Oblivious HTTP
draft-thomson-http-oblivious-00
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| Authors | Martin Thomson , Christopher A. Wood | ||
| Last updated | 2021-01-27 | ||
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draft-thomson-http-oblivious-00
HTTPBIS M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track C.A. Wood
Expires: 1 August 2021 Cloudflare
28 January 2021
Oblivious HTTP
draft-thomson-http-oblivious-00
Abstract
This document describes a system for the forwarding of encrypted HTTP
messages. This allows clients to make requests of servers without
the server being able to link requests to other requests from the
same client.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the HTTP Working Group
mailing list (http@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/http/.
Source for this draft and an issue tracker can be found at
https://github.com/unicorn-wg/oblivious-http.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 1 August 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Key Configuration . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Key Configuration Encoding . . . . . . . . . . . . . . . 7
4.2. Key Configuration Media Type . . . . . . . . . . . . . . 7
5. HPKE Encapsulation . . . . . . . . . . . . . . . . . . . . . 8
5.1. HPKE Encapsulation of Requests . . . . . . . . . . . . . 9
5.2. HPKE Encapsulation of Responses . . . . . . . . . . . . . 11
6. HTTP Usage . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Informational Responses . . . . . . . . . . . . . . . . . 13
6.2. Errors . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Media Types . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. message/ohttp-req Media Type . . . . . . . . . . . . . . 14
7.2. message/ohttp-res Media Type . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.2. Proxy Responsibilities . . . . . . . . . . . . . . . . . 17
8.2.1. Denial of Service . . . . . . . . . . . . . . . . . . 18
8.2.2. Linkability Through Traffic Analysis . . . . . . . . 18
8.3. Server Responsibilities . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Complete Example of a Request and Response . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
The act of making a request using HTTP reveals information about the
client identity to a server. Though the content of requests might
reveal information, that is information under the control of the
client. In comparison, the source address on the connection reveals
information that a client has only limited control over.
Even where an IP address is not directly attributed to an individual,
the use of an address over time can be used to correlate requests.
Servers are able to use this information to assemble profiles of
client behavior, from which they can make inferences about the people
involved. The use of persistent connections to make multiple
requests improves performance, but provides servers with additional
certainty about the identity of clients in a similar fashion.
Use of an HTTP proxy can provide a degree of protection against
servers correlating requests. Systems like virtual private networks
or the Tor network [Dingledine2004], provide other options for
clients.
Though the overhead imposed by these methods varies, the cost for
each request is significant. Preventing request linkability requires
that each request use a completely new TLS connection to the server.
At a minimum, this requires an additional round trip to the server in
addition to that required by the request. In addition to having high
latency, there are significant secondary costs, both in terms of the
number of additional bytes exchanged and the CPU cost of
cryptographic computations.
This document describes a method of encapsulation for binary HTTP
messages [BINARY] using Hybrid Public Key Encryption (HPKE; [HPKE]).
This protects the content of both requests and responses and enables
a deployment architecture that can separate the identity of a
requester from the request.
Though this scheme requires that servers and proxies explicitly
support it, this design represents a performance improvement over
options that perform just one request in each connection. With
limited trust placed in the proxy (see Section 8), clients are
assured that requests are not uniquely attributed to them or linked
to other requests.
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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.
Encapsulated Request: An HTTP request that is encapsulated in an
HPKE-encrypted message; see Section 5.1.
Encapsulated Response: An HTTP response that is encapsulated in an
HPKE-encrypted message; see Section 5.2.
Oblivious Proxy Resource: An intermediary that forwards requests and
responses between clients and a single oblivious request resource.
Oblivious Request Resource: A resource that can receive an
encapsulated request, extract the contents of that request,
forward it to an oblivious target resource, receive a response,
encapsulate that response, then return that response.
Oblivious Target Resource: The resource that is the target of an
encapsulated request. This resource logically handles only
regular HTTP requests and responses and so might be ignorant of
the use of oblivious HTTP to reach it.
This draft includes pseudocode that uses the functions and
conventions defined in [HPKE].
Encoding an integer to a sequence of bytes in network byte order is
described using the function "encode(n, v)", where "n" is the number
of bytes and "v" is the integer value. The function "len()" returns
the length of a sequence of bytes.
Formats are described using notation from Section 1.3 of [QUIC].
3. Overview
A client learns the following:
* The identity of an oblivious request resource. This might include
some information about oblivious target resources that the
oblivious request resource supports.
* The details of an HPKE public key that the oblivious request
resource accepts, including an identifier for that key and the
HPKE algorithms that are used with that key.
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* The identity of an oblivious proxy resource that will forward
encapsulated requests and responses to the oblivious request
resource.
This information allows the client to make a request of an oblivious
target resource without that resource having only a limited ability
to correlate that request with the client IP or other requests that
the client might make to that server.
+---------+ +----------+ +----------+ +----------+
| Client | | Proxy | | Request | | Target |
| | | Resource | | Resource | | Resource |
+---------+ +----------+ +----------+ +----------+
| | | |
| Encapsulated | | |
| Request | | |
|----------------->| Encapsulated | |
| | Request | |
| |------------------>| Request |
| | |-------------->|
| | | |
| | | Response |
| | Encapsulated |<--------------|
| | Response | |
| Encapsulated |<------------------| |
| Response | | |
|<-----------------| | |
| | | |
Figure 1: Overview of Oblivious HTTP
In order to make a request to an oblivious target resource, the
following steps occur, as shown in Figure 1:
1. The client constructs an HTTP request for an oblivious target
resource.
2. The client encodes the HTTP request in a binary HTTP message and
then encapsulates that message using HPKE and the process from
Section 5.1.
3. The client sends a POST request to the oblivious proxy resource
with the encapsulated request as the content of that message.
4. The oblivious proxy resource forwards this request to the
oblivious request resource.
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5. The oblivious request resource receives this request and removes
the HPKE protection to obtain an HTTP request.
6. The oblivious request resource makes an HTTP request that
includes the target URI, method, fields, and content of the
request it acquires.
7. The oblivious target resource answers this HTTP request with an
HTTP response.
8. The oblivious request resource encapsulates the HTTP response
following the process in Section 5.2 and sends this in response
to the request from the oblivious proxy resource.
9. The oblivious proxy resource forwards this response to the
client.
10. The client removes the encapsulation to obtain the response to
the original request.
4. Key Configuration
A client needs to acquire information about the key configuration of
the oblivious request resource in order to send encapsulated
requests.
In order to ensure that clients do not encapsulate messages that
other entities can intercept, the key configuration MUST be
authenticated and have integrity protection. One way to ensure
integrity for key configuration is for the oblivious request resource
to serve content to the client directly, using HTTPS and the
"application/ohttp-keys" media type; see Section 4.2.
Specifying a format for expressing the information a client needs to
construct an encapsulated request ensures that different client
implementations can be configured in the same way. This also enables
advertising key configurations in a consistent format.
A client might have multiple key configurations to select from when
encapsulating a request. Clients are responsible for selecting a
preferred key configuration from those it supports. Clients need to
consider both the key encapsulation method (KEM) and the combinations
of key derivation function (KDF) and authenticated encryption with
associated data (AEAD) in this decision.
Evolution of the key configuration format is supported through the
definition of new formats that are identified by new media types.
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4.1. Key Configuration Encoding
A single key configuration consists of a key identifier, a public
key, an identifier for the KEM that the public key uses, and a set
HPKE symmetric algorithms. Each symmetric algorithm consists of an
identifier for a KDF and an identifier for an AEAD.
Figure 2 shows a single key configuration, KeyConfig, that is
expressed using the TLS syntax; see Section 3 of [TLS].
opaque HpkePublicKey<1..2^16-1>;
uint16 HpkeKemId;
uint16 HpkeKdfId;
uint16 HpkeAeadId;
struct {
HpkeKdfId kdf_id;
HpkeAeadId aead_id;
} HpkeSymmetricAlgorithms;
struct {
uint8 key_id;
HpkeKemId kem_id;
HpkePublicKey public_key;
HpkeSymmetricAlgorithms cipher_suites<4..2^16-4>;
} KeyConfig;
Figure 2: A Single Key Configuration
The types HpkeKemId, HpkeKdfId, and HpkeAeadId identify a KEM, KDF,
and AEAD respectively. The definitions for these identifiers and the
semantics of the algorithms they identify can be found in [HPKE].
4.2. Key Configuration Media Type
The "application/ohttp-keys" format is a media type that identifies a
serialized collection of key configurations. The content of this
media type comprises one or more key configuration encodings (see
Section 4.1) that are concatenated.
Type name: application
Subtype name: ohttp-keys
Required parameters: N/A
Optional parameters: None
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Encoding considerations: only "8bit" or "binary" is permitted
Security considerations: see Section 8
Interoperability considerations: N/A
Published specification: this specification
Applications that use this media type: N/A
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: IESG
5. HPKE Encapsulation
HTTP message encapsulation uses HPKE for request and response
encryption. An encapsulated HTTP message includes the following
values:
1. A binary-encoded HTTP message; see [BINARY].
2. Padding of arbitrary length which MUST contain all zeroes.
The encoding of an HTTP message is as follows:
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Plaintext Message {
Message Length (i),
Message (..),
Padding Length (i),
Padding (..),
}
An Encapsulated Request is comprised of a length-prefixed key
identifier and a HPKE-protected request message. HPKE protection
includes an encapsulated KEM shared secret (or "enc"), plus the AEAD-
protected request message. An Encapsulated Request is shown in
Figure 3. Section 5.1 describes the process for constructing and
processing an Encapsulated Request.
Encapsulated Request {
Key Identifier (8),
KDF Identifier (16),
AEAD Identifier (16),
Encapsulated KEM Shared Secret (..),
AEAD-Protected Request (..),
}
Figure 3: Encapsulated Request
Responses are bound to responses and so consist only of AEAD-
protected content. Section 5.2 describes the process for
constructing and processing an Encapsulated Response.
Encapsulated Response {
Nonce (Nk),
AEAD-Protected Response (..),
}
Figure 4: Encapsulated Response
The size of the Nonce field in an Encapsulated Response corresponds
to the size of an AEAD key for the corresponding HPKE ciphersuite.
5.1. HPKE Encapsulation of Requests
Clients encapsulate a request "request" using values from a key
configuration:
* the key identifier from the configuration, "keyID",
* the public key from the configuration, "pkR", and
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* a selected combination of KDF, identified by "kdfID", and AEAD,
identified by "aeadID".
The client then constructs an encapsulated request, "enc_request", as
follows:
1. Compute an HPKE context using "pkR", yielding "context" and
encapsulation key "enc".
2. Construct associated data, "aad", by concatenating the values of
"keyID", "kdfID", and "aeadID", as 8-, 16- and 16-bit integers
respectively, each in network byte order.
3. Encrypt (seal) "request" with "aad" as associated data using
"context", yielding ciphertext "ct".
4. Concatenate the values of "aad", "enc", and "ct", yielding an
Encapsulated Request "enc_request".
Note that "enc" is of fixed-length, so there is no ambiguity in
parsing this structure.
In pseudocode, this procedure is as follows:
enc, context = SetupBaseS(pkR, "request")
aad = concat(encode(1, keyID),
encode(2, kdfID),
encode(2, aeadID))
ct = context.Seal(aad, request)
enc_request = concat(aad, enc, ct)
Servers decrypt an Encapsulated Request by reversing this process.
Given an Encapsulated Request "enc_request", a server:
1. Parses "enc_request" into "keyID", "kdfID", "aeadID", "enc", and
"ct" (indicated using the function "parse()" in pseudocode). The
server is then able to find the HPKE private key, "skR",
corresponding to "keyID".
a. If "keyID" does not identify a key, the server returns an
error.
b. If "kdfID" and "aeadID" identify a combination of KDF and
AEAD that the server is unwilling to use with "skR", the server
returns an error.
2. Compute an HPKE context using "skR" and the encapsulated key
"enc", yielding "context".
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3. Construct additional associated data, "aad", from "keyID",
"kdfID", and "aeadID" or as the first five bytes of
"enc_request".
4. Decrypt "ct" using "aad" as associated data, yielding "request"
or an error on failure. If decryption fails, the server returns
an error.
In pseudocode, this procedure is as follows:
keyID, kdfID, aeadID, enc, ct = parse(enc_request)
aad = concat(encode(1, keyID),
encode(2, kdfID),
encode(2, aeadID))
context = SetupBaseR(enc, skR, "request")
request, error = context.Open(aad, ct)
5.2. HPKE Encapsulation of Responses
Given an HPKE context "context", a request message "request", and a
response "response", servers generate an Encapsulated Response
"enc_response" as follows:
1. Export a secret "secret" from "context", using the string
"response" as context. The length of this secret is "max(Nn,
Nk)", where "Nn" and "Nk" are the length of AEAD key and nonce
associated with "context".
2. Generate a random value of length "max(Nn, Nk)" bytes, called
"response_nonce".
3. Extract a pseudorandom key "prk" using the "Extract" function
provided by the KDF algorithm associated with "context". The
"ikm" input to this function is "secret"; the "salt" input is the
concatenation of "enc" (from "enc_request") and "response_nonce"
4. Use the "Expand" function provided by the same KDF to extract an
AEAD key "key", of length "Nk" - the length of the keys used by
the AEAD associated with "context". Generating "key" uses a
label of "key".
5. Use the same "Expand" function to extract a nonce "nonce" of
length "Nn" - the length of the nonce used by the AEAD.
Generating "nonce" uses a label of "nonce".
6. Encrypt "response", passing the AEAD function Seal the values of
"key", "nonce", empty "aad", and a "pt" input of "request", which
yields "ct".
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7. Concatenate "response_nonce" and "ct", yielding an Encapsulated
Response "enc_response". Note that "response_nonce" is of fixed-
length, so there is no ambiguity in parsing either
"response_nonce" or "ct".
In pseudocode, this procedure is as follows:
secret = context.Export("response", Nk)
response_nonce = random(max(Nn, Nk))
salt = concat(enc, response_nonce)
prk = Extract(salt, secret)
aead_key = Expand(secret, "key", Nk)
aead_nonce = Expand(secret, "nonce", Nn)
ct = Seal(aead_key, aead_nonce, "", response)
enc_response = concat(response_nonce, ct)
Clients decrypt an Encapsulated Request by reversing this process.
That is, they first parse "enc_response" into "response_nonce" and
"ct". They then follow the same process to derive values for
"aead_key" and "aead_nonce".
The client uses these values to decrypt "ct" using the Open function
provided by the AEAD. Decrypting might produce an error, as follows:
reponse, error = Open(aead_key, aead_nonce, "", ct)
6. HTTP Usage
A client interacts with the oblivious proxy resource by constructing
an encapsulated request. This encapsulated request is included as
the content of a POST request to the oblivious proxy resource. This
request MUST only contain those fields necessary to carry the
encapsulated request: a method of POST, a target URI of the oblivious
proxy resource, a header field containing the content type (see
(Section 7), and the encapsulated request as the request content.
Clients MAY include fields that do not reveal information about the
content of the request, such as Alt-Used [ALT-SVC], or information
that it trusts the oblivious proxy resource to remove, such as fields
that are listed in the Connection header field.
The oblivious proxy resource interacts with the oblivious request
resource by constructing a request using the same restrictions as the
client request, except that the target URI is the oblivious request
resource. The content of this request is copied from the client.
The oblivious proxy resource MUST NOT add information about the
client to this request.
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When a response is received from the oblivious request resource, the
oblivious proxy resource forwards the response according to the rules
of an HTTP proxy; see Section 7.6 of [HTTP].
An oblivious request resource, if it receives any response from the
oblivious target resource, sends a single 200 response containing the
encapsulated response. Like the request from the client, this
response MUST only contain those fields necessary to carry the
encapsulated response: a 200 status code, a header field indicating
the content type, and the encapsulated response as the response
content. As with requests, additional fields MAY be used to convey
information that does not reveal information about the encapsulated
response.
An oblivious request resource acts as a gateway for requests to the
oblivious target resource (see Section 7.6 of [HTTP]). The one
exception is that any information it might forward in a response MUST
be encapsulated, unless it is responding to errors it detects before
removing encapsulation of the request; see Section 6.2.
6.1. Informational Responses
This encapsulation does not permit progressive processing of
responses. Though the binary HTTP response format does support the
inclusion of informational (1xx) status codes, the AEAD encapsulation
cannot be removed until the entire message is received.
In particular, the Expect header field with 100-continue (see
Section 10.1.1 of [HTTP]) cannot be used. Clients MUST NOT construct
a request that includes a 100-continue expectation; the oblivious
request resource MUST generate an error if a 100-continue expectation
is received.
6.2. Errors
A server that receives an invalid message for any reason MUST
generate an HTTP response with a 4xx status code.
Errors detected by the oblivious proxy resource and errors detected
by the oblivious request resource before removing protection
(including being unable to remove encapsulation for any reason)
result in the status code being sent without protection in response
to the POST request made to that resource.
Errors detected by the oblivious request resource after successfully
removing encapsulation and errors detected by the oblivious target
resource MUST be sent in an encapsulated response.
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7. Media Types
Media types are used to identify encapsulated requests and responses.
Evolution of the format of encapsulated requests and responses is
supported through the definition of new formats that are identified
by new media types.
7.1. message/ohttp-req Media Type
The "message/ohttp-req" identifies an encapsulated binary HTTP
request. This is a binary format that is defined in Section 5.1.
Type name: message
Subtype name: ohttp-req
Required parameters: N/A
Optional parameters: None
Encoding considerations: only "8bit" or "binary" is permitted
Security considerations: see Section 8
Interoperability considerations: N/A
Published specification: this specification
Applications that use this media type: N/A
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
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Author: see Authors' Addresses section
Change controller: IESG
7.2. message/ohttp-res Media Type
The "message/ohttp-res" identifies an encapsulated binary HTTP
response. This is a binary format that is defined in Section 5.2.
Type name: message
Subtype name: ohttp-res
Required parameters: N/A
Optional parameters: None
Encoding considerations: only "8bit" or "binary" is permitted
Security considerations: see Section 8
Interoperability considerations: N/A
Published specification: this specification
Applications that use this media type: N/A
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: IESG
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8. Security Considerations
In this design, a client wishes to make a request of a server that is
authoritative for the oblivious target resource. The client wishes
to make this request without linking that request with either:
1. The identity at the network and transport layer of the client
(that is, the client IP address and TCP or UDP port number the
client uses to create a connection).
2. Any other request the client might have made in the past or might
make in the future.
In order to ensure this, the client selects a proxy (that serves the
oblivious proxy resource) that it trusts will protect this
information by forwarding the encapsulated request and response
without passing the server (that serves the oblivious request
resource).
In this section, a deployment where there are three entities is
considered:
* A client makes requests and receives responses
* A proxy operates the oblivious proxy resource
* A server operates both the oblivious request resource and the
oblivious target resource
To achieve the stated privacy goals, the oblivious proxy resource
cannot be operated by the same entity as the oblivious request
resource. However, colocation of the oblivious request resource and
oblivious target resource simplifies the interactions between those
resources without affecting client privacy.
8.1. Client
Clients MUST ensure that the key configuration they select for
generating encapsulated requests is integrity protected and
authenticated so that it can be attributed to the oblivious request
resource; see Section 4.
Clients MUST NOT include identifying information in the request that
is encapsulated.
Clients cannot carry connection-level state between requests as they
only establish direct connections to the proxy responsible for the
oblivious proxy resource. However, clients need to ensure that they
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construct requests without any information gained from previous
requests. Otherwise, the server might be able to use that
information to link requests. Cookies [COOKIES] are the most obvious
feature that MUST NOT be used by clients. However, clients need to
include all information learned from requests, which could include
the identity of resources.
Clients MUST generate a new HPKE context for every request, using a
good source of entropy ([RANDOM]) for generating keys. Key reuse not
only risks requests being linked, reuse could expose request and
response contents to the proxy.
The request the client sends to the oblivious proxy resource only
requires minimal information; see Section 6. The request that
carries the encapsulated request and is sent to the oblivious proxy
resource MUST NOT include identifying information unless the client
ensures that this information is removed by the proxy. A client MAY
include information only for the oblivious proxy resource in header
fields identified by the Connection header field if it trusts the
proxy to remove these as required by Section 7.6.1 of [HTTP]. The
client needs to trust that the proxy does not replicate the source
addressing information in the request it forwards.
Clients rely on the oblivious proxy resource to forward encapsulated
requests and responses. However, the proxy can only refuse to
forward messages, it cannot inspect or modify the contents of
encapsulated requests or responses.
8.2. Proxy Responsibilities
The proxy that serves the oblivious proxy resource has a very simple
function to perform. For each request it receives, it makes a
request of the oblivious request resource that includes the same
content. When it receives a response, it sends a response to the
client that includes the content of the response from the oblivious
request resource. When generating a request, the proxy MUST follow
the forwarding rules in Section 7.6 of [HTTP].
A proxy can also generate responses, though it assumed to not be able
to examine the content of a request (other than to observe the choice
of key identifier, KDF, and AEAD), so it is also assumed that it
cannot generate an encapsulated response.
A proxy MUST NOT add information about the client identity when
forwarding requests. This includes the Via field, the Forwarded
field [FORWARDED], and any similar information.
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8.2.1. Denial of Service
As there are privacy benefits from having a large rate of requests
forwarded by the same proxy (see Section 8.2.2), servers that operate
the oblivious request resource might need an arrangement with
proxies. This arrangement might be necessary to prevent having the
large volume of requests being classified as an attack by the server.
If a server does accept a large volume of requests from a proxy, it
needs to trust that the proxy does not allow abusive levels of
request volumes from clients. That is, if a server allows requests
from the proxy to be exempt from rate limits, the server might want
to ensure that the proxy applies similar rate limiting when receiving
requests from clients.
Servers that enter into an agreement with a proxy that enables a
higher request rate might choose to authenticate the proxy to enable
the higher rate.
8.2.2. Linkability Through Traffic Analysis
As the time at which encapsulated request or response messages are
sent can reveal information to a network observer. Though messages
exchanged between the oblivious proxy resource and the oblivious
request resource might be sent in a single connection, traffic
analysis could be used to match messages that are forwarded by the
proxy.
A proxy could, as part of its function, add delays in order to
increase the anonymity set into which each message is attributed.
This could latency to the overall time clients take to receive a
response, which might not what some clients want.
A proxy can use padding to reduce the effectiveness of traffic
analysis.
A proxy that forwards large volumes of exchanges can provide better
privacy by providing larger sets of messages that need to be matched.
8.3. Server Responsibilities
A server that operates both oblivious request and oblivious target
resources is responsible for removing request encapsulation,
generating a response the encapsulated request, and encapsulating the
response.
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Servers should account for traffic analysis based on response size or
generation time. Techniques such as padding or timing delays can
help protect against such attacks; see Section 8.2.2.
If separate entities provide the oblivious request resource and
oblivious target resource, these entities might need an arrangement
similar to that between server and proxy for managing denial of
service; see Section 8.2.1. It is also necessary to provide
confidentiality protection for the unprotected requests and
responses, plus protections for traffic analysis; see Section 8.2.2.
An oblivious request resource needs to have a plan for replacing
keys. This might include regular replacement of keys, which can be
assigned new key identifiers. If an oblivious request resource
receives a request that contains a key identifier that it does not
understand or that corresponds to a key that has been replaced, the
server can respond with an HTTP 422 (Unprocessable Content) status
code.
A server can also use a 422 status code if the server has a key that
corresponds to the key identifier, but the encapsulated request
cannot be successfully decrypted using the key.
9. IANA Considerations
Please update the "Media Types" registry at
https://www.iana.org/assignments/media-types
(https://www.iana.org/assignments/media-types) with the registration
information in Section 7 for the media types "message/ohttp-req",
"message/ohttp-res", and "application/ohttp-keys".
10. References
10.1. Normative References
[BINARY] Thomson, M., "Binary Representation of HTTP Messages",
Work in Progress, Internet-Draft, draft-ietf-http-binary-
message-latest, 28 January 2021,
<https://tools.ietf.org/html/draft-ietf-http-binary-
message-latest>.
[HPKE] Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
Public Key Encryption", Work in Progress, Internet-Draft,
draft-irtf-cfrg-hpke-07, 16 December 2020,
<http://www.ietf.org/internet-drafts/draft-irtf-cfrg-hpke-
07.txt>.
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[HTTP] Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-14, 12 January 2021,
<http://www.ietf.org/internet-drafts/draft-ietf-httpbis-
semantics-14.txt>.
[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Work in Progress, Internet-Draft,
draft-ietf-quic-transport-34, 14 January 2021,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-34.txt>.
[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/info/rfc2119>.
[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/info/rfc8174>.
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
10.2. Informative References
[ALT-SVC] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>.
[COOKIES] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[Dingledine2004]
Dingledine, R., Mathewson, N., and P. Syverson, "Tor: The
Second-Generation Onion Router", August 2004,
<https://svn.torproject.org/svn/projects/design-paper/tor-
design.html>.
[FORWARDED]
Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
RFC 7239, DOI 10.17487/RFC7239, June 2014,
<https://www.rfc-editor.org/info/rfc7239>.
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[ODOH] Kinnear, E., McManus, P., Pauly, T., and C. Wood,
"Oblivious DNS Over HTTPS", Work in Progress, Internet-
Draft, draft-pauly-dprive-oblivious-doh-04, 26 January
2021, <http://www.ietf.org/internet-drafts/draft-pauly-
dprive-oblivious-doh-04.txt>.
[RANDOM] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[X25519] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
Appendix A. Complete Example of a Request and Response
A single request and response exchange is shown here. Binary values
(key configuration, secret keys, the content of messages, and
intermediate values) are shown in hexadecimal. The request and
response here are absolutely minimal; the purpose of this example is
to show the cryptographic operations.
The oblivious request resource generates a key pair. In this example
the server chooses DHKEM(X25519, HKDF-SHA256) and generates an X25519
key pair [X25519]. The X25519 secret key is:
15ce887006e079dcc7d67e73e5c13e31a55083f816eca9ebcf523b90ea2ab7b0
The oblivious request resource constructs a key configuration that
includes the corresponding public key as follows:
0100200020f21c612398e4384c21b7f2a759133c6c2d1b9ce6d033613dfad2c7
3d4826214900080001000100010003
This key configuration is somehow obtained by the client. Then when
a client wishes to send an HTTP request of a GET request to
"https://example.com", it constructs the following binary HTTP
message:
00034745540568747470730b6578616d706c652e636f6d012f
The client then reads the oblivious request resource key
configuration and selects a mutually supported KDF and AEAD. In this
example, the client selects HKDF-SHA256 and AES-128-GCM. The client
then generates an HPKE context that uses the server public key. This
results in the following encapsulated key:
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a7e0dffe93bc9ed807b51588f10669cd2f09ceb7f6a71153658275993eb88276
The corresponding private key is:
3b04f76e7ea1313484dd73c343adb94c23671f98cb66fc7ecc6127f38e1d4431
Applying the Seal operation from the HPKE context produces an
encrypted message, allowing the client to construct the following
encapsulated request:
0100010001a7e0dffe93bc9ed807b51588f10669cd2f09ceb7f6a71153658275
993eb88276b497362592977a4abc7857cf9892377af698ca0ee31fe72d21ca12
6ff2074eb9292ce7a83fc53158ff
The client then sends this to the oblivious proxy resource in a POST
request, which might look like the following HTTP/1.1 request:
POST /request.example.net/proxy HTTP/1.1
Host: proxy.example.org
Content-Type: message/ohttp-req
Content-Length: 78
<content is the encapsulated request above>
The oblivious proxy resource receives this request and forwards it to
the oblivious request resource, which might look like:
POST /oblivious/request HTTP/1.1
Host: example.com
Content-Type: message/ohttp-req
Content-Length: 78
<content is the encapsulated request above>
The oblivous request resource receives this request, selects the key
it generated previously using the key identifier from the message,
and decrypts the message. As this request is directed to the same
server, the oblivious request resource does not need to initiate an
HTTP request to the oblivious target resource. The request can be
served directly by the oblivious target resource, which generates a
minimal response (consisting of just a 200 status code) as follows:
0140c8
The response is constructed by extracting a secret from the HPKE
context:
861c67eefd91906068ee3208a9102274
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The key derivation for the encapsulated response uses both the
encapsulated KEM key from the request and a randomly selected nonce.
This produces a salt of:
a7e0dffe93bc9ed807b51588f10669cd2f09ceb7f6a71153658275993eb88276
55645b6626d6e8b1163d739c6cd3d94a
The salt and secret are both passed to the Extract function of the
selected KDF (HKDF-SHA256) to produce a pseudorandom key of:
ac29e0e1df48de4b349bb97db74da7f6732557a2fa6d12019ddadd8bf1e9cd99
The pseudorandom key is used with the Expand function of the KDF and
an info field of "key" to produce a 16-byte key for the selected AEAD
(AES-128-GCM):
0d81ce961eb6e61d510e18053418f316
With the same KDF and pseudorandom key, an info field of "nonce" is
used to generate a 12-byte nonce:
ba2d3637b5f041cc15ef73b2
The AEAD Seal function is then used to encrypt the response, which is
added to the randomized nonce value to produce the encapsulated
response:
55645b6626d6e8b1163d739c6cd3d94ac7e0cfd48adf8057517241ee220d20bc
e00a4a
The oblivious request resource then constructs a response:
HTTP/1.1 200 OK
Date: Wed, 27 Jan 2021 04:45:07 GMT
Cache-Control: private, no-store
Content-Type: message/ohttp-res
Content-Length: 38
<content is the encapsulated response>
The same response might then be generated by the oblivious proxy
resource which might change as little as the Date header. The client
is then able to use the HPKE context it created and the nonce from
the encapsulated response to construct the AEAD key and nonce and
decrypt the response.
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Acknowledgments
This design is based on a design for oblivious DoH, described in
[ODOH]. Eric Rescorla helped unify the structure of the key format.
Authors' Addresses
Martin Thomson
Mozilla
Email: mt@lowentropy.net
Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net
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