Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                             P. Beverloo
Expires: August 3, 2016                                           Google
                                                        January 31, 2016

        Voluntary Application Server Identification for Web Push


   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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   provisions of BCP 78 and BCP 79.

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

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   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Voluntary Identification  . . . . . . . . . . . . . . . .   3
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Application Server Self-Identification  . . . . . . . . . . .   4
     2.1.  Application Server Contact Information  . . . . . . . . .   4
     2.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  WebPush Authentication Scheme . . . . . . . . . . . . . . . .   5
   4.  Public Key Representation . . . . . . . . . . . . . . . . . .   6
   5.  Subscription Restriction  . . . . . . . . . . . . . . . . . .   6
     5.1.  Creating a Restricted Push Subscription . . . . . . . . .   6
     5.2.  Using Restricted Subscriptions  . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  WebPush Authentication Scheme . . . . . . . . . . . . . .   8
     7.2.  p256ecdsa Parameter for Crypto-Key Header Field . . . . .   9
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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.

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   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

   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].

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   The terms "push message", "push service", "push subscription",
   "application server", and "user agent" are used as defined in

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:

   o  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.

   o  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"

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

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   Section 3 and a Crypto-Key header field that includes its public key
   Section 4.

   POST /p/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1
   Content-Type: text/plain;charset=utf8
   Content-Length: 36
   Authorization: Bearer
   Crypto-Key: p256ecdsa=BA1Hxzyi1RUM1b5wjxsn7nGxAszw2u61m164i3MrAIxH


            Figure 1: Requesting Push Message Delivery with JWT

   Note that the header fields shown in Figure 1 don't include line
   wrapping.  Extra whitespace is added to meet formatting constraints.

   This equates to a JWT with the header and body shown in Figure 2.
   This JWT would be valid until 2016-01-21T01:53:25Z [RFC3339].

   header = {"typ":"JWT","alg":"ES256"}
   body = { "aud":"",
            "sub":"" }

                   Figure 2: Example JWT Header and Body

   Issue:  The first part of the JWT is effectively fixed.  Would be it
      acceptable to require that that segment is omitted from the header

3.  WebPush Authentication Scheme

   A new "WebPush" HTTP authentication scheme [RFC7235] is defined.
   This authentication scheme carries a signed JWT, as described in
   Section 2.

   This authentication scheme is for origin-server authentication only.
   Therefore, this authentication scheme MUST NOT be used with The
   Proxy-Authenticate or Proxy-Authorization header fields.

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   This authentication scheme does not require a challenge.  Clients are
   able to generate the Authorization header field without any
   additional information from a server.  Therefore, a challenge for
   this authentication scheme MUST NOT be sent in a WWW-Authenticate
   header field.

   All unknown or unsupported parameters to "WebPush" authentication
   credentials MUST be ignored.  The "realm" parameter is ignored for
   this authentication scheme.

4.  Public Key Representation

   In order for the push service to be able to validate the JWT, it
   needs to learn the public key of the application server.  A
   "p256ecdsa" parameter is defined for the Crypto-Key header field
   [I-D.ietf-httpbis-encryption-encoding] to carry this information.

   The "p256ecdsa" parameter includes an elliptic curve digital
   signature algorithm (ECDSA) public key [FIPS186] in uncompressed form
   [X9.62] that is encoded using the URL- and filename-safe variant of
   base-64 [RFC4648] with padding removed.

   Note that with push message encryption [I-D.ietf-webpush-encryption],
   this results in two values in the Crypto-Key header field, one with
   the a "p256dh" key and another with a "p256ecdsa" key.

   Editor's Note:  JWK [RFC7517] seems like the obvious choice here.
      However, JWK doesn't define a compact representation for public
      keys, which complicates the representation of JWK in a header

5.  Subscription Restriction

   The public key of the application server serves as a stable
   identifier for the server.  This key can be used to restrict a push
   subscription to a specific application server.

   Subscription restriction reduces the reliance on endpoint secrecy by
   requiring proof of possession to be demonstrated by an application
   server when requesting delivery of a push message.  This provides an
   additional level of protection against leaking of the details of the
   push subscription.

5.1.  Creating a Restricted Push Subscription

   The user agent includes the public key of the application server when
   requesting that a push subscription.  This restricts use of the

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   resulting push subscription to application servers that are able to
   provide proof of possession for the corresponding private key.

   This public key is then added to the request to create a push
   subscription as described in Section 4.  The Crypto-Key header field
   includes exactly one public key.  For example:

   POST /subscribe/ HTTP/1.1
   Crypto-Key: p256ecdsa=BBa22H8qaZ-iDMH9izb4qE72puwyvfjH2RxoQr5oiS4b

                    Figure 3: Example Subscribe Request

   An application might use the Web Push API [API] to include this
   information.  For example, the API might permit an application to
   provide a public key as part of a new field on the
   "PushSubscriptionOptions" dictionary.

   Editor's Note:  Allowing the inclusion of multiple keys when creating
      a subscription would allow a subscription to be associated with
      multiple application servers or application server instances.
      This might be more flexible, but it also would require more state
      to be maintained by the push service for each subscription.

5.2.  Using Restricted Subscriptions

   When a push subscription has been associated with an application
   server, the request for push message delivery MUST include proof of
   possession for the associated private key or token that was used when
   creating the push subscription.

   A push service MUST reject a message that includes omits mandatory
   credentials with a 401 (Unauthorized) status code.  A push service
   MAY reject a message that includes invalid credentials with a 403
   (Forbidden) status code.  Credentials are invalid if:

   o  either the authentication credentials or public key are not
      included in the request,

   o  the signature on the JWT cannot be successfully verified using the
      included public key,

   o  the current time is later than the time identified in the "exp"
      (Expiry) claim or more than 24 hours before the expiry time,

   o  the origin of the push resource is not included in the "aud"
      (Audience) claim, or

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   o  the public key used to sign the doesn't match the one that was
      included in the creation of the push message.

   A push subscription that is not restricted to a particular key MAY
   still validate a token that is present, except for the last check.  A
   push service MAY then reject a request if the token is found to be

   Editor's Note:  In theory, since the push service was given a public
      key, the push message request could omit the public key.  On
      balance, this keeps things simple and it allows push services to
      compress the public key (by hashing it, for example).  In any
      case, the relatively minor space savings aren't particularly
      important on the connection between the application server and
      push service.

   A push service does not need to forward the JWT or public key to the
   user agent when delivering the push message.

6.  Security Considerations

   This authentication scheme is vulnerable to replay attacks if an
   attacker can acquire a valid JWT.  Applying narrow limits to the
   period over which a replayable token can be reused limits the
   potential value of a stolen token to an attacker and can increase the
   difficulty of stealing a token.

   An application server might offer falsified contact information.  A
   push service operator therefore cannot use the presence of
   unvalidated contact information as input to any security-critical
   decision-making process.

   Validation of a signature on the JWT requires a non-trivial amount of
   computation.  For something that might be used to identify legitimate
   requests under denial of service attack conditions, this is not
   ideal.  Application servers are therefore encouraged to reuse a JWT,
   which permits the push service to cache the results of signature

7.  IANA Considerations

7.1.  WebPush Authentication Scheme

   This registers the "WebPush" authentication scheme in the "Hypertext
   Transfer Protocol (HTTP) Authentication Scheme Registry" established
   in [RFC7235].

   Authentication Scheme Name:  WebPush

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   Pointer to specification text:  Section 3 of this document

   Notes:  This scheme is origin-server only and does not define a

7.2.  p256ecdsa Parameter for Crypto-Key Header Field

   This registers a "p256ecdsa" parameter for the Crypto-Key header
   field in the "Hypertext Transfer Protocol (HTTP) Crypto-Key
   Parameters" established in [I-D.ietf-httpbis-encryption-encoding].

   Parameter Name:  p256ecdsa

   Purpose:  Conveys a public key for that is used to generate an ECDSA

   Reference:  Section 4 of this document

8.  Acknowledgements

   This document would have been much worse than it currently is if not
   for the contributions of Benjamin Bangert, Chris Karlof, Costin
   Manolache, and others.

9.  References

9.1.  Normative References

   [FIPS186]  National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", NIST PUB 186-4 , July

              Thomson, M., "Encrypted Content-Encoding for HTTP", draft-
              ietf-httpbis-encryption-encoding-00 (work in progress),
              December 2015.

              Thomson, M., Damaggio, E., and B. Raymor, "Generic Event
              Delivery Using HTTP Push", draft-ietf-webpush-protocol-02
              (work in progress), November 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [X9.62]    ANSI, "Public Key Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62 , 1998.

9.2.  Informative References

   [API]      van Ouwerkerk, M. and M. Thomson, "Web Push API", 2015,

              Thomson, M., "Message Encryption for Web Push", draft-
              ietf-webpush-encryption-01 (work in progress), October

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,

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   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,

Authors' Addresses

   Martin Thomson


   Peter Beverloo


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