WebPush Encryption using HPKE

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Table of Contents

1. Introduction

Message Encryption for Web Push [RFC8291] defines a hybrid encryption system that depends on elliptic curve cryptography. This system is known to be vulnerable to compromise in the event that a cryptographically-relevant quantum computer (CRQC) is created. The risk of this resulting in the compromise of Web Push encryption will be increasingly likely as time passes [PQC-GUIDE].¶

This document defines a message encryption design that can integrate a post-quantum KEM. This design is a generic use of Hybrid Public Key Encryption (HPKE) [HPKE] that can use any defined set of algorithms.¶

At the same time, the use of HPKE addresses a minor, but inconsequential, vulnerability in the design of [RFC8291] that would allow an application to construct a single ciphertext that two parties would successfully authenticate and decrypt to different plaintexts. This attack is described in Section 8.2.¶

This document obsoletes RFC 8291 [RFC8291].¶

This document is one of a pair. Its companion, [WEBPUSH-SYM] is an alternative to this approach. [WEBPUSH-SYM] describes how to encode messages using symmetric cryptography.¶

2. Conventions and Definitions

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.¶

3. Using HPKE in Web Push

Web Push [WEBPUSH] provides a messaging capability that allows applications to send small messages to user agents via a push service. The push service enables efficient message delivery, even when user agents have constrained connectivity.¶

Typical interactions for Web Push are shown in Figure 1.¶

Encryption of push messages ensures that the push service is unable to read or alter the messages it handles.¶

Web Push uses a hybrid encryption mode that combines a key-encapsulation mechanism (KEM) and pre-shared key (PSK) with authenticated encryption with additional data (AEAD) encryption. This document describes how this can be provided by [HPKE].¶

The overall encryption design is composed from two parts:¶

1. A user agent key configuration, where the user agent provides the information necessary to construct a message it will accept. This is provided by the user agent to the application during the establishment of a Web Push subscription. Details of the format of this key configuration are provided in Section 4.¶

2. A message protection format, used to encode push messages. This format is defined in Section 5.¶

4. User Agent Key Configuration

In order for an application server to send a push message using HPKE, it requires knowledge of:¶

• the HPKE algorithms that the user agent accepts,¶

• the PSK the user agent has chosen, and¶

• the KEM public key (or keys) the user agent holds secret keys for.¶

This information is formatted into a key configuration, similar in shape to that used in the Encrypted Client Hello (ECH) HpkeKeyConfig (Section 4 of [ECH]) or the Oblivious HTTP key configuration (Section 3 of [OHTTP]).¶

Figure 2 shows the format of the key configuration, using the format described in Section 1.3 of [QUIC].¶

Key Config {
  Key Config Item With Length (..) ...,
}

Key Config Item With Length {
  Key Config Item Length (16),
  Key Config Item (..),
}

Key Config Item {
  Key Identifier (8),
  PSK (128),
  HPKE KEM ID (16),
  HPKE Public Key (Npk * 8),
  HPKE Symmetric Algorithms Length (16) = 4..65532,
  HPKE Symmetric Algorithms (32) ...,
}

HPKE Symmetric Algorithms {
  HPKE KDF ID (16),
  HPKE AEAD ID (16),
}

The format consists of one or more Key Config Items, each prefixed with a 16 bit length, in network byte order.¶

Each Key Config Item contains:¶

Key Identifier:

An 8 bit value that will be used in protected messages to identify the user agent's PSK and KEM secret key.¶

PSK:

A 16 byte sequence of bytes of the pre-shared key.¶

HPKE KEM ID:

A 16 bit value that identifies the Key Encapsulation Method (KEM) used for the identified key as defined in Section 7.1 of [HPKE] or the HPKE KDF IANA registry.¶

HPKE Public Key:

The public key used by the user agent. The length of the public key is Npk, which is determined by the choice of HPKE KEM as defined in Section 4 of [HPKE]. (As a result, knowledge of the KEM is necessary to process subsequent fields.)¶

HPKE Symmetric Algorithms Length:

A 16 bit integer in network byte order that encodes the length, in bytes, of the HPKE Symmetric Algorithms field that follows.¶

HPKE Symmetric Algorithms:

One or more pairs of identifiers for the different combinations of HPKE KDF and AEAD that the user agent supports:¶

HPKE KDF ID:

A 16 bit HPKE KDF identifier as defined in Section 7.2 of [HPKE] or the HPKE KDF IANA registry.¶

HPKE AEAD ID:

A 16 bit HPKE AEAD identifier as defined in Section 7.3 of [HPKE] or the HPKE AEAD IANA registry.¶

5. HPKE-Encrypted Push Message Format

The format of push messages when using HPKE is illustrated in Figure 3.¶

HPKE-Protected Push Message {
  Key Identifier (8),
  HPKE KEM ID (16),
  HPKE KDF ID (16),
  HPKE AEAD ID (16),
  Encapsulated KEM Shared Secret (8 * Nenc),
  HPKE-Protected Message Contents (..),
}

This format is identical to that in Section 4.1 of [OHTTP] and it uses a similar construction following the guidance of Section 4.6 of [OHTTP], differing only in that it includes a pre-shared key. Processes for the application server that sends messages are included in Section 5.1; processes for the user agent that receives messages are included in Section 5.2.¶

hdr = concat(encode(1, key_id),
             encode(2, kem_id),
             encode(2, kdf_id),
             encode(2, aead_id))
info = concat(encode_str("webpush message"),
              encode(1, 0),
              hdr)
enc, sctxt = SetupPSKS(pkR, info, psk, [key_id])
ct = sctxt.Seal("", msg)
push_message = concat(hdr, enc, ct)

key_id, kem_id, kdf_id, aead_id, enc, ct = parse(enc_request)
info = concat(encode_str("message/bhttp request"),
              encode(1, 0),
              encode(1, key_id),
              encode(2, kem_id),
              encode(2, kdf_id),
              encode(2, aead_id))
rctxt = SetupBaseR(enc, skR, info, psk, [key_id])
msg, error = rctxt.Open("", ct)

6. Mandatory Algorithms

All implementations of this specification MUST implement the following algorithms:¶

• the MLKEM768-X25519 KEM (0x647a) Section 8.2 of [HPKE-PQ]¶

• the TurboSHAKE128 KDF (0x0012) Section 5 of [HPKE-PQ]¶

• the AES-GCM AEAD (0x0001) Section 7.3 of [HPKE]¶

7. Upgrading from RFC 8291

The encryption scheme in [RFC8291] identifies itself through the use of the encrypted "aes128gcm" content coding [RFC8188]. Though it is possible to use this message encryption system and the "aes128gcm" content coding at the same time, this is not generally necessary. A user agent can therefore use the absence of the encrypted content coding to indicate the use of this scheme.¶

This document defines a media type for HPKE-protected push messages; see Section 9. This media type MAY be omitted if the "aes128gcm" content coding is not present.¶

Unlike RFC 8291, no native padding facility is provided by this encryption format; see Section 8 for details.¶

8. Security Considerations

A Web Push user agent relies to some degree on two values remaining confidential to protect it from unwanted messages:¶

• the URL at which push messages are delivered, knowledge of which is necessary for messages to reach the user agent at all, and¶

• the pre-shared key that is used to authenticate the sender, which is the primary defense.¶

Knowledge of these values, along with the other parameters in the key configuration, allows an attacker to send any message it chooses to the user agent.¶

Like the design in RFC 8291 (see Section 7 of [RFC8291]), this mechanism cannot obscure the presence, timing, and size of messages from the push service. Though communication with the push service is protected by TLS, without additional traffic analysis protection, network observers might also be able to observe these attributes of messages.¶

Padding of the payload of messages can be used to obscure the precise size of the message, but no facility is provided in this document for padding messages. This differs from the RFC 8291 design, which was able to use the padding provided by the [RFC8188] encoding.¶

This document exercises the options described in [RFC8192] for replacing the encryption scheme. Any replacement of this encryption scheme can use those same techniques.¶

9. IANA Considerations

This document registers the "application/webpush-message" content type to identify a push message that is protected with this scheme, as defined in Section 5.¶

Type name:

application¶

Subtype name:

webpush-message¶

Required parameters:

N/A¶

Optional parameters:

N/A¶

Encoding considerations:

"binary"¶

Security considerations:

see Section 8.3¶

Interoperability considerations:

N/A¶

Published specification:

this specification¶

Applications that use this media type:

This type identifies an encrypted web push message.¶

Fragment identifier considerations:

N/A¶

Additional information:

Magic number(s):

N/A¶

Deprecated alias names for this type:

N/A¶

File extension(s):

N/A¶

Macintosh file type code(s):

N/A¶

Person and email address to contact for further information:

see Authors' Addresses section¶

Intended usage:

COMMON¶

Restrictions on usage:

N/A¶

Author:

see Authors' Addresses section¶

Change controller:

IETF¶

10. References

Acknowledgments

TODO¶

Author's Address