Internet DRAFT - draft-schwartz-ohai-consistency-doublecheck
draft-schwartz-ohai-consistency-doublecheck
ohai B. M. Schwartz
Internet-Draft Google LLC
Intended status: Standards Track 19 October 2022
Expires: 22 April 2023
Key Consistency by Double-Checking via a Semi-Trusted Proxy
draft-schwartz-ohai-consistency-doublecheck-03
Abstract
Several recent IETF privacy protocols require clients to acquire
bootstrap information for a service in a way that guarantees both
authenticity and consistency, e.g., encrypting to the same key as
many other users. This specification defines a procedure for
transferring arbitrary HTTP resources in a manner that provides these
guarantees. The procedure relies on access to a semi-trusted HTTP
proxy, under the same security assumptions as an Oblivious HTTP
Relay.
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-schwartz-ohai-consistency-
doublecheck/.
Source for this draft and an issue tracker can be found at
https://github.com/bemasc/access-services.
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-
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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 22 April 2023.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Origin . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Proxy . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Client . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Negative responses . . . . . . . . . . . . . . . . . . . 7
5. Example: Oblivious DoH . . . . . . . . . . . . . . . . . . . 8
6. Performance Implications . . . . . . . . . . . . . . . . . . 10
6.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Thundering Herds . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7.1. In scope . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1.1. Authenticity . . . . . . . . . . . . . . . . . . . . 12
7.1.2. Consistency . . . . . . . . . . . . . . . . . . . . . 12
7.1.3. Temporal Correlation Attacks . . . . . . . . . . . . 12
7.1.4. Abusive traffic . . . . . . . . . . . . . . . . . . . 13
7.2. Out of scope . . . . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 14
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
Oblivious HTTP [I-D.ietf-ohai-ohttp] enables an HTTP client to send
requests to a target in such a way that no single party can determine
both the identity of the client and the contents of its request.
Oblivious HTTP identifies four parties to each exchange: the Client,
Relay, Gateway and Target.
In order for Oblivious HTTP's privacy guarantees to hold, several
preconditions must be met:
1. The Client must be one of many users who might be using the
Relay. Otherwise, use of the Relay reveals the user's identity
to the Gateway.
2. The Client must hold an authentic KeyConfig for the Gateway.
Otherwise, the Client could be speaking to the Relay,
impersonating the Gateway.
3. All users of the Relay must be equally likely to use this
Gateway, KeyConfig, and Target, regardless of their prior
activity. Otherwise, the encrypted request identifies the Client
to the Gateway.
4. (optional) The Gateway should not learn the IP addresses of the
Clients, collectively. Otherwise, the Gateway might be able to
deanonymize requests by correlating them with external
information about the Clients.
The Privacy Pass protocol [I-D.ietf-privacypass-protocol] allows a
Client to retrieve tokens from an Issuer, and present them to an
Origin, in such a way that the Origin can verify the validity of the
tokens but cannot identify the Client, even if it colludes with the
Issuer. Privacy Pass requires a similar set of preconditions for its
privacy guarantees:
1. The Client's transport to the Origin must not reveal the client's
identity (e.g. via the client IP address).
2. The Client must hold an authentic Issuer Directory Object.
Otherwise, issuance will fail (resulting in a denial of service,
not a privacy violation).
3. All Clients with the same transport metadata must be using the
same Issuer Directory Object for this Issuer. Otherwise, these
factors together uniquely identify the client.
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4. (optional) The Origin should not learn the IP addresses of the
Clients collectively, as above.
Pre-requisite 1 on this list can be achieved by employing a shared
transport proxy, on the assumption that at least one of the proxy or
the Origin is non-malicious. (This is the same assumption that an
Oblivious HTTP Client makes about its Relay).
This specification assumes that a shared, "semi-trusted" proxy is
available (fulfilling precondition 1 on each list). It defines a
three-party protocol for a Client, Proxy, and Origin to fetch an HTTP
resource in a manner that ensures authenticity and consistency among
Clients of a single Proxy. When used to initialize Oblivious HTTP or
Privacy Pass, it guarantees preconditions 2, 3, and optionally 4.
The input to this protocol is a Desired Resource URI: an "https" URI
on the Origin that is assumed to have been distributed to Clients in
a globally consistent fashion. For example, this URI might be the
default value of a software setting, or it might be published on a
third party's website. This specification allows Clients to convert
the static, long-lived Desired Resource URI into a fresh copy of that
resource, i.e. to fetch the resource.
In principle, the Desired Resource could have been distributed
directly through the assumed globally consistent channel. However,
these ad hoc publication channels may not be fast enough to support
frequent updates (e.g., key rotations), especially if updates require
user intervention.
This draft is an instantiation of the Shared Proxy with Key
Confirmation strategy defined in Section 4.3 of
[I-D.wood-key-consistency].
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. Overview
In the Key Consistency Double-Check procedure, the Client emits two
HTTP GET requests for the Desired Resource. One uses the Proxy as an
HTTP request proxy (terminating TLS and HTTP), and the other
optionally uses it as a transport proxy (using TLS end-to-end with
the Origin). The Proxy will forward the first request to the Origin
only if the response is not in cache.
+--------+ +-------+ +--------+
| |<=====>| |<----->| |
| Client | | Proxy | | Origin |
| |<=====================>| |
+--------+ +-------+ +--------+
Figure 1: Overview of Key-Consistency Double-Check
The Proxy caches the response, ensuring that all clients share it
during its freshness lifetime. The client checks this against the
authenticated response from the Origin, preventing forgeries.
4. Requirements
4.1. Origin
The Desired Resource MUST include a "strong validator" ETag
(Section 2 of [RFC7232]) in any response to a GET request, and MUST
support the "If-Match" HTTP request header (Section 3 of [RFC7232]).
The response MUST indicate "Cache-Control: public, no-transform,
s-maxage=(...), immutable" [RFC9111][RFC8246]. For efficiency
reasons, the max age SHOULD be at least 60 seconds, and preferably
much longer.
If the Desired Resource changes, and the Origin receives a request
for the resource whose "If-Match" header identifies a previously
served version that has not yet expired, it MUST return a success
response containing the previous version. This response MAY indicate
"Cache-Control: private".
4.2. Proxy
The Proxy MUST offer HTTP request proxy capability, and SHOULD also
offer TCP proxy, UDP proxy, and encrypted DNS resolution
capabilities. (An associated encrypted DNS resolver enables reliable
use of HTTPS records [SVCB], improves metadata confidentiality, and
allows EDNS Client Subnet to be disabled reliably.) For example, a
proxy in the recommended configuration could be described by the
following Access Service Description
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[I-D.schwartz-masque-access-descriptions]:
{
"http": {
"template": "https://relay.example.org/http{?target_uri}"
},
"tcp": {
"template":
"https://proxy.example.org/tcp{?target_host,tcp_port}"
},
"udp": {
"template":
"https://proxy.example.org/masque{?target_host,target_port}"
},
"dns": {
"template": "https://doh.example.com/dns-query{?dns}",
}
}
Figure 2: Example Proxy Access Service Description
The Proxy MUST allow use of the GET method to proxy small responses,
and SHOULD make ample cache space available in order to avoid
eviction of Desired Resources. The proxy SHOULD share cache state
among all clients, to ensure that they observe the same resource
contents. If the cache must be partitioned for architectural or
performance reasons, operators SHOULD keep the number of users in
each partition as large as possible.
Proxies MUST preserve the ETag response header on cached responses,
and MUST add an Age header ([RFC9111], Section 5.1) to all proxied
responses. Proxies MUST respect the "Cache-Control: immutable"
directive, and MUST NOT revalidate fresh immutable cache entries in
response to any incoming requests. (Note that this is different from
the general recommendation in Section 2.1 of [RFC8246]). Proxies
also MUST NOT accept PUSH_PROMISE frames from the target.
Proxies SHOULD employ defenses against malicious attempts to fill the
cache. Some possible defenses include:
* Rate-limiting each client's use of GET requests.
* Prioritizing preservation of cache entries that have been served
to many clients, if eviction is required.
Proxies that are not intended for general-purpose use MAY impose
strict transfer limits or rate limits on transport proxy usage.
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If the Proxy offers an Encrypted DNS service, it MUST NOT enable EDNS
Client Subnet support [RFC7871].
4.3. Client
The Client is assumed to know the "https" URI of the Desired
Resource. To retrieve that resource, it MUST perform the following
"double-check" procedure:
1. Send a GET request for the Desired Resource URL via the Proxy's
HTTP request proxy function.
2. Record the response (A).
3. Check that response A's "Cache-Control" values indicate "public"
and "immutable".
4. Establish a transport tunnel through the Proxy to the Origin
(OPTIONAL).
5. Fetch the Desired Resource (through this tunnel if available),
using a GET request with "If-Match" set to response A's ETag.
6. Record the response (B).
7. Check that responses A and B were successful and the contents are
identical, otherwise fail.
This procedure ensures that the retrieved copy of the Desired
Resource is authentic and is identical to the one held by any other
users of this proxy. Once response A or B expires, the client MUST
refresh it before continuing to use this resource, and MUST repeat
the "double-check" process if either response changes.
Clients MUST perform each fetch to the Origin (step 4) as a fully
isolated request. Any state related to this Origin (e.g., cached DNS
records, CONNECT-UDP tunnels, QUIC transport state, TLS session
tickets, HTTP cookies) MUST NOT be shared with prior or subsequent
requests.
4.4. Negative responses
If the Desired Resource does not exist, the Double Check requirements
apply without modification. The Cache-Control headers ensure that
the negative response (e.g., HTTP 404) is cacheable regardless of its
status code. If Double Check succeeds with a negative response, the
Client can be confident that any other Clients of this proxy are
holding the same negative response.
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5. Example: Oblivious DoH
In this example, the client has been configured with a Proxy via the
following Access Service Description:
{
"dns": {
"template": "https://proxy.example.org/dns-query{?dns}",
},
"udp": {
"template":
"https://proxy.example.org/masque{?target_host,target_port}"
},
"http": {
"template": "https://relay.example.org/http{?target_uri}"
}
}
The client has recently been instructed to use an Encrypted DNS
server identified as "doh.example.com" that might support Oblivious
DoH. To discover and access this Oblivious DoH service, the client
attempts to retrieve two Desired Resources on the Origin
"https://doh.example.com":
* The DNS Server's Access Service Description, at "/.well-known/
access-services" (Section 5 of
[I-D.schwartz-masque-access-descriptions]).
- This conveys the DoH URI template associated with this origin.
* The Oblivious Gateway KeyConfig, at "/.well-known/oblivious-
gateway" (Section 5 of [I-D.pauly-ohai-svcb-config]).
- This conveys the public keys to use for Oblivious HTTP.
To prevent client-targeting attacks, the client retrieves both of
these resources via DoubleCheck. The HTTP requests for the Access
Service Description appears as follows:
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HEADERS
:method = GET
:scheme = https
:authority = relay.example.org
:path = /http?target_uri=
https%3A%2F%2Fdoh.example.com%2F.well-known%2Faccess-services
accept: application/access-services+json
HEADERS
:status = 200
cache-control: public, immutable, \
no-transform, s-maxage=86400
age: 80000
etag: ABCD1234
content-type: application/access-services+json
{
"dns": {
"template":
"https://doh.example.com/foo{?dns}"
}
}
HEADERS
:method = CONNECT
:protocol = connect-udp
:scheme = https
:authority = proxy.example.org
:path = /masque?target_host=doh.example.com,target_port=443
capsule-protocol = ?1
HEADERS
:status = 200
capsule-protocol = ?1
The client now has a CONNECT-UDP tunnel to doh.example.com, over
which it performs the following GET request using HTTP/3:
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HEADERS
:method = GET
:scheme = https
:authority = doh.example.com
:path = /.well-known/access-services
accept: application/access-services+json
if-match = ABCD1234
HEADERS
:status = 200
cache-control: public, immutable, \
no-transform, s-maxage=86400
etag: ABCD1234
content-type: application/access-services+json
{
"dns": {
"template":
"https://doh.example.com/foo{?dns}"
}
}
Having successfully fetched the DoH Service Description from both
locations, the client confirms that:
* The responses are identical.
* The cache-control response from the proxy contained the "public"
and "immutable" directives.
This concludes the DoubleCheck procedure. The Oblivious Gateway
KeyConfig is retrieved by a similar procedure. These two DoubleCheck
procedures can run in parallel, and MAY share a single transport
proxy tunnel for efficiency.
Once the client has double-checked the DoH Service Description and
the KeyConfig, it can use the Oblivious DoH service by forming DNS-
over-HTTPS requests for "https://doh.example.com/foo{?dns}" as Binary
HTTP requests, encrypting them to the KeyConfig, and POSTing the
ciphertext to "https://relay.example.org/
http?target_uri=https%3A%2F%2Fdoh.example.com%2F.well-
known%2Foblivious-gateway".
6. Performance Implications
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6.1. Latency
Suppose that the Client-Proxy Round-Trip Time (RTT) is A, and the
Proxy-Origin RTT is B. Suppose additionally that the Client has a
persistent connection to the Proxy that is already running. Then the
procedure described in Section 4.3, with a CONNECT-UDP transport
proxy, requires:
* A for the GET request to the Proxy
- +B if the Desired Resource is not in cache
- +B if the Proxy does not have a TLS session ticket for the
Origin
* A for the CONNECT-UDP request to the Proxy
* A + B for a QUIC handshake to the Origin
* A + B for the GET request to the Origin
This is a total of 4A + 4B in the worst case. However, clients can
reduce the latency by issuing the requests to the Proxy in parallel,
and by using CONNECT-UDP's "false start" support. The Origin can
also optimize performance, by issuing long-lived TLS session tickets.
With these optimizations, the expected total time is 2A + 2B.
This procedure only needs to be repeated if the Desired Resource has
expired. To enable regular key rotation and operational adjustments,
a cache lifetime of 24 hours may be suitable. Clients MAY perform
this procedure in advance of an expiration to avoid a delay.
6.2. Thundering Herds
All clients of the same Proxy and Desired Resource will have locally
cached copies with the same expiration time. When this copy expires,
all active clients will send refresh GET requests to the Proxy at
their next request. Proxies SHOULD use "request coalescing" to avoid
duplicate cache-refresh requests to the Origin.
If the Desired Resource has changed, these clients will all initiate
GET requests to the Origin (via transport proxy if applicable) to
double-check the new contents. Proxies and Origins MAY use an HTTP
503 response with a "Retry-After" header to manage load spikes.
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7. Security Considerations
7.1. In scope
7.1.1. Authenticity
A malicious Proxy could attempt to forge the Desired Resource by
replacing the response body (e.g., returning an Oblivious HTTP
KeyConfig containing a Public Key controlled by the Relay). This is
prevented by the Client's requirement that the Desired Resource be
served to it by the Origin over HTTPS (Section 4.3).
7.1.2. Consistency
A malicious Origin could attempt to break the consistency guarantee
by issuing each Client a unique, persistent variant of the Desired
Resource. This attack is prevented by the Client's requirement that
the resource be fresh according to the Proxy's cache (Section 4.3).
A malicious Origin could attempt to rotate its entry in the Proxy's
cache in several ways:
* Using HTTP PUSH_PROMISE frames. This attack is prevented by
disabling PUSH_PROMISE at the Proxy (Section 4.2).
* By also acting as a Client and sending requests designed to
replace the Desired Resource in the cache before it expires:
- By sending GET requests with a "Cache-Control: no-cache" or
similar directive. This is prevented by the response's "Cache-
Control: public, immutable" directives, which are verified by
the Client (Section 4.3), and by the Proxy's obligation to
respect these directives strictly (Section 4.2).
- By filling the cache with new entries, causing the cached copy
of the resource to be evicted. Section 4.2 describes some
possible mitigations.
7.1.3. Temporal Correlation Attacks
A malicious Origin could attempt to identify or link different
requests for the Desired Resource, in order to link proxied requests
that follow shortly after. If a transport proxy is used (as
recommended), this is prevented by fully isolating each request
(Section 4.3), and by disabling EDNS Client Subnet (Section 4.2).
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7.1.4. Abusive traffic
A malicious Client could use the Proxy to send abusive traffic to any
destination on the internet. Abuse concerns can be mitigated by
imposing a rate limit at the proxy (Section 4.2).
7.2. Out of scope
This specification assumes that the Client starts with identities of
the Proxy and the Desired Resource that are authentic and widely
shared. If these identities are inauthentic, or are unique to the
client, then the security goals of this specification are not
achieved.
This specification assumes that at most a small fraction of Clients
are acting on behalf of a malicious Origin. If a large fraction of
the Clients are malicious, they could conspire to flood the Proxy's
cache with entries that seem popular, leading to rapid eviction of
any cached resources. Similar concerns apply if a malicious Origin
can compel naive Clients to fetch a very large number of distinct
resources through the Proxy.
Even when a transport proxy is used, the Client's requests for the
Desired Resource may become linkable if they have distinctive TLS
ClientHellos, QUIC Initials, HTTP/3 Settings, RTT, or other protocol
features observable through the transport proxy. This specification
does not offer specific mitigations for protocol fingerprinting.
8. IANA Considerations
No IANA action is requested.
9. References
9.1. Normative References
[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>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<https://www.rfc-editor.org/rfc/rfc7232>.
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[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<https://www.rfc-editor.org/rfc/rfc7871>.
[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>.
[RFC8246] McManus, P., "HTTP Immutable Responses", RFC 8246,
DOI 10.17487/RFC8246, September 2017,
<https://www.rfc-editor.org/rfc/rfc8246>.
[RFC9111] 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>.
9.2. Informative References
[I-D.ietf-ohai-ohttp]
Thomson, M. and C. A. Wood, "Oblivious HTTP", Work in
Progress, Internet-Draft, draft-ietf-ohai-ohttp-05, 26
September 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-ohai-ohttp-05>.
[I-D.ietf-privacypass-protocol]
Celi, S., Davidson, A., Faz-Hernández, A., Valdez, S., and
C. A. Wood, "Privacy Pass Issuance Protocol", Work in
Progress, Internet-Draft, draft-ietf-privacypass-protocol-
06, 6 July 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-privacypass-protocol-06>.
[I-D.pauly-ohai-svcb-config]
Pauly, T. and T. Reddy.K, "Discovery of Oblivious Services
via Service Binding Records", Work in Progress, Internet-
Draft, draft-pauly-ohai-svcb-config-03, 27 July 2022,
<https://datatracker.ietf.org/doc/html/draft-pauly-ohai-
svcb-config-03>.
[I-D.schwartz-masque-access-descriptions]
Schwartz, B. M., "HTTP Access Service Description
Objects", Work in Progress, Internet-Draft, draft-
schwartz-masque-access-descriptions-03, 18 October 2022,
<https://datatracker.ietf.org/doc/html/draft-schwartz-
masque-access-descriptions-03>.
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[I-D.wood-key-consistency]
Davidson, A., Finkel, M., Thomson, M., and C. A. Wood,
"Key Consistency and Discovery", Work in Progress,
Internet-Draft, draft-wood-key-consistency-03, 17 August
2022, <https://datatracker.ietf.org/doc/html/draft-wood-
key-consistency-03>.
[SVCB] Schwartz, B. M., Bishop, M., and E. Nygren, "Service
binding and parameter specification via the DNS (DNS SVCB
and HTTPS RRs)", Work in Progress, Internet-Draft, draft-
ietf-dnsop-svcb-https-11, 11 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
svcb-https-11>.
Acknowledgments
TODO acknowledge.
Author's Address
Benjamin M. Schwartz
Google LLC
Email: bemasc@google.com
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