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Checking Resource Consistency with HTTP Mirrors
draft-ietf-privacypass-consistency-mirror-00

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Document	Type	Active Internet-Draft (privacypass WG)
	Authors	Benjamin Beurdouche , Matthew Finkel , Steven Valdez , Christopher A. Wood , Tommy Pauly
	Last updated	2024-04-07 (Latest revision 2024-01-30)
	Replaces	draft-group-privacypass-consistency-mirror
	RFC stream	Internet Engineering Task Force (IETF)
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draft-ietf-privacypass-consistency-mirror-00

Privacy Pass B. Beurdouche
Internet-Draft Mozilla
Intended status: Standards Track M. Finkel
Expires: 2 August 2024 Apple Inc.
S. Valdez
Google LLC
C. A. Wood
Cloudflare
T. Pauly
Apple Inc.
30 January 2024

Checking Resource Consistency with HTTP Mirrors
draft-ietf-privacypass-consistency-mirror-00

Abstract

This document describes the mirror protocol, an HTTP-based protocol
for fetching mirrored HTTP resources. The primary use case for the
mirror protocol is to support HTTP resource consistency checks in
protocols that require clients have a consistent view of some
protocol-specific resource (typically, a public key) for security or
privacy reasons, including Privacy Pass and Oblivious HTTP. To that
end, this document also describes how to use the mirror protocol to
implement these consistency checks.

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-privacypass-consistency-
mirror/.

Discussion of this document takes place on the Privacy Pass Working
Group mailing list (mailto:privacy-pass@ietf.org), which is archived
at https://mailarchive.ietf.org/arch/browse/privacy-pass/. Subscribe
at https://www.ietf.org/mailman/listinfo/privacy-pass/.

Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-privacypass/draft-ietf-privacypass-
consistency-mirror.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 2 August 2024.

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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Mirror Protocol . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Mirror Request and Response Example . . . . . . . . . . . 6
5. Using Mirrors for Consistency Checks . . . . . . . . . . . . 7
5.1. Handling Consistency Failures . . . . . . . . . . . . . . 8
5.2. Selecting Mirror Servers . . . . . . . . . . . . . . . . 8
5.3. Consistency Validity Period . . . . . . . . . . . . . . . 9
5.4. Privacy Pass Profile . . . . . . . . . . . . . . . . . . 10
5.5. Oblivious HTTP Profile . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13

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1. Introduction

Privacy-enhancing protocols such as Privacy Pass [PRIVACYPASS] and
Oblivious HTTP [OHTTP] require clients to obtain and use a public key
for execution. In Privacy Pass, public keys are used by clients when
issuing and redeeming tokens for anonymous authorization. In
Oblivious HTTP (OHTTP), clients use public keys to encrypt messages
to a gateway server.

Deployments of protocols such as Privacy Pass and OHTTP requires that
very large sets of clients share the same key, or even that all
clients globally share the same key. This is because the privacy
properties depend on the client anonymity set size. In other words,
the key that's used determines the set to which a particular client
belongs. Using a unique, client-specific key would yield an
anonymity set of size one, therefore violating the desired privacy
goals of the system. Clients that use the same key as one another
are said to have a consistent view of the key.

[CONSISTENCY] describes this notion of consistency in more detail.
It also outlines several designs that can be used as the basis for
consistency systems. This document is a concrete instantiation of
one of those designs, "Shared Cache Discovery". In particular, this
document describes the mirror protocol, which is a protocol for
fetching a cached copy of an HTTP resource from so-called mirrors.
In this context, a mirror is an HTTP resource that fetches and caches
copies of other HTTP resources that can be returned to clients. In
turn, clients can then use these cached resource copies for
consistency checks, i.e., to compare their expected representation of
the resource against that which the mirrors provide.

The mirror protocol can be run one or more times by an application to
achieve the desired consistency criteria. For example, the mirror
protocol can be run once with a trusted mirror, or more than once
with many, potentially less trusted mirrors, and used for determining
consistency. Section 5 provides general guidance for using the
mirror protocol for consistency checks, along with specific profiles
of the protocol for Privacy Pass and OHTTP in Section 5.4 and
Section 5.5, respectively.

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.

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3. Terminology

The following terms are used throughout this document:

* Resource: A HTTP resource identified by a URL.

* Normalized resource representation: A unique or otherwise
protocol-specific representation that is derived from an HTTP
resource. The process of normalization is specific to a protocol
and the resource in question.

* Mirror: A HTTP resource that fetches and caches HTTP resources.

4. Mirror Protocol

The mirror protocol is a simple HTTP-based protocol similar to a
reverse proxy. Each mirror resource, henceforth referred to as a
mirror, is identified by a Mirror URI Template [RFC6570]. The scheme
for the Mirror URI Template MUST be "https". The Mirror URI Template
uses the Level 3 encoding defined Section 1.2 of [RFC6570] and
contains one variables: "target", which is the percent-encoded URL of
a HTTP resource to be mirrored. Example Mirror URI Templates are
shown below.

https://mirror.example/mirror{?target}
https://mirror.example/{target}

The Mirror URI Template MUST contain the "target" variable exactly
once. The variable MUST be within the path or query components of
the URI.

In addition, each mirror is configured with a MIN_VALIDITY_WINDOW
parameter, which is an integer indicating the minimum time for
resources the mirror will cache according to their "max-age" response
directive. We refer to the validity window of the mirror response as
the period of time determined by the Cache-Control headers as the
response.

Clients send requests to mirror resources after being configured with
their corresponding Mirror URI Template. Clients MUST ignore
configurations that do not conform to this template.

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Upon receipt of a mirror request, mirrors validate the incoming
request. If the request is invalid or malformed, e.g., the "target"
parameter is not a correctly encoded URL, the mirror aborts and
returns a a 4xx (Client Error) to the client. The mirror SHOULD
check that the target resource identified by the "target" parameter
is allowed by policy, e.g., so that it is not abused to fetch
arbitrary resources. One way to implement this check is via an
allowlist of target URLs.

If the request is valid and allowed, the mirror checks to see if it
has a cached version of the resource identified by the target URL.
Mirrors can provide a cached response to a client request if the
following criteria are met:

1. The target URL matches that of a cached response.

2. The cached response is fresh according to its Cache-Control
header (see Section 4.2 of [CACHING]).

If both criteria are met, the mirror encodes the cached response
using Binary HTTP [BHTTP] and returns it to the client in a response.
The mirror response includes a Cache-Control header with "max-age"
directive set to that of the cached response.

Otherwise, mirrors send a GET request to the target resource URL,
copying the Accept header from the client request if present. If
this request fails, the mirror returns a 4xx error to the client. If
this request suceeeds, the mirror checks it for validity. The
response is considered valid and stored in the mirror's cache if the
following criteria are met:

1. The response can be cached according to the rules in Section 3 of
[CACHING]. In particular, if the request had a Vary header, this
is used in determining whether the mirror's response is valid.

2. The Cache-Control header is present, has a "max-age" response
directive that is greater than or equal to MIN_VALIDITY_WINDOW,
and does not have a "no-store" or "private" directive.

Mirrors purge this cache when the response is no longer valid
according to the Cache-Control headers.

To complete the client request, the mirror then encodes the response
using Binary HTTP [BHTTP] and returns it to the client in a response.
The mirror response incldues a Cache-Control header with "max-age"
directive set to that of the cached response.

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Clients recover the target's mirrored response by Binary HTTP
decoding the mirror response content.

4.1. Mirror Request and Response Example

The following example shows two mirror request and response examples.
The first one yields a mirror cache miss and the second one yields a
mirror cache hit. The Mirror URI Template is
"https://mirror.example/mirror{?target}", and the target URL is
"https://issuer.example/.well-known/private-token-issuer-directory".

The first client request to the mirror might be the following.

:method = GET
:scheme = https
:authority = mirror.example
:path = /mirror?target=https%3A%2F%2Fissuer.example%2F.well-known%2Fprivate-token-issuer-directory
accept = application/private-token-issuer-directory

Upon receipt, the mirror decodes the "target" parameter, inspects its
cache for a copy of the resource, and then constructs a HTTP request
to the target URL to fetch the content. If present, the relay copies
the Accept header from the client request to the request sent to the
target. This mirror request to the target might be the following.

:method = GET
:scheme = https
:authority = target.example
:path = /.well-known/private-token-issuer-directory
accept = application/private-token-issuer-directory

The target response is then returned to the mirror, like so:

:status = 200
content-type = application/private-token-issuer-directory
content-length = ...
cache-control: max-age=3600

The mirror caches this response content for the target URL, encodes
it using Binary HTTP [BHTTP], and then returns the response to the
client:

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:status = 200
content-length = ...
cache-control: max-age=3600

When a second client asks for the same request by the mirror it can
be served with the cached copy. The second client's request might be
the following:

:method = GET
:scheme = https
:authority = mirror.example
:path = /mirror?target=https%3A%2F%2Fissuer.example%2F.well-known%2Fprivate-token-issuer-directory

The mirror validates the request, locates the cached copy of the
"https://issuer.example/.well-known/private-token-issuer-directory"
content, and then returns it to the client without updating its
cached copy.

:status = 200
content-length = ...
cache-control: max-age=3600

5. Using Mirrors for Consistency Checks

Clients can use mirrors to implement consistency checks for a
candidate HTTP resource. In particular, in possession of the target
URL at which the resource was obtained, as well as an authoritative
representation of the resource, clients can check to see if this
resource is consistent with that of the mirror's as follows:

1. Send a mirror request to the mirror for the target URL. If the
request fails, fail this consistency check.

2. Otherwise, compute the first valid representation of the resource
based on the mirror's response.

3. Compare the computed representation to the input resource
representation. If they do not match, fail this consistency
check. Otherwise, this consistency check succeeds.

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The benefits of using the mirror protocol to check consistency depend
on a multitude of factors, including, but not limited to, the number
of clients interacting with a particular mirror, whether or not the
mirror is trustworthy, and application requirements for dealing with
consistency check failures.

5.1. Handling Consistency Failures

If a consistency check fails because the mirrored resource did not
match, the client MUST NOT use the original resource. For cases
where the check failed because the client was unable to communicate
with the mirror, client policy dictates whether or not to assume the
resource is consistent. Client behavior for what to do in the case
of inconsistency can vary depending on the protocol, availability of
alternative services, and client policy.

If the client has multiple options for equivalent services, it can
choose to fall back from a service that failed a consistency check to
one that passed all consistency checks. For example, if a client has
the option of using one of a set of Privacy Pass token issuers, it
can choose an issuer that passes all consistency checks.

If the service that failed the consistency check is an optional
optimization for the client, the client can simply choose to not use
the service. For example, if a Privacy Pass token is used to avoid
showing the user a CAPTCHA, but the Privacy Pass token issuer fails
the consistency check, the client can fall back to showing the user a
CAPTCHA.

For cases where the client has no alternate services to use, and the
service is required in order to perform user-facing functionality,
the client SHOULD report the error in a visible way that presents the
error to the user or an administrator. This functionality can be
similar to how invalid TLS certificates are reported.

5.2. Selecting Mirror Servers

In many of these systems where the mirror protocol might be used,
including common configurations for Privacy Pass and OHTTP, there is
already a party who is necessarily trusted to protect the user's
privacy, and whose operational availability is already a prerequisite
for using the system. In OHTTP, this is the Relay; in Privacy Pass
it might be the Attester (in Split Mode) or a transport proxy.

When such a party exists, it is RECOMMENDED that they operate a
mirror service for their users, and that clients do not use any other
mirrors for the purposes of consistency checks. This avoids
revealing any metadata about the client's activity to additional

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parties and reduces the likelihood of an outage. More information
for implementing this check in the context of Privacy Pass and OHTTP
is provided in Section 5.4 and Section 5.5, respectively.

In some cases, this trusted party can provide consistency enforcement
through a protocol-specific mechanism (e.g.,
[I-D.pw-privacypass-in-band-consistency] for Privacy Pass in Split
Mode). Protocol-specific consistency mechanisms may be preferable to
protocol-agnostic consistency checks based on the mirror protocol,
especially if they provide equivalent consistency guarantees with
better performance or reliability.

5.3. Consistency Validity Period

Executing a consistency check provides a client with some assurance
that other clients may be using the same resource Section 6.
However, this result only reflects a mirror's view of a resource at a
particular point in time. The client should periodically re-check
consistency. The frequency of re-checking depends on the
requirements of the client Section 5. Clients should re-confirm
consistency of a cached resource if it is not fresh (see Section 4.2
of [CACHING]).

Two strategies for maintaining consistency are: 1. Pre-emptively
execute a consistency check for a resource that is expiring soon; and
1. Execute a consistency check of an expired resource at the time of
its next use.

For strategy 1, clients that execute consistency checks at the same
time can induce a thundering herd that overwhelms the mirror
resource. Since coordinating consistency checks across clients is
difficult, clients can instead execute consistency checks at random
times before the resource expires. Clients that have more
information about a mirror's available capacity can choose different
implementations for strategy 1.

Strategy 2 might be preferrable for a service and resource that is
infrequently used. However, a consequence of this strategy is that
it can reveal a client's usage patterns to the mirror.

When the origin server has multiple versions of a resource
corresponding to a URL, it should respond with the resource that is
both currently valid and will remain fresh for the longest amount of
time in the future.

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5.4. Privacy Pass Profile

Clients are given as input an issuer token key from an origin server
and want to check whether it is consistent with the key that is given
to other clients. Let the input key be denoted token_key and its
identifier be token_key_id. Clients are also given as input the name
of the issuer, from which they can construct the target URL for the
issuer directory. If clients have already checked this issuer’s
token key, i.e., they’ve previously run a consistency check, they can
simply reuse the result up to its expiration. Otherwise, clients
invoke a mirror-based consistency check in parallel with the issuance
protocol.

Each issuer directory can yield one or more normalized
representations that clients use in the consistency check. For
example, given a mirrored token directory resource like the
following:

{
"issuer-request-uri": "https://issuer.example.net/request",
"token-keys": [
{
"token-type": 2,
"token-key": "MI...AB",
"not-before": 1686913811,
},
{
"token-type": 2,
"token-key": "MI...AQ",
}
]
}

Clients compute the first valid representation of this directory,
i.e., the first entry in the list that the client can use, which
might be the key ID of the first key in the "token-keys" list
(depending on the "not-before" value), or the key ID of the second
key in the "token-keys" list. The key ID is computed as defined in
Section 6.5 of [PRIVACYPASS-ISSUANCE].

5.5. Oblivious HTTP Profile

Clients can run consistency checks for OHTTP in several ways
depending on the deployment. In practice, common deployments are as
follows:

1. Clients are configured with gateway configurations; and

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2. Clients fetch gateway configurations before use.

In both cases, clients begin with a gateway configuration and want to
check it for consistency. In OHTTP, there is exactly one
representation for a gateway configuration – the configuration
itself. Before using the configuration to encrypt a binary HTTP
message to the gateway, clients can run a consistency check with
their configured mirror(s) to ensure that this configuration is
correct for the given gateway.

6. Security Considerations

Consistency checks assume that the client-configured set of mirrors
is honest. Under this assumption, the consistency properties of
consistency checks based on the mirror protocol are as follows:

1. With honest mirrors, clients that successfully check a resource
are assured that they share the same copy of the resource with
the union of mirror clients for each configured mirror.

2. Consistency only holds for the period of time of the minimum
mirror validity window.

3. With at least one dishonest mirror, the probability of
discovering an inconsistency is 1 - (1 / 2^(k-1)), where k is the
number of disjoint consistency checks. This is the probability
that each individual consistency check succeeds.

Unless all clients share the same configured mirrors, consistency
checks using the mirror protocol do not achieve global consistency as
is defined in [CONSISTENCY].

7. IANA Considerations

This document has no IANA actions.

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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[CONSISTENCY]
Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
"Key Consistency and Discovery", Work in Progress,
Internet-Draft, draft-ietf-privacypass-key-consistency-01,
10 July 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-privacypass-key-consistency-01>.

[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>.

[PRIVACYPASS]
Davidson, A., Iyengar, J., and C. A. Wood, "The Privacy
Pass Architecture", Work in Progress, Internet-Draft,
draft-ietf-privacypass-architecture-16, 25 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
privacypass-architecture-16>.

[PRIVACYPASS-ISSUANCE]
Celi, S., Davidson, A., Valdez, S., and C. A. Wood,
"Privacy Pass Issuance Protocol", Work in Progress,
Internet-Draft, draft-ietf-privacypass-protocol-16, 3
October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-privacypass-protocol-16>.

[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>.

[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/rfc/rfc6570>.

[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

[CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/rfc/rfc9111>.

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[DOUBLE-CHECK]
Schwartz, B. M., "Key Consistency by Double-Checking via a
Semi-Trusted Proxy", Work in Progress, Internet-Draft,
draft-schwartz-ohai-consistency-doublecheck-03, 19 October
2022, <https://datatracker.ietf.org/doc/html/draft-
schwartz-ohai-consistency-doublecheck-03>.

[I-D.pw-privacypass-in-band-consistency]
Pauly, T. and C. A. Wood, "Privacy Pass In-Band Key
Consistency Checks", Work in Progress, Internet-Draft,
draft-pw-privacypass-in-band-consistency-00, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-pw-
privacypass-in-band-consistency-00>.

Acknowledgments

This document is based on the [DOUBLE-CHECK] protocol from Benjamin
Schwartz.

Authors' Addresses

Benjamin Beurdouche
Mozilla
Email: ietf@beurdouche.com

Matthew Finkel
Apple Inc.
Email: sysrqb@apple.com

Steven Valdez
Google LLC
Email: svaldez@chromium.org

Christopher A. Wood
Cloudflare
Email: caw@heapingbits.net

Tommy Pauly
Apple Inc.
Email: tpauly@apple.com

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Checking Resource Consistency with HTTP Mirrors
draft-ietf-privacypass-consistency-mirror-00