Network Working Group M. Nottingham
Internet-Draft December 11, 2013
Intended status: Standards Track
Expires: June 14, 2014
Opportunistic Encryption for HTTP URIs
draft-nottingham-http2-encryption-02
Abstract
This document proposes two changes to HTTP/2.0; first, it suggests
using ALPN Protocol Identifies to identify the specific stack of
protocols in use, including TLS, and second, it proposes a way to
opportunistically encrypt HTTP/2.0 using TLS for HTTP URIs.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Goals and Non-Goals . . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. Proposal: Indicating Security Properties in Protocol
Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Proposal: The "h2r" Protocol . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
3.1. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . . 5
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Normative References . . . . . . . . . . . . . . . . . . . 6
4.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 7
Appendix B. Recent History and Background . . . . . . . . . . . . 7
Appendix C. Frequently Asked Questions . . . . . . . . . . . . . . 8
C.1. Will this make encryption mandatory in HTTP/2.0? . . . . . 9
C.2. No certificate checks? Really? . . . . . . . . . . . . . . 9
C.3. Why do this if a downgrade attack is so easy? . . . . . . . 9
C.4. Why Have separate relaxed protocol identifiers? . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
In discussion at IETF87, it was proposed that the current means of
bootstrapping encryption in HTTP [I-D.ietf-httpbis-p1-messaging] -
using the "HTTPS" URI scheme - unintentionally gives the server
disproportionate power in determining whether encryption (through use
of TLS [RFC6246]) is used.
This document proposes using the new "alternate services" layer
described in [I-D.nottingham-httpbis-alt-svc] to decouple the URI
scheme from the use and configuration of underlying encryption,
allowing a "http://" URI to be upgraded to use TLS opportunistically.
Additionally, because using TLS requires acquiring and configuring a
valid certificate, some deployments may find supporting it difficult.
Therefore, this document also proposes a "relaxed" profile of
HTTP/2.0 over TLS that does not require strong server authentication,
specifically for use with "http://" URIs.
1.1. Goals and Non-Goals
The immediate goal is to make HTTP URIs more robust in the face of
passive monitoring.
Such passive attacks are often opportunistic; they rely on sensitive
information being available in the clear. Furthermore, they are
often broad, where all available data is collected en masse, being
analyzed separately for relevant information.
It is not a goal of this document to address active or targeted
attacks, although future solutions may be complementary.
Other goals include ease of implementation and deployment, with
minimal impact upon performance (in keeping with the goals of
HTTP/2.0).
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. Proposal: Indicating Security Properties in Protocol Identifiers
In past discussions, there has been general agreement to reusing the
ALPN protocol identifier [I-D.ietf-tls-applayerprotoneg] for all
negotiation mechanisms in HTTP/2.0, not just TLS.
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This document proposes putting additional information into them to
identify the use of encryption as well as configuration of that
encryption, independent of the URI scheme in use.
Thus, we won't have just one protocol identifier for HTTP/2.0, but
two; one with and one without the use of TLS. As such, the following
identifiers are recommended if this approach is adopted:
o h1 - http/1.x over TCP
o h1t - http/1.x over TLS over TCP (as per [RFC2818])
o h2 - http/2.x over TCP
o h2t - http/2.x over TLS over TCP (as per [RFC2818])
o h2r - http/2.x over TLS over TCP (see Section 2.1)
Draft implementations could be indicated with a suffix; e.g., h2t-
draft10.
Most of these are already latently defined by HTTP/2.0, with the
exception being h2r, defined below. Note that the focus of this
proposal is on the semantics of the identifiers; an exact syntax for
them is not part of it.
By indicating the use of TLS in the protocol identifier allows a
client and server to negotiate the use of TLS for "http://" URIs; if
the server offers h2t, the client can select that protocol, start TLS
and use it.
Note that, as discussed in Section 3.1, there may be situations
(e.g,. ALPN) where advertising some of these profiles are
inapplicable or inadvisable. For example, in an ALPN negotiation for
a "https://" URI, it is only sensible to offer h1t and h2t.
If adopted, this proposal would be effected by adjusting the text in
Section 3 of [I-D.ietf-httpbis-http2] ("Starting HTTP/2.0") along the
lines described above. Note that the specific protocol identifiers
above are suggestions only.
2.1. Proposal: The "h2r" Protocol
If the proposal above is adopted, a separate proposal is to define a
separate protocol identifier for "relaxed" TLS operation.
Servers that support the "h2r" protocol indicate that they support
TLS for access to URIs with the "http" URI scheme using HTTP/2.0 or
greater.
Servers MAY advertise the "h2r" profile for resources with a "http"
origin scheme; they MUST NOT advertise it for resources with a
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"https" origin.
When a client connects to an "h2r" alternate service, it MUST use
TLS1.1 or greater, and MUST use HTTP/2.x. HTTP/2.0 SHOULD be used as
soon as TLS negotiation is completed; i.e., the "Upgrade dance"
SHOULD NOT be performed.
When connecting to an "h2r" service, the algorithm for authenticating
the server described in [RFC2818] Section 3.1 changes; the client
does not necessarily validate its certificate for expiry, hostname
match or relationship to a known certificate authority (as it would
with "normal" HTTPS).
However, the client MAY perform additional checks on the certificate
and make a decision as to its validity before using the server.
Definition of such additional checks are out of scope for this
specification.
Upon initial adoption of this proposal, it is expected that no such
additional checks will be performed. Therefore, the client MUST NOT
use the "h2r" profile to connect to alternate services whose host
does not match that of the origin (as per
[I-D.nottingham-httpbis-alt-svc]), unless additional checks are
performed.
Servers SHOULD use the same certificate consistently over time, to
aid future extensions for building trust and adding other services.
[TODO: define "same"; likely not the same actual certificate. ]
When the h2r protocol is in use, User Agents MUST NOT indicate the
connection has the same level of security as https:// (e.g. using a
"lock device").
If this proposal is adopted, the "h2r" protocol could be defined in
[I-D.ietf-httpbis-http2] (most likely, Section 3), or in a separate
document.
3. Security Considerations
3.1. Downgrade Attacks
A downgrade attack against the negotiation for TLS is possible,
depending upon the properties of the negotiation mechanism.
For example, because the Alt-Svc header field
[I-D.nottingham-httpbis-alt-svc] appears in the clear for "http://"
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URIs, it is subject to downgrade by attackers that are able to Man-
in-the-Middle the network connection; in its simplest form, an
attacker that wants the connection to remain in the clear need only
strip the Alt-Svc header from responses.
This proposal does not offer a remedy for this risk. However, it's
important to note that it is no worse than current use of unencrypted
HTTP in the face of such active attacks.
Future proposals might attempt to address this risk.
4. References
4.1. Normative References
[I-D.ietf-httpbis-http2]
Belshe, M., Peon, R., Thomson, M., and A. Melnikov,
"Hypertext Transfer Protocol version 2.0",
draft-ietf-httpbis-http2-08 (work in progress),
November 2013.
[I-D.ietf-tls-applayerprotoneg]
Friedl, S., Popov, A., Langley, A., and S. Emile,
"Transport Layer Security (TLS) Application Layer Protocol
Negotiation Extension", draft-ietf-tls-applayerprotoneg-03
(work in progress), October 2013.
[I-D.nottingham-httpbis-alt-svc]
Nottingham, M., "HTTP Alternate Services",
draft-nottingham-httpbis-alt-svc-00 (work in progress),
October 2013.
[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.
4.2. Informative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-25 (work in progress),
November 2013.
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[I-D.mbelshe-httpbis-spdy]
Belshe, M. and R. Peon, "SPDY Protocol",
draft-mbelshe-httpbis-spdy-00 (work in progress),
February 2012.
[RFC2804] IAB and IESG, "IETF Policy on Wiretapping", RFC 2804,
May 2000.
[RFC3365] Schiller, J., "Strong Security Requirements for Internet
Engineering Task Force Standard Protocols", BCP 61,
RFC 3365, August 2002.
[RFC6246] Sajassi, A., Brockners, F., Mohan, D., and Y. Serbest,
"Virtual Private LAN Service (VPLS) Interoperability with
Customer Edge (CE) Bridges", RFC 6246, June 2011.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
July 2013.
[firesheep]
Butler, E., "Firesheep", 2010,
.
[streetview]
Kravets, D., "The Anatomy of Google's Wi-Fi Sniffing
Debacle", 2012, .
[xkeyscore]
Greenwald, G., "NSA tool collects 'nearly everything a
user does on the internet'", 2013, .
Appendix A. Acknowledgements
Thanks to Patrick McManus, Eliot Lear, Stephen Farrell, Guy Podjarny,
Stephen Ludin, Erik Nygren, Paul Hoffman and Adam Langley for their
feedback and suggestions.
Appendix B. Recent History and Background
One of the design goals for SPDY [I-D.mbelshe-httpbis-spdy] was
increasing the use of encryption on the Web, achieved by only
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supporting the protocol over a connection protected by TLS [RFC5246].
This was done, in part, because sensitive information - including not
only login credentials, but also personally identifying information
(PII) and even patterns of access - are increasingly prevalent on the
Web, being evident in potentially every HTTP request made.
Attacks such as FireSheep [firesheep] showed how easy it is to gather
such information when it is sent in the clear, and incidents such as
Google's collection of unencrypted data by its StreetView Cars
[streetview] further illustrated the risks.
In adopting SPDY as the basis of HTTP/2 [I-D.ietf-httpbis-http2], the
HTTPbis Working Group agreed not to make TLS mandatory to implement
(MtI) or mandatory to use (MtU) in our charter, despite an IETF
policy to prefer the "best security available" [RFC3365].
There were a variety of reasons for this, but most significantly,
HTTP is used for much more than the traditional browsing case, and
encryption is not needed for all of these uses. Making encryption
MtU or MtI was seen as unlikely to succeed because of the wide
deployment of HTTP URIs.
However, since making that decision, there have been developments
that have caused the Working Group to discuss these issues again:
1. Active contributors to some browser implementations have stated
that their products will not use HTTP/2 over unencrypted
connections. If this eventuates, it will prevent wide deployment
of the new protocol (i.e., it couldn't be used with those
products for HTTP URIs; only HTTPS URIs).
2. It has been reported that surveillance of HTTP traffic takes
place on a broad scale [xkeyscore]. While the IETF does not take
a formal, moral position on wiretapping, we do have a strongly
held belief "that both commercial development of the Internet and
adequate privacy for its users against illegal intrusion requires
the wide availability of strong cryptographic technology"
[RFC2804]. This requirement for privacy is further reinforced by
[RFC6973].
As a result, we decided to revisit the issue of how encryption is
used in HTTP/2.0 at IETF87.
Appendix C. Frequently Asked Questions
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C.1. Will this make encryption mandatory in HTTP/2.0?
Not in the sense that this proposal would have it required (with a
MUST) in the specification.
What might happen, however, is that some browser implementers will
take the flexibility that this approach grants and decide to not
negotiate for HTTP/2.0 without one of the encryption profiles. That
means that servers would need to implement one of the encryption-
enabling profiles to interoperate using HTTP/2.0 for HTTP URIs.
C.2. No certificate checks? Really?
h2r has the effect of relaxing certificate checks on "http://" - but
not "https://" - URIs when TLS is in use. Since TLS isn't in use for
any "http://" URIs today, there is no net loss of security, and we
gain some privacy from passive attacks.
This makes TLS significantly simpler to deploy for servers; they are
able to use a self-signed certificate.
Additionally, it is possible to detect some attacks by remembering
what certificate is used in the past "pinning" or third-party
verification of the certificate in use. This may offer a way to gain
stronger authentication of the origin server's identity, and mitigate
downgrade attacks (although doing so is out of the scope of this
document).
C.3. Why do this if a downgrade attack is so easy?
There are many attack scenarios (e.g., third parties in coffee shops)
where active attacks are not feasible, or much more difficult.
Additionally, active attacks can often be detected, because they
change protocol interactions; as such, they bring a risk of
discovery.
C.4. Why Have separate relaxed protocol identifiers?
If all implementations agree that using TLS for "http://" URIs always
means that the certificate checks are "relaxed", it could be that
there is no need for a separate protocol identifier. However, this
needs to be discussed.
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Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
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