| Internet-Draft | Key Consistency Double Check | June 2022 |
| Schwartz | Expires 30 December 2022 | [Page] |
The assurances provided by Oblivious HTTP depend on the client's ability to verify that it is using the same Request Resource and KeyConfig as many other users. This specification defines a protocol to enable this verification.¶
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.¶
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 30 December 2022.¶
Copyright (c) 2022 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
Oblivious HTTP [I-D.ietf-ohai-ohttp] presumes at least three parties to each exchange: the client, the proxy, and the target (formally, the Oblivious Request Resource). When used properly, Oblivious HTTP enables the client to send requests to the target in such a way that the target cannot tell whether two requests came from the same client and the proxy cannot see the contents of the requests.¶
Oblivious HTTP's threat model assumes that at least one of the proxy and the target is acting properly, i.e. complying with the protocol and keeping certain information confidential. If either proxy or target misbehaves, the only effect must be a denial of service.¶
In order for these security guarantees to hold, several preconditions must be met:¶
This specification defines behaviors for the client, proxy, and target that achieve preconditions 2-4. (This specification does not address precondition 1.)¶
This draft is an instantiation of the "Single Proxy Discovery" architecture for key consistency, defined in Section 4.2 of [I-D.wood-key-consistency].¶
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.¶
In the Key Consistency Double-Check procedure, the Client emits two HTTP GET requests: one to the Proxy, and one through the Proxy to the Target. The Proxy will forward the first request to the Target if the response is not in cache.¶
+--------+ +-------+ +--------+
| |<=====>| |<---->| |
| Client | | Proxy | | Target |
| |<====================>| |
+--------+ +-------+ +--------+
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 Target, preventing forgeries.¶
The Oblivious Request Resource MUST publish an Access Description [I-D.schwartz-masque-access-descriptions] containing the "ohttp.request" key, e.g.:¶
{
"ohttp": {
"request": {
"uri": "https://example.com/ohttp/",
"key": "(KeyConfig in Base64)"
}
}
}
¶
This resource MUST be available over HTTP/3 [RFC9114], so that it can be accessed via the proxy's CONNECT-UDP service (see Section 4.2).¶
The Oblivious Request Resource MUST include a "strong validator" ETag (Section 2 of [RFC7232]) in any response to a GET request for this access description, 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 this Access Description changes, and the resource receives a request 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".¶
The Oblivious Proxy MUST publish an Access Description that includes the "ohttp.proxy" and "udp" keys, indicating support for CONNECT-UDP [I-D.ietf-masque-connect-udp]. It SHOULD also contain the "dns" key, indicating support for DNS over HTTPS [RFC8484], to enable the use of HTTPS records [SVCB] with CONNECT-UDP.¶
{
"dns": {
"template": "https://doh.example.com/dns-query{?dns}",
},
"udp": {
"template":
"https://proxy.example.org/masque{?target_host,target_port}"
},
"ohttp": {
"proxy": {
"template": "https://proxy.example.org/ohttp{?request_uri}"
}
}
}
The Oblivious Proxy Resources MUST allow use of the GET method to retrieve small JSON responses, and SHOULD make ample cache space available in order to avoid eviction of Access Descriptions. The proxy SHOULD share cache state among all clients, to ensure that they use the same Access Descriptions for each Oblivious Request Resource. 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.¶
Oblivious Proxies MUST preserve the ETag response header on cached responses, and MUST add an Age header ([RFC9111], Section 5.1) to all proxied responses. Oblivious 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]). Oblivious 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:¶
Oblivious Proxies that are not intended for general-purpose proxy usage MAY impose strict transfer limits or rate limits on HTTP CONNECT and CONNECT-UDP usage.¶
If the proxy offers a DNS over HTTPS resolver, it MUST NOT enable EDNS Client Subnet support [RFC7871].¶
The Client is assumed to know an "https" URI of an Oblivious Request Resource's Access Description. To use that Request Resource, it MUST perform the following "double-check" procedure:¶
ohttp.proxy.template) with request_uri set to the Access Description URI.¶
This procedure ensures that the Access Description is authentic and will be shared by all users of this proxy. Once response A or B expires, the client MUST refresh it before continuing to use this Access Description, 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.¶
In this example, the client has been configured with an Oblivious DoH server and an Oblivious Proxy. The Oblivious DoH server is identified by an Access Description at "https://doh.example.com/config.json" with the following contents:¶
{
"dns": {
"template": "https://doh.example.com/dns-query{?dns}",
},
"ohttp": {
"request": {
"uri": "https://example.com/ohttp/",
"key": "(KeyConfig in Base64)"
}
}
}
¶
The Oblivious Proxy is identified as "proxy.example.org", which implies an Access Description at "https://proxy.example.org/.well-known/access-services". This resource's contents are:¶
{
"dns": {
"template": "https://proxy.example.org/dns-query{?dns}",
},
"udp": {
"template":
"https://proxy.example.org/masque{?target_host,target_port}"
},
"ohttp": {
"proxy": {
"template": "https://proxy.example.org/ohttp{?request_uri}"
}
}
}
¶
The following exchanges then occur between the client and the proxy:¶
HEADERS
:method = GET
:scheme = https
:authority = proxy.example.org
:path = /ohttp?request_uri=https%3A%2F%2Fdoh.example.com%2Fconfig.json
accept: application/json
HEADERS
:status = 200
cache-control: public, immutable, \
no-transform, s-maxage=86400
age: 80000
etag: ABCD1234
content-type: application/json
[Access Description contents here]
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:¶
HEADERS
:method = GET
:scheme = https
:authority = doh.example.com
:path = /config.json
if-match = ABCD1234
HEADERS
:status = 200
cache-control: public, immutable, \
no-transform, s-maxage=86400
etag: ABCD1234
content-type: application/json
[Access Description contents here]
¶
Having successfully fetched the Access Description from both locations, the client confirms that:¶
The client can now use the KeyConfig in this Access Description to reach the Oblivious DoH server, by forming Binary HTTP requests for "https://doh.example.com/dns-query" and delivering the encapsulated requests to "https://example.com/ohttp/" via the proxy.¶
Suppose that the client-proxy Round-Trip Time (RTT) is A, and the proxy-target 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 requires:¶
A for the GET request to the proxy¶
A for the CONNECT-UDP request to the proxy¶
A + B for the QUIC handshake to the target¶
A + B for the GET request to the target¶
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 target 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 Access Description has expired. Access Descriptions do not need to change frequently, so a cache lifetime of 1 day may be suitable. Clients MAY perform this procedure in advance of an expiration to avoid a delay.¶
All clients of the same proxy and target will have locally cached Access Descriptions with the same expiration time. When this entry 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 target.¶
If the Access Description has changed, these clients will initiate GET requests through the proxy to the target to double-check the new contents. Proxies and targets MAY use an HTTP 503 response with a "Retry-After" header to manage load spikes.¶
A malicious proxy could attempt to learn the contents of the oblivious request by forging an Access Description containing its own KeyConfig. This is prevented by the client's requirement that the KeyConfig be served to it by the configured origin over HTTPS (Section 4.3).¶
A malicious target could attempt to link multiple Oblivious HTTP requests together by issuing each user a unique, persistent KeyConfig. This attack is prevented by the client's requirement that the KeyConfig be fresh according to the proxy's cache (Section 4.3).¶
A malicious target could attempt to rotate its entry in the proxy's cache in several ways:¶
By also acting as a client and sending requests designed to replace the Access Description in the cache before it expires:¶
A malicious target could attempt to link different requests for the Access Description, in order to link the Oblivious HTTP requests that follow shortly after. This is prevented by fully isolating each request (Section 4.3), and by disabling EDNS Client Subnet (Section 4.2).¶
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).¶
This specification assumes that the client starts with identities of the proxy and target that are authentic and widely shared. If these identities are inauthentic, or are unique to the user, 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 target. If a large fraction of the clients are malicious, they could conspire to flood the proxy cache with entries that seem popular, leading to rapid eviction of the malicious target's Access Descriptions. Similar concerns apply if a malicious target can compel naive clients to fetch a very large number of Access Descriptions.¶
No IANA action is requested.¶
TODO acknowledge.¶