Internet DRAFT - draft-nottingham-http2-encryption
draft-nottingham-http2-encryption
Network Working Group M. Nottingham
Internet-Draft
Intended status: Standards Track M. Thomson
Expires: November 21, 2014 Mozilla
May 20, 2014
Opportunistic Encryption for HTTP URIs
draft-nottingham-http2-encryption-03
Abstract
This describes how "http" URIs can be accessed using Transport Layer
Security (TLS) to mitigate pervasive monitoring attacks.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Goals and Non-Goals . . . . . . . . . . . . . . . . . . . 2
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Using HTTP URIs over TLS . . . . . . . . . . . . . . . . . . 3
3. Server Authentication . . . . . . . . . . . . . . . . . . . . 3
4. Interaction with "https" URIs . . . . . . . . . . . . . . . . 4
5. Requiring Use of TLS . . . . . . . . . . . . . . . . . . . . 4
5.1. The HTTP-TLS Header Field . . . . . . . . . . . . . . . . 5
5.2. Operational Considerations . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6.1. Security Indicators . . . . . . . . . . . . . . . . . . . 7
6.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 7
6.3. Privacy Considerations . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document describes a use of HTTP Alternative Services
[I-D.ietf-httpbis-alt-svc] to decouple the URI scheme from the use
and configuration of underlying encryption, allowing a "http" URI to
be accessed using TLS [RFC5246] opportunistically.
Currently, "https" URIs requires acquiring and configuring a valid
certificate, which means that some deployments find supporting TLS
difficult. Therefore, this document describes a usage model whereby
sites can serve "http" URIs over TLS without being required to
support strong server authentication.
A mechanism for limiting the potential for active attacks is
described in Section 5. This provides clients with additional
protection against them for a period after successfully connecting to
a server using TLS. This does not offer the same level of protection
as afforded to "https" URIs, but increases the likelihood that an
active attack be detected.
1.1. Goals and Non-Goals
The immediate goal is to make the use of HTTP more robust in the face
of pervasive passive monitoring [RFC7258].
A secondary goal is to limit the potential for active attacks. It is
not intended to offer the same level of protection as afforded to
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"https" URIs, but instead to increase the likelihood that an active
attack can be detected.
A final (but significant) goal is to provide for ease of
implementation, deployment and operation. This mechanism should have
a minimal impact upon performance, and should not require extensive
administrative effort to configure.
1.2. Notational Conventions
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 [RFC2119].
2. Using HTTP URIs over TLS
An origin server that supports the resolution of HTTP URIs can
indicate support for this specification by providing an alternative
service advertisement [I-D.ietf-httpbis-alt-svc] for a protocol
identifier that uses TLS, such as "h2" [I-D.ietf-httpbis-http2].
A client that receives such an advertisement MAY direct future
requests for the associated origin to the identified service (as
specified by [I-D.ietf-httpbis-alt-svc]).
A client that places the importance of passive protections over
performance might choose to withold requests until an encrypted
connection is available. However, if such a connection cannot be
successfully established, the client MAY 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. Clients with
expired alternative services information could make a similar request
in parallel to an attempt to contact an alternative service, to
minimize the delays that might be incurred by failing to contact the
alternative service.
3. Server Authentication
There are no existing expectations with respect to cryptographically
strong server authentication when it comes to resolving HTTP URIs.
Establishing it, as described in [RFC2818], creates a number of
operational challenges. For these reasons, server authentication is
not mandatory for HTTP URIs when using the mechanism described in
this specification.
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When connecting to an alternative service for an "http" URI, clients
are required to perform the server authentication procedure described
in Section 3.1 of [RFC2818]. The server certificate, if one is
proffered by the alternative service, is not necessarily checked for
validity, expiration, issuance by a trusted certificate authority or
matched against the name in the URI. Therefore, the alternative
service MAY provide any certificate, or even select TLS cipher suites
that do not include authentication.
A client MAY perform additional checks on the certificate that it is
offered (if the server does not select an unauthenticated TLS cipher
suite). For instance, a client could examine the certificate to see
if it has changed over time.
In order to retain the authority properties of "http" URIs, and as
stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use
alternative services that identify a host other than that of the
origin, unless the alternative service indication itself is strongly
authenticated. This is not currently possible for "http" URIs on
cleartext transports.
4. Interaction with "https" URIs
An alternative service that is discovered to support "http" URIs
might concurrently support "https" URIs, because HTTP/2 permits the
sending of requests for multiple origins (see [RFC6454]) on the one
connection. Therefore, when using alternative services, both HTTP
and HTTPS URIs might be sent on the same connection.
"https" URIs rely on server authentication. Therefore, if a
connection is initially created without authenticating the server,
requests for "https" resources cannot be sent over that connection
until the server certificate is successfully authenticated.
Section 3.1 of [RFC2818] describes the basic mechanism, though the
authentication considerations in [I-D.ietf-httpbis-alt-svc] could
also apply.
Connections that are established without any means of server
authentication (for instance, the purely anonymous TLS cipher
suites), cannot be used for "https" URIs.
5. Requiring Use of TLS
Editors' Note: this is a very rough take on an approach that would
provide a limited form of protection against downgrade attack. It's
unclear at this point whether the additional effort (and modest
operational cost) is worthwhile.
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The mechanism described in this specification is trival to mount an
active attack against, for two reasons:
o A client that doesn't perform authentication an easy victim of
server impersonation, through man-in-the-middle attacks.
o A client that is willing to use cleartext to resolve the resource
will do so if access to any TLS-enabled alternative services is
blocked at the network layer.
Given that the primary goal of this specification is to prevent
passive attacks, these are not critical failings (especially
considering the alternative - HTTP over cleartext). However, a
modest form of protection against active attacks can be provided for
clients on subsequent connections.
When an alternate service is able to commit to providing service for
a particular origin over TLS for a bounded period of time, clients
can choose to rely upon its avilability, failing when it cannot be
contacted. Effectively, this makes the alternative service "sticky"
in the client.
One drawback with this approach is that clients need to strongly
authenticate the alternative service to act upon such a commitment;
otherwise, an attacker could create a persistent denial of service.
5.1. The HTTP-TLS Header Field
A alternative service can make this commitment by sending a "HTTP-
TLS" header field:
HTTP-TLS = 1#parameter
When it appears in a HTTP response from a strongly authenticated
alternative service, this header field indicates that the
availability of the origin through TLS-protected alternative services
is "sticky", and that the client MUST NOT fall back to cleartext
protocols while this information is considered fresh.
For example:
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HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: 600
Age: 30
Date: Thu, 1 May 2014 16:20:09 GMT
HTTP-TLS: ma=3600
Note that the commitment is not bound to a particular alternative
service; clients SHOULD use other alternative services that they
become aware of, as long as the requirements regarding authentication
and avoidance of cleartext protocols are met.
When this header field appears in a response, clients MUST strongly
authenticate the alternative service, as described in Section 3.1 of
[RFC2818], noting the additional requirements in
[I-D.ietf-httpbis-alt-svc]. The header field MUST be ignored if
strong authentication fails.
Persisted information expires after a period determined by the value
of the "ma" parameter. See Section 4.2.3 of
[I-D.ietf-httpbis-p6-cache] for details of determining response age.
ma-parameter = delta-seconds
Requests for an origin that has a persisted, unexpired value for
"HTTP-TLS" MUST fail if they cannot be made over an authenticated TLS
connection.
5.2. Operational Considerations
To avoid situations where a persisted value of "HTTP-TLS" causes a
client to be unable to contact a site, clients SHOULD limit the time
that a value is persisted for a given origin. A hard limit might be
set to a month. A lower limit might be appropriate for initial
observations of "HTTP-TLS"; the certainty that a site has set a
correct value - and the corresponding limit on persistence - can
increase as the value is seen more over time.
Once a server has indicated that it will support authenticated TLS, a
client MAY use key pinning [I-D.ietf-websec-key-pinning] or any other
mechanism that would otherwise be restricted to use with HTTPS URIs,
provided that the mechanism can be restricted to a single HTTP
origin.
6. Security Considerations
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6.1. Security Indicators
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., using a "lock device").
6.2. Downgrade Attacks
A downgrade attack against the negotiation for TLS is possible. With
the "HTTP-TLS" header field, this is limited to occasions where
clients have no prior information (see Section 6.3), or when
persisted commitments have expired.
For example, because the "Alt-Svc" header field
[I-D.ietf-httpbis-alt-svc] 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.
As long as a client is willing to use cleartext TCP to contact a
server, these attacks are possible. The "HTTP-TLS" header field
provides an imperfect mechanism for establishing a commitment. The
advantage is that this only works if a previous connection is
established where an active attacker was not present. A continuously
present active attacker can either prevent the client from ever using
TLS, or offer a self-signed certificate. This would prevent the
client from ever seeing the "HTTP-TLS" header field, or if the header
field is seen, from successfully validating and persisting it.
6.3. Privacy Considerations
Clients that persist state for origins can be tracked over time based
on their use of this information. Persisted information can be
cleared to reduce the ability of servers to track clients. A browser
client MUST clear persisted all alternative service information when
clearing other origin-based state (i.e., cookies).
7. References
7.1. Normative References
[I-D.ietf-httpbis-alt-svc]
Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", draft-ietf-httpbis-alt-svc-01 (work
in progress), April 2014.
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[I-D.ietf-httpbis-http2]
Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol version 2", draft-ietf-httpbis-http2-12 (work in
progress), April 2014.
[I-D.ietf-httpbis-p6-cache]
Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
Transfer Protocol (HTTP/1.1): Caching", draft-ietf-
httpbis-p6-cache-26 (work in progress), February 2014.
[I-D.ietf-websec-key-pinning]
Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", draft-ietf-websec-key-pinning-13
(work in progress), May 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
7.2. Informative References
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
Appendix A. Acknowledgements
Thanks to Patrick McManus, Eliot Lear, Stephen Farrell, Guy Podjarny,
Stephen Ludin, Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla
and Richard Barnes for their feedback and suggestions.
Authors' Addresses
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
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Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
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