]> Google LLC

dschinazi.ietf@gmail.com

Web and Internet Transport HTTP oblivious HTTP configuration extension Oblivious HTTP (OHTTP) is a protocol for forwarding encrypted HTTP messages. It does not provide Perfect Forward Secrecy (PFS). Chunked OHTTP expands OHTTP to be suitable for longer-lived streams, but still does not offer PFS. Combined, this is leading sensitive traffic to de deployed at scale without PFS. This document proposes a solution. 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 HTTP Working Group mailing list (), which is archived at . Working Group information can be found at . Source for this draft and an issue tracker can be found at .

Introduction Oblivious HTTP () is a protocol for forwarding encrypted HTTP messages. It does not provide Perfect Forward Secrecy (PFS). Chunked OHTTP () expands OHTTP to be suitable for longer-lived streams, but still does not offer PFS. Unfortunately, providing a streaming abstraction over OHTTP makes it an attractive tool to provide privacy. This is leading application designers to build Remote Procedure Call (RPC) systems over this bidirectional stream, without realizing the security cost of losing PFS. When PFS is needed, a simpler solution is to avoid OHTTP entirely and instead use TLS () over HTTP CONNECT (see ). However, completing the TLS handshake takes a full round trip. For latency-sensitive applications, TLS 0-RTT (see ) can be used; that has the advantage of allowing non-PFS data for the client's first flight and then PFS for all other data. TLS 0-RTT does however have one relevant limitation: it allows a TLS server to link distinct 0-RTT requests by encoding identifying information in TLS session tickets. That makes TLS 0-RTT unsuitable for use cases that require unlinkability between requests, which Chunked OHTTP provides. This document proposes a solution that offers PFS to all data sent over OHTTP apart from the client's first flight. This provides privacy and security properties similar to TLS 0-RTT over HTTP CONNECT without losing the performance nor request-correlation-prevention properties of OHTTP. This mechanism is designed to be backwards compatible with unextended OHTTP.

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. This document uses SerializePublicKey(), DeserializePublicKey(), GenerateKeyPair(), SetupBaseS(), SetupBaseR(), Export(), Seal(), and Open() from .

Mechanism This mechanism relies on the generation of two more ephemeral key pairs per OHTTP request: one for the client (denoted skC, pkC) and one for the gateway (not denoted since it only exists inside of SetupBaseS()). The client starts by generating a second ephemeral key pair, using the same KEM it has selected for this request: The client then adds the serialized public key SerializePublicKey(pkC) to its Binary HTTP () request headers using the "OHTTP-PFS" header. That header is a Structured Header Field Item of type Byte Sequence as defined in . For example: The client then encrypts the Binary HTTP request following the procedure in . The gateway follows the procedure in that same section to recover the Binary HTTP request. The gateway then checks the request for the presence of the "ohttp-pfs" header to determine whether this extension is in use. If it is, it uses DeserializePublicKey() to parse the key then uses the HPKE receiver context (rctxt) from the OHTTP request as follows:

1. Export a secret, req_secret, from rctxt, using the string "OHTTP PFS Request Derivation" as the exporter_context parameter to context.Export; see . The length of this secret is Nsecret, the length of the shared secret for the KEM associated with context.
2. Build info2 by concatenating the ASCII-encoded string "OHTTP PFS Response", a zero byte, 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, and req_secret.
3. Create a sending HPKE context by invoking SetupBaseS() () with the public key of the client pkE and info2. This yields the context sctxt2 and an encapsulation key enc2.
4. Encrypt response by invoking the Seal() method on sctxt2 () with empty associated data aad, yielding ciphertext ct2.
5. Concatenate enc2 and ct2, yielding an Encapsulated Response, enc_response.

In pseudocode, this procedure is as follows: The client then reverses this process to extract the response. To decapsulate an Encapsulated Response, enc_response, it uses the HPKE sender context (sctxt) from the OHTTP request as follows:

1. Parses enc_request into enc2 and ct2 (indicated using the function parse() in pseudocode). Note that enc2 is of fixed-length, so there is no ambiguity in parsing this structure.
2. Export a secret, req_secret, from sctxt, using the string "OHTTP PFS Request Derivation" as the exporter_context parameter to context.Export; see . The length of this secret is Nsecret, the length of the shared secret for the KEM associated with context.
3. Build info2 by concatenating the ASCII-encoded string "OHTTP PFS Response", a zero byte, 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, and req_secret.
4. Create a receiving HPKE context, rctxt2, by invoking SetupBaseR() () with skE, enc2, and info2.
5. Decrypt ct2 by invoking the Open() method on rctxt2 (), with an empty associated data aad, yielding response or an error on failure.

In pseudocode, this procedure is as follows:

Media Type This mechanism uses a media type to differentiate the response from OHTTP responses without this extension. The "message/ohttp-res-pfs" is used to indicate that this extension is in use for the response. Gateways MAY respond with "message/ohttp-res" responses and use the response encoding described in if it does not PFS to be useful for the response. This allows saving computational resources, for example, if the response is a generic response with an error status code where no part of the response is tied to information in the request. If chunked OHTTP is in use, the response media type for this extension is "message/ohttp-chunked-res-pfs".

Chunked OHTTP Chunked OHTTP () is different in that the client can send more data after its first flight. For the reponse, all chunks are encrypted by the gateway using the sctxt2 context and decrypted by the client using the rctxt2 context, and using the AAD construction described in . For the request, there is a distinction between the first flight and subsequent chunks. The first chunks are encrypted using sctxt as described in . The client keeps using sctxt to send subsequent chunks until is receives the first chunk from the gateway. Once the client receives that, it creates rctxt2 (see ), and performs the following steps to switch to it for sending:

1. Encrypt an empty chunk, the encrypted last first flight chunk elff_chunk with sctxt using empty associated data aad. Send this chunk with a variable-length integer length prefix.
2. Use that as the associated data aad to encrypt the first PFS chunk sealed_pfs_chunk1 using sctxt2. Send that chunk with a variable-length integer length prefix.
3. Encrypt all subsequent non-final chunks with an empty AAD and a variable-length integer length prefix, using sctxt2.
4. Encrypt the final chunk with the AAD set to the ASCII-encoded string "final" and prefixed with a zero length, using sctxt2.

In pseudocode, this procedure is as follows: The gateway needs to handle this transition. It initially handles chunks using the procedure from . When the gateway decrypts a chunk with a zero-length plaintext, this means that the client is transitioning to the PFS context (zero-length plaintext non-final chunks are explicitly disallowed in chunked OHTTP without PFS, see ). When the gateway decrypts its first zero-length plaintext using the rctxt context, it acts on the ciphertext elff_chunk as follows:

1. Save elff_chunk until the next chunk is received.
2. Wait for and parse the next variable-length integer length-prefixed chunk, sealed_pfs_chunk1.
3. Decrypt sealed_pfs_chunk1 using rctxt2 with the associated data aad set to elff_chunk.
4. Keep decrypting variable-length integer length-prefixed chunks with the associated data aad empty.
5. When encoutering a variable-length integer length-prefix set to zero, decrypt the final chunk using the associated data aad set to the ASCII-encoded string "final".

Security Considerations The security considerations described in and apply to this document as well. As mentioned in , interactivity can cause the gateway to learn the round trip to the client. The PFS mechanism can increase that risk if the client sends its last first flight chunk (see ) immediately when it receives the gateway's first flight. To avoid this, clients MUST NOT send their last first flight chunk until they have more application data to send to the gateway.

IANA Considerations

OHTTP-PFS HTTP Header Field This document requests IANA to register the following entry in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" maintained at <>:

Field Name:

OHTTP-PFS

Template:

None

Status:

provisional (permanent if this document is approved)

Reference:

This document

Comments:

None

Media Types This document requests IANA to register the "message/ohttp-res-pfs" and "message/ohttp-chunked-res-pfs" media types in the "Media Types Registry" maintained at <>. The subtype names are "ohttp-res-pfs" and "ohttp-chunked-res-pfs", respectively. The other fields have the following values:

Type name:

message

Required parameters:

N/A

Optional parameters:

N/A

Encoding considerations:

"binary"

Security considerations:

see

Interoperability considerations:

N/A

Published specification:

This document

Applications that use this media type:

Oblivious HTTP and applications that use Oblivious HTTP use this media type to identify encapsulated binary HTTP responses when using the PFS extension.

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 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. Apple Mozilla This document defines a variant of the Oblivious HTTP message format that allows chunks of requests and responses to be encrypted and decrypted before the entire request or response is processed. This allows incremental processing of Oblivious HTTP messages, which is particularly useful for handling large messages or systems that process messages slowly. 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 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 three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide 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 SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. This document defines a binary format for representing HTTP messages. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields. This document obsoletes RFC 8941. Informative References This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230.

Acknowledgments Thank you to Martin Thomson for asking me to write this draft, and to Chris Wood for supporting it more than two years before it was written. Thanks to Martin Thomson for reviewing this document.