Chunked Oblivious HTTP Messages
draft-ietf-ohai-chunked-ohttp-00

OHAI Working Group                                              T. Pauly
Internet-Draft                                                     Apple
Intended status: Standards Track                              M. Thomson
Expires: 13 August 2024                                          Mozilla
                                                        10 February 2024

                    Chunked Oblivious HTTP Messages
                    draft-ietf-ohai-chunked-ohttp-00

Abstract

   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.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ohai-chunked-ohttp/.

   Discussion of this document takes place on the OHAI Working Group
   mailing list (mailto:ohai@ietf.org), which is archived at
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Chunked Request and Response Media Types  . . . . . . . . . .   4
   4.  Request Format  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Response Format . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Encapsulation of Chunks . . . . . . . . . . . . . . . . . . .   6
     6.1.  Request Encapsulation . . . . . . . . . . . . . . . . . .   6
     6.2.  Response Encapsulation  . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     7.1.  Interactivity and Privacy . . . . . . . . . . . . . . . .   8
     7.2.  message/ohttp-chunked-req Media Type  . . . . . . . . . .   9
     7.3.  message/ohttp-chunked-res Media Type  . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Oblivious HTTP [OHTTP] defines a system for sending HTTP requests and
   responses as encrypted messages.  Clients send requests via a relay
   to a gateway, which is able to decrypt and forward the request to a
   target server.  Responses are encrypted with an ephemeral symmetric
   key by the gateway and sent back to the client via the relay.  The
   messages are protected with Hybrid Public Key Encryption (HPKE;
   [HPKE]), and are intended to prevent the gateway from linking any two
   independent requests to the same client.

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   The definition of Oblivious HTTP in [OHTTP] encrypts messages such
   that entire request and response bodies need to be received before
   any of the content can be decrypted.  This is well-suited for many of
   the use cases of Oblivious HTTP, such as DNS queries or metrics
   reporting.

   However, some applications of Oblivious HTTP can benefit from being
   able to encrypt and decrypt parts of the messages in chunks.  If a
   request or response can be processed by a receiver in separate parts,
   and is particularly large or will be generated slowly, then sending a
   series of encrypted chunks can improve the performance of
   applications.

   Incremental delivery of responses allows an Oblivious Gateway
   Resource to provide Informational (1xx) responses (Section 15.2 of
   [HTTP]).

   This document defines an optional message format for Oblivious HTTP
   that supports the progressive creation and processing of both
   requests and responses.  New media types are defined for this
   purpose.

1.1.  Applicability

   Like the non-chunked variant, chunked Oblivious HTTP has limited
   applicability as described in Section 2.1 of [OHTTP], and requires
   the use of a willing Oblivious Relay Resource and Oblivious Gateway
   Resource.

   Chunked Oblivious HTTP is intended to be used in cases for where the
   privacy properties of Oblivious HTTP are needed — specifically,
   removing linkage at the transport layer between separate HTTP
   requests — but incremental processing is also needed for performance
   or functionality.

   One specific functional capability that requires chunked Oblivious
   HTTP is support for Informational (1xx) responses (Section 15.2 of
   [HTTP]).

   In order to be useful, the content of chunked Oblivious HTTP needs to
   be possible to process incrementally.  Since incremental processing
   means that the message might end up being truncated, for example in
   the case of an error on the underlying transport, applications also
   need to be prepared to safely handle incomplete messages (see
   Section 7 for more discussion).  Applications that use the
   Indeterminate format of Binary HTTP (Section 3.2 of [BHTTP]) are
   well-suited to using chunked Oblivious HTTP.

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   Chunked Oblivious HTTP is not intended to be used for long-lived
   sessions between clients and servers that might build up state, or as
   a replacement for a proxied TLS session.

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.

   Notational conventions from [OHTTP] are used in this document.

3.  Chunked Request and Response Media Types

   Chunked Oblivious HTTP defines different media than the non-chunked
   variant.  These media types are "message/ohttp-chunked-req" (defined
   in Section 7.2) and "message/ohttp-chunked-res" (defined in
   Section 7.3).

4.  Request Format

   Chunked OHTTP requests start with the same header as used for the
   non-chunked variant, which consists of a key ID, algorithm IDs, and
   the KEM shared secret.  This header is followed by chunks of data
   protected with HPKE, each of which is preceded by a variable-length
   integer (as defined in Section 16 of [QUIC]) that indicates the
   length of the chunk.  The final chunk is preceded by a length field
   with the value 0, which means the chunk extends to the end of the
   outer stream.

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   Chunked Encapsulated Request {
     Chunked Request Header (56 + 8 * Nenc),
     Chunked Request Chunks (..),
   }

   Chunked Request Header {
     Key Identifier (8),
     HPKE KEM ID (16),
     HPKE KDF ID (16),
     HPKE AEAD ID (16),
     Encapsulated KEM Shared Secret (8 * Nenc),
   }

   Chunked Request Chunks {
     Non-Final Request Chunk (..),
     Final Request Chunk Indicator (i) = 0,
     HPKE-Protected Final Chunk (..),
   }

   Non-Final Request Chunk {
     Length (i) = 1..,
     HPKE-Protected Chunk (..),
   }

               Figure 1: Chunked Encapsulated Request Format

   The content of the HPKE-protected chunks is defined in Section 6.1.

5.  Response Format

   Chunked OHTTP responses start with a nonce, followed by chunks of
   data protected with an AEAD.  Each chunk is preceded by a variable-
   length integer that indicates the length of the chunk.  The final
   chunk is preceded by a length field with the value 0, which means the
   chunk extends to the end of the outer stream.

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   Chunked Encapsulated Response {
     Response Nonce (Nk),
     Chunked Response Chunks (..),
   }

   Chunked Response Chunks {
     Non-Final Response Chunk (..),
     Final Response Chunk Indicator (i) = 0,
     AEAD-Protected Final Response Chunk (..),
   }

   Non-Final Response Chunk {
     Length (i) = 1..,
     AEAD-Protected Chunk (..),
   }

               Figure 2: Chunked Encapsulated Response Format

6.  Encapsulation of Chunks

   The encapsulation of chunked Oblivious HTTP requests and responses
   uses the same approach as the non-chunked variant, with the
   difference that the body of requests and responses are sealed and
   opened in chunks, instead of as a whole.

   The AEAD that protects both requests and responses protects
   individual chunks from modification or truncation.  Additionally,
   chunk authentication protects two other pieces of information:

   1.  the order of the chunks (the sequence number of each chunk),
       which is included in the nonce of each chunk.

   2.  which chunk is the final chunk, which is indicated by a sentinel
       in the AAD of the final chunk.

   The format of the outer packaging that carries the chunks (the length
   prefix for each chunk specifically) is not explicitly authenticated.
   This allows the chunks to be transported by alternative means, and
   still be valid as long as the order and finality are preserved.  In
   particular, the variable-length encoding used for lengths allows for
   different expressions of the same value, where the choice between
   equivalent encodings is not authenticated.

6.1.  Request Encapsulation

   For requests, the setup of the HPKE context and the encrypted request
   header is the same as the non-chunked variant.  This is the Chunked
   Request Header defined in Section 4.

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   hdr = concat(encode(1, key_id),
                encode(2, kem_id),
                encode(2, kdf_id),
                encode(2, aead_id))
   info = concat(encode_str("message/bhttp chunked request"),
                 encode(1, 0),
                 hdr)
   enc, sctxt = SetupBaseS(pkR, info)
   enc_request_hdr = concat(hdr, enc)

   Each chunk is sealed using the HPKE context.  For non-final chunks,
   the AAD is empty.

   sealed_chunk = sctxt.Seal("", chunk)
   sealed_chunk_len = varint_encode(len(sealed_chunk))
   non_final_chunk = concat(sealed_chunk_len, sealed_chunk)

   The final chunk in a request uses an AAD of the string "final".

   sealed_final_chunk = sctxt.Seal("final", chunk)
   sealed_final_chunk_len = varint_encode(len(sealed_final_chunk))
   final_chunk = concat(sealed_final_chunk_len, sealed_final_chunk)

   HPKE already maintains a sequence number for sealing operations as
   part of the context, so the order of chunks is protected.

6.2.  Response Encapsulation

   For responses, the first piece of data sent back is the response
   nonce, as in the non-chunked variant.

   entropy_len = max(Nn, Nk)
   response_nonce = random(entropy_len)

   Each chunk is sealed using the same AEAD key and AEAD nonce that are
   derived for the non-chunked variant, which are calculated as follows:

  secret = context.Export("message/bhttp chunked response", entropy_len)
  salt = concat(enc, response_nonce)
  prk = Extract(salt, secret)
  aead_key = Expand(prk, "key", Nk)
  aead_nonce = Expand(prk, "nonce", Nn)

   The sender also maintains a counter of chunks, which is set to 0 for
   the first chunk an incremented by 1 after encoding each chunk.

   counter = 0

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   The AEAD nonce is XORed with the counter for encrypting (and
   decrypting) each chunk.  For non-final chunks, the AAD is empty.

   chunk_nonce = aead_nonce XOR encode(Nn, counter)
   sealed_chunk = Seal(aead_key, chunk_nonce, "", chunk)
   sealed_chunk_len = varint_encode(len(sealed_chunk))
   non_final_chunk = concat(sealed_chunk_len, sealed_chunk)
   counter++

   The final chunk in a response uses an AAD of the string "final".

   chunk_nonce = aead_nonce XOR encode(Nn, counter)
   sealed_final_chunk = Seal(aead_key, chunk_nonce, "final", chunk)
   sealed_final_chunk_len = varint_encode(len(sealed_final_chunk))
   final_chunk = concat(sealed_final_chunk_len, sealed_final_chunk)

7.  Security Considerations

   The primary advantage of a chunked encoding is that chunked requests
   or responses can be generated or processed incrementally.  However,
   for a recipient in particular, processing an incomplete message can
   have security consequences.

   The potential for message truncation is not a new concern for HTTP.
   All versions of HTTP provide incremental delivery of messages.  For
   this use of Oblivious HTTP, incremental processing that might result
   in side-effects demands particular attention as Oblivious HTTP does
   not provide strong protection against replay attacks; see Section 6.5
   of [OHTTP].  Truncation might be the result of interference at the
   network layer, or by a malicious Oblivious Relay Resource.

   Endpoints that receive chunked messages can perform early processing
   if the risks are understood and accepted.  Conversely, endpoints that
   depend on having a complete message MUST ensure that they do not
   consider a message complete until having received a chunk with a
   0-valued length prefix, which was successfully decrypted using the
   expected sentinel value, "final", in the AAD.

7.1.  Interactivity and Privacy

   Without chunking, Oblivious HTTP involves a single request and
   response, with no further interactivity.  Using a chunked variant at
   both Client and Oblivious Gateway Resource creates the possibility
   that an exchange could lead to multiple rounds of interaction.
   Information from early chunks from a peer could influence how an
   endpoint constructs later chunks of their message.

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   An Oblivious Gateway Resource could be able to observe the round trip
   time to the Client if the Client conditions the timing or content of
   chunks on what it receives in a response.

   Client implementations therefore need to be aware of the possibility
   that processing chunks might result in observable interactivity that
   could reduces the privacy protection that the protocol could
   otherwise provide.  Interactivity that is deliberate might be
   acceptable.  For instance, the 100-continue feature in HTTP, which
   has the client withhold the body of a request until it receives a 100
   Informational response, is not possible without chunked encoding.
   This highlights the risks involved in the use of this chunked
   encoding to adapt an existing HTTP-based interaction to use Oblivious
   HTTP as such an adaptation might not achieve expected privacy
   outcomes.  In order to prevent the Oblivious Gateway Resource from
   observing the round trip time to the client, client implementations
   can choose to not base the sending of request chunks based on
   received response chunks.  These interactions can still benefit from
   chunked processing, without incurring additional observability risks.
   # IANA Considerations

   This document updates the "Media Types" registry at
   https://iana.org/assignments/media-types
   (https://iana.org/assignments/media-types) to add the media types
   "message/ohttp-chunked-req" (Section 7.2), and "message/ohttp-
   chunked-res" (Section 7.3), following the procedures of [RFC6838].

7.2.  message/ohttp-chunked-req Media Type

   The "message/ohttp-chunked-req" identifies an encrypted binary HTTP
   request that is transmitted or processed in chunks.  This is a binary
   format that is defined in Section 4.

   Type name:  message
   Subtype name:  ohttp-chunked-req
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  "binary"
   Security considerations:  see Section 7
   Interoperability considerations:  N/A
   Published specification:  this specification
   Applications that use this media type:  Oblivious HTTP and
      applications that use Oblivious HTTP use this media type to
      identify encapsulated binary HTTP requests that are incrementally
      generated or processed.
   Fragment identifier considerations:  N/A
   Additional information:  Magic number(s):  N/A
                            Deprecated alias names for this type:  N/A

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                            File extension(s):  N/A
                            Macintosh file type code(s):  N/A
   Person and email address to contact for further information:  see Aut
      hors' Addresses section
   Intended usage:  COMMON
   Restrictions on usage:  N/A
   Author:  see Authors' Addresses section
   Change controller:  IETF

7.3.  message/ohttp-chunked-res Media Type

   The "message/ohttp-chunked-res" identifies an encrypted binary HTTP
   response that is transmitted or processed in chunks.  This is a
   binary format that is defined in Section 5.

   Type name:  message
   Subtype name:  ohttp-chunked-res
   Required parameters:  N/A
   Optional parameters:  N/A
   Encoding considerations:  "binary"
   Security considerations:  see Section 7
   Interoperability considerations:  N/A
   Published specification:  this specification
   Applications that use this media type:  Oblivious HTTP and
      applications that use Oblivious HTTP use this media type to
      identify encapsulated binary HTTP responses that are incrementally
      generated or processed.
   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 Aut
      hors' Addresses section
   Intended usage:  COMMON
   Restrictions on usage:  N/A
   Author:  see Authors' Addresses section
   Change controller:  IETF

8.  References

8.1.  Normative References

   [BHTTP]    Thomson, M. and C. A. Wood, "Binary Representation of HTTP
              Messages", RFC 9292, DOI 10.17487/RFC9292, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9292>.

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   [HPKE]     Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

   [OHTTP]    Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
              Progress, Internet-Draft, draft-ietf-ohai-ohttp-10, 25
              August 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-ohai-ohttp-10>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [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/rfc/rfc2119>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/rfc/rfc6838>.

   [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/rfc/rfc8174>.

8.2.  Informative References

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

Acknowledgments

   TODO acknowledgements.

Authors' Addresses

   Tommy Pauly
   Apple
   Email: tpauly@apple.com

   Martin Thomson
   Mozilla

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   Email: mt@lowentropy.net

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