| Network Working Group | M. Thomson |
| Internet-Draft | Mozilla |
| Intended status: Standards Track | February 14, 2017 |
| Expires: August 18, 2017 |
Message Encryption for Web Push
draft-ietf-webpush-encryption-08
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.
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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.
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].
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.
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.
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.
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.
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].
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
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.
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 some of the length 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”.
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]). Absent header (86 octets), padding (minimum 2 octets), and expansion for AEAD_AES_128_GCM (16 octets), this equates to at most 3992 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.
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 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.
[[RFC EDITOR: please remote this section before publication.]] This document makes no request of IANA.
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.
| [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. |
The intermediate values calculated for the example in Section 5 are shown here. The following are inputs to the calculation:
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:
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.