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Versions: (draft-thomson-webpush-encryption) 00 01 02 03 04 05 06 07 08 09 RFC 8291

Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                        October 19, 2015
Expires: April 21, 2016


                    Message Encryption for Web Push
                    draft-ietf-webpush-encryption-01

Abstract

   A message encryption scheme is described for the Web Push protocol.
   This scheme provides confidentiality and integrity for messages sent
   from an Application Server to a User Agent.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2016.

Copyright Notice

   Copyright (c) 2015 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
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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Key Generation and Agreement  . . . . . . . . . . . . . . . .   3
     2.1.  Diffie-Hellman Group Information  . . . . . . . . . . . .   3
     2.2.  Key Distribution  . . . . . . . . . . . . . . . . . . . .   4
   3.  Message Encryption  . . . . . . . . . . . . . . . . . . . . .   4
   4.  Message Decryption  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Mandatory Group and Public Key Format . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The Web Push protocol [I-D.thomson-webpush-protocol] is an
   intermediated protocol by necessity.  Messages from an Application
   Server are delivered to a User Agent via a Push Service.

    +-------+           +--------------+       +-------------+
    |  UA   |           | Push Service |       | Application |
    +-------+           +--------------+       +-------------+
        |                      |                      |
        |        Setup         |                      |
        |<====================>|                      |
        |           Provide Subscription              |
        |-------------------------------------------->|
        |                      |                      |
        :                      :                      :
        |                      |     Push Message     |
        |    Push Message      |<---------------------|
        |<---------------------|                      |
        |                      |                      |

   This document describes how messages sent using this protocol can be
   secured against inspection or modification by a Push Service.

   Web Push messages are the payload of an HTTP message [RFC7230].
   These messages are encrypted using an encrypted content encoding
   [I-D.thomson-http-encryption].  This document describes how this
   content encoding is applied and describes a recommended key
   management scheme.





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   For efficiency reasons, multiple users of Web Push often share a
   central agent that aggregates push functionality.  This agent can
   enforce the use of this encryption scheme by applications that use
   push messaging.  An agent that only delivers messages that are
   properly encrypted strongly encourages the end-to-end protection of
   messages.

   A web browser that implements the Web Push API [API] can enforce the
   use of encryption by forwarding only those messages that were
   properly encrypted.

1.1.  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].

2.  Key Generation and Agreement

   For each new subscription that the User Agent generates for an
   application, it also generates an asymmetric key pair for use in
   Diffie-Hellman (DH) [DH] or elliptic-curve Diffie-Hellman (ECDH)
   [ECDH].  The public key for this key pair can then be distributed by
   the application to the Application Server along with the URI of the
   subscription.  The private key MUST remain secret.

   This key pair is used with the Diffie-Hellman key exchange as
   described in Section 4.2 of [I-D.thomson-http-encryption].

   A User Agent MUST generate and provide a public key for the scheme
   described in Section 5.

   The public key MUST be accompanied by a key identifier that can be
   used in the "keyid" parameter to identify which key is in use.  Key
   identifiers need only be unique within the context of a subscription.

2.1.  Diffie-Hellman Group Information

   As described in [I-D.thomson-http-encryption], use of Diffie-Hellman
   for key agreement requires that the receiver provide clear
   information about it's chosen group and the format for the "dh"
   parameter with each potential sender.

   This document only describes a single ECDH group and point format,
   described in Section 5.  A specification that defines alternative
   groups or formats MUST provide a means of indicating precisely which
   group and format is in use for every public key that is provided.




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2.2.  Key Distribution

   The application using the subscription distributes the key identifier
   and public key along with other subscription information, such as the
   subscription URI and expiration time.

   The communication medium by which an application distributes the key
   identifier and public key MUST be confidentiality protected for the
   reasons described in [I-D.thomson-webpush-protocol].  Most
   applications that use push messaging have a pre-existing relationship
   with an Application Server.  Any existing communication mechanism
   that is authenticated and provides confidentiality and integrity,
   such as HTTPS [RFC2818], is sufficient.

3.  Message Encryption

   An Application Server that has the key identifier, public key, group
   and format information can encrypt a message for the User Agent.

   The Application Server generates a new DH or ECDH key pair in the
   same group as the value generated by the User Agent.

   From the newly generated key pair, the Application Server performs a
   DH or ECDH computation with the public key provided by the User Agent
   to find the shared secret.  The Application Server then generates 16
   octets of salt that is unique to the message.  A random [RFC4086]
   salt is acceptable.  These values are used to calculate the content
   encryption key as defined in Section 3.2 of
   [I-D.thomson-http-encryption].

   The Application Server then encrypts the payload.  Header fields are
   populated with URL-safe base-64 encoded [RFC4648] values:

   o  the salt is added to the "salt" parameter of the Encryption header
      field; and

   o  the public key for its DH or ECDH key pair is placed in the "dh"
      parameter of the Encryption-Key header field.

   An application server MUST encrypt a push message with a single
   record.  This allows for a minimal receiver implementation that
   handles a single record.  If the message is 4096 octets long, or
   longer, this means that the "rs" parameter MUST be set to a value
   that is longer than the encrypted push message length.

   Note that a push service is not required to support more than 4096
   octets of payload body, which equates to 4080 octets of cleartext, so




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   the "rs" parameter can be omitted for messages that fit within this
   limit.

   An application server MUST NOT use other content encodings for push
   messages.  In particular, content encodings that compress could
   result in leaking of push message contents.  The Content-Encoding
   header field therefore has exactly one value, which is "aesgcm128".
   Multiple "aesgcm128" values are not permitted.

   An application server MUST include exactly one entry in each of the
   Encryption and Encryption-Key header fields.  This allows the "keyid"
   parameter to be omitted from both header fields.

   An application server MUST NOT include a "key" parameter in the
   Encryption header field.

4.  Message Decryption

   A User Agent decrypts messages are decrypted as described in
   [I-D.thomson-http-encryption].  The value of the "keyid" parameter is
   used to identify the correct key pair, if there is more than one
   possible value for the corresponding subscription.

   A receiver is not required to support multiple records.  Such a
   receiver MUST check that the record size is large enough to contain
   the entire payload body in a single record.  The "rs" parameter MUST
   NOT be exactly equal to the length of the payload body minus the
   length of the authentication tag (16 octets); that length indicates
   that the message has been truncated.

5.  Mandatory Group and Public Key Format

   User Agents that enforce encryption MUST expose an elliptic curve
   Diffie-Hellman share on the P-256 curve [FIPS186].  Public keys, such
   as are encoded into the "dh" parameter, MUST be in the form of an
   uncompressed point as described in [X.692].

6.  IANA Considerations

   This document has no IANA actions.

7.  Security Considerations

   The security considerations of [I-D.thomson-http-encryption] describe
   the limitations of the content encoding.  In particular, any HTTP
   header fields are not protected by the content encoding scheme.  A
   User Agent MUST consider HTTP header fields to have come from the
   Push Service.  An application on the User Agent that uses information



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   from header fields to alter their processing of a push message is
   exposed to a risk of attack by the Push Service.

   The timing and length of communication cannot be hidden from the Push
   Service.  While an outside observer might see individual messages
   intermixed with each other, the Push Service will see what
   Application Server is talking to which User Agent, and the
   subscription they are talking about.  Additionally, the length of
   messages could be revealed unless the padding provided by the content
   encoding scheme is used to obscure length.

8.  References

8.1.  Normative References

   [DH]       Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory,
              V.IT-22 n.6 , June 1977.

   [ECDH]     SECG, "Elliptic Curve Cryptography", SEC 1 , 2000,
              <http://www.secg.org/>.

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

   [I-D.thomson-http-encryption]
              Thomson, M., "Encrypted Content-Encoding for HTTP", draft-
              thomson-http-encryption-01 (work in progress), July 2015.

   [I-D.thomson-webpush-protocol]
              Thomson, M., Damaggio, E., and B. Raymor, "Generic Event
              Delivery Using HTTP Push", draft-thomson-webpush-
              protocol-00 (work in progress), April 2015.

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

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [X.692]    ANSI, "Public Key Cryptography For The Financial Services
              Industry: The Elliptic Curve Digital Signature Algorithm
              (ECDSA)", ANSI X9.62 , 1998.



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8.2.  Informative References

   [API]      Sullivan, B., Fullea, E., and M. van Ouwerkerk, "Web Push
              API", 2015, <https://w3c.github.io/push-api/>.

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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [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,
              <http://www.rfc-editor.org/info/rfc7230>.

Author's Address

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com



























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