhat different constructs.
Consider giving them more unique names.

[Ballot comment]
Section 4: It seems like Intel's New Instruction Set either needs a citation or needs to be removed as a specific example unless it's expected to continue to be a meaningful example for as long as this document remains in force.

Section 6: "NIST document" seems like it should have a more descriptive title

Section 9.1: The link to the NIST document is broken.

[Ballot comment]
From §3:
  If authentication is implemented, then AES-CMAC as specified in RFC
  4493 [RFC4493] SHOULD be computed over all fields in the NTP header,
  and any extension fields that are present in the NTP packet as
  described in RFC 5905 [RFC5905].

When would AES-CMAC not be computer over all fields, or when would it not be used?  IOW, why is a MUST not used?  The text already indicates that it is talking about "if authentication is implemented..."

[Ballot comment]
From §3:
  If authentication is implemented, then AES-CMAC as specified in RFC
  4493 [RFC4493] SHOULD be computed over all fields in the NTP header,
  and any extension fields that are present in the NTP packet as
  described in RFC 5905 [RFC5905].

When would AES-CMAC not be computer over all fields, or when would it not be used?  IOW, why is a MUST not used?  The text already indicates that is is talking about "if authentication is implemented..."

[Ballot comment]
It would be useful to know if this is actually implemented / deployed. I spent some time looking around, and found something saying that this was slated for ntp-4.4.0, but it looks like ntp-4.2.8p12 was just recently released? There was some mumbling about new numbering formats, but I didn't explore that rabbithole.
https://github.com/ntp-project/ntp/blob/stable/ntpd/ntp.conf.html has refernces to MD-5, but not AES, so, um, perhaps it isn't?

[Ballot comment]
Please consider using the RFC 8174 version of the BCP14 boilerplate.

It's somewhat surprising to me that this document seems to focus on the
"md5 is broken" aspect of the previous ntp-md5 construction, with no
mention that there is additionally a cryptographic flaw: it is subject to
length-extension attacks.  (I'm told that no practical attacks of this
nature are known, but it still seems prudent to note the additional flaw in
the mechanism being replaced.)

Similarly, to avoid concerns about the risk of extension attacks with the
new construction, it may be worth explicitly noting that the CMAC
construction is secure even in the presence of variable length messages
(see, e.g., [OMAC1a] from RFC 4493).  (In other contexts the message length
is explicitly included as an input to the MAC, though given the above that
does not seem necessary here.)

Section 3

  If authentication is implemented, then AES-CMAC as specified in RFC

Maybe "NTP authentication"?

  described in RFC 5905 [RFC5905].  The MAC key for NTP MUST be at
  least 128 bits long AES-128 key and the resulting MAC tag MUST be at
  least 128 bits long as stated in section 2.4 of RFC 4493 [RFC4493].

I'm not sure I understand why these are "at least" -- if AES-128 is used,
exact matching should be fine.

[Ballot comment]
Rich version of this review at:
https://mozphab-ietf.devsvcdev.mozaws.net/D3591



COMMENTS
S 3.
>      If authentication is implemented, then AES-CMAC as specified in RFC
>      4493 [RFC4493] SHOULD be computed over all fields in the NTP header,
>      and any extension fields that are present in the NTP packet as
>      described in RFC 5905 [RFC5905].  The MAC key for NTP MUST be at
>      least 128 bits long AES-128 key and the resulting MAC tag MUST be at
>      least 128 bits long as stated in section 2.4 of RFC 4493 [RFC4493].

Is there a way for either of these values to be > 128 bits? If not,
maube just say "must be 128 buts:

[Ballot comment]
I guess the assumption is that all hosts configured to use the same authentication scheme? Or would makse sense to add a short note on backward compatibility?

(Via drafts-lastcall@iana.org): IESG/Authors/WG Chairs:

The IANA Functions Operator has reviewed draft-ietf-ntp-mac-05, which is currently in Last Call, and has the following comments:

We understand that this document doesn't require any registry actions.

While it's often helpful for a document's IANA Considerations section to remain in place upon publication even if there are no actions, if the authors strongly prefer to remove it, we do not object.

If this assessment is not accurate, please respond as soon as possible.

Thank you,

Sabrina Tanamal
Senior IANA Services Specialist

The following Last Call announcement was sent out (ends 2018-10-19):

From: The IESG
To: IETF-Announce
CC: ntp@ietf.org, odonoghue@isoc.org, Karen O'Donoghue , ntp-chairs@ietf.org, draft-ietf-ntp-mac@ietf.org, suresh@kaloom.com
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (Message Authentication Code for the Network Time Protocol) to Proposed Standard


The IESG has received a request from the Network Time Protocol WG (ntp) to
consider the following document: - 'Message Authentication Code for the
Network Time Protocol'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2018-10-19. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the beginning of
the Subject line to allow automated sorting.

Abstract


  RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets
  should be authenticated by appending a 128-bit key to the NTP data,
  and hashing the result with MD5 to obtain a 128-bit tag.  This
  document deprecates MD5-based authentication, which is considered to
  be too weak, and recommends the use of AES-CMAC [RFC4493] as a
  replacement.




The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-ntp-mac/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-ntp-mac/ballot/


No IPR declarations have been submitted directly on this I-D.

This is the publication request and document shepherd write up for:

Message Authentication Code for the Network Time Protocol
draft-ietf-ntp-mac

Prepared by: Karen O’Donoghue, 29 May 2018

(1) What type of RFC is being requested (BCP, Proposed Standard, Internet Standard, Informational, Experimental, or Historic)? Why is this the proper type of RFC? Is this type of RFC indicated in the title page header?

This document is being requested for publication as a Proposed Standard given that it updates an existing Proposed Standard (RFC 5905).

(2) The IESG approval announcement includes a Document Announcement Write-Up. Please provide such a Document Announcement Write-Up. Recent examples can be found in the "Action" announcements for approved documents. The approval announcement contains the following sections:

Technical Summary:

RFC 5905 [RFC5905] states that Network Time Protocol (NTP) packets    should be authenticated by appending a 128-bit key to the NTP data, and hashing the result with MD5 to obtain a 128-bit tag.  This document deprecates MD5-based authentication, which is considered to be too weak, and recommends the use of AES-CMAC [RFC4493] as a replacement.

Working Group Summary:

The document has clear working group consensus for publication, and has been reviewed by several WG participants since its initial adoption as a working group item.

Document Quality:
 
This document has been reviewed and revised several times during its development. There were no specific external expert reviews conducted.
 
Personnel: 

Karen O'Donoghue is acting as the Document Shepherd.  Suresh Krishnan is the Responsible Area Director.

(3) Briefly describe the review of this document that was performed by the Document Shepherd. If this version of the document is not ready for publication, please explain why the document is being forwarded to the IESG.
 
The document shepherd has followed the working group process and reviewed the final document and feels this document is ready for IESG review.

(4) Does the document Shepherd have any concerns about the depth or breadth of the reviews that have been performed?
 
The document shepherd does not have any concerns about the reviews that were performed.

(5) Do portions of the document need review from a particular or from broader perspective, e.g., security, operational complexity, AAA, DNS, DHCP, XML, or internationalization? If so, describe the review that took place.

This document does not require any special reviews beyond those planned during the IESG review process.
 
(6) Describe any specific concerns or issues that the Document Shepherd has with this document that the Responsible Area Director and/or the IESG should be aware of? For example, perhaps he or she is uncomfortable with certain parts of the document, or has concerns whether there really is a need for it. In any event, if the WG has discussed those issues and has indicated that it still wishes to advance the document, detail those concerns here.
 
The Document Shepherd is comfortable with this document as a simple update to RFC 5905 aligned with the long held position of the IETF to deprecate MD5.

(7) Has each author confirmed that any and all appropriate IPR disclosures required for full conformance with the provisions of BCP 78 and BCP 79 have already been filed. If not, explain why?

The authors have confirmed that they have dealt with all appropriate IPR disclosures.
 
(8) Has an IPR disclosure been filed that references this document? If so, summarize any WG discussion and conclusion regarding the IPR disclosures.

There is no IPR disclosures for this document.
 
(9) How solid is the WG consensus behind this document? Does it represent the strong concurrence of a few individuals, with others being silent, or does the WG as a whole understand and agree with it?

The document represents strong WG consensus.
 
(10) Has anyone threatened an appeal or otherwise indicated extreme discontent? If so, please summarize the areas of conflict in separate email messages to the Responsible Area Director.

There have been no threats of anyone appealing the documents.
 
(11) Identify any ID nits the Document Shepherd has found in this document. (See http://www.ietf.org/tools/idnits/ and the Internet-Drafts Checklist). Boilerplate checks are not enough; this check needs to be thorough.

Two errors have been found in the ID nits tool. Both can be fixed easily and will be discussed with the AD.

idnits 2.15.01
/tmp/draft-ietf-ntp-mac-04.txt:

  ** The abstract seems to contain references ([RFC5905], [RFC4493]), which
    it shouldn't.  Please replace those with straight textual mentions of the
    documents in question.

  ** Downref: Normative reference to an Informational RFC: RFC 4493

(12) Describe how the document meets any required formal review criteria, such as the MIB Doctor, media type, and URI type reviews.

There are no formal review criteria for this document.
 
(13) Have all references within this document been identified as either normative or informative?

All references are tagged as normative or informative.
 
(14) Are there normative references to documents that are not ready for advancement or are otherwise in an unclear state? If such normative references exist, what is the plan for their completion?

All normative references are completed.
 
(15) Are there downward normative references (see RFC 3967)? If so, list these downward references to support the Area Director in the Last Call procedure.

There is one downward normative reference to the Information RFC (4493) that specifies the AES-CMAC cryptographic algorithm used in this document.

(16) Will publication of this document change the status of any existing RFCs? Are those RFCs listed on the title page header, listed in the abstract, and discussed in the introduction? If the RFCs are not listed in the Abstract and Introduction, explain why, and point to the part of the document where the relationship of this document to the other RFCs is discussed. If this information is not in the document, explain why the WG considers it unnecessary.

This document updates RFC 5905.
 
(17) Describe the Document Shepherd's review of the IANA considerations section, especially with regard to its consistency with the body of the document. Confirm that all protocol extensions that the document makes are associated with the appropriate reservations in IANA registries. Confirm that any referenced IANA registries have been clearly identified. Confirm that newly created IANA registries include a detailed specification of the initial contents for the registry, that allocations procedures for future registrations are defined, and a reasonable name for the new registry has been suggested (see RFC 5226).

There are no new IANA considerations contained in this document.

(18) List any new IANA registries that require Expert Review for future allocations. Provide any public guidance that the IESG would find useful in selecting the IANA Experts for these new registries.

There are no new IANA considerations contained in this document.
 
(19) Describe reviews and automated checks performed by the Document Shepherd to validate sections of the document written in a formal language, such as XML code, BNF rules, MIB definitions, etc.

There are no formal language sections in this document.