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Versions: 00 01

SAAG                                                              Y. Nir
Internet-Draft                                                  Dell EMC
Intended status: Informational                                T. Fossati
Expires: April 17, 2018                                            Nokia
                                                              Y. Sheffer
                                                        October 14, 2017

            Considerations For Using Short Term Certificates


   Recently there has been renewed interest in an old idea: Issue
   certificates with short validity periods and forego revocation
   processing, reasoning that expiration is a sufficient replacement for
   revocation as long as that expiration is not too far off.

   This document covers considerations, both security and operational,
   for using such Short Term Auto Renewed (STAR) certificates for
   various scenarios where Using a revocation protocol is considered

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 17, 2018.

Copyright Notice

   Copyright (c) 2017 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

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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
   2.  Short Term Auto Renewed Certificates  . . . . . . . . . . . .   4
     2.1.  Alternative Design: OCSP Stapling . . . . . . . . . . . .   5
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Data Center Network Hosts . . . . . . . . . . . . . . . .   5
     3.2.  Distributed System Installed in One Or More Data Centers    5
       3.2.1.  Distributed Network Security Functions  . . . . . . .   5
     3.3.  Certificate Delegation for Content Delivery Networks  . .   6
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
     4.1.  Certificate Lifetime and Renewal Schedule . . . . . . . .   6
     4.2.  Availability of the Certificate Authority . . . . . . . .   7
     4.3.  Clock Skew and the notBefore Field  . . . . . . . . . . .   7
     4.4.  Automatic Renewal . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Certificate Transparency  . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     5.1.  Reasons for Revocation  . . . . . . . . . . . . . . . . .  10
     5.2.  Longevity and Revocation  . . . . . . . . . . . . . . . .  10
     5.3.  Clock Skew and Security . . . . . . . . . . . . . . . . .  11
     5.4.  CA availability . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Certificates ([RFC5280]) are used in multiple protocols such as the
   Internet Key Exchange (IKEv2-[RFC7296]) and the Transport Layer
   Security protocol (TLS-[RFC5246]).  Certificates are used to
   authenticate communicating parties to each other.  Certificates are
   issued by Certificate Authorities (CAs) to End Entities (EE) to be
   used to authenticate them to Relying Parties (RPs) in security
   protocols.  Systems that use secure communications typically include
   certificate authorities, end entities and relying parties, with some
   nodes in the network having more than one of these roles.

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   When deploying a system involving secure communications, one of the
   challenges is how to deal with an End Entity losing control of its
   private key or having its secrecy potentially compromised.  The
   standardized ways of dealing with this is adding a protocol layer for
   revocation such as CRLs ([RFC5280]) or OCSP ([RFC6960]).

   Such revocation protocols have drawbacks.  Although caching of CRLs
   and OCSP responses is allowed, each setup of a secure channel may
   require accessing the CRL distribution point (DP) or the OCSP
   responder.  This is both time consuming and provides the system with
   a few more modes of failure.  Assuring reliability of the revocation
   service increases the cost, and overcoming the latency issue requires
   changes to the security protocols.

   For these reasons it is attractive to forego revocation checking.
   Some deployed systems do this by either eliminating the CRL DP and
   OCSP extensions from the certificates, or ignoring network and
   timeout errors in fetching revocation information.  Both practices
   reduce the efficacy of revocation.

   An alternative solution to the revocation problem is to issue
   certificates with a short validity period.  Normally certificates are
   issued with a validity period of between a few months and a few
   years.  With a shorter validity period if the private key is
   compromised the potential for abuse is lower because the certificate
   and its private key expire within a short period of time - a few
   hours to a few days.

   The rest of this document describes operational and security
   considerations with using short term certificates.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Throughout this document we will use the term DP to denote a server
   for revocation information, either a CRL distribution point or an
   OCSP Responder.  For our purposes they are the same.

   We use the term longevity for the period of time between certificate
   issuance and the time of its expiration as indicated in the notAfter
   field of the certificate.  Note that issuance time may be different
   from the notBefore field in the certificate.

   The text describes end entities as renewing their certificates
   because the usual operational model for certificates is one of

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   "pull": end entities create certificate requests and send them to CAs
   for signature.  Some systems are designed around a "push" operation
   where either the CA or a management function generates a new
   certificate and installs it on the end entity.  The text in the
   document uses pull terminology, but is equally relevant for push

2.  Short Term Auto Renewed Certificates

   Short term certificates are like any other [RFC5280] certificates
   except that the period of time between their issuance and their
   notAfter date is relatively short.  Whereas normally certificates are
   issued for a period of time between a few months and a few years,
   short term certificates usually expire after a few hours, a few days,
   or at a limit a couple of weeks.

   While it is not a part of the definition, short term certificates
   typically have neither a CRL DP extension nor an OCSP
   authorityInformationAccess extension.  In other words such
   certificates cannot be revoked.  Instead, they are valid until they

   Automatic certificate renewal is getting ever more popular with
   enrollment protocols such as EST ([RFC7030]) or ACME
   ([I-D.ietf-acme-acme]).  For short term certificates automatic
   renewal is essential as a human cannot be expected to flawlessly
   perform a manual renewal every few days or hours.  This document does
   not recommend any particular automatic renewal method, but does
   recommend (Section 4.4) that some such method be used.  Automatic
   renewal processing can roll over the keys from one certificate to its
   successor, or it can generate new keys with each Certificate
   generation.  As revocation may not exist, multiple certificates for
   the same EE may be valid at any given time.

   The solution for revocation in this scheme is to stop the automatic
   renewal.  The existing compromised certificate will remain valid
   until it expires.  See the considerations in Section 5.1 about

   [Topalovic] describes the design of a system involving STAR
   certificates for the web, and analyzes its security and efficacy.  It
   concludes that STAR certificates can be as secure as certificates
   with OCSP revocation.

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2.1.  Alternative Design: OCSP Stapling

   Relying parties can also avoid the need for contacting the DP at
   connection setup by having the End Entities implement OCSP stapling.
   This feature has the EEs rather than the RPs retrieve the OCSP
   response and send it as part of the protocol.  OCSP stapling is
   described for TLS in [RFC6961] and [RFC6066], and for IKE in

   STAR has two advantages over OCSP stapling:

   o  A CA that only signs certificates is simpler than a CA that both
      signs certificates and issues OCSP responses.  In fact, a CA for
      STAR does not need to keep any record of issued certificates.

   o  OCSP stapling in TLS works only for the server as end entity.
      There is no provision for sending the OCSP response for a client
      certificate in the protocol.

3.  Use Cases

   This section lists some use cases where STAR certificates seem to be
   more appropriate than long-lived certificates with revocation
   checking.  The purpose of this section is only motivational.  None of
   the following sections are intended to be a definition of the use
   case or the standard by which future documents or implementations
   will be measured for sufficiency.

3.1.  Data Center Network Hosts


3.2.  Distributed System Installed in One Or More Data Centers

   This is a system installed in multiple hosts in one or more data
   centers that fulfills some task and requires mutual authentication of
   its components.  An example of such a system is a Storage Area
   Network (SAN).

3.2.1.  Distributed Network Security Functions

   This example of a distributed system is multiple network security
   functions (NSF) [RFC8192] where the SDN controller needs to
   authenticate the NSFs with which it communicates, and some NSFs need
   to communicate with each other.

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3.3.  Certificate Delegation for Content Delivery Networks


4.  Operational Considerations

   The motivations for using short-term certificates are operational.
   We don't want the latency introduced by fetching the CRL from the DP;
   we don't want the cost of making the DP 99.999% reliable, and we
   don't want the cost of making the network paths from all RPs to the
   DP always available.

   Deploying short term certificates comes with its own set of
   operational considerations, and some of these are enumerated in the
   following sub-sections.

4.1.  Certificate Lifetime and Renewal Schedule

   Since we do not assume the CA to be close to 100% available it makes
   sense for End Entities to renew their certificates well in advance.
   While the security considerations in Section 5.2 set an upper limit
   on the longevity of a STAR certificate, operational necessity sets
   the frequency of renewal.  It is necessary to strike a balance
   between renewing too often which leads to increased load on the CA
   and renewing too seldom which increases the risk of having the
   certificate expire while either the CA or the End Entity are down.

   Individual system properties play a significant role here.  Systems
   where both the CA and the EEs are expected to be up all of the time
   absent a fault may choose to renew a day or even an hour before
   expiration, while systems with nodes that are only up infrequently
   and for short periods of time may choose to renew the certificates
   whenever the EEs happen to be up.

   As a general rule of thumb for systems where the CA is mostly
   available it makes sense for the EE to make the first attempt to
   renew its certificate about half-way through its lifetime.  If that
   attempt fails because the CA is not available an EE SHOULD retry at
   regular intervals until it succeeds.  Shortly before expiration, the
   EE SHOULD increase the frequency of retires.

   For example, suppose a STAR certificate is issued for 8 days.  The EE
   will first attempt to renew the certificate 4 days before expiration.
   If that fails it will retry every three hours until only six hours
   are left before expiration.  At that point it will increase the
   frequency and retry every five minutes.  If this is part of the
   system design, at this point it should also alert the user that
   something is wrong.

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4.2.  Availability of the Certificate Authority

   While the STAR design does not require 99.999% availability, the CA
   does need to be available for renewing certificates.  Downtimes of
   more than a quarter of the certificate longevity SHOULD NOT happen.
   For most modern hardware this is entirely possible even without
   exotic clustering solutions, but when configuring the system
   administrators should consider that the longevity of the certificates
   constrains the required availability of the CA.

   When setting the longevity for certificates administrators SHOULD
   consider how long it takes to recover from a failure of the CA.  That
   length of time can be seconds with a good clustering solution, but
   can span hours or days without one, especially if the fault happens
   at a bad time.  A failure of a CA should be considered a conceivable
   occurrence, and longevity should be set so that such a failure does
   not lead to expiration and outage.

   Conversely, if short longevity is required by security targets, the
   CA should be made more reliable with clustering solutions.

4.3.  Clock Skew and the notBefore Field

   Despite NTP ([RFC5905]) being over thirty years old and implemented
   in every major operating system clock skew is a fact of life and many
   deployed systems don't have the right time.  It is also not possible
   to just mandate the use of NTP because the systems that use STAR
   certificates are often installed on hosts and networks where NTP is
   either not configured or blocked.  We cannot assume that these
   systems can enable NTP at will.

   Skewed clocks have always been a problem for certificates.  Because
   STAR certificates are always just a few days or hours from expiration
   they are more sensitive to clock skew.  A sufficiently skewed clock
   can cause three different disfunctions and for STAR certificate such
   disfunction happens with considerably less skew than with long term

   o  A valid certificate may be rejected as not yet valid if the
      current system time is earlier than its notBefore time.
      Fortunately this issue can be safely mitigated by setting the
      notBefore field to a time earlier than the time of issuance.

   o  A valid certificate may be rejected as expired if the current
      system time is later than its notAfter time.  As long as the clock
      skew is not too great this is solved by a sensible renewal policy.
      If as in the example in Section 4.1 the certificate is renewed 4

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      days before expiration or within a few hours after that, a clock
      skew of up to 3 days will not be a problem.

   o  An expired certificate may be accepted if the current system time
      is earlier than its notAfter time.  This is a security issue that
      is discussed in Section 5.3.

   There are several common modes of clock skew:

   o  The system that doesn't have its clock set at all.  These systems
      might be set to January 1st, 1970 or to some date that was
      interesting for the hardware vendor.  Such systems are
      incompatible with certificates and MUST NOT be used for STAR

   o  The system has its timezone set wrong, and the system time was set
      so that local time looks good.  This limits the clock skew to 24
      hours and is generally workable.

   o  A system that has the time set right but the date set wrong.
      These are also not usable with certificates.

   o  A system that was set to the correct time once but has since
      drifted away.  Computer hardware varies wildly between systems
      with quartz clocks that drift only a few seconds a month and
      systems that can lose or gain minutes a day.  The former are quite
      usable, the latter are not.

   Because of the prevalence of systems with a relatively small skew it
   is RECOMMENDED to set the notBefore field to a time 72 hours before
   the actual issuance date.

   End Entities MUST NOT use expired certificates and Relying Parties
   SHOULD alert whenever an expired certificate is presented.  This will
   help the users keep their host clocks set or encourage them to enable

4.4.  Automatic Renewal

   Automatic enrollment and renewal is recommended for any system using
   certificates.  While it is possible to renew certificates manually on
   time, even organizations with the best of IT departments occasionally
   miss this: [cert-expires]

   With short term certificates, this becomes even more important.
   Renewing a certificate manually every few days or hours is extremely
   labor intensive, especially when the system contains hundreds,

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   thousands or more end entities, and the risk of outages becomes a

   This document does not mandate any particular enrollment or renewal
   mechanism.  Any of a myriad of standard and proprietary methods can
   be used and systems with proprietary methods have been shipping for
   years.  The IETF is in the process of standardizing the ACME protocol
   for enrollment and renewal ([I-D.ietf-acme-acme]) and an extension is
   proposed to make it more suitable for STAR certificates

4.5.  Certificate Transparency

   Certificate Transparency (CT), [RFC6962] is about keeping a log of
   all issued certificates.

   A system that issues a certificate every few days to thousands or end
   entities will create more records for a CT log than a web host that
   gets one certificate every year.

   TBA: Discussion about this.

5.  Security Considerations

   STAR certificates eliminate an important security feature of PKI
   which is the ability to revoke certificates.  Revocation allows the
   administrator to limit the damage done by a rogue node or an
   adversary who has control of the private key.  With STAR certificates
   expiration replaces revocation so there is a timeliness issue.

   It should be noted that revocation also has timeliness issues,
   because both CRLs and OCSP responses have nextUpdate fields that tell
   RPs how long they should trust this revocation data.  These fields
   are typically set to hours, days, or even weeks in the future.  Any
   revocation that happens before the time in nextUpdate goes unnoticed
   by the RP.

   Section 5.1 discusses the reasons why a certificate would be revoked
   if revocation was available and how STAR certificates do the same.
   Section 5.2 discusses considerations for setting the longevity of a
   certificate, and Section 5.3 discusses how longevity should be
   adjusted to deal with clock skew.

   More discussion of the security of STAR certificates is available in

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5.1.  Reasons for Revocation

   There are two types of compromise that require administrators to
   revoke a certificate:

   o  A host has lost control of the private key.  There are many ways
      that this can happen: a host can be hacked and a file containing
      the private key may or may not have been copied; a disk may be
      replaced and the old one has not been securely disposed of; a
      fault causes the private key to be erased.  In all these cases we
      would like to revoke the certificate to make sure an adversary
      cannot use the private key for nefarious purposes.  For STAR
      certificates the only solution is to wait for the certificate to
      expire and the system is vulnerable until that happens.  Longevity
      should be set so that this risk is acceptable.

   o  A host may begin doing unintended things, either due to a software
      fault or due to a malicious takeover.  Again without revocation
      RPs will continue to trust this node until its certificate

   When a node "goes rogue" or an adversary gets control of the private
   key it is important to block renewal or these certificates or else
   the attack can persist forever.  No matter how short-term these short
   term certificates are, there is a certain window of time when the
   attacker can use the certificate.  This can often be mitigated with
   application-level measures.

   With most systems relying parties are configured with the names of
   nodes with which they are allowed to communicate.  When revocation is
   not available changing the configuration so that the rogue node
   cannot connect is RECOMMENDED.  This is useful even when revocation
   is available because timeliness issues are common to both revocation
   and expiration.

5.2.  Longevity and Revocation

   There is always a period of time between when a compromise is
   discovered and when RPs stop trusting the certificate.  With
   revocation this has to do with the time it takes to process the
   revocation and the span of time between the thisUpdate and nextUpdate
   fields.  With STAR certificates this is controlled by the time it
   takes to inhibit renewals and the longevity of the certificates.

   For this reason it makes sense to set the longevity to a period of
   time similar to the span of time that we would set for the CRL or
   OCSP updates.  Typically a few days is an appropriate time.  For some
   cases this can be as low as a few hours.  Setting the renewal time

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   too short may cause operational problems as discussed in Section 4.3
   and Section 4.2.  In general longevity should not be set shorter than
   the availability of the CA allows.

   Fortunately modern hardware is powerful enough and reliable enough
   that even a system with tens of thousands of end entities with
   longevity of 1-2 days should not suffer an outage because of expired

5.3.  Clock Skew and Security

   As discussed in Section 4.3 clock skew can lead to expired
   certificates being treated as valid.  While even the use of NTP may
   leave clocks with a few seconds of inaccuracy, all installations MUST
   take steps to limit the clock skew on their hosts.

   An upper bound for the amount of skew allowed for hosts in a
   particular system is one of the parameters for such a system.  For
   systems using NTP this can be 2 seconds.  For systems where the
   clocks are set manually, this tends to be far greater, but without an
   upper bound no guarantees can be made about the security of
   certificate use.

   This upper bound is also a limit on the target certificate longevity.
   For example, if hosts and CAs can each have a clock skew of 24 hours
   then it is impossible to achieve a longevity of under 48 hours.  With
   a reasonable skew and a reasonable target longevity we can achieve
   our security targets by reducing the certificate longevity by twice
   the upper bound for skew.  So if skew is bounded by 24 hours (the bad
   timezone case) and target longevity is 7 days, it makes sense to set
   the longevity on the CA to 5 days.

5.4.  CA availability

   A successful Denial of Service (DoS) attack against a CA prevents it
   from issuing certificates.  With short-term certificates this could
   quickly lead to outages as certificates expire.

   The important period of time here is the time between when the EE
   first attempts to renew the certificate and the time that the
   certificate expires.  For example, if the EE attempts to renew the
   certificates a mere five minutes before expiration, then a five-
   minute CA outage can lead to an invalid certificate and failed

   This issue is no different from DoS attacks against the DP for
   certificates with revocation.  The methods of protection are also

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   o  Certificate renewal should first be attempted plenty of time in
      advance as recommended in Section 4.1.  This will leave enough
      time for administrators to deal with the attack.

   o  As for all important infrastructure, network defenses SHOULD be
      deployed to mitigate DoS attacks.

6.  IANA Considerations

   There are no requests to IANA in this document.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

7.2.  Informative References

              Lennon, M., "Google Lets SMTP Certificate Expire", April
              2015, <http://www.securityweek.com/

              Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
              Certificate Management Environment (ACME)", draft-ietf-
              acme-acme-07 (work in progress), June 2017.

              Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
              Perales, A., and T. Fossati, "Use of Short-Term,
              Automatically-Renewed (STAR) Certificates to Delegate
              Authority over Web Sites", draft-ietf-acme-acme-07 (work
              in progress), June 2017.

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   [RFC4806]  Myers, M. and H. Tschofenig, "Online Certificate Status
              Protocol (OCSP) Extensions to IKEv2", RFC 4806,
              DOI 10.17487/RFC4806, February 2007,

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

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [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,

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

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

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   [RFC8192]  Hares, S., Lopez, D., Zarny, M., Jacquenet, C., Kumar, R.,
              and J. Jeong, "Interface to Network Security Functions
              (I2NSF): Problem Statement and Use Cases", RFC 8192,
              DOI 10.17487/RFC8192, July 2017,

              Topalovic, E., Saeta, B., Huang, L., Jackson, C., and D.
              Boneh, "Towards Short-Lived Certificates", 2012,

Authors' Addresses

   Yoav Nir
   Dell EMC
   9 Andrei Sakharov St
   Haifa  3190500

   EMail: ynir.ietf@gmail.com

   Thomas Fossati

   EMail: thomas.fossati@nokia.com

   Yaron Sheffer

   EMail: yaronf.ietf@gmail.com

Nir, et al.              Expires April 17, 2018                [Page 14]

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