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Applications and Real-Time Web-Based Push Notifications spam transdimensional annoyance This document defines how to use Hybrid Public Key Encryption (HPKE) with Web Push. This document obsoletes RFC 8291. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Web-Based Push Notifications Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Message Encryption for Web Push 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 . 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) 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 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 . This document obsoletes RFC 8291 . This document is one of a pair. Its companion, is an alternative to this approach. describes how to encode messages using symmetric cryptography.

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 when, and only when, they appear in all capitals, as shown here.

Using HPKE in Web Push Web Push 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 .

Web Push Overview | | | | | | Provide Subscription | |-------------------------------------------->| | | | ~~~~~~~~~~~~~~~~~~~~~~~ (Later) ~~~~~~~~~~~~~~~~~~~~~~~ | | | | | Push Message | | Push Message |<---------------------| |<---------------------| | | | | ]]>

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

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 () or the Oblivious HTTP key configuration (). shows the format of the key configuration, using the format described in .

Configuration Format

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 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 . (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 or the HPKE KDF IANA registry.

HPKE AEAD ID:

A 16 bit HPKE AEAD identifier as defined in or the HPKE AEAD IANA registry.

HPKE-Encrypted Push Message Format The format of push messages when using HPKE is illustrated in .

HPKE-Protected Push Message Format

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

Push Message Application Server Processing Applications encapsulate a push message, msg, using values from the key configuration:

• the key identifier from the configuration, key_id, with the corresponding KEM identified by kem_id,
• the pre-shared key from the configuration, psk,
• the public key from the configuration, pkR, and
• a combination of KDF, identified by kdf_id, and AEAD, identified by aead_id, that the application selects from those in the key configuration.

The application then constructs an encrypted push message, push_message, from the plaintext of the message, msg, as follows:

1. Construct a message header, hdr, by concatenating the values of key_id, kem_id, kdf_id, and aead_id, as one 8-bit integer and three 16-bit integers, respectively, each in network byte order.
2. Build info by concatenating the ASCII-encoded string "webpush message", a zero byte, and the header.
3. Create a sending HPKE context by invoking SetupPSKS() () with the public key of the receiver pkR, info as constructed, the pre-shared key psk, a single byte pre-shared key identifier containing a single key_id. This yields the context sctxt and an encapsulation key enc.
4. Encrypt msg by invoking the Seal() method on sctxt () with empty associated data aad, yielding ciphertext ct.
5. Concatenate the values of hdr, enc, and ct, yielding an encrypted push message push_message.

Note that enc is of fixed-length, so there is no ambiguity in parsing this structure. In pseudocode, this procedure is as follows:

Push Message Receiver Processing An user agent decrypts a push message by reversing this process. To decapsulate the encrypted push message, push_message:

1. Parses push_message into key_id, kem_id, kdf_id, aead_id, enc, and ct (indicated using the function parse() in pseudocode). The user agent is then able to find the HPKE private key, skR, and pre-shared key, psk. a. If key_id does not identify a key matching the type of kem_id, the user agent discards the message. b. If kdf_id and aead_id identify a combination of KDF and AEAD that the user agent is unwilling to use with skR, the user agent discards the message.
2. Build info by concatenating the ASCII-encoded string "message/bhttp request", a zero byte, key_id as an 8-bit integer, plus kem_id, kdf_id, and aead_id as three 16-bit integers.
3. Create a receiving HPKE context, rctxt, by invoking SetupPSKR() () with enc, skR, info, psk, and a one byte pre-shared key ID containing key_id.
4. Decrypt ct by invoking the Open() method on rctxt (), with an empty associated data aad, yielding msg or an error on failure. If an error is returned, the user agent discards the message.

In pseudocode, this procedure is as follows:

Mandatory Algorithms All implementations of this specification MUST implement the following algorithms:

• the MLKEM768-X25519 KEM (0x647a)
• the TurboSHAKE128 KDF (0x0012)
• the AES-GCM AEAD (0x0001)

Upgrading from RFC 8291 The encryption scheme in identifies itself through the use of the encrypted "aes128gcm" content coding . 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 . 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 for details.

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 ), 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 encoding. This document exercises the options described in for replacing the encryption scheme. Any replacement of this encryption scheme can use those same techniques.

Handling Denial of Service A user agent that receives multiple unwanted messages, including messages that are discarded, might be the target of a denial of service attack. Such an attack might seek to drain batteries by activating the device radio or by creating messages that cannot be decrypted. A user agent can disable the associated subscription if they receive too many unwanted or invalid messages.

Multi-Target Ciphertexts in RFC 8291 RFC 8291 is vulnerable to an attack where a malicious application can construct a single ciphertext that will be decrypted successfully (that is, the AEAD tag is accepted) by multiple recipients, each of whom can receive a different plaintext. This format is also vulnerable to this attack.It is possible to apply the techniques from to achieve this outcome with only modest amounts of computation. This attack has no practical consequence, as it requires that the attacker have information that would allow it to send any message without any need to a share a ciphertext.

Media Type Security Push messages contain arbitrary content chosen by an application server. Though the format does not provide explicit signaling for the encapsulated media type, applications can include arbitrary data. How data is handled is determined by the application, which both sends and receives the data.

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 .

Type name:

application

Subtype name:

webpush-message

Required parameters:

N/A

Optional parameters:

N/A

Encoding considerations:

"binary"

Security considerations:

see

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

References Normative References Inria Rosenpass This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes a variant that authenticates possession of a pre-shared key. HPKE works for any combination of an asymmetric Key Encapsulation Mechanism (KEM), key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. This document provides instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA-2. This document obsoletes RFC 9180. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes a simple protocol for the delivery of real- time events to user agents. This scheme uses HTTP/2 server push. Cisco Selkie Cryptography Updating key exchange and public-key encryption protocols to resist attack by quantum computers is a high priority given the possibility of "harvest now, decrypt later" attacks. Hybrid Public Key Encryption (HPKE) is a widely-used public key encryption scheme based on combining a Key Encapsulation Mechanism (KEM), a Key Derivation Function (KDF), and an Authenticated Encryption with Associated Data (AEAD) scheme. In this document, we define KEM algorithms for HPKE based on both post-quantum KEMs and hybrid constructions of post- quantum KEMs with traditional KEMs, as well as a KDF based on SHA-3 that is suitable for use with these KEMs. When used with these algorithms, HPKE is resilient with respect to attacks by a quantum computer. This memo introduces a content coding for HTTP that allows message payloads to be encrypted. Informative References This document describes a message encryption scheme for the Web Push protocol. This scheme provides confidentiality and integrity for messages sent from an application server to a user agent. The advent of a cryptographically relevant quantum computer (CRQC) would render state-of-the-art, traditional public key algorithms deployed today obsolete, as the mathematical assumptions underpinning their security would no longer hold. To address this, protocols and infrastructure must transition to post-quantum algorithms, which are designed to resist both traditional and quantum attacks. This document explains why engineers need to be aware of and understand post-quantum cryptography (PQC), and it details the impact of CRQCs on existing systems and the challenges involved in transitioning to post-quantum algorithms. Unlike previous cryptographic updates, this shift may require significant protocol redesign due to the unique properties of post-quantum algorithms. This document describes a mechanism in Transport Layer Security (TLS) for encrypting a message under a server public key. This document describes Oblivious HTTP, a protocol for forwarding encrypted HTTP messages. Oblivious HTTP allows a client to make multiple requests to an origin server without that server being able to link those requests to the client or to identify the requests as having come from the same client, while placing only limited trust in the nodes used to forward the messages. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. This document sets out the problem statement for Interface to Network Security Functions (I2NSF) and outlines some companion use cases.

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