Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track January 30, 2017
Expires: August 3, 2017

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

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

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Table of Contents

1. Introduction

The Web Push protocol [I-D.ietf-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, modification and falsification 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.ietf-httpbis-encryption-encoding]. This document describes how this content encoding is applied and describes a recommended key management scheme.

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. 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 dh parameter of the Crypto-Key header field; the salt is encoded into message payload. The ECDH key pair can be discarded after encrypting the message.

The content of the push message is encrypted or decrypted using a content encryption key and nonce that is derived using all of these inputs and 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 [I-D.ietf-webpush-protocol], this ensures that the authentication secret is not revealed to unauthorized entities, which can be used to generate push messages that will be accepted by the User Agent.

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. Push Message Encryption

Push message encryption happens in four phases:

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

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 elliptic-curve Diffie-Hellman (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 [I-D.ietf-httpbis-encryption-encoding].

An Application 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 dh 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 shown 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 HMAC-based key derivation function (HKDF) described in [RFC5869]. This produces the input keying material used by [I-D.ietf-httpbis-encryption-encoding].

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

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

salt:
the authentication secret
IKM:
the shared secret derived using ECDH
info:
the concatenation of the ASCII-encoded string “WebPush: info”, a zero octet, the X9.62 encoding of the User Agent ECDH public key, and X9.62 encoding of the Application Server ECDH public key; that is
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 the 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)

   -- For an Application Server:
   ecdh_secret = ECDH(as_private, ua_public)
   auth_secret = <from User Agent>

   -- For both:
   PRK_key = HMAC-SHA-256(auth_secret, ecdh_secret)
   key_info = "WebPush: info" || 0x00 || ua_public || as_public
   IKM = HMAC-SHA-256(PRK_cek, key_info || 0x01)

   salt = random(16)
   PRK = HMAC-SHA-256(salt, IKM)
   cek_info = "Content-Encoding: aes128gcm" || 0x00
   CEK = HMAC-SHA-256(PRK, cek_info || 0x01)[0..15]
   nonce_info = "Content-Encoding: nonce" || 0x00
   NONCE = HMAC-SHA-256(PRK, nonce_info || 0x01)[0..11]

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 at least the length of the plaintext, plus the padding delimiter (1 octet) and the AEAD_AES_128_GCM expansion (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”.

A push service is not required to support more than 4096 octets of payload body (see Section 7.2 of [I-D.ietf-webpush-protocol]). Accounting for the header (86 octets), padding delimiter (1 octet), and the expansion of AEAD_AES_128_GCM (16 octets), this equates to at most 3993 octets of plaintext and padding.

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 field. A User Agent MUST check that the padding delimiter has a value of 2, indicating that the record is the last record.

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: 144
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 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: <the value of the "dh" parameter>

Intermediate values for this example are included in Appendix A.

6. IANA Considerations

[[RFC EDITOR: please remote this section before publication.]] This document makes no request of IANA.

7. Security Considerations

The security considerations of [I-D.ietf-httpbis-encryption-encoding] 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 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 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.

8. References

8.1. Normative References

[ECDH] SECG, "Elliptic Curve Cryptography", SEC 1 , 2000.
[FIPS180-4] Department of Commerce, National., "NIST FIPS 180-4, Secure Hash Standard", March 2012.
[FIPS186] National Institute of Standards and Technology (NIST), "Digital Signature Standard (DSS)", NIST PUB 186-4 , July 2013.
[I-D.ietf-httpbis-encryption-encoding] Thomson, M., "Encrypted Content-Encoding for HTTP", Internet-Draft draft-ietf-httpbis-encryption-encoding-06, December 2016.
[I-D.ietf-webpush-protocol] Thomson, M., Damaggio, E. and B. Raymor, "Generic Event Delivery Using HTTP Push", Internet-Draft draft-ietf-webpush-protocol-12, October 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4086] Eastlake 3rd, D., Schiller, J. and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010.
[X9.62] ANSI, "Public Key Cryptography For The Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI X9.62 , 1998.

8.2. Informative References

[API] van Ouwerkerk, M. and M. Thomson, "Web Push API", 2015.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014.

Appendix A. Intermediate Values for Encryption

The intermediate values calculated for the example in Section 5 are shown here. 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 sufficient. 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
Pseudo-random key for key combining (PRK_key):
Snr3JMxaHVDXHWJn5wdC52WjpCtd2EIEGBykDcZW32k
Info for key combining (key_info):
V2ViUHVzaDogaW5mbwAEJXGyvs3942BVGq8e0PTNNmwR zr5VX4m8t7GGpTM5FzFo7OLr4BhZe9MEebhuPI-OztV3 ylkYfpJGmQ22ggCLDgT-M_SrDepxkU21WCP3O1SUj0Ew bZIHMtu5pZpTKGSCIA5Zent7wmC6HCJ5mFgJkuk5cwAv MBKiiujwa7t45ewP
Input keying material for content encryption key derivation (IKM):
S4lYMb_L0FxCeq0WhDx813KgSYqU26kOyzWUdsXYyrg
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 DGv6ra1nlYgDCS1FRnbzlwAAEABBBP4z9KsN6nGR TbVYI_c7VJSPQTBtkgcy27mlmlMoZIIgDll6e3vC YLocInmYWAmS6TlzAC8wEqKK6PBru3jl7A8.

The push message plaintext has a padding delimiter octet added (and no additional padding) to produce V2hlbiBJIGdyb3cgdXAsIEkgd2FudCB0byBiZSBh IHdhdGVybWVsb24C. The plaintext is then encrypted with AES-GCM, which emits ciphertext of 8pfeW0KbunFT06SuDKoJH9Ql87S1QUrdirN6GcG7 sFz1y1sqLgVi1VhjVkHsUoEsbI_0LpXMuGvnzQ.

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

Author's Address

Martin Thomson Mozilla EMail: martin.thomson@gmail.com
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