Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 8292                                       Mozilla
Category: Standards Track                                    P. Beverloo
ISSN: 2070-1721                                                   Google
                                                           November 2017

Voluntary Application Server Identification (VAPID) for Web Push

Abstract

   An application server can use the Voluntary Application Server
   Identification (VAPID) method described in this document to
   voluntarily identify itself to a push service.  The "vapid"
   authentication scheme allows a client to include its identity in a
   signed token with requests that it makes.  The signature can be used
   by the push service to attribute requests that are made by the same
   application server to a single entity.  The identification
   information can allow the operator of a push service to contact the
   operator of the application server.  The signature can be used to
   restrict the use of a push message subscription to a single
   application server.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8292.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   1. Introduction ....................................................3
      1.1. Voluntary Identification ...................................3
      1.2. Notational Conventions .....................................4
   2. Application Server Self-Identification ..........................4
      2.1. Application Server Contact Information .....................5
      2.2. Additional Claims ..........................................5
      2.3. Cryptographic Agility ......................................5
      2.4. Example ....................................................5
   3. VAPID Authentication Scheme .....................................6
      3.1. Token Parameter ("t") ......................................7
      3.2. Public Key Parameter ("k") .................................7
   4. Subscription Restriction ........................................7
      4.1. Creating a Restricted Push Message Subscription ............8
      4.2. Using Restricted Subscriptions .............................9
   5. Security Considerations .........................................9
   6. IANA Considerations ............................................10
      6.1. VAPID Authentication Scheme Registration ..................10
      6.2. VAPID Authentication Scheme Parameters ....................10
      6.3. application/webpush-options+json Media Type Registration ..11
   7. References .....................................................12
      7.1. Normative References ......................................12
      7.2. Informative References ....................................14
   Acknowledgements ..................................................14
   Authors' Addresses ................................................14

1. Introduction

   The Web Push protocol [RFC8030] 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.  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, [RFC8030] does not define
   a mandatory system for authentication of application servers.

   An unfortunate consequence of the design of [RFC8030] is that a push
   service is exposed to a greater risk of denial-of-service attacks.
   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, the design of [RFC8030] relies on maintaining the
   secrecy of push message subscription URIs.  Any application server in
   possession of a push message subscription URI is able to send
   messages to the user agent.  If use of a subscription could be
   limited to a single application server, this would reduce the impact
   of the push message subscription URI being learned by an unauthorized
   party.

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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The terms "push message", "push service", "push message
   subscription", "application server", and "user agent" are used as
   defined in [RFC8030].

2. Application Server Self-Identification

   Application servers that wish to self-identify generate and maintain
   a signing key pair.  This key pair MUST be usable with the Elliptic
   Curve Digital Signature Algorithm (ECDSA) over the P-256 curve
   [FIPS186].  Use of this key when sending push messages establishes an
   identity for the application server that is consistent across
   multiple messages.

   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 and
      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 over which a token is
      valid.  An "exp" claim MUST NOT be more than 24 hours from the

      time of the request.  Limiting this to 24 hours balances the need
      for reuse against the potential cost and likelihood of theft of a
      valid token.

   This JWT is included in an Authorization header field, using an
   authentication scheme of "vapid".  A push service MAY reject a
   request with a 403 (Forbidden) status code [RFC7231] if the JWT
   signature or its claims are invalid.  A push service MUST NOT use
   information from an invalid token.

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

2.1. Application Server Contact Information

   If the application server wishes to provide contact details, it MAY
   include a "sub" (Subject) claim in the JWT.  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. Additional Claims

   An application server MAY include additional claims using public or
   private names (see Sections 4.2 and 4.3 of [RFC7519]).  Since the JWT
   is in a header field, the size of additional claims SHOULD be kept as
   small as possible.

2.3. Cryptographic Agility

   The "vapid" HTTP authentication scheme (Section 3) is used to
   identify the specific profile of JWT defined in this document.  A
   different authentication scheme is needed to update the signature
   algorithm or other parameters.  This ensures that existing mechanisms
   for negotiating authentication schemes can be used rather than
   defining new parameter negotiation mechanisms.

2.4. Example

   An application server requests the delivery of a push message as
   described in [RFC8030].  If the application server wishes to self-
   identify, it includes an Authorization header field with credentials
   that use the "vapid" authentication scheme.

   POST /p/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1
   Host: push.example.net
   TTL: 30
   Content-Length: 136
   Content-Encoding: aes128gcm
   Authorization: vapid
      t=eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiJ9.eyJhdWQiOiJodHRwczovL3
        B1c2guZXhhbXBsZS5uZXQiLCJleHAiOjE0NTM1MjM3NjgsInN1YiI6Im1ha
        Wx0bzpwdXNoQGV4YW1wbGUuY29tIn0.i3CYb7t4xfxCDquptFOepC9GAu_H
        LGkMlMuCGSK2rpiUfnK9ojFwDXb1JrErtmysazNjjvW2L9OkSSHzvoD1oA,
      k=BA1Hxzyi1RUM1b5wjxsn7nGxAszw2u61m164i3MrAIxHF6YK5h4SDYic-dR
        uU_RCPCfA5aq9ojSwk5Y2EmClBPs
   { encrypted push message }

            Figure 1: Requesting Push Message Delivery with JWT

   Note that the example header fields in this document include extra
   line wrapping to meet formatting constraints.

   The "t" parameter of the Authorization header field contains a JWT;
   the "k" parameter includes the base64url-encoded key that signed that
   token.  The JWT input values and the JSON Web Key (JWK) [RFC7517]
   corresponding to the signing key are shown in Figure 2 with
   additional whitespace added for readability purposes.  This JWT would
   be valid until 2016-01-23T04:36:08Z.

   JWT header = { "typ": "JWT", "alg": "ES256" }
   JWT body = { "aud": "https://push.example.net",
                "exp": 1453523768,
                "sub": "mailto:push@example.com" }
   JWK = { "crv":"P-256",
           "kty":"EC",
           "x":"DUfHPKLVFQzVvnCPGyfucbECzPDa7rWbXriLcysAjEc",
           "y":"F6YK5h4SDYic-dRuU_RCPCfA5aq9ojSwk5Y2EmClBPs" }

                     Figure 2: Decoded Example Values

3. VAPID Authentication Scheme

   This document defines a new HTTP authentication scheme [RFC7235]
   named "vapid".  This authentication scheme carries a signed JWT, as
   described in Section 2, plus the key that signed that JWT.

   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.

   The challenge for the "vapid" authentication scheme contains only the
   "auth-scheme" production.  No parameters are currently defined.

   Two parameters are defined for this authentication scheme: "t" and
   "k".  All unknown or unsupported parameters to "vapid" authentication
   credentials MUST be ignored.  The "realm" parameter is ignored for
   this authentication scheme.

   This authentication scheme is intended for use by an application
   server when using the Web Push protocol [RFC8030].

3.1. Token Parameter ("t")

The "t" parameter of the "vapid" authentication scheme carries a JWT
as described in Section 2.

3.2. Public Key Parameter ("k")

   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 "k"
   parameter is defined for the "vapid" authentication scheme to carry
   this information.

   The "k" parameter includes an ECDSA public key [FIPS186] in
   uncompressed form [X9.62] that is encoded using base64url encoding
   [RFC7515].

   Note:  X9.62 encoding is used over JWK [RFC7517] for two reasons.  A
      JWK does not have a canonical form, so X9.62 encoding makes it
      easier for the push service to handle comparison of keys from
      different sources.  Secondarily, the X9.62 encoding is also
      considerably smaller.

   Some elliptic curve implementations permit the same P-256 key to be
   used for signing and key exchange.  An application server MUST select
   a different private key for the key exchange [RFC8291] and signing
   the authentication token.  Though a push service is not obligated to
   check either parameter for every push message, a push service SHOULD
   reject push messages that have identical values for these parameters
   with a 400 (Bad Request) status code.

4. 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
   message subscription to a specific application server.

   Subscription restriction reduces the reliance on endpoint secrecy by
   requiring that an application server provide a signed token when
   requesting delivery of a push message.  This provides an additional
   level of protection against leaking of the details of the push
   message subscription.

4.1. Creating a Restricted Push Message Subscription

   A user agent that wishes to create a restricted subscription includes
   the public key of the application server when requesting the creation
   of a push message subscription.  This restricts use of the resulting
   subscription to application servers that are able to provide a valid
   JWT signed by the corresponding private key.

   The user agent then adds the public key to the request to create a
   push message subscription.  The push message subscription request is
   extended to include a body.  The body of the request is a JSON object
   as described in [RFC7159].  The user agent adds a "vapid" member to
   this JSON object that contains a public key on the P-256 curve,
   encoded in the uncompressed form [X9.62] and base64url encoded
   [RFC7515].  The media type of the body is set to "application/
   webpush-options+json" (see Section 6.3 for registration of this media
   type).

   A push service MUST ignore the body of a request that uses a
   different media type.  For the "application/webpush-options+json"
   media type, a push service MUST ignore any members on this object
   that it does not understand.

   The example in Figure 3 shows a restriction to the key used in
   Figure 1.  Extra whitespace is added to meet formatting constraints.

   POST /subscribe/ HTTP/1.1
   Host: push.example.net
   Content-Type: application/webpush-options+json
   Content-Length: 104
   { "vapid": "BA1Hxzyi1RUM1b5wjxsn7nGxAszw2u61m164i3MrAIxH
               F6YK5h4SDYic-dRuU_RCPCfA5aq9ojSwk5Y2EmClBPs" }

                    Figure 3: Example Subscribe Request

   An application might use the Push API [API] to provide the user agent
   with a public key.

4.2. Using Restricted Subscriptions

   When a push message subscription has been restricted to an
   application server, the request for push message delivery MUST
   include a JWT signed by the private key that corresponds to the
   public key used when creating the subscription.

   A push service MUST reject a message sent to a restricted push
   message subscription if that message includes no "vapid"
   authentication or invalid "vapid" authentication.  A 401
   (Unauthorized) status code might be used if the authentication is
   absent; a 403 (Forbidden) status code might be used if authentication
   is invalid.

   "vapid" authentication is invalid if:

   o  either the authentication token or public key is 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

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

   A push service MUST NOT forward the JWT or public key to the user
   agent when delivering the push message.

   An application server that needs to replace its signing key needs to
   request the creation of a new subscription by the user agent that is
   restricted to the updated key.  Application servers need to remember
   the key that was used when requesting the creation of a subscription.

5. Security Considerations

   This authentication scheme is vulnerable to replay attacks if an
   attacker can acquire a valid JWT.  Sending requests using HTTPS as
   required by [RFC8030] provides confidentiality.  Additionally,
   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.  The
   application server asserts its email address or contact URI without
   any evidence to support the claim.  A push service operator 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 tokens,
   which permits the push service to cache the results of signature
   validation.

   An application server that changes its signing key breaks linkability
   between push messages that it sends under different keys.  A push
   service that relies on a consistent identity for application servers
   might categorize requests made with new keys differently.  Gradual
   migration to a new signing key reduces the chances that requests that
   use the new key will be categorized as abusive.

6. IANA Considerations

This document registers a new authentication scheme, a registry for
parameters of that scheme, and a media type for push options.

6.1. VAPID Authentication Scheme Registration

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

   Authentication Scheme Name:  vapid

   Pointer to specification text:  Section 3 of this document

6.2. VAPID Authentication Scheme Parameters

   This document creates a "VAPID Authentication Scheme Parameters"
   registry for parameters to the "vapid" authentication scheme.  These
   parameters are defined for use in requests (in the Authorization
   header field) and for challenges (in the WWW-Authenticate header
   field).  This registry is under the "Web Push Parameters" grouping.
   The registry operates on the "Specification Required" policy
   [RFC8126].

6.3. application/webpush-options+json Media Type Registration

   This document registers the "application/webpush-options+json" media
   type in the "Media Types" registry following the process described in
   [RFC6838].

   Type name:  application

   Subtype name:  webpush-options+json

   Required parameters:  none

   Optional parameters:  none

   Encoding considerations:  binary (JSON is UTF-8-encoded text)

   Security considerations:  See [RFC7159] for security considerations
      specific to JSON.

   Interoperability considerations:  See [RFC7159] for interoperability
      considerations specific to JSON.

   Published specification:  [RFC8292]

   Applications that use this media type:  Web browsers, via the Web
      Push protocol [RFC8030]

   Fragment identifier considerations:  none

   Additional information:

      Deprecated alias names for this type:  n/a

      Magic number(s):  n/a

      File extension(s):  .json

      Macintosh file type code(s):  TEXT

   Person & email address to contact for further information:  Martin
      Thomson (martin.thomson@gmail.com)

   Intended usage:  LIMITED USE

   Restrictions on usage:  For use with the Web Push protocol [RFC8030]

   Author:  See "Authors' Addresses" section of [RFC8292].

   Change controller:  Internet Engineering Task Force

7. References

7.1. Normative References

   [FIPS186]  National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", FIPS PUB 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
              <https://www.rfc-editor.org/info/rfc6068>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://www.rfc-editor.org/info/rfc7518>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC8030]  Thomson, M., Damaggio, E., and B. Raymor, Ed., "Generic
              Event Delivery Using HTTP Push", RFC 8030,
              DOI 10.17487/RFC8030, December 2016,
              <https://www.rfc-editor.org/info/rfc8030>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8291]  Thomson, M., "Message Encryption for Web Push", RFC 8291,
              DOI 10.17487/RFC8291, November 2017,
              <http://www.rfc-editor.org/info/rfc8291>.

   [X9.62]    ANSI, "Public Key Cryptography for the Financial Services
              Industry: the Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62, 2005.

7.2. Informative References

   [API]      Beverloo, P., Thomson, M., van Ouwerkerk, M., Sullivan,
              B., and E. Fullea, "Push API", October 2017,
              <https://www.w3.org/TR/push-api/>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/info/rfc7517>.

Acknowledgements

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

Authors' Addresses

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com

   Peter Beverloo
   Google

   Email: beverloo@google.com