draft-ietf-httpbis-http2-encryption-03.txt   draft-ietf-httpbis-http2-encryption-04.txt 
HTTP Working Group M. Nottingham HTTP Working Group M. Nottingham
Internet-Draft Internet-Draft
Intended status: Experimental M. Thomson Intended status: Experimental M. Thomson
Expires: June 19, 2016 Mozilla Expires: September 18, 2016 Mozilla
December 17, 2015 March 17, 2016
Opportunistic Security for HTTP Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-03 draft-ietf-httpbis-http2-encryption-04
Abstract Abstract
This document describes how "http" URIs can be accessed using This document describes how "http" URIs can be accessed using
Transport Layer Security (TLS) to mitigate pervasive monitoring Transport Layer Security (TLS) to mitigate pervasive monitoring
attacks. attacks.
Note to Readers
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 .
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Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Goals and Non-Goals . . . . . . . . . . . . . . . . . . . 3 1.1. Goals and Non-Goals . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Using HTTP URIs over TLS . . . . . . . . . . . . . . . . . . 3 2. Using HTTP URIs over TLS . . . . . . . . . . . . . . . . . . 3
3. Server Authentication . . . . . . . . . . . . . . . . . . . . 4 3. Server Authentication . . . . . . . . . . . . . . . . . . . . 4
4. Interaction with "https" URIs . . . . . . . . . . . . . . . . 4 4. Interaction with "https" URIs . . . . . . . . . . . . . . . . 5
5. Requiring Use of TLS . . . . . . . . . . . . . . . . . . . . 5 5. Requiring Use of TLS . . . . . . . . . . . . . . . . . . . . 5
5.1. The HTTP-TLS Header Field . . . . . . . . . . . . . . . . 5 5.1. Opportunistic Commitment . . . . . . . . . . . . . . . . 6
5.2. Operational Considerations . . . . . . . . . . . . . . . 7 5.2. Client Handling of A Commitment . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5.3. Operational Considerations . . . . . . . . . . . . . . . 7
6.1. Security Indicators . . . . . . . . . . . . . . . . . . . 7 6. The "http-opportunistic" well-known URI . . . . . . . . . . . 7
6.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.3. Privacy Considerations . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.4. Confusion Regarding Request Scheme . . . . . . . . . . . 8 8.1. Security Indicators . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8 8.3. Privacy Considerations . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9 8.4. Confusion Regarding Request Scheme . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
This document describes a use of HTTP Alternative Services This document describes a use of HTTP Alternative Services
[I-D.ietf-httpbis-alt-svc] to decouple the URI scheme from the use [I-D.ietf-httpbis-alt-svc] to decouple the URI scheme from the use
and configuration of underlying encryption, allowing a "http" URI to and configuration of underlying encryption, allowing a "http" URI
be accessed using TLS [RFC5246] opportunistically. [RFC7230] to be accessed using TLS [RFC5246] opportunistically.
Serving "https" URIs require acquiring and configuring a valid Serving "https" URIs require acquiring and configuring a valid
certificate, which means that some deployments find supporting TLS certificate, which means that some deployments find supporting TLS
difficult. This document describes a usage model whereby sites can difficult. This document describes a usage model whereby sites can
serve "http" URIs over TLS without being required to support strong serve "http" URIs over TLS without being required to support strong
server authentication. server authentication.
Opportunistic Security [RFC7435] does not provide the same guarantees Opportunistic Security [RFC7435] does not provide the same guarantees
as using TLS with "https" URIs; it is vulnerable to active attacks, as using TLS with "https" URIs; it is vulnerable to active attacks,
and does not change the security context of the connection. and does not change the security context of the connection.
skipping to change at page 4, line 7 skipping to change at page 4, line 14
A client can also explicitly probe for an alternative service A client can also explicitly probe for an alternative service
advertisement by sending a request that bears little or no sensitive advertisement by sending a request that bears little or no sensitive
information, such as one with the OPTIONS method. Likewise, clients information, such as one with the OPTIONS method. Likewise, clients
with existing alternative services information could make such a with existing alternative services information could make such a
request before they expire, in order minimize the delays that might request before they expire, in order minimize the delays that might
be incurred. be incurred.
3. Server Authentication 3. Server Authentication
By their nature, "http" URIs do not require cryptographically strong [I-D.ietf-httpbis-alt-svc] requires that an alternative service only
server authentication; that is only implied by "https" URIs. be used when there are "reasonable assurances" that it is under
Furthermore, doing so (as per [RFC2818]) creates a number of control of and valid for the whole origin.
operational challenges. For these reasons, server authentication is
not mandatory for "http" URIs when using the mechanism described in
this specification.
When connecting to an alternative service for an "http" URI, clients As defined in that specification, one way of establishing this is
are not required to perform the server authentication procedure using a TLS-based protocol with the certificate checks defined in
described in Section 3.1 of [RFC2818]. The server certificate, if [RFC2818]. Clients MAY impose additional criteria for establishing
one is proffered by the alternative service, is not necessarily reasonable assurances.
checked for validity, expiration, issuance by a trusted certificate
authority or matched against the name in the URI. Therefore, the
alternative service can provide any certificate, or even select TLS
cipher suites that do not include authentication.
A client MAY perform additional checks on the offered certificate if For the purposes of this specification, an additional way of
the server does not select an unauthenticated TLS cipher suite. This establishing reasonable assurances is available when the alternative
document doesn't define any such checks, though clients could be is on the same host as the origin, using the "http-opportunistic"
configured with a policy that defines what is acceptable. well-known URI defined in Section 6.
As stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use This allows deployment without the use of valid certificates, to
alternative services with a host other than the origin's, unless the encourage deployment of opportunistic security. When it is in use,
alternative service itself is strongly authenticated (as the origin's the alternative service can provide any certificate, or even select
host); for example, using TLS with a certificate that validates as TLS cipher suites that do not include authentication.
per [RFC2818].
When the client has a valid http-opportunistic response for an
origin, it MAY consider there to be reasonable assurances when:
o The origin and alternative service's hostnames are the same when
compared in a case-insensitive fashion, and
o The chosen alternative service returns the same response as above.
For example, this request/response pair would constitute reasonable
assurances for the origin "http://www.example.com" for any
alternative service also on "www.example.com":
GET /.well-known/http-opportunistic HTTP/1.1
Host: www.example.com
HTTP/1.1 200 OK
Content-Type: application/json
Connection: close
{
"origins": ["http://example.com", "http://www.example.com:81"]
}
Note that this mechanism is only defined to establish reasonable
assurances for the purposes of this specification; it does not apply
to other uses of alternative services unless they explicitly invoke
it.
4. Interaction with "https" URIs 4. Interaction with "https" URIs
When using alternative services, requests for resources identified by When using alternative services, requests for resources identified by
both "http" and "https" URIs might use the same connection, because both "http" and "https" URIs might use the same connection, because
HTTP/2 permits requests for multiple origins on the same connection. HTTP/2 permits requests for multiple origins on the same connection.
Since "https" URIs rely on server authentication, a connection that Since "https" URIs rely on server authentication, a connection that
is initially created for "http" URIs without authenticating the is initially created for "http" URIs without authenticating the
server cannot be used for "https" URIs until the server certificate server cannot be used for "https" URIs until the server certificate
is successfully authenticated. Section 3.1 of [RFC2818] describes is successfully authenticated. Section 3.1 of [RFC2818] describes
the basic mechanism, though the authentication considerations in the basic mechanism, though the authentication considerations in
[I-D.ietf-httpbis-alt-svc] also apply. [I-D.ietf-httpbis-alt-svc] also apply.
Connections that are established without any means of server Connections that are established without any means of server
authentication (for instance, the purely anonymous TLS cipher authentication (for instance, the purely anonymous TLS cipher
suites), cannot be used for "https" URIs. suites), cannot be used for "https" URIs.
5. Requiring Use of TLS 5. Requiring Use of TLS
Editors' Note: this is a very rough take on an approach that would Even when the alternative service is strongly authenticated,
provide a limited form of protection against downgrade attack. It's opportunistically upgrading cleartext HTTP connections to use TLS is
unclear at this point whether the additional effort (and modest subject to active attacks. In particular:
operational cost) is worthwhile.
The mechanism described in this specification is trivial to mount an
active attack against, for two reasons:
o A client that doesn't perform authentication is an easy victim of o Because the original HTTP connection is in cleartext, it is
server impersonation, through man-in-the-middle attacks. vulnerable to man-in-the-middle attacks, and
o A client that is willing to use HTTP over cleartext to resolve the o By default, if clients cannot reach the alternative service, they
resource will do so if access to any TLS-enabled alternative will fall back to using the original cleartext origin.
services is blocked at the network layer.
Given that the primary goal of this specification is to prevent Given that the primary goal of this specification is to prevent
passive attacks, these are not critical failings (especially passive attacks, these are not critical failings (especially
considering the alternative - HTTP over cleartext). However, a considering the alternative - HTTP over cleartext). However, a
modest form of protection against active attacks can be provided for modest form of protection against active attacks can be provided for
clients on subsequent connections. clients on subsequent connections.
When an alternative service is able to commit to providing service When an origin is able to commit to providing service for a
for a particular origin over TLS for a bounded period of time, particular origin over TLS for a bounded period of time, clients can
clients can choose to rely upon its availability, failing when it choose to rely upon its availability, failing when it cannot be
cannot be contacted. Effectively, this makes the choice to use a contacted. Effectively, this makes the choice to use a secured
secured protocol "sticky" in the client. protocol "sticky".
5.1. The HTTP-TLS Header Field 5.1. Opportunistic Commitment
A alternative service can make this commitment by sending a "HTTP- An origin can reduce the risk of attacks on opportunistically secured
TLS" header field, described here using the '#' ABNF extension connections by committing to provide an secured, authenticated
defined in Section 7 of [RFC7230]: alternative service. This is done by including the optional "commit"
member in the http-opportunistic well-known resource (see Section 6).
This feature is optional due to the requirement for server
authentication and the potential risk entailed (see Section 5.3).
HTTP-TLS = 1#parameter The value of the "commit" member is a number ([RFC7159], Section 6)
indicating the duration of the commitment interval in seconds.
When it appears in a HTTP response from a strongly authenticated {
alternative service, this header field indicates that the "origins": ["http://example.com", "http://www.example.com:81"],
availability of the origin through TLS-protected alternative services "commit": 86400
is "sticky", and that the client MUST NOT fall back to cleartext }
protocols while this information is considered fresh.
For example: Including "commit" creates a commitment to provide a secured
alternative service for the advertised period. Clients that receive
this commitment can assume that a secured alternative service will be
available for the indicated period. Clients might however choose to
limit this time (see Section 5.3).
GET /index.html HTTP/1.1 5.2. Client Handling of A Commitment
Host: example.com
HTTP/1.1 200 OK The value of the "commit" member MUST be ignored unless the
Content-Type: text/html alternative service can be strongly authenticated. The same
Cache-Control: max-age=600 authentication requirements that apply to "https://" resources SHOULD
Age: 30 be applied to authenticating the alternative. Minimum authentication
Date: Thu, 1 May 2014 16:20:09 GMT requirements for HTTP over TLS are described in Section 2.1 of
HTTP-TLS: ma=3600 [I-D.ietf-httpbis-alt-svc] and Section 3.1 of [RFC2818]. As noted in
[I-D.ietf-httpbis-alt-svc], clients can impose other checks in
addition to this minimum set. For instance, a client might choose to
apply key pinning [RFC7469].
This header field creates a commitment from the origin [RFC6454] of A client that receives a commitment and that successfully
the associated resource (in the example, "http://example.com"). For authenticates the alternative service can assume that a secured
the duration of the commitment, clients SHOULD strongly authenticate alternative will remain available for the commitment interval. The
the server for all subsequent requests made to that origin, though commitment interval starts when the commitment is received and
this creates some risks for clients (see Section 5.2). authenticated and runs for a number of seconds equal to value of the
"commit" member, less the current age of the http-opportunistic
response (as defined in Section 4.2.3 of [RFC7234]). A client SHOULD
avoid sending requests via cleartext protocols or to unauthenticated
alternative services for the duration of the commitment interval,
except to discover new potential alternatives.
Authentication for HTTP over TLS is described in Section 3.1 of A commitment only applies to the origin of the http-opportunistic
[RFC2818], noting the additional requirements in Section 2.1 of well-known resource that was retrieved; other origins listed in the
[I-D.ietf-httpbis-alt-svc]. The header field MUST be ignored if "origins" member MUST be independently discovered and authenticated.
strong authentication fails; otherwise, an attacker could create a
persistent denial of service by falsifying a commitment.
The commitment to use authenticated TLS persists for a period A commitment is not bound to a particular alternative service.
determined by the value of the "ma" parameter. See Section 4.2.3 of Clients are able to use alternative services that they become aware
[RFC7234] for details of determining response age. of. However, once a valid and authenticated commitment has been
received, clients SHOULD NOT use an unauthenticated alternative
service. Where there is an active commitment, clients SHOULD ignore
advertisements for unsecured alternative services. A client MAY send
requests to an unauthenticated origin in an attempt to discover
potential alternative services, but these requests SHOULD be entirely
generic and avoid including credentials.
ma-parameter = delta-seconds 5.3. Operational Considerations
The commitment made by the "HTTP-TLS" header field applies only to Errors in configuration of commitments has the potential to render
the origin of the resource that generates the "HTTP-TLS" header even the unsecured origin inaccessible for the duration of a
field. commitment. Initial deployments are encouraged to use short duration
commitments so that errors can be detected without causing the origin
to become inaccessible to clients for extended periods.
Requests for an origin that has a persisted, unexpired value for To avoid situations where a commitment causes errors, clients MAY
"HTTP-TLS" MUST fail if they cannot be made over an authenticated TLS limit the time over which a commitment is respected for a given
connection. origin. A lower limit might be appropriate for initial commitments;
the certainty that a site has set a correct value - and the
corresponding limit on persistence - might increase as a commitment
is renewed multiple times.
Note that the commitment is not bound to a particular alternative 6. The "http-opportunistic" well-known URI
service. Clients SHOULD use alternative services that they become
aware of. However, clients MUST NOT use an unauthenticated
alternative service for an origin with this commitment. Where there
is an active commitment, clients MAY instead ignore advertisements
for unsecured alternatives services.
5.2. Operational Considerations This specification defines the "http-opportunistic" well-known URI
[RFC5785]. An origin is said to have a valid http-opportunistic
resource when:
To avoid situations where a persisted value of "HTTP-TLS" causes a o The client has obtained a 200 (OK) response for the well-known URI
client to be unable to contact a site, clients SHOULD limit the time from the origin, or refreshed one in cache [RFC7234], and
that a value is persisted for a given origin. 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 o That response has the media type "application/json", and
client MAY use key pinning [RFC7469] or any other mechanism that o That response's payload, when parsed as JSON [RFC7159], contains
would otherwise be restricted to use with "https" URIs, provided that an object as the root.
the mechanism can be restricted to a single HTTP origin.
6. Security Considerations o The "origins" member of the root object has a value of an array of
strings, one of which is a case-insensitive character-for-
character match for the origin in question, serialised into
Unicode as per [RFC6454], Section 6.1, and
6.1. Security Indicators This specification defines one additional, optional member of the
root object, "commit" in Section 5. Unrecognised members MUST be
ignored.
7. IANA Considerations
This specification registers a Well-known URI [RFC5785]:
o URI Suffix: http-opportunistic
o Change Controller: IETF
o Specification Document(s): [this specification]
o Related Information:
8. Security Considerations
8.1. Security Indicators
User Agents MUST NOT provide any special security indicia when an User Agents MUST NOT provide any special security indicia when an
"http" resource is acquired using TLS. In particular, indicators "http" resource is acquired using TLS. In particular, indicators
that might suggest the same level of security as "https" MUST NOT be that might suggest the same level of security as "https" MUST NOT be
used (e.g., using a "lock device"). used (e.g., a "lock device").
6.2. Downgrade Attacks 8.2. Downgrade Attacks
A downgrade attack against the negotiation for TLS is possible. With A downgrade attack against the negotiation for TLS is possible. With
the "HTTP-TLS" header field, this is limited to occasions where commitment Section 5, this is limited to occasions where clients have
clients have no prior information (see Section 6.3), or when no prior information (see Section 8.3), or when persisted commitments
persisted commitments have expired. have expired.
For example, because the "Alt-Svc" header field For example, because the "Alt-Svc" header field
[I-D.ietf-httpbis-alt-svc] likely appears in an unauthenticated and [I-D.ietf-httpbis-alt-svc] likely appears in an unauthenticated and
unencrypted channel, it is subject to downgrade by network attackers. unencrypted channel, it is subject to downgrade by network attackers.
In its simplest form, an attacker that wants the connection to remain In its simplest form, an attacker that wants the connection to remain
in the clear need only strip the "Alt-Svc" header field from in the clear need only strip the "Alt-Svc" header field from
responses. responses.
Downgrade attacks can be partially mitigated using the "HTTP-TLS" Downgrade attacks can be partially mitigated using the "commit"
header field, because when it is used, a client can avoid using member of the http-opportunistic well-known resource, because when it
cleartext to contact a supporting server. However, this only works is used, a client can avoid using cleartext to contact a supporting
when a previous connection has been established without an active server. However, this only works when a previous connection has been
attacker present; a continuously present active attacker can either established without an active attacker present; a continuously
prevent the client from ever using TLS, or offer its own certificate. present active attacker can either prevent the client from ever using
TLS, or offer its own certificate.
6.3. Privacy Considerations 8.3. Privacy Considerations
Cached alternative services can be used to track clients over time; Cached alternative services can be used to track clients over time;
e.g., using a user-specific hostname. Clearing the cache reduces the e.g., using a user-specific hostname. Clearing the cache reduces the
ability of servers to track clients; therefore clients MUST clear ability of servers to track clients; therefore clients MUST clear
cached alternative service information when clearing other origin- cached alternative service information when clearing other origin-
based state (i.e., cookies). based state (i.e., cookies).
6.4. Confusion Regarding Request Scheme 8.4. Confusion Regarding Request Scheme
Many existing HTTP/1.1 implementations use the presence or absence of Many existing HTTP/1.1 implementations use the presence or absence of
TLS in the stack to determine whether requests are for "http" or TLS in the stack to determine whether requests are for "http" or
"https" resources. This is necessary in many cases because the most "https" resources. This is necessary in many cases because the most
common form of an HTTP/1.1 request does not carry an explicit common form of an HTTP/1.1 request does not carry an explicit
indication of the URI scheme. indication of the URI scheme.
HTTP/1.1 MUST NOT be used for opportunistically secured requests. HTTP/1.1 MUST NOT be used for opportunistically secured requests.
Some HTTP/1.1 implementations use ambient signals to determine if a Some HTTP/1.1 implementations use ambient signals to determine if a
request is for an "https" resource. For example, implementations request is for an "https" resource. For example, implementations
might look for TLS on the stack or a port number of 443. An might look for TLS on the stack or a port number of 443. An
implementation that supports opportunistically secured requests implementation that supports opportunistically secured requests
SHOULD suppress these signals if there is any potential for SHOULD suppress these signals if there is any potential for
confusion. confusion.
7. References 9. References
7.1. Normative References 9.1. Normative References
[I-D.ietf-httpbis-alt-svc] [I-D.ietf-httpbis-alt-svc]
mnot, m., McManus, P., and J. Reschke, "HTTP Alternative mnot, m., McManus, P., and J. Reschke, "HTTP Alternative
Services", draft-ietf-httpbis-alt-svc-09 (work in Services", draft-ietf-httpbis-alt-svc-14 (work in
progress), November 2015. progress), March 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>. <http://www.rfc-editor.org/info/rfc2818>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>. <http://www.rfc-editor.org/info/rfc5246>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<http://www.rfc-editor.org/info/rfc5785>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011, DOI 10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>. <http://www.rfc-editor.org/info/rfc6454>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>. <http://www.rfc-editor.org/info/rfc7230>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014, RFC 7234, DOI 10.17487/RFC7234, June 2014,
<http://www.rfc-editor.org/info/rfc7234>. <http://www.rfc-editor.org/info/rfc7234>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>. <http://www.rfc-editor.org/info/rfc7540>.
7.2. Informative References 9.2. Informative References
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>. 2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435, Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>. December 2014, <http://www.rfc-editor.org/info/rfc7435>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <http://www.rfc-editor.org/info/rfc7469>.
Appendix A. Acknowledgements Appendix A. Acknowledgements
Thanks to Patrick McManus, Eliot Lear, Stephen Farrell, Guy Podjarny, Thanks to Patrick McManus, Eliot Lear, Stephen Farrell, Guy Podjarny,
Stephen Ludin, Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla Stephen Ludin, Erik Nygren, Paul Hoffman, Adam Langley, Eric Rescorla
and Richard Barnes for their feedback and suggestions. and Richard Barnes for their feedback and suggestions.
Authors' Addresses Authors' Addresses
Mark Nottingham Mark Nottingham
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