draft-ietf-httpbis-http2-encryption-00.txt   draft-ietf-httpbis-http2-encryption-01.txt 
Network Working Group M. Nottingham HTTPbis Working Group M. Nottingham
Internet-Draft Internet-Draft
Intended status: Experimental M. Thomson Intended status: Experimental M. Thomson
Expires: December 14, 2014 Mozilla Expires: June 18, 2015 Mozilla
June 12, 2014 December 15, 2014
Opportunistic Encryption for HTTP URIs Opportunistic Security for HTTP
draft-ietf-httpbis-http2-encryption-00 draft-ietf-httpbis-http2-encryption-01
Abstract Abstract
This describes how "http" URIs can be accessed using Transport Layer This document describes how "http" URIs can be accessed using
Security (TLS) to mitigate pervasive monitoring attacks. Transport Layer Security (TLS) to mitigate pervasive monitoring
attacks.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 14, 2014. This Internet-Draft will expire on June 18, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Goals and Non-Goals . . . . . . . . . . . . . . . . . . . 2 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 . . . . . . . . . . . . . . . . . . . . 3 3. Server Authentication . . . . . . . . . . . . . . . . . . . . 4
4. Interaction with "https" URIs . . . . . . . . . . . . . . . . 4 4. Interaction with "https" URIs . . . . . . . . . . . . . . . . 4
5. Requiring Use of TLS . . . . . . . . . . . . . . . . . . . . 4 5. Requiring Use of TLS . . . . . . . . . . . . . . . . . . . . 5
5.1. The HTTP-TLS Header Field . . . . . . . . . . . . . . . . 5 5.1. The HTTP-TLS Header Field . . . . . . . . . . . . . . . . 5
5.2. Operational Considerations . . . . . . . . . . . . . . . 6 5.2. Operational Considerations . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6.1. Security Indicators . . . . . . . . . . . . . . . . . . . 7 6.1. Security Indicators . . . . . . . . . . . . . . . . . . . 7
6.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 7 6.2. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 7
6.3. Privacy Considerations . . . . . . . . . . . . . . . . . 7 6.3. Privacy Considerations . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.4. Confusion Regarding Request Scheme . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8 7.1. Normative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8 7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 to
be accessed using TLS [RFC5246] opportunistically. be accessed using TLS [RFC5246] opportunistically.
Currently, "https" URIs requires acquiring and configuring a valid Currently, "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. Therefore, this document describes a usage model whereby difficult. Therefore, this document describes a usage model whereby
sites can serve "http" URIs over TLS without being required to sites can serve "http" URIs over TLS without being required to
support strong server authentication. support strong server authentication.
A mechanism for limiting the potential for active attacks is Opportunistic Security [I-D.dukhovni-opportunistic-security] does not
described in Section 5. This provides clients with additional provide the same guarantees as using TLS with "https" URIs; it is
protection against them for a period after successfully connecting to vulnerable to active attacks, and does not change the security
a server using TLS. This does not offer the same level of protection context of the connection. Normally, users will not be able to tell
as afforded to "https" URIs, but increases the likelihood that an that it is in use (i.e., there will be no "lock icon").
active attack be detected.
By its nature, this technique is vulnerable to active attacks. A
mechanism for partially mitigating them is described in Section 5.
It 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 1.1. Goals and Non-Goals
The immediate goal is to make the use of HTTP more robust in the face The immediate goal is to make the use of HTTP more robust in the face
of pervasive passive monitoring [RFC7258]. of pervasive passive monitoring [RFC7258].
A secondary goal is to limit the potential for active attacks. It is 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 not intended to offer the same level of protection as afforded to
"https" URIs, but instead to increase the likelihood that an active "https" URIs, but instead to increase the likelihood that an active
attack can be detected. attack can be detected.
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administrative effort to configure. administrative effort to configure.
1.2. Notational Conventions 1.2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. Using HTTP URIs over TLS 2. Using HTTP URIs over TLS
An origin server that supports the resolution of HTTP URIs can An origin server that supports the resolution of "http" URIs can
indicate support for this specification by providing an alternative indicate support for this specification by providing an alternative
service advertisement [I-D.ietf-httpbis-alt-svc] for a protocol service advertisement [I-D.ietf-httpbis-alt-svc] for a protocol
identifier that uses TLS, such as "h2" [I-D.ietf-httpbis-http2]. identifier that uses TLS, such as "h2" [I-D.ietf-httpbis-http2].
A client that receives such an advertisement MAY direct future A client that receives such an advertisement MAY make future requests
requests for the associated origin to the identified service (as intended for the associated origin ([RFC6454]) to the identified
specified by [I-D.ietf-httpbis-alt-svc]). service (as specified by [I-D.ietf-httpbis-alt-svc]).
A client that places the importance of passive protections over A client that places the importance of protection against passive
performance might choose to withold requests until an encrypted attacks over performance might choose to withhold requests until an
connection is available. However, if such a connection cannot be encrypted connection is available. However, if such a connection
successfully established, the client MAY resume its use of the cannot be successfully established, the client MAY resume its use of
cleartext connection. the cleartext connection.
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. Clients with information, such as one with the OPTIONS method. Likewise, clients
expired alternative services information could make a similar request with existing alternative services information could make such a
in parallel to an attempt to contact an alternative service, to request before they expire, in order minimize the delays that might
minimize the delays that might be incurred by failing to contact the be incurred.
alternative service.
3. Server Authentication 3. Server Authentication
There are no existing expectations with respect to cryptographically By their nature, "http" URIs do not require cryptographically strong
strong server authentication when it comes to resolving HTTP URIs. server authentication; that is only implied by "https" URIs.
Establishing it, as described in [RFC2818], creates a number of Furthermore, doing so (as per [RFC2818]) creates a number of
operational challenges. For these reasons, server authentication is operational challenges. For these reasons, server authentication is
not mandatory for HTTP URIs when using the mechanism described in not mandatory for "http" URIs when using the mechanism described in
this specification. this specification.
When connecting to an alternative service for an "http" URI, clients When connecting to an alternative service for an "http" URI, clients
are required to perform the server authentication procedure described are not required to perform the server authentication procedure
in Section 3.1 of [RFC2818]. The server certificate, if one is described in Section 3.1 of [RFC2818]. The server certificate, if
proffered by the alternative service, is not necessarily checked for one is proffered by the alternative service, is not necessarily
validity, expiration, issuance by a trusted certificate authority or checked for validity, expiration, issuance by a trusted certificate
matched against the name in the URI. Therefore, the alternative authority or matched against the name in the URI. Therefore, the
service MAY provide any certificate, or even select TLS cipher suites alternative service MAY provide any certificate, or even select TLS
that do not include authentication. cipher suites that do not include authentication.
A client MAY perform additional checks on the certificate that it is A client MAY perform additional checks on the offered certificate if
offered (if the server does not select an unauthenticated TLS cipher the server does not select an unauthenticated TLS cipher suite. This
suite). For instance, a client could examine the certificate to see document doesn't define any such checks, though clients could be
if it has changed over time. configured with a policy that defines what is acceptable.
In order to retain the authority properties of "http" URIs, and as As stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use
stipulated by [I-D.ietf-httpbis-alt-svc], clients MUST NOT use alternative services with a host other than the origin's, unless the
alternative services that identify a host other than that of the alternative service itself is strongly authenticated (as the origin's
origin, unless the alternative service itself is strongly host); for example, using TLS with a certificate that validates as
authenticated (as the origin's host). This is not currently possible per [RFC2818].
for "http" URIs on cleartext transports.
4. Interaction with "https" URIs 4. Interaction with "https" URIs
An alternative service that is discovered to support "http" URIs When using alternative services, both "http" and "https" URIs might
might concurrently support "https" URIs, because HTTP/2 permits the use the same connection, because HTTP/2 permits requests for multiple
sending of requests for multiple origins (see [RFC6454]) on the one origins on the same connection.
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 Since "https" URIs rely on server authentication, a connection that
connection is initially created without authenticating the server, is initially created for "http" URIs without authenticating the
requests for "https" resources cannot be sent over that connection server cannot be used for "https" URIs until the server certificate
until the server certificate is successfully authenticated. is successfully authenticated. Section 3.1 of [RFC2818] describes
Section 3.1 of [RFC2818] describes the basic mechanism, though the the basic mechanism, though the authentication considerations in
authentication considerations in [I-D.ietf-httpbis-alt-svc] could [I-D.ietf-httpbis-alt-svc] also apply.
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 Editors' Note: this is a very rough take on an approach that would
provide a limited form of protection against downgrade attack. It's provide a limited form of protection against downgrade attack. It's
unclear at this point whether the additional effort (and modest unclear at this point whether the additional effort (and modest
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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 alternative service is able to commit to providing service
for a particular origin over TLS for a bounded period of time, for a particular origin over TLS for a bounded period of time,
clients can choose to rely upon its avilability, failing when it clients can choose to rely upon its avilability, failing when it
cannot be contacted. Effectively, this makes the alternative service cannot be contacted. Effectively, this makes the choice to use a
"sticky" in the client. secured protocol "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 5.1. The HTTP-TLS Header Field
A alternative service can make this commitment by sending a "HTTP- A alternative service can make this commitment by sending a "HTTP-
TLS" header field: TLS" header field:
HTTP-TLS = 1#parameter HTTP-TLS = 1#parameter
When it appears in a HTTP response from a strongly authenticated When it appears in a HTTP response from a strongly authenticated
alternative service, this header field indicates that the alternative service, this header field indicates that the
availability of the origin through TLS-protected alternative services availability of the origin through TLS-protected alternative services
is "sticky", and that the client MUST NOT fall back to cleartext is "sticky", and that the client MUST NOT fall back to cleartext
protocols while this information is considered fresh. protocols while this information is considered fresh.
For example: For example:
GET /index.html HTTP/1.1
Host: example.com
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: text/html Content-Type: text/html
Cache-Control: 600 Cache-Control: 600
Age: 30 Age: 30
Date: Thu, 1 May 2014 16:20:09 GMT Date: Thu, 1 May 2014 16:20:09 GMT
HTTP-TLS: ma=3600 HTTP-TLS: ma=3600
Note that the commitment is not bound to a particular alternative This header field creates a commitment from the origin [RFC6454] of
service; clients SHOULD use other alternative services that they the associated resource (in the example, "http://example.com"). For
become aware of, as long as the requirements regarding authentication the duration of the commitment, clients SHOULD strongly authenticate
and avoidance of cleartext protocols are met. the server for all subsequent requests made to that origin, though
this creates some risks for clients Section 5.2.
When this header field appears in a response, clients MUST strongly Authentication for HTTP over TLS is described in Section 3.1 of
authenticate the alternative service, as described in Section 3.1 of
[RFC2818], noting the additional requirements in [RFC2818], noting the additional requirements in
[I-D.ietf-httpbis-alt-svc]. The header field MUST be ignored if [I-D.ietf-httpbis-alt-svc]. The header field MUST be ignored if
strong authentication fails. strong authentication fails; otherwise, an attacker could create a
persistent denial of service by falsifying a commitment.
Persisted information expires after a period determined by the value The commitment to use authenticated TLS persists for a period
of the "ma" parameter. See Section 4.2.3 of 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. [RFC7234] for details of determining response age.
ma-parameter = delta-seconds ma-parameter = delta-seconds
Requests for an origin that has a persisted, unexpired value for The commitment made by the "HTTP-TLS" header field applies only to
"HTTP-TLS" MUST fail if they cannot be made over an authenticated TLS the origin of the resource that generates the "HTTP-TLS" header
connection. field. 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.
Note that the commitment is not bound to a particular alternative
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 5.2. Operational Considerations
To avoid situations where a persisted value of "HTTP-TLS" causes a 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 client to be unable to contact a site, clients SHOULD limit the time
that a value is persisted for a given origin. A lower limit might be that a value is persisted for a given origin. A lower limit might be
appropriate for initial observations of "HTTP-TLS"; the certainty appropriate for initial observations of "HTTP-TLS"; the certainty
that a site has set a correct value - and the corresponding limit on that a site has set a correct value - and the corresponding limit on
persistence - can increase as the value is seen more over time. persistence - can increase as the value is seen more over time.
Once a server has indicated that it will support authenticated TLS, a 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 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, mechanism that would otherwise be restricted to use with "https"
provided that the mechanism can be restricted to a single HTTP URIs, provided that the mechanism can be restricted to a single HTTP
origin. origin.
6. Security Considerations 6. Security Considerations
6.1. Security Indicators 6.1. 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., using a "lock device").
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clients have no prior information (see Section 6.3), or when clients have no prior information (see Section 6.3), or when
persisted commitments have expired. persisted commitments 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.
As long as a client is willing to use cleartext TCP to contact a Downgrade attacks can be partially mitigated using the "HTTP-TLS"
server, these attacks are possible. The "HTTP-TLS" header field header field, because when it is used, a client can avoid using
provides an imperfect mechanism for establishing a commitment. The cleartext to contact a supporting server. However, this only works
advantage is that this only works if a previous connection is when a previous connection has been established without an active
established where an active attacker was not present. A continuously attacker present; a continuously present active attacker can either
present active attacker can either prevent the client from ever using prevent the client from ever using TLS, or offer its own certificate.
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 6.3. Privacy Considerations
Clients that persist state for origins can be tracked over time based Cached alternative services can be used to track clients over time;
on their use of this information. Persisted information can be e.g., using a user-specific hostname. Clearing the cache reduces the
cleared to reduce the ability of servers to track clients. Clients ability of servers to track clients; therefore clients MUST clear
MUST clear persisted alternative service information when clearing cached alternative service information when clearing other origin-
other origin-based state (i.e., cookies). based state (i.e., cookies).
6.4. Confusion Regarding Request Scheme
Many existing HTTP/1.1 implementations use the presence or absence of
TLS in the stack to determine whether requests are for "http" or
"https" resources. This is necessary in many cases because the most
common form of an HTTP/1.1 request does not carry an explicit
indication of the URI scheme.
HTTP/1.1 MUST NOT be sent over HTTP/1.1 or earlier versions of the
protocol. Opportunistically secured HTTP requests MUST include an
explicit scheme identifier.
7. References 7. References
7.1. Normative References 7.1. Normative References
[I-D.ietf-httpbis-alt-svc] [I-D.ietf-httpbis-alt-svc]
Nottingham, M., McManus, P., and J. Reschke, "HTTP Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", draft-ietf-httpbis-alt-svc-01 (work Alternative Services", draft-ietf-httpbis-alt-svc-05 (work
in progress), April 2014. in progress), December 2014.
[I-D.ietf-httpbis-http2] [I-D.ietf-httpbis-http2]
Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol version 2", draft-ietf-httpbis-http2-12 (work in Protocol version 2", draft-ietf-httpbis-http2-16 (work in
progress), April 2014. progress), November 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] [I-D.ietf-websec-key-pinning]
Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", draft-ietf-websec-key-pinning-13 Extension for HTTP", draft-ietf-websec-key-pinning-21
(work in progress), May 2014. (work in progress), October 2014.
[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, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[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, August 2008. (TLS) Protocol Version 1.2", RFC 5246, August 2008.
7.2. Informative References
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011. 2011.
[RFC7234] Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June
2014.
7.2. Informative References
[I-D.dukhovni-opportunistic-security]
Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", draft-dukhovni-opportunistic-
security-06 (work in progress), November 2014.
[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, May 2014. Attack", BCP 188, RFC 7258, May 2014.
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
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