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Versions: (draft-housley-web-pki-problems) 00 01 02 03 04 05

Internet Architecture Board                                   R. Housley
Internet-Draft                                            Vigil Security
Intended status: Informational                             K. O'Donoghue
Expires: March 26, 2017                                 Internet Society
                                                      September 22, 2016

Problems with the Public Key Infrastructure (PKI) for the World Wide Web


   The Public Key Infrastructure (PKI) used for the World Wide Web (Web
   PKI) is a vital component of trust in the Internet.  In recent years,
   there have been a number of improvements made to this infrastructure,
   including improved certificate status checking, automation, and
   transparency of governance.  However, additional improvements
   necessary.  This document identifies continuing areas of concerns and
   provides recommendations to the Internet community for additional
   improvements, moving toward a more robust and secure Web PKI.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 26, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Very Brief Description of the Web PKI . . . . . . . . . . . .   2
   3.  Improvements to the Web PKI . . . . . . . . . . . . . . . . .   3
     3.1.  Weak Cryptographic Algorithms or Short Public Keys  . . .   3
     3.2.  Support for Enterprise PKIs . . . . . . . . . . . . . . .   4
     3.3.  Web PKI in the Home . . . . . . . . . . . . . . . . . . .   6
     3.4.  Browser Error Messages  . . . . . . . . . . . . . . . . .   8
     3.5.  Governance Improvements to the Web PKI  . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  13
   Appendix B.  IAB Members at the Time of Approval  . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   There are many interrelated problems with the current Public Key
   Infrastructure (PKI) used for the World Wide Web (Web PKI).  The Web
   PKI makes use of certificates as described in RFC 5280 [RFC5280].
   These certificates are primarily used with Transport Layer Security
   (TLS) as described in RFC 5246 [RFC5246].

   The economics of the Web PKI value chain are discussed in [VFBH],
   [AV], and [AVAV].  This document does not discuss the economic issues
   further, but these economic issues provide motivation for correcting
   the other problems that are discussed in this document.

   Over the years, many technical improvements have been made to the Web
   PKI, but several challenges remain.  This document offers a general
   set of recommendations to the Internet community that might be
   helpful in addressing these remaining challenges.

2.  Very Brief Description of the Web PKI

   Certificates are specified in RFC 5280 [RFC5280].  Certificates
   contain, among other things, a subject name, a public key, a limited
   valid lifetime, and the digital signature of the Certification
   Authority (CA).  Certificate users require confidence that the

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   private key associated with the certified public key is owned by the
   named subject.

   The architectural model used in the Web PKI includes:

   EE:   End Entity -- the subject of a certificate -- certificates are
         issued to end entities including Web servers and clients that
         need mutual authentication.

   CA:   Certification Authority -- the issuer of a certificate --
         issues certificates for end entities including Web servers and

   RA:   Registration Authority -- an optional system to which a CA
         delegates some management functions such as identity validation
         or physical credential distribution.

   A valid certificate may be revoked for any number of reasons.  CAs
   are responsible for indicating the revocation status of the
   certificates that they issue throughout the lifetime of the
   certificate.  Revocation status information may be provided using the
   Online Certificate Status Protocol (OCSP) [RFC6960], certificate
   revocation lists (CRLs) [RFC5280], or some other mechanism.  In
   general, when revocation status information is provided using CRLs,
   the CA is also the CRL issuer.  However, a CA may delegate the
   responsibility for issuing CRLs to a different entity.

   The enrollment process used by a CA makes sure that the subject name
   in the certificate is appropriate and that the subject actually holds
   the private key.  Proof of possession of the private key is often
   accomplished through a challenge-response protocol.

3.  Improvements to the Web PKI

   Over the years, many technical improvements have been made to the Web
   PKI.  Despite this progress, several challenges remain.  This section
   discusses several unresolved problems, and it suggests general
   directions for tackling them.

3.1.  Weak Cryptographic Algorithms or Short Public Keys

   Several digital signature algorithms, one-way hash functions, and
   public key sizes that were once considered strong are no longer
   considered adequate.  This is not a surprise.  Cryptographic
   algorithms age; they become weaker over time.  As new cryptanalysis
   techniques are developed and computing capabilities increase, the
   amount of time needed to break a particular cryptographic algorithm

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   will decrease.  For this reason, the algorithms and key sizes used in
   the Web PKI need to migrate over time.

   Browser vendors have been trying to manage algorithm and key size
   transitions.  It is a significant challenge to maintain a very high
   degree of interoperability across the world wide web while phasing
   out aged cryptographic algorithm or too small key size.  When these
   appear in a long-lived trust anchor or intermediate CA certificate,
   refusal to accept them can impact a very large certificate subtree.
   In addition, if a certificate for a web site with a huge amount of
   traffic is in that subtree, rejecting that certificate may impact too
   many users.

   Despite this situation, the MD5 and SHA-1 one-way hash functions have
   been almost completely eliminated from the Web PKI, and 1024-bit RSA
   public keys are essentially gone [MB2015] [MB2016].  It took a very
   long time to make this happen, and trust anchors and certificates
   that used these cryptographic algorithms were considered valid long
   after they were widely known to be too weak.

   Obviously, additional algorithm transitions will be needed in the
   future.  The algorithms and key sizes that are acceptable today will
   become weaker with time.  [RFC7696] offers some guidelines regarding
   cryptographic algorithm agility.

   The Web PKI can prepare for the next transition by:

   1.  Having experts periodically evaluate the current choices of
       algorithm and key size.  While it is not possible to predict when
       a new cryptanalysis technique will be discovered, the end of the
       useful lifetime of most algorithms and key sizes is known many
       years in advance.

   2.  Planning for a smooth and orderly transition from a weak
       algorithm or key size.  Experience has shown that many years are
       needed produce to specifications, develop implementations, and
       then deploy replacements.

   3.  Reducing the lifetime of certificates, especially intermediate CA
       certificates, to create frequent opportunities to change an
       algorithm or key size.

3.2.  Support for Enterprise PKIs

   Many enterprises operate their own PKI.  These enterprises do not
   want to be part of the traditional Web PKI, but they face many
   challenges in order to achieve a similar user experience and level of

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   Users must install the trust anchor or trust anchors for the
   enterprise PKI in their browser.  There is not a standard way to
   accomplish trust anchor installation, and users are often faced with
   scary warnings in the process of the installation.

   Enterprise PKI users often experience greater latency than tradition
   Web PKI users.  Some browser vendors provide a proprietary revocation
   checking mechanism that obtains revocation status for the entire Web
   PKI in a very compact form.  This mechanism eliminates latency since
   no network traffic is generated at the time that a certificate is
   being validated.  However, these mechanisms cover only the trust
   anchor store for that browser vendor, excluding all enterprise PKIs.
   In addition, measurements in 2015 [IMC2015] show that these
   mechanisms do not currently provide adequate coverage of the Web PKI.

   RFC 6961 [RFC6961] defines the TLS Multiple Certificate Status
   Request extension, which allows the web server to provide status
   information about its own certificate and also the status of
   intermediate certificates in the certification path.  By including
   this extension in the TLS handshake, the browser asks the web server
   to provide OCSP responses in addition to the server certificate.
   This approach greatly reduces the latency since the browser does not
   need to generate any OCSP requests or wait for any OCSP responses.

   The provision of the time-stamped OCSP response in the TLS handshake
   is referred to as "stapling" the OCSP response to the TLS handshake.
   If the browser does not receive a stapled OCSP response, it can
   contact the OCSP responder directly.  In addition, the MUST_STAPLE
   feature [TLSFEATURE] can be used to insist that OCSP stapling be

   When every browser that connects to a high volume website performs
   its own OCSP lookup, the OCSP responder must handle a large volume of
   OCSP requests in real-time.  OCSP stapling can avoid enormous volumes
   of OCSP requests for certificates of popular websites, so stapling
   can significantly reduce the cost of providing an OCSP service.

   OCSP stapling can also improve user privacy, since the web server,
   not the browser, contacts the OCSP responder.  In this way, the OCSP
   responder is not able to determine which browsers are checking the
   validity of certificate for particular websites.

   Several enterprises issue certificates to all of their employees, and
   among other uses, these certificates are used in TLS client
   authentication.  There is not a common way to import the private key
   and the client certificate into browsers.  In fact, the private key
   can be stored in many different formats depending on the software
   used to generate the public/private key pair.  PKCS#12 [RFC7292]

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   seems to be the most popular format at the moment.  A standard way to
   import the needed keying material and a standard format will make
   this task much easier, and the World Wide Web might enjoy an increase
   in mutual authentication.

   Enterprise PKIs can be better supported if:

   1.  Each enterprise PKI offers an OCSP Responder, and web sites with
       certificates from the enterprise PKI make use of OCSP Stapling.

   2.  Browser vendors support a standard way for the trust anchors for
       the enterprise PKI to be installed.

   3.  Browser vendors support a standard way to install private keys
       and certificates for use in client authentication.

   4.  In the event that browser vendors continue to offer latency-free
       proprietary revocation status checking mechanisms, then these
       mechanisms need to expand the coverage to all of the Web PKI and
       offer a means to include enterprise PKIs in the coverage.

3.3.  Web PKI in the Home

   More and more, web protocols are being used to manage devices in the
   home.  For example, homeowners can use a web browser to connect to a
   web site that is embedded in their home router to adjust various
   settings.  The router allows the browser to access web pages to
   adjust these setting as long as the connection originates from the
   home network and the proper password is provided.  However, there is
   no way for the browser to authenticate to the embedded web site.
   Authentication of the web site is normally performed during the TLS
   handshake, but the Web PKI is not equipped to issue certificates to
   home routers or the many other home devices that employ embedded web
   sites for homeowner management.

   A solution in this environment must not depend on the homeowner to
   perform duties that are normally associated with a web site
   administrator.  However, some straightforward tasks could be done at
   the time the device is installed in the home.  These task cannot be
   more complex that the initial setup of a new printer in the home,
   otherwise they will be skipped or done incorrectly.

   There are two very different approaches to certificates for home
   devices that have been discussed over the years.  In the first
   approach, a private key and certificate are installed in the device
   at the factory.  The certificate has an unlimited lifetime.  Since it
   never expires, no homeowner action is needed to renew it.  Also,
   since the certificate never changes, the algorithms are selected by

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   the factory for the lifetime of the device.  The subject name in the
   certificate is quite generic, as it must be comprised of information
   that is known in the factory.  The subject name is often based on
   some combination of the manufacturer, model, serial number, and MAC
   address.  While these do uniquely identify the device, they have
   little meaning to the homeowner.

   In the second approach, like the first one, a private key and a
   certificate that are installed in the device at the factory, but the
   homeowner is unaware of them.  This factory-installed certificate is
   used only to authenticate to a CA operated by the manufacturer.  At
   the time the device is installed, the homeowner can provide a portion
   of the subject name for the device, and the manufacturer CA can issue
   a certificate that includes a subject name that the homeowner will
   recognize.  The certificate can be renewed without any action by the
   homeowner at appropriate intervals.  Also, following a software
   update, the algorithms used in the TLS handshake and the certificate
   can be updated.

   Section 3.1 of this document calls for the ability to transition from
   weak cryptographic algorithms over time.  For this reason, and the
   ability to use a subject name that the homeowner will recognize, the
   second approach is preferred.

   One potential problem with the second approach is continuity of
   operations of the manufacturer CA.  After the device is deployed, the
   manufacturer might go out of business, and then come time for renewal
   of the certificate, there will not be a CA to issue the new
   certificate.  One possible solution might be modeled on the domain
   name business, where other parties will continue to provide needed
   services if the original provider goes out of business.

   The Web PKI can prepare for the vast number of home devices that need
   certificates by:

   1.  Building upon the work being done in the IETF ACME Working Group
       [ACMEWG] to facilitate the automatic renewal of certificates for
       home devices without any actions by the homeowner beyond the
       initial device setup.

   2.  Working with device manufacturers to establish scalable CAs that
       will continue to issue certificates for the deployed devices even
       if the manufacture goes out of business.

   3.  Working with device manufacturers to establish OCSP Responders so
       that the web sites that are embedded in the devices can provide
       robust authentication and OCSP stapling in a manner that is
       compatible with traditional web sites.

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3.4.  Browser Error Messages

   Many users find browser error messages related to certificates
   confusing.  Good man-machine interfaces are always difficult, but in
   this situation users are unable to fully understand the risks that
   they are accepting, and as a result they do not make informed
   decisions about when to proceed and when to stop.  This aspect of
   browser usability needs to be improved for users to make better
   security choices.

   Browser users have been trained to ignore warnings, making them
   ineffective.  Further, solving the error message situation in
   isolation is probably not possible; instead, it needs to be
   considered along side the other issues raised in this document.
   Browser users have been trained to find a way around any error, and
   very often, the error is the result of web server misconfiguration,
   and it does not actually present a risk to the browser user.

   If the risk to the user is high, then the session should be closed
   with a clear description of the problem that was encountered.  Doing
   so will lead to improved management of the overall Web infrastructure
   over time, especially as the tools that are being developed for web
   server administrators are rolled out.

   In some enterprises, avoiding these errors requires the addition of
   enterprise-specific trust anchors to the browser.  Additional tools
   are needed to allow administrations to easily add appropriate trust
   anchors, especially browsers on consumer devices as more and more
   enterprises are embracing bring-your-own-device policies.

   The Web PKI can simplify user choice and improve user actions by:

   1.  Browser vendors providing additional intelligence to eliminate
       the majority of certificate warnings, only prompting the user
       when there is likely to be a risk.

   2.  Browser vendors improving error messages to clearly indicate the
       risk that the user is accepting if they proceed.

3.5.  Governance Improvements to the Web PKI

   As with many other technologies, Web PKI technical issues are tangled
   up with policy and process issues.  Policy and process issues have
   evolved over time, sometimes eroding confidence and trust in the Web
   PKI.  Governance structures are needed that increase transparency and

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   Web PKI users are by definition asked to trust CAs.  This includes
   what CAs are trusted to do properly, and what they are trusted not to
   do.  The system for determining which CAs are added to or removed
   from the trust anchor store in browsers is opaque and confusing to
   most Web PKI users.  The CA/Browser Forum has developed baseline
   requirements for the management and issuance of certificates
   [CAB2014] for individual CAs.  However, the process by which an
   individual CA gets added to the trust anchor store by each of the
   browsers vendors is not straightforward.  The individual browser
   vendors determine what should and should not be trusted by including
   the CA certificate in their trust anchor store.  They do this by
   leveraging the AICPA/CICA WebTrust Program for Certification
   Authorities [WEBTRUST].  This program provides auditing requirements
   and a trust mark for CAs that meet them.  Failure to pass an audit
   can result in the CA being removed from the trust store.

   Once the browser has shipped, how does a user know which CAs are
   trusted or what has changed recently?  For an informed user,
   information about which CAs have been added to or deleted from the
   browser trust anchor store can be found in the browser release notes.
   Users can also examine the policies of the various CAs that have been
   developed and posted for the WebTrust Program.  However, this is a
   very high barrier for the average user.  There are options to make
   local modifications by educated users, but there is little
   understanding about the implications of these choices.  How does an
   individual, organization, or enterprise really determine if a
   particular CA is trustworthy?  Do the default choices inherited from
   the browser vendors truly represent the organization's trust model?
   What constitutes sufficiently bad behavior by a CA to cause removal
   from the trust anchor store?

   In addition, it can be hazardous for users to remove CAs from the
   browser trust anchor store.  If a user removes a CA from the browser
   trust anchor store, some web sites may become completely inaccessible
   or require the user to take explicit action to accept warnings or
   bypass browser protections related to untrusted certificates.

   The guidelines provided by the WebTrust program [WEBTRUST] provide a
   framework for removing a CA from the trust anchor store.  There may
   be a few very large CAs that are critical to significant portions of
   World Wide Web infrastructure.  Removing one of these CAs can have a
   significant impact on a huge number of websites.  As discussed in
   Section 3.4, users are already struggling to understand the
   implications of untrusted certificates, so they often ignore warnings
   presented by the browser.

   There are a number of organizations that play significant roles in
   the operation of the Web PKI, including the CA/Browser Forum, the

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   WebTrust Program, and the browser vendors.  These organizations act
   on behalf of the entire Internet community; therefore, transparency
   in these operations is fundamental to confidence and trust in the Web
   PKI.  Recently the CA/Browser Forum made some changes to their
   operational procedures to make it easier for people to participate
   and to improve visibility into their process [CAB1.2].  This is a
   significant improvement, but these processes need to continue to
   evolve in an open, inclusive, and transparent manner.  Currently, as
   the name implies, the CA/Browser Forum members primarily represent
   CAs and browser vendors.  It would be better if relying parties also
   have a voice in this forum.

   Since the Web PKI is widespread, applications beyond the World Wide
   Web are making use of the Web PKI.  For example, the Web PKI is used
   to secure connections between SMTP servers.  In these environments,
   the browser-centric capabilities are unavailable.  The current
   governance structure does not provide a way for the relying parties
   in these applications to participate.

   The Web PKI governance structures can be made more open and
   transparent by:

   1.  Browser vendors providing additional visibility and tools to
       support the management of the trust anchor store.

   2.  Governance organizations providing a way for all relying parties,
       including ones associated with non-browser applications, to

4.  Security Considerations

   This document considers the weaknesses of the current Web PKI system
   and provides recommendations for improvements.  Some of the risks
   associated with doing nothing or continuing down the current path are
   articulated.  The Web PKI is a vital component of a trusted Internet,
   and as such needs to be improved to sustain continued growth of the

5.  IANA Considerations


   {{{ RFC Editor: Please remove this section prior to publication. }}}

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6.  References

6.1.  Normative References

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

6.2.  Informative References

   [ACMEWG]   IETF, "Charter for Automated Certificate Management
              Environment (acme) Working Group", June 2015,

   [AV]       Arnbak, A. and N. van Eijk, "Certificate Authority
              Collapse: Regulating Systemic Vulnerabilities in the HTTPS
              Value Chain", 2012 TRPC , August 2012,

   [AVAV]     Asghari, H., van Eeten, M., Arnbak, A., and N. van Eijk,
              "Security Economics in the HTTPS Value Chain", Workshop on
              Economics of Information Security (WEIS) 2013 , 2013,

   [CAB1.2]   CA/Browser Forum, "Bylaws of the CA/Browser Forum",
              October 2014, <https://cabforum.org/wp-content/uploads/CA-

   [CAB2014]  CA/Browser Forum, "CA/Browser Forum Baseline Requirements
              for the Issuance and Management of Publicly-Trusted
              Certificates, v.1.2.2", October 2014,

   [IMC2015]  Liu, Y., Tome, W., Zhang, L., Choffnes, D., Levin, D.,
              Maggs, B., Mislove, A., Schulman, A., and C. Wilson, "An
              End-to-End Measurement of Certificate Revocation in the
              Web's PKI", October 2015,

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   [MB2015]   Wilson, K., "Phase 2: Phasing out Certificates with
              1024-bit RSA Keys", January 2015,

   [MB2016]   Barnes, R., "Payment Processors Still Using Weak Crypto",
              February 2016,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,

   [RFC7292]  Moriarty, K., Ed., Nystrom, M., Parkinson, S., Rusch, A.,
              and M. Scott, "PKCS #12: Personal Information Exchange
              Syntax v1.1", RFC 7292, DOI 10.17487/RFC7292, July 2014,

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,

              Hallam-Baker, P., "X.509v3 TLS Feature Extension", draft-
              hallambaker-tlsfeature-10 (work in progress), July 2015.

   [VFBH]     Vratonjic, N., Freudiger, J., Bindschaedler, V., and J.
              Hubaux, "The Inconvenient Truth About Web Certificates",
              Workshop on Economics of Information Security (WEIS)
              2011 , 2011,

              CPA Canada, "WebTrust Program for Certification
              Authorities", August 2015, <http://www.webtrust.org/

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Appendix A.  Acknowledgements

   This document has been developed within the IAB Privacy and Security
   Program.  The authors greatly appreciate the review and suggestions
   provided by Rick Andrews, Mary Barnes, Richard Barnes, Marc Blanchet,
   Alissa Cooper, Nick Doty, Stephen Farrell, Joe Hall, Ted Hardie,
   Ralph Holz, Lee Howard, Christian Huitema, Eliot Lear, Xing Li, Lucy
   Lynch, Gervase Markham, Andrei Robachevsky, Thomas Roessler, Jeremy
   Rowley, Christine Runnegar, Jakob Schlyter, Wendy Seltzer, Brian
   Trammell, and Juan Carlos Zuniga.

Appendix B.  IAB Members at the Time of Approval

   {{{ RFC Editor: Please add the names to the IAB members at the time
   that this document is put into the RFC Editor queue. }}}

Authors' Addresses

   Russ Housley
   Vigil Security
   918 Spring Knoll Drive
   Herndon, VA  20170

   Email: housley@vigilsec.com

   Karen O'Donoghue
   Internet Society
   1775 Wiehle Ave #201
   Reston, VA  20190

   Email: odonoghue@isoc.org

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