Internet DRAFT - draft-thomson-webpush-encryption
draft-thomson-webpush-encryption
Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track July 2, 2015
Expires: January 3, 2016
Message Encryption for Web Push
draft-thomson-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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 January 3, 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
Provisions Relating to IETF Documents
(http://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 Expires January 3, 2016 [Page 1]
Internet-Draft Web Push Encryption July 2015
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 . . . . . . . . . . . . . . . . . 6
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.
Thomson Expires January 3, 2016 [Page 2]
Internet-Draft Web Push Encryption July 2015
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.
Thomson Expires January 3, 2016 [Page 3]
Internet-Draft Web Push Encryption July 2015
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 "keyid" from the User Agent is added to both the Encryption-
Key and Encryption header fields;
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.
Thomson Expires January 3, 2016 [Page 4]
Internet-Draft Web Push Encryption July 2015
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
the "rs" parameter can be omitted for messages that fit within this
limit.
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
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.
Thomson Expires January 3, 2016 [Page 5]
Internet-Draft Web Push Encryption July 2015
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, March 1997.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[X.692] 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] 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, May 2000.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
Thomson Expires January 3, 2016 [Page 6]
Internet-Draft Web Push Encryption July 2015
Author's Address
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
Thomson Expires January 3, 2016 [Page 7]
