[Webpush] AD Review of draft-ietf-webpush-vapid

Adam Roach <adam@nostrum.com> Wed, 14 June 2017 00:21 UTC

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From: Adam Roach <adam@nostrum.com>
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Date: Tue, 13 Jun 2017 19:21:00 -0500
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Subject: [Webpush] AD Review of draft-ietf-webpush-vapid
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VAPID authors --

This is my AD review for draft-ietf-webpush-vapid-02.

I think the document may be ready for IETF last call, but I'd like some 
clarifications before sending it out.

The first issue I'd like to have clarified regards agility of the crypto 
suite. This document hard-codes the signing algorithm as ES256. I'm 
familiar with the fact that documents will frequently do this kind of 
hardcoding, and then normatively update allowed algorithms in future 
documents; however, for all such cases I'm familiar with, there is a 
negotiation involved that allows for bilateral transition from one 
algorithm to another. For example, RFC7616 was able to include new 
algorithms because of the challenge/response nature of its 
authentication scheme. Since VAPID specifically does not include a 
challenge, application servers have no way of reasonably knowing what 
the push server might be capable of handling. I think the security 
section really needs a treatment of how this can be handled, since the 
obvious solutions with the current design are less than elegant.

The second issue entails application server key rotation, especially 
when used without the Subscription Restriction mechanism described in 
section 4. My reading of the document (and I'll note, as a nit, that 
this is implied but not particularly explicit) is that the public key 
used to identify a server is a TOFU token; and that the only means a 
server has to detect impersonation is verifying that the public key is 
identical to previous uses. What is the system behavior intended to be 
when an application server needs to change its VAPID key gracefully; and 
what is the behavior intended to be when an application server loses its 
key unexpectedly? For subscription restriction purposes, I can see how 
servers could just change the key used for the subscription request; but 
when this mechanism is being used solely for the purposes of continuity 
of an application server's identity, it seems problematic.

Finally, I'm curious about whether there was any discussion regarding 
the use of "application/json" rather than using the syntax defined by 
RFC6839 (e.g., "application/vapid+json"). My concern is that the use of 
a semantic-free syntactic designation here makes it more complicated to 
use push subscription requests for anything *other* than conveying a 
VAPID key in the future. If the intention is that push subscription 
bodies generically contain a JSON object with potentially myriad keys 
for a variety of unrelated purposes, please spell that out clearly (and 
I would still encourage the use of something less generic than 
"application/json" even in such a case).

Nits:

Although "vapid" appears in the protocol elements and section headers, 
it is never put next to its expansion in the document. Please consider 
changing the title to "Voluntary Application Server Identification 
(VAPID) for Web Push".

The abstract's phrasing of "This can used to reduce the secrecy for push 
subscription URLs..." seems quite awkward. While this is necessarily one 
result of employing the described technique, it could hardly be 
construed as a goal. I suggest rephrasing.

The JWT shown in the Authorization header field has an expiration date 
of 1453523768, while the JSON that claims to be its expansion shows an 
expiration date of 1453341205. Please make these match: implementors are 
likely to use the examples as part of testing their implementations, and 
this may cause unnecessary confusion. Please also double-check that the 
signature in the example is valid after performing this adjustment.

Thanks!

/a