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Internet Engineering Task Force (IETF)                        M. ThomsonRequest for Comments: 8291                                       MozillaCategory: Standards Track                                  November 2017ISSN: 2070-1721Message Encryption for Web PushAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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.Thomson                      Standards Track                    [Page 1]

RFC 8291                   Web Push Encryption             November 2017Table of Contents1. Introduction ....................................................21.1. Notational Conventions .....................................32. Push Message Encryption Overview ................................32.1. Key and Secret Distribution ................................43. Push Message Encryption .........................................43.1. Diffie-Hellman Key Agreement ...............................53.2. Push Message Authentication ................................53.3. Combining Shared and Authentication Secrets ................53.4. Encryption Summary .........................................64. Restrictions on Use of "aes128gcm" Content Coding ...............75. Push Message Encryption Example .................................86. IANA Considerations .............................................87. Security Considerations .........................................88. References .....................................................108.1. Normative References ......................................108.2. Informative References ....................................11Appendix A.  Intermediate Values for Encryption ...................12   Author's Address ..................................................131.  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.Thomson                      Standards Track                    [Page 2]

RFC 8291                   Web Push Encryption             November 2017   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 inBCP14 [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 (seeSection 3.1) and a symmetric secret for authentication (seeSection 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 (seeSection 2.1 of [RFC8188]).  The ECDH key pair can be discarded after   encrypting the message.Thomson                      Standards Track                    [Page 3]

RFC 8291                   Web Push Encryption             November 2017   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 inSection 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] (seeSection 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] (seeSection 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 inSection 3.4.   Restrictions on the use of the encrypted content coding are described   inSection 4.Thomson                      Standards Track                    [Page 4]

RFC 8291                   Web Push Encryption             November 20173.1.  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 (seeSection 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 inSection 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 ECDHThomson                      Standards Track                    [Page 5]

RFC 8291                   Web Push Encryption             November 2017   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 fromRFC 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]Thomson                      Standards Track                    [Page 6]

RFC 8291                   Web Push Encryption             November 2017   Note that this omits the exclusive-OR of the final nonce with the   record sequence number, since push messages contain only a single   record (seeSection 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 (seeSection 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.Thomson                      Standards Track                    [Page 7]

RFC 8291                   Web Push Encryption             November 20175.  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 inAppendix 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.Thomson                      Standards Track                    [Page 8]

RFC 8291                   Web Push Encryption             November 2017   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.Thomson                      Standards Track                    [Page 9]

RFC 8291                   Web Push Encryption             November 20178.  References8.1.  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 inRFC2119 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.Thomson                      Standards Track                   [Page 10]

RFC 8291                   Web Push Encryption             November 20178.2.  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>.Thomson                      Standards Track                   [Page 11]

RFC 8291                   Web Push Encryption             November 2017Appendix A.  Intermediate Values for Encryption   The intermediate values calculated for the example inSection 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_L0FxCeq0WhDx813KgSYqU26kOyzWUdsXYyrgThomson                      Standards Track                   [Page 12]

RFC 8291                   Web Push Encryption             November 2017   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 inSection 5.Author's Address   Martin Thomson   Mozilla   Email: martin.thomson@gmail.comThomson                      Standards Track                   [Page 13]