





Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 8291                                       Mozilla
Category: Standards Track                                  November 2017
ISSN: 2070-1721


                    Message Encryption for Web Push

Abstract

   This document describes a message encryption scheme 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 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/rfc8291.

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.









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Table of Contents

   1. Introduction ....................................................2
      1.1. Notational Conventions .....................................3
   2. Push Message Encryption Overview ................................3
      2.1. Key and Secret Distribution ................................4
   3. Push Message Encryption .........................................4
      3.1. Diffie-Hellman Key Agreement ...............................5
      3.2. Push Message Authentication ................................5
      3.3. Combining Shared and Authentication Secrets ................5
      3.4. Encryption Summary .........................................6
   4. Restrictions on Use of "aes128gcm" Content Coding ...............7
   5. Push Message Encryption Example .................................8
   6. IANA Considerations .............................................8
   7. Security Considerations .........................................8
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................11
   Appendix A.  Intermediate Values for Encryption ...................12
   Author's Address ..................................................13

1.  Introduction

   The Web Push protocol [RFC8030] is an intermediated protocol by
   necessity.  Messages from an application server are delivered to a
   user agent (UA) via a push service, as shown in Figure 1.

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

                                 Figure 1

   This document describes how messages sent using this protocol can be
   secured against inspection, modification, and forgery by a push
   service.




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   Web Push messages are the payload of an HTTP message [RFC7230].
   These messages are encrypted using an encrypted content encoding
   [RFC8188].  This document describes how this content encoding is
   applied and describes a recommended key management scheme.

   Multiple users of Web Push at the same user agent 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 Push API [API] can enforce the use
   of encryption by forwarding only those messages that were properly
   encrypted.

1.1.  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.

   This document uses the terminology from [RFC8030], primarily "user
   agent", "push service", and "application server".

2.  Push Message Encryption Overview

   Encrypting a push message uses Elliptic Curve Diffie-Hellman (ECDH)
   [ECDH] on the P-256 curve [FIPS186] to establish a shared secret (see
   Section 3.1) and a symmetric secret for authentication (see
   Section 3.2).

   A user agent generates an ECDH key pair and authentication secret
   that it associates with each subscription it creates.  The ECDH
   public key and the authentication secret are sent to the application
   server with other details of the push subscription.

   When sending a message, an application server generates an ECDH key
   pair and a random salt.  The ECDH public key is encoded into the
   "keyid" parameter of the encrypted content coding header, and the
   salt is encoded into the "salt" parameter of that same header (see
   Section 2.1 of [RFC8188]).  The ECDH key pair can be discarded after
   encrypting the message.






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   The content of the push message is encrypted or decrypted using a
   content encryption key and nonce.  These values are derived by taking
   the "keyid" and "salt" as input to the process described in
   Section 3.

2.1.  Key and Secret Distribution

   The application using the subscription distributes the subscription
   public key and authentication secret to an authorized application
   server.  This could be sent along with other subscription information
   that is provided by the user agent, such as the push subscription
   URI.

   An application MUST use an authenticated, confidentiality-protected
   communications medium for this purpose.  In addition to the reasons
   described in [RFC8030], this use ensures that the authentication
   secret is not revealed to unauthorized entities, which would allow
   those entities to generate push messages that will be accepted by the
   user agent.

   Most applications that use push messaging have a preexisting
   relationship with an application server that can be used for
   distribution of subscription data.  An authenticated communication
   mechanism that provides adequate confidentiality and integrity
   protection, such as HTTPS [RFC2818], is sufficient.

3.  Push Message Encryption

   Push message encryption happens in four phases:

   o  A shared secret is derived using ECDH [ECDH] (see Section 3.1 of
      this document).

   o  The shared secret is then combined with the authentication secret
      to produce the input keying material (IKM) used in [RFC8188] (see
      Section 3.3 of this document).

   o  A content encryption key and nonce are derived using the process
      in [RFC8188].

   o  Encryption or decryption follows according to [RFC8188].

   The key derivation process is summarized in Section 3.4.
   Restrictions on the use of the encrypted content coding are described
   in Section 4.






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3.1.  Diffie-Hellman Key Agreement

   For each new subscription that the user agent generates for an
   application, it also generates a P-256 [FIPS186] key pair for use in
   ECDH [ECDH].

   When sending a push message, the application server also generates a
   new ECDH key pair on the same P-256 curve.

   The ECDH public key for the application server is included as the
   "keyid" parameter in the encrypted content coding header (see
   Section 2.1 of [RFC8188]).

   An application server combines its ECDH private key with the public
   key provided by the user agent using the process described in [ECDH];
   on receipt of the push message, a user agent combines its private key
   with the public key provided by the application server in the "keyid"
   parameter in the same way.  These operations produce the same value
   for the ECDH shared secret.

3.2.  Push Message Authentication

   To ensure that push messages are correctly authenticated, a symmetric
   authentication secret is added to the information generated by a user
   agent.  The authentication secret is mixed into the key derivation
   process described in Section 3.3.

   A user agent MUST generate and provide a hard-to-guess sequence of 16
   octets that is used for authentication of push messages.  This SHOULD
   be generated by a cryptographically strong random number generator
   [RFC4086].

3.3.  Combining Shared and Authentication Secrets

   The shared secret produced by ECDH is combined with the
   authentication secret using the HMAC-based key derivation function
   (HKDF) [RFC5869].  This produces the input keying material used by
   [RFC8188].

   The HKDF function uses the SHA-256 hash algorithm [FIPS180-4] with
   the following inputs:

   salt: the authentication secret

   IKM:  the shared secret derived using ECDH






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   info: the concatenation of the ASCII-encoded string "WebPush: info"
         (this string is not NUL-terminated), a zero octet, the user
         agent ECDH public key, and the application server ECDH public
         key, (both ECDH public keys are in the uncompressed point form
         defined in [X9.62].  That is:

   key_info = "WebPush: info" || 0x00 || ua_public || as_public

   L:    32 octets (i.e., the output is the length of the underlying
         SHA-256 HMAC function output)

3.4.  Encryption Summary

   This results in a final content encryption key and nonce generation
   using the following sequence, which is shown here in pseudocode with
   HKDF expanded into separate discrete steps using HMAC with SHA-256:

      -- For a user agent:
      ecdh_secret = ECDH(ua_private, as_public)
      auth_secret = random(16)
      salt = <from content coding header>

      -- For an application server:
      ecdh_secret = ECDH(as_private, ua_public)
      auth_secret = <from user agent>
      salt = random(16)

      -- For both:

      ## Use HKDF to combine the ECDH and authentication secrets
      # HKDF-Extract(salt=auth_secret, IKM=ecdh_secret)
      PRK_key = HMAC-SHA-256(auth_secret, ecdh_secret)
      # HKDF-Expand(PRK_key, key_info, L_key=32)
      key_info = "WebPush: info" || 0x00 || ua_public || as_public
      IKM = HMAC-SHA-256(PRK_key, key_info || 0x01)

      ## HKDF calculations from RFC 8188
      # HKDF-Extract(salt, IKM)
      PRK = HMAC-SHA-256(salt, IKM)
      # HKDF-Expand(PRK, cek_info, L_cek=16)
      cek_info = "Content-Encoding: aes128gcm" || 0x00
      CEK = HMAC-SHA-256(PRK, cek_info || 0x01)[0..15]
      # HKDF-Expand(PRK, nonce_info, L_nonce=12)
      nonce_info = "Content-Encoding: nonce" || 0x00
      NONCE = HMAC-SHA-256(PRK, nonce_info || 0x01)[0..11]






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   Note that this omits the exclusive-OR of the final nonce with the
   record sequence number, since push messages contain only a single
   record (see Section 4) and the sequence number of the first record is
   zero.

4.  Restrictions on Use of "aes128gcm" Content Coding

   An application server MUST encrypt a push message with a single
   record.  This allows for a minimal receiver implementation that
   handles a single record.  An application server MUST set the "rs"
   parameter in the "aes128gcm" content coding header to a size that is
   greater than the sum of the lengths of the plaintext, the padding
   delimiter (1 octet), any padding, and the authentication tag (16
   octets).

   A push message MUST include the application server ECDH public key in
   the "keyid" parameter of the encrypted content coding header.  The
   uncompressed point form defined in [X9.62] (that is, a 65-octet
   sequence that starts with a 0x04 octet) forms the entirety of the
   "keyid".  Note that this means that the "keyid" parameter will not be
   valid UTF-8 as recommended in [RFC8188].

   A push service is not required to support more than 4096 octets of
   payload body (see Section 7.2 of [RFC8030]).  Absent header (86
   octets), padding (minimum 1 octet), and expansion for
   AEAD_AES_128_GCM (16 octets), this equates to, at most, 3993 octets
   of plaintext.

   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 "aes128gcm".
   Multiple "aes128gcm" values are not permitted.

   A user agent is not required to support multiple records.  A user
   agent MAY ignore the "rs" parameter.  If a record size is unchecked,
   decryption will fail with high probability for all valid cases.  The
   padding delimiter octet MUST be checked; values other than 0x02 MUST
   cause the message to be discarded.












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5.  Push Message Encryption Example

   The following example shows a push message being sent to a push
   service.

   POST /push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1
   Host: push.example.net
   TTL: 10
   Content-Length: 145
   Content-Encoding: aes128gcm

   DGv6ra1nlYgDCS1FRnbzlwAAEABBBP4z9KsN6nGRTbVYI_c7VJSPQTBtkgcy27ml
   mlMoZIIgDll6e3vCYLocInmYWAmS6TlzAC8wEqKK6PBru3jl7A_yl95bQpu6cVPT
   pK4Mqgkf1CXztLVBSt2Ks3oZwbuwXPXLWyouBWLVWGNWQexSgSxsj_Qulcy4a-fN

   This example shows the ASCII-encoded string, "When I grow up, I want
   to be a watermelon".  The content body is shown here with line
   wrapping and URL-safe base64url [RFC4648] encoding to meet
   presentation constraints.

   The keys used are shown below using the uncompressed form [X9.62]
   encoded using base64url.

      Authentication Secret: BTBZMqHH6r4Tts7J_aSIgg
      Receiver:
         private key: q1dXpw3UpT5VOmu_cf_v6ih07Aems3njxI-JWgLcM94
         public key: BCVxsr7N_eNgVRqvHtD0zTZsEc6-VV-JvLexhqUzORcx
                     aOzi6-AYWXvTBHm4bjyPjs7Vd8pZGH6SRpkNtoIAiw4
      Sender:
         private key: yfWPiYE-n46HLnH0KqZOF1fJJU3MYrct3AELtAQ-oRw
         public key: BP4z9KsN6nGRTbVYI_c7VJSPQTBtkgcy27mlmlMoZIIg
                     Dll6e3vCYLocInmYWAmS6TlzAC8wEqKK6PBru3jl7A8

   Intermediate values for this example are included in Appendix A.

6.  IANA Considerations

   This document does not require any IANA actions.

7.  Security Considerations

   The privacy and security considerations of [RFC8030] all apply to the
   use of this mechanism.

   The Security Considerations section of [RFC8188] describes the
   limitations of the content encoding.  In particular, no HTTP header
   fields are protected by the content encoding scheme.  A user agent
   MUST consider HTTP header fields to have come from the push service.



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   Though header fields might be necessary for processing an HTTP
   response correctly, they are not needed for correct operation of the
   protocol.  An application on the user agent that uses information
   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 which
   application server is talking to which user agent and the
   subscription that is used.  Additionally, the length of messages
   could be revealed unless the padding provided by the content encoding
   scheme is used to obscure length.

   The user agent and application MUST verify that the public key they
   receive is on the P-256 curve.  Failure to validate a public key can
   allow an attacker to extract a private key.  The appropriate
   validation procedures are defined in Section 4.3.7 of [X9.62] and,
   alternatively, in Section 5.6.2.3 of [KEYAGREEMENT].  This process
   consists of three steps:

   1.  Verify that Y is not the point at infinity (O),

   2.  Verify that for Y = (x, y), both integers are in the correct
       interval,

   3.  Ensure that (x, y) is a correct solution to the elliptic curve
       equation.

   For these curves, implementers do not need to verify membership in
   the correct subgroup.

   In the event that this encryption scheme would need to be replaced, a
   new content coding scheme could be defined.  In order to manage
   progressive deployment of the new scheme, the user agent can expose
   information on the content coding schemes that it supports.  The
   "supportedContentEncodings" parameter of the Push API [API] is an
   example of how this might be done.













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8.  References

8.1.  Normative References

   [ECDH]     SECG, "SEC 1: Elliptic Curve Cryptography", Version 2.0,
              May 2009, <http://www.secg.org/>.

   [FIPS180-4]
              National Institute of Standards and Technology (NIST),
              "Secure Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015.

   [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>.

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

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [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>.

   [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>.

   [RFC8188]  Thomson, M., "Encrypted Content-Encoding for HTTP",
              RFC 8188, DOI 10.17487/RFC8188, June 2017,
              <https://www.rfc-editor.org/info/rfc8188>.

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




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8.2.  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/>.

   [KEYAGREEMENT]
              Barker, E., Chen, L., Roginsky, A., and M. Smid,
              "Recommendation for Pair-Wise Key Establishment Schemes
              Using Discrete Logarithm Cryptography", NIST Special
              Publication 800-56A, Revision 2,
              DOI 10.6028/NIST.SP.800-56Ar2, May 2013.

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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7230>.


























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Appendix A.  Intermediate Values for Encryption

   The intermediate values calculated for the example in Section 5 are
   shown here.  The base64url values in these examples include
   whitespace that can be removed.

   The following are inputs to the calculation:

   Plaintext:  V2hlbiBJIGdyb3cgdXAsIEkgd2FudCB0byBiZSBhIHdhdGVybWVsb24

   Application server public key (as_public):
      BP4z9KsN6nGRTbVYI_c7VJSPQTBtkgcy27mlmlMoZIIg
      Dll6e3vCYLocInmYWAmS6TlzAC8wEqKK6PBru3jl7A8

   Application server private key (as_private):
      yfWPiYE-n46HLnH0KqZOF1fJJU3MYrct3AELtAQ-oRw

   User agent public key (ua_public):  BCVxsr7N_eNgVRqvHtD0zTZsEc6-VV-
      JvLexhqUzORcx aOzi6-AYWXvTBHm4bjyPjs7Vd8pZGH6SRpkNtoIAiw4

   User agent private key (ua_private):
      q1dXpw3UpT5VOmu_cf_v6ih07Aems3njxI-JWgLcM94

   Salt:  DGv6ra1nlYgDCS1FRnbzlw

   Authentication secret (auth_secret):  BTBZMqHH6r4Tts7J_aSIgg

   Note that knowledge of just one of the private keys is necessary.
   The application server randomly generates the salt value, whereas
   salt is input to the receiver.

   This produces the following intermediate values:

   Shared ECDH secret (ecdh_secret):
      kyrL1jIIOHEzg3sM2ZWRHDRB62YACZhhSlknJ672kSs

   Pseudorandom key (PRK) for key combining (PRK_key):
      Snr3JMxaHVDXHWJn5wdC52WjpCtd2EIEGBykDcZW32k

   Info for key combining (key_info):  V2ViUHVzaDogaW5mbwAEJXGyvs3942BVG
      q8e0PTNNmwR zr5VX4m8t7GGpTM5FzFo7OLr4BhZe9MEebhuPI-OztV3
      ylkYfpJGmQ22ggCLDgT-M_SrDepxkU21WCP3O1SUj0Ew
      bZIHMtu5pZpTKGSCIA5Zent7wmC6HCJ5mFgJkuk5cwAv MBKiiujwa7t45ewP

   Input keying material for content encryption key derivation (IKM):
      S4lYMb_L0FxCeq0WhDx813KgSYqU26kOyzWUdsXYyrg





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   PRK for content encryption (PRK):
      09_eUZGrsvxChDCGRCdkLiDXrReGOEVeSCdCcPBSJSc

   Info for content encryption key derivation (cek_info):
      Q29udGVudC1FbmNvZGluZzogYWVzMTI4Z2NtAA

   Content encryption key (CEK):  oIhVW04MRdy2XN9CiKLxTg

   Info for content encryption nonce derivation (nonce_info):
      Q29udGVudC1FbmNvZGluZzogbm9uY2UA

   Nonce (NONCE):  4h_95klXJ5E_qnoN

   The salt, record size of 4096, and application server public key
   produce an 86-octet header of:

   DGv6ra1nlYgDCS1FRnbzlwAAEABBBP4z 9KsN6nGRTbVYI_c7VJSPQTBtkgcy27ml
   mlMoZIIgDll6e3vCYLocInmYWAmS6Tlz AC8wEqKK6PBru3jl7A8

   The push message plaintext has the padding delimiter octet (0x02)
   appended to produce:

   V2hlbiBJIGdyb3cgdXAsIEkgd2FudCB0 byBiZSBhIHdhdGVybWVsb24C

   The plaintext is then encrypted with AES-GCM, which emits ciphertext
   of:

   8pfeW0KbunFT06SuDKoJH9Ql87S1QUrd irN6GcG7sFz1y1sqLgVi1VhjVkHsUoEs
   bI_0LpXMuGvnzQ

   The header and ciphertext are concatenated and produce the result
   shown in Section 5.

Author's Address

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com












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