Voluntary Application Server Identification for Web Push
draft-thomson-webpush-vapid-02

Abstract

An application server can voluntarily identify itself to a push service using the described technique. This identification information can be used by the push service to attribute requests that are made by the same application server to a single entity. This can used to reduce the secrecy for push subscription URLs by being able to restrict subscriptions to a specific application server. An application server is further able include additional information the operator of a push service can use to contact the operator of the application server.

Status of This Memo

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This Internet-Draft will expire on August 3, 2016.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Voluntary Identification
• 1.2. Notational Conventions
• 2. Application Server Self-Identification
• 2.1. Application Server Contact Information
• 2.2. Example
• 3. WebPush Authentication Scheme
• 4. Public Key Representation
• 5. Subscription Restriction
• 5.1. Creating a Restricted Push Subscription
• 5.2. Using Restricted Subscriptions
• 6. Security Considerations
• 7. IANA Considerations
• 7.1. WebPush Authentication Scheme
• 7.2. p256ecdsa Parameter for Crypto-Key Header Field
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

The Web Push protocol [I-D.ietf-webpush-protocol] describes how an application server is able to request that a push service deliver a push message to a user agent.

As a consequence of the expected deployment architecture, there is no basis for an application server to be known to a push service prior to requesting delivery of a push message. By the same measure, requesting the creation of a subscription for push message receipts has no prior authentication. Requiring that the push service be able to authenticate application servers places an unwanted constraint on the interactions between user agents and application servers, who are the ultimate users of a push service. That constraint would also degrade the privacy-preserving properties the protocol provides. For these reasons, [I-D.ietf-webpush-protocol] does not define a mandatory system for authentication of application servers.

An unfortunate consequence of this design is that a push service is exposed to a greater risk of denial of service attack. While requests from application servers can be indirectly attributed to user agents, this is not always efficient or even sufficient. Providing more information about the application server directly to a push service allows the push service to better distinguish between legitimate and bogus requests.

Additionally, this design also relies on endpoint secrecy as any application server in possession of the endpoint is able to send messages, albeit without payloads. In situations where usage of a subscription can be limited to a single application server, the ability to associate a subscription with the application server could reduce the impact of a data leak.

1.1. Voluntary Identification

This document describes a system whereby an application server can volunteer information about itself to a push service. At a minimum, this provides a stable identity for the application server, though this could also include contact information, such as an email address.

A consistent identity can be used by a push service to establish behavioral expectations for an application server. Significant deviations from an established norm can then be used to trigger exception handling procedures.

Voluntarily-provided contact information can be used to contact an application server operator in the case of exceptional situations.

Experience with push service deployment has shown that software errors or unusual circumstances can cause large increases in push message volume. Contacting the operator of the application server has proven to be valuable.

Even in the absence of usable contact information, an application server that has a well-established reputation might be given preference over an unidentified application server when choosing whether to discard a push message.

1.2. Notational Conventions

The words “MUST”, “MUST NOT”, “SHOULD”, and “MAY” are used in this document. It’s not shouting, when they are capitalized, they have the special meaning described in [RFC2119].

The terms “push message”, “push service”, “push subscription”, “application server”, and “user agent” are used as defined in [I-D.ietf-webpush-protocol].

2. Application Server Self-Identification

Application servers SHOULD generate and maintain a signing key pair usable with elliptic curve digital signature (ECDSA) over the P-256 curve [FIPS186]. Use of this key when sending push messages establishes a continuous identity for the application server.

When requesting delivery of a push message, the application includes a JSON Web Token (JWT) [RFC7519], signed using its signing key. The token includes a number of claims as follows:

• An “aud” (Audience) claim in the token MUST include the unicode serialization of the origin (Section 6.1 of [RFC6454]) of the push resource URL. This binds the token to a specific push service. This ensures that the token is reusable for all push resource URLs that share the same origin.
• An “exp” (Expiry) claim MUST be included with the time after which the token expires. This limits the time that a token over which a token is valid. An “exp” claim MUST NOT be more than 24 hours from the time of the request.

This JWT is included in an Authorization header field, using an auth-scheme of “WebPush”. A push service MAY reject a request with a 403 (Forbidden) status code [RFC7235] if the JWT signature or its claims are invalid.

The JWT MUST use a JSON Web Signature (JWS) [RFC7515]. The signature MUST use ECDSA on the NIST P-256 curve [FIPS186], that is “ES256” [RFC7518].

2.1. Application Server Contact Information

If the application server wishes to provide the JWT MAY include an “sub” (Subject) claim. The “sub” claim SHOULD include a contact URI for the application server as either a “mailto:” (email) [RFC6068] or an “https:” [RFC2818] URI.

2.2. Example

An application server requests the delivery of a push message as described in [I-D.ietf-webpush-protocol]. If the application server wishes to self-identify, it includes an Authorization header field with credentials that use the “WebPush” authentication scheme Section 3 and a Crypto-Key header field that includes its public key Section 4.

POST /p/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1
Host: push.example.net
Push-Receipt: https://push.example.net/r/3ZtI4YVNBnUUZhuoChl6omU
Content-Type: text/plain;charset=utf8
Content-Length: 36
Authorization: Bearer
    eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiJ9.eyJhdWQiOiJodHRwczovL3B
    1c2guZXhhbXBsZS5uZXQiLCJleHAiOjE0NTM1MjM3NjgsInN1YiI6Im1haWx
    0bzpwdXNoQGV4YW1wbGUuY29tIn0.i3CYb7t4xfxCDquptFOepC9GAu_HLGk
    MlMuCGSK2rpiUfnK9ojFwDXb1JrErtmysazNjjvW2L9OkSSHzvoD1oA
Crypto-Key: p256ecdsa=BA1Hxzyi1RUM1b5wjxsn7nGxAszw2u61m164i3MrAIxH
                      F6YK5h4SDYic-dRuU_RCPCfA5aq9ojSwk5Y2EmClBPs

iChYuI3jMzt3ir20P8r_jgRR-dSuN182x7iB

header = {"typ":"JWT","alg":"ES256"}
body = { "aud":"https://push.example.net",
         "exp":1453341205,
         "sub":"mailto:push@example.com" }

POST /subscribe/ HTTP/1.1
Host: push.example.net
Crypto-Key: p256ecdsa=BBa22H8qaZ-iDMH9izb4qE72puwyvfjH2RxoQr5oiS4b
                      KImoRwJm5xK9hLrbfIik20g31z8MpLFMCMr8y2cu6gY