Re: [Webpush] Application server authentication new years edition

Benjamin Bangert <bbangert@mozilla.com> Thu, 07 January 2016 20:56 UTC

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Date: Thu, 7 Jan 2016 12:56:11 -0800
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From: Benjamin Bangert <bbangert@mozilla.com>
To: Costin Manolache <costin@gmail.com>
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Cc: Costin Manolache <costin@google.com>, Martin Thomson <martin.thomson@gmail.com>, "webpush@ietf.org" <webpush@ietf.org>
Subject: Re: [Webpush] Application server authentication new years edition
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On Thu, Jan 7, 2016 at 10:59 AM, Costin Manolache <costin@gmail.com>; wrote:

>
>
> On Wed, Jan 6, 2016 at 11:53 PM, Martin Thomson <martin.thomson@gmail.com>;
> wrote:
>
>> On 7 January 2016 at 16:43, Costin Manolache <costin@gmail.com>; wrote:
>> > That leaves 2.3 only.
>>
>> It looks like we're mostly in agreement for something like 2.3.
>> Should I take some time and flesh out the details?  Does anyone
>> disagree?
>>
>
> Sounds good, but I have few questions/comments on the details.
>
> 1. Key identifiers ('kid') - for normal JWT, the key is known from the
> issuer or
> 'sub'. Since in our case we'll have self-generated keys, and we want to
> avoid
> having each key registered with all providers - all that a push service
> will know
> when seeing a JWT token the first time will be the content of the token.
> So we need a way to find the public key needed to verify the content - and
> the only one I know is to use something like the SHA of the public key.
> On subscribe - the provider can store the public key, with the kid as
> lookup key, than
> on send it can verify. For example kid == (SHA1 or SHA256 truncated to
> 64bits) would work.
>

If an app-server has a private key leak and needs all its users to rotate
sender keys, that will also tie up a substantial amount of server resources
having such a large unsubscribe/subscribe cycle.

Key rotation for private keys is common in many cases already, it'd be nice
if such a thing wasn't expensive for the server.


> 2. 'aud' field - not sure what would be the right value, maybe the domain
> of the push provider ?
>

I know it doesn't seem popular to include stronger authentication at this
layer.... but if 'aud' was the domain (or sub-domain for per-push-provider
public keys), and there was a DNS TXT record that contained the JWT public
key then the push service would have a way to lookup the key. It'd also
allow the app-server to rotate keys used without all the push clients
needing to resubscribe.

This would bump the authentication to domain verified, while remedying the
issue of how to get the public key to the push service and reducing server
resources needed to handle sender key rotation.


> 3. Going back to the 'voluntary'/optional part: I'm not sure if encryption
> is going to be required or
> optional in the final version ?
>
> 4. I think we can have the sender key optional in the subscribe operation,
> in particular for cases
> like low end IOT - not including it would give permission to anyone with
> the URL and pubkey
> of the device to send messages. There is another case where it helps -
> pairing, in cases of D2D -
> but with restrictions ( either time, or binding the subscription to the
> sender at first use ).
> In other words: I'm not opposed to having 'optional' on the subscribe (and
> on contact info).
>
> 5. This may be a bit controversial :-), if we require the developers to
> use a JOSE library to
> generate JWT tokens, signed with a ES256 - wouldn't make sense to also
> allow them to use
> the same library to encrypt the payload for the e2e ? There is already a
> content type defined, so easy to
> differentiate.
>

+1 on simplifying this for developers.

Cheers,
Ben