Last Call Review of draft-ietf-websec-key-pinning-20

Request Review of draft-ietf-websec-key-pinning
Requested rev. no specific revision (document currently at 21)
Type Last Call Review
Team General Area Review Team (Gen-ART) (genart)
Deadline 2014-08-01
Requested 2014-07-31
Authors Chris Evans, Chris Palmer, Ryan Sleevi
Draft last updated 2014-07-31
Completed reviews Genart Last Call review of -19 by Elwyn Davies (diff)
Genart Last Call review of -20 by Elwyn Davies (diff)
Assignment Reviewer Elwyn Davies 
State Completed
Review review-ietf-websec-key-pinning-20-genart-lc-davies-2014-07-31
Reviewed rev. 20 (document currently at 21)
Review result Ready with Issues
Review completed: 2014-07-31


Hi, Yoav and authors.

Thanks for the responses and -20 updates.  Apologies for the delay in 

replying (expedition to rural parts for my birthday!)

These have fixed several of the issues/nits but some remain as noted below.

On 04/08/14 16:49, Yoav Nir wrote:

[ with no hats on ]


On Jul 31, 2014, at 10:27 PM, Yoav Nir <ynir.ietf at> wrote:

On Jul 31, 2014, at 4:05 PM, Elwyn Davies <elwynd at> wrote:

I am the assigned Gen-ART reviewer for this draft. For background on
Gen-ART, please see the FAQ at


Please resolve these comments along with any other Last Call comments
you may receive.

Document: draft-ietf-websec-key-pinning-19.txt
Reviewer: Elwyn Davies
Review Date: 31 July 2014
IETF LC End Date: 1 August 2014
IESG Telechat date: (if known) -

Almost ready.  There are some minor issues some of which may be as a result of some
misunderstanding on my part.  The descriptive text in the early part of s2 is missing
some definitions that make it unclear until they appear later on.  This makes the early
descriptions more confusing that illuminating in places.  Suggestions in the detailed
comments below.

[YN] I believe the missing definitions you’re talking about are Pin, Pin Validation. I think anyone who reads this specification is familiar enough with HTTP to know what request, response, UA and directive mean. If so, I suggest that these can be defined in 1-2 sentences in section 1, even if they’re better explained later on.

One thing that is not fully clear to me and could probably be explained to help others
is the start up of the pinning mechanisms for a given host domain.  AFAICS Pin validation
would possibly not be carried out on a first connection to a domain when there are no
preconfigured Pins.  I am not clear if this adds anything to the risk of a MITM attack or
does it in any way negate the value of the whole pinning process?  I was not clear if
an effective Pin validation should be carried out during the first connection when the
UA receives the host's Pins for the first time and decides that it is now a Pinned Host,
in that the document doesn't state that the connection is terminated if the setting up
of the Pinned host fails because the certs don't validate.

Key pinning is a TOFU (trust on first use) mechanism. As such, the first time a UA connects to a domain there is no validation. A MitM attacking such a first connection will not be discovered. Worse, such a MitM can inject its own PKP header into the HTTP stream, and pin the UA to its own keys. Note that in order to pin false information, the attacker would have to be able to produce an error-free connection. Without compromising a trusted CA, this should not be possible, and the best that attackers can accomplish is to use an invalid TLS certificate, leading to an interstitial warning page.

This is apparent from section 2.3.1, but perhaps deserves a short paragraph in the introduction.

Fine - done.

Major issues:

Minor issues:
s1: The term "Pin" (as a noun) is not explicitly defined. The definition doesn't appear
until s2.4.


s2.1.1: I'm not sure if this could be an issue.. should a maximum possible value for max_age
be specified to avoid UA's being cluttered up with old Pins - this might possible be a DoS
attack vehicle but I am not sure (yet) if it could be. OK.. so s2.3.3 talks about limits.
A pointer to this discussion would be useful here

A pointer to the discussion in s2.3.3 would still help.

s2.2.1: Does this response behaviour apply to all possible request types? Once a server has sent a
Pin header should it send it again on all subsequent requests on the same TLS connection or is
that a choice?  Given the "SHOULD" in s2.2.1, what are the circumstances in which the server should
refrain from sending the Pins? [I first thought about 'Redirects' but realized that that was probably
a really good time to send Pins!]

This has been discussed, and because of all kinds of weird deployments, such as with multiplexed servers
behind a load-balancer the only viable way was to send the PKP in every response. Some (me!) wereconcerned

about the overhead of these large-ish headers, but we heard from people 

who actually run servers

that a header this small is inconsequential. The draft for HTTP/2 makes 

this less of an issue, as

repeated headers are efficiently encoded by referring back to previous 

header sets.

Ok.  Accepting that the host is expected normally to send the headers on 

all responses, when SHOULD it not send the headers? (from the previous 

comments a server that knows it is not multiplexed might be a candidate 

- or if it knows it has already responded on this connection?)

s2.3.1/s2.4: S2.4 states that hash algorithms might be deprecated in future.  Presumably a
deprecated algorithm should be treated as an unrecognized directive or some such to avoid
downgrade attacks.  Probably worth being explicit about this.  Also this is potentially
incompatible with the statement that 'UAs MUST recognize "sha256"' (Para 3 of s2.4).
This results in a potential downgrade attack when and if SHA256 is deemed to be no longer
cryptographically effective. I think this statement can be removed as presently a UA
has no other option if it is to implement the specification.

Not addressed as yet.  At least the statement in s2.4 needs to be removed.


   Note that the UA MUST perform Pin Validation when setting up the TLS
   session, before beginning an HTTP conversation over the TLS channel.

I suspect I am confused: If a UA is making its first connection to a host for which it doesn't
have a preconfigured Pin, then it won't get the Pin(s) from the host until it has set up the
TLS connection and received the response to the request at the HTTP protocol level.  In that case
Pin validation will pass by default (subject to local policy perhaps) since the cache doesn't have
entries for the host.  Presumably the UA should then perform Pin validation if it has passed by
default during TLS setup (assuming that this is possible given the layering) or does the UA have to
terminate the session and restart it so that Pin validation can be performed?  The second case may
give scope for a DoS attack.  Or is it the case that Pin validation is not needed on the first
connection... I don't see why this shouldn't be done but I may not understand the problem.  I think
some clarification about the startup of the process is needed.

Ok. Done.

Nits/editorial comments:

s1, last para: s/toegether/together/, s/but is possible/but it is possible/
s2.1: It would be good to expand the term OWS.

s2.1, Figure 1 caption: The acronym HPKP needs expanding.

s2.1, 2nd para after numbered bullets:
It is not the definition of hash algorithms that is relevant, but allowing them to be
used in pin-directives thus:
additional algorithms may be defined in the future
additional algorithms may be allowed for use in this context in future

Also the implication of the "sha256" name should be explained precisely -
i.e, that the SHA256 hash algorithm will be used, and a suitable reference
for SHA256 should be given. (Again this doesn't happen until s2.4).

And finally the "Fingerprint" of what SPKI? Defining Pin formally as noted above would help!

s2.1, last para: s/hahs/hash/

s4.2/Figure 8: The first line of text is too wide.

s5, para 1: Is it really "HSTS or HPKP"?  I thought it would be "HSTS combined with HPKP".

s5, 2nd top level bullet: Expand SNI acronym.

All nits above sorted.

s6: Needs to be more precise about *which* message headers repository is to be updated! Presumably
the permanent one at



Also there may be some of the questions in s8.3.1 of RFC 7231 that need specific answers.

There are still some of the questions that ought to be explicitly answered.

One extra nit:
s2, para after bullets: s/reistry/registry/