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Applications and Real-TimeHTTP Working GroupInternet-DraftThis document describes how http URIs can be accessed using Transport Layer Security (TLS) to mitigate pervasive monitoring attacks.Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.Working Group information can be found at http://httpwg.github.io/; source code and issues list for this draft can be found at https://github.com/httpwg/http-extensions/labels/opp-sec.

This document describes a use of HTTP Alternative Services to decouple the URI scheme from the use and configuration of underlying encryption, allowing a http URI to be accessed using Transport Layer Security (TLS) opportunistically.Serving https URIs requires avoiding Mixed Content , which is problematic in many deployments. This document describes a usage model whereby sites can serve http URIs over TLS, thereby avoiding these issues, while still providing protection against passive attacks.Opportunistic Security does not provide the same guarantees as using TLS with https URIs; it is vulnerable to active attacks, and does not change the security context of the connection. Normally, users will not be able to tell that it is in use (i.e., there will be no “lock icon”).

The immediate goal is to make the use of HTTP more robust in the face of pervasive passive monitoring .A secondary (but significant) goal is to provide for ease of implementation, deployment and operation. This mechanism is expected to have a minimal impact upon performance, and require a trivial administrative effort to configure.Preventing active attacks (such as a Man-in-the-Middle) is a non-goal for this specification. Furthermore, this specification is not intended to replace or offer an alternative to https, since it both prevents active attacks and invokes a more stringent security model in most clients.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in .

An origin server that supports the resolution of http URIs can indicate support for this specification by providing an alternative service advertisement for a protocol identifier that uses TLS, such as h2 .A client that receives such an advertisement MAY make future requests intended for the associated origin () to the identified service (as specified by ), provided that the alternative service opts in as described in .A client that places the importance of protection against passive attacks over performance might choose to withhold requests until an encrypted connection is available. However, if such a connection cannot be successfully established, the client can resume its use of the cleartext connection.A client can also explicitly probe for an alternative service advertisement by sending a request that bears little or no sensitive information, such as one with the OPTIONS method. Likewise, clients with existing alternative services information could make such a request before they expire, in order minimize the delays that might be incurred.Client certificates are not meaningful for URLs with the http scheme, and therefore clients creating new TLS connections to alternative services for the purposes of this specification MUST NOT present them. Established connections with client certificates MAY be reused, however.

It is possible that the server might become confused about whether requests’ URLs have a http or https scheme, for various reasons; see . To ensure that the alternative service has opted into serving http URLs over TLS, clients are required to perform additional checks before directing http requests to it.Clients MUST NOT send http requests over a connection with the h2 protocol identifier, unless they have obtained a valid http-opportunistic response for an origin (as per ), and:The chosen alternative service presents a certificate that is valid for the origin, as per (this also establishes “reasonable assurances” for the purposes of {RFC7838}}), andThe origin object of the http-opportunistic response has a `tls-ports’ member, whose value is an array of numbers, one of which matches the port of the alternative service in question, andThe chosen alternative service returns the same representation as the origin did for the http-opportunistic resource.For example, this request/response pair would allow reqeusts for the origin “http://www.example.com” to be sent to an alternative service on port 443 or 8000 of the host “www.example.com”:

When using alternative services, requests for resources identified by both http and https URIs might use the same connection, because HTTP/2 permits requests for multiple origins on the same connection.Because of the risk of server confusion about individual requests’ schemes (see ), clients MUST NOT send http requests on a connection that has previously been used for https requests, unless the http-opportunistic origin object fetched over that connection has a “mixed-scheme” member whose value is “true”.

This specification defines the “http-opportunistic” well-known URI . A client is said to have a valid http-opportunistic response for a given origin when:The client has obtained a 200 (OK) response for the well-known URI from the origin, and it is fresh (potentially through revalidation ), andThat response has the media type “application/json”, andThat response’s payload, when parsed as JSON , contains an object as the root, andThe root object contains a member whose name is a case-insensitive character-for-character match for the origin in question, serialised into Unicode as per Section 6.1 of , and whose value is an object (hereafter, the “origin object”),The origin object has a “lifetime” member, whose value is a number indicating the number of seconds which the origin object is valid for (hereafter, the “origin object lifetime”), andThe origin object lifetime is greater than the current_age (as per , Section 4.2.3).Note that origin object lifetime might differ from the freshness lifetime of the response.

This specification registers a Well-Known URI :URI Suffix: http-opportunisticChange Controller: IETFSpecification Document(s): of [this specification]Related Information:

User Agents MUST NOT provide any special security indicia when an http resource is acquired using TLS. In particular, indicators that might suggest the same level of security as https MUST NOT be used (e.g., a “lock device”).

A downgrade attack against the negotiation for TLS is possible.For example, because the Alt-Svc header field likely appears in an unauthenticated and unencrypted channel, it is subject to downgrade by network attackers. In its simplest form, an attacker that wants the connection to remain in the clear need only strip the Alt-Svc header field from responses.

Cached alternative services can be used to track clients over time; e.g., using a user-specific hostname. Clearing the cache reduces the ability of servers to track clients; therefore clients MUST clear cached alternative service information when clearing other origin-based state (i.e., cookies).

HTTP implementations and applications sometimes use ambient signals to determine if a request is for an https resource; for example, they might look for TLS on the stack, or a server port number of 443.This might be due to limitations in the protocol (the most common HTTP/1.1 request form does not carry an explicit indication of the URI scheme), or it may be because how the server and application are implemented (often, they are two separate entities, with a variety of possible interfaces between them).Any security decisions based upon this information could be misled by the deployment of this specification, because it violates the assumption that the use of TLS (or port 443) means that the client is accessing a HTTPS URI, and operating in the security context implied by HTTPS.Therefore, servers need to carefully examine the use of such signals before deploying this specification.

This specification requires that a server send both an Alternative Service advertisement and host content in a well-known location to send HTTP requests over TLS. Servers SHOULD take suitable measures to ensure that the content of the well-known resource remains under their control. Likewise, because the Alt-Svc header field is used to describe policies across an entire origin, servers SHOULD NOT permit user content to set or modify the value of this header.

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.This memo describes how to use Transport Layer Security (TLS) to secure Hypertext Transfer Protocol (HTTP) connections over the Internet. This memo provides information for the Internet community.This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes. [STANDARDS-TRACK]This document defines the concept of an "origin", which is often used as the scope of authority or privilege by user agents. Typically, user agents isolate content retrieved from different origins to prevent malicious web site operators from interfering with the operation of benign web sites. In addition to outlining the principles that underlie the concept of origin, this document details how to determine the origin of a URI and how to serialize an origin into a string. It also defines an HTTP header field, named "Origin", that indicates which origins are associated with an HTTP request. [STANDARDS-TRACK]JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data.This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance.The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations.The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients.This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.This document specifies "Alternative Services" for HTTP, which allow an origin's resources to be authoritatively available at a separate network location, possibly accessed with a different protocol configuration.Pervasive monitoring is a technical attack that should be mitigated in the design of IETF protocols, where possible.This document defines the concept "Opportunistic Security" in the context of communications protocols. Protocol designs based on Opportunistic Security use encryption even when authentication is not available, and use authentication when possible, thereby removing barriers to the widespread use of encryption on the Internet.This document defines a new HTTP header that allows web host operators to instruct user agents to remember ("pin") the hosts' cryptographic identities over a period of time. During that time, user agents (UAs) will require that the host presents a certificate chain including at least one Subject Public Key Info structure whose fingerprint matches one of the pinned fingerprints for that host. By effectively reducing the number of trusted authorities who can authenticate the domain during the lifetime of the pin, pinning may reduce the incidence of man-in-the-middle attacks due to compromised Certification Authorities.

Mike Bishop contributed significant text to this document.Thanks to Patrick McManus, Stefan Eissing, Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin, Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla, Julian Reschke, Kari Hurtta, and Richard Barnes for their feedback and suggestions.