Internet DRAFT - draft-ietf-webpush-encryption
draft-ietf-webpush-encryption
Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track September 04, 2017
Expires: March 8, 2018
Message Encryption for Web Push
draft-ietf-webpush-encryption-09
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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This Internet-Draft will expire on March 8, 2018.
Copyright Notice
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Intermediate Values for Encryption . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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.
+-------+ +--------------+ +-------------+
| 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 forgery by a push
service.
Web Push messages are the payload of an HTTP message [RFC7230].
These messages are encrypted using an encrypted content encoding
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[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 Web 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, the salt in
the "salt" parameter of that same header (see Section 2.1 of
[RFC8188]). 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.
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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 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 pre-existing
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 elliptic-curve Diffie-Hellman
[ECDH] (Section 3.1).
o The shared secret is then combined with the authentication secret
to produce the input keying material used in [RFC8188]
(Section 3.3).
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.
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].
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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 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 the Hashed Message Authentication Code
(HMAC)-based key derivation function (HKDF) [RFC5869]. This produces
the input keying material used by [RFC8188].
The HKDF function uses SHA-256 hash algorithm [FIPS180-4] with the
following inputs:
salt: the authentication secret
IKM: the shared secret derived using ECDH
info: the concatenation of the ASCII-encoded string "WebPush: info"
(this string is not NUL-terminated), a zero octet, and the user
agent ECDH public key and the application server ECDH public key,
both in the uncompressed point form defined in [X9.62]; that is:
key_info = "WebPush: info" || 0x00 || ua_public || as_public
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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)
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]
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.
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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" field. 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.
5. Push Message Encryption Example
The following example shows a push message being sent to a push
service.
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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
[[RFC EDITOR: please remove this section before publication.]] This
document makes no request of IANA.
7. Security Considerations
The privacy and security considerations of [RFC8030] all apply to the
use of this mechanism.
The security considerations of [RFC8188] describe 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. 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
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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 5.6.2.6 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.
8. References
8.1. Normative References
[ECDH] SECG, "Elliptic Curve Cryptography", SEC 1 , 2000,
<http://www.secg.org/>.
[FIPS180-4]
Department of Commerce, National., "NIST FIPS 180-4,
Secure Hash Standard", March 2012,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
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[FIPS186] National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", NIST PUB 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 , 1998.
8.2. Informative References
[API] Beverloo, P. and M. Thomson, "Web Push API", 2015,
<https://w3c.github.io/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 800-38D, May 2013.
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[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>.
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-n46HLnH0KqZOF1f
JJU3MYrct3AELtAQ-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): kyrL1jIIOHEzg3sM2ZWRHDRB62YACZhhSl
knJ672kSs
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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
PRK for content encryption (PRK): 09_eUZGrsvxChDCGRCdkLiDXrReGOEVeSC
dCcPBSJSc
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 cipher text 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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