BGPsec key rollover: BGPsec Router Certificate Rollover

Internet Engineering Task Force (IETF)                           B. Weis
Request for Comments: 8634                                   Independent
BCP: 224                                                     R. Gagliano
Category: Best Current Practice                            Cisco Systems
ISSN: 2070-1721                                                 K. Patel
                                                            Arrcus, Inc.
                                                             August 2019


                   BGPsec Router Certificate Rollover

Abstract

   Certification Authorities (CAs) within the Resource Public Key
   Infrastructure (RPKI) manage BGPsec router certificates as well as
   RPKI certificates.  The rollover of BGPsec router certificates must
   be carefully performed in order to synchronize the distribution of
   router public keys with BGPsec UPDATE messages verified with those
   router public keys.  This document describes a safe rollover process,
   and it discusses when and why the rollover of BGPsec router
   certificates is necessary.  When this rollover process is followed,
   the rollover will be performed without routing information being
   lost.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8634.

Copyright Notice

   Copyright (c) 2019 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   4
   3.  Key Rollover in BGPsec  . . . . . . . . . . . . . . . . . . .   4
     3.1.  Rollover Process  . . . . . . . . . . . . . . . . . . . .   5
   4.  BGPsec Router Key Rollover as a Measure against Replay
       Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  BGP UPDATE Window of Exposure Requirement . . . . . . . .   7
     4.2.  BGPsec Key Rollover as a Mechanism to Protect against
           Replay Attacks  . . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   In BGPsec, a key rollover (or re-key) is the process of changing a
   router's BGPsec key pair (or key pairs), issuing the corresponding
   new BGPsec router certificate, and (if the old certificate is still
   valid) revoking the old certificate.  This process will need to
   happen at regular intervals, normally due to policies of the local
   network.  This document describes a safe rollover process that
   results in a BGPsec receiver always having the needed verification
   keys.  Certification Practice Statement (CPS) documents may reference
   this memo.  This memo only addresses changing of a router's BGPsec
   key pair within the RPKI.  Refer to [RFC6489] for a procedure to roll
   over RPKI Certification Authority key pairs.

   When a router receives or creates a new key pair (using a key
   provisioning mechanism), this key pair will be used to sign new
   BGPsec UPDATE messages [RFC8205] that are originated at or that
   transit through the BGP speaker.  Additionally, the BGP speaker will
   refresh its outbound BGPsec UPDATE messages to include a signature
   using the new key (replacing the old key).  When the rollover process
   finishes, the old BGPsec router certificate (and its key) will no
   longer be valid; thus, any BGPsec UPDATE message that includes a
   signature performed by the old key will be invalid.  Consequently, if
   the router does not refresh its outbound BGPsec UPDATE messages,
   previously sent routing information may be treated as unauthenticated
   after the rollover process is finished.  Therefore, it is extremely
   important that new BGPsec router certificates have been distributed
   throughout the RPKI before the router begins signing BGPsec UPDATE
   messages with a new private key.

   It is also important for an AS to minimize the BGPsec router key-
   rollover interval (i.e., the period between the time when an AS
   distributes a BGPsec router certificate with a new public key and the
   time a BGPsec router begins to use its new private key).  This can be
   due to a need for a BGPsec router to distribute BGPsec UPDATE
   messages signed with a new private key in order to invalidate BGPsec
   UPDATE messages signed with the old private key.  In particular, if
   the AS suspects that a stale BGPsec UPDATE message is being
   distributed instead of the most recently signed attribute, it can
   cause the stale BGPsec UPDATE messages to be invalidated by
   completing a key-rollover procedure.  The BGPsec router rollover
   interval can be minimized when an automated certificate provisioning
   process such as Enrollment over Secure Transport (EST) [RFC7030] is
   used.

   "Security Requirements for BGP Path Validation" [RFC7353] also
   describes the need for protecting against suppression of BGP UPDATE
   messages with Withdrawn Routes or replay of BGP UPDATE messages, such
   as controlling BGPsec's window of exposure to such attacks.  The
   BGPsec router certificate rollover method in this document can be
   used to achieve this goal.

   In [RFC8635], the "operator-driven" method is introduced, in which a
   key pair can be shared among multiple BGP speakers.  In this
   scenario, the rollover of the corresponding BGPsec router certificate
   will impact all the BGP speakers sharing the same private key.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Key Rollover in BGPsec

   A BGPsec router certificate SHOULD be replaced when the following
   events occur, and it can be replaced for any other reason at the
   discretion of the AS responsible for the BGPsec router certificate.

   Scheduled rollover:  BGPsec router certificates have an expiration
         date (NotValidAfter) that requires a frequent rollover process
         to refresh certificates or issue new certificates.  The
         validity period for these certificates is typically expressed
         in the CA's CPS document.

   Router certificate field changes:  Information contained in a BGPsec
         router certificate (such as the Autonomous System Number (ASN)
         or the Subject) may need to be changed.

   Emergency router key rollover:  Some special circumstances (such as a
         compromised key) may require the replacement of a BGPsec router
         certificate.

   Protection against withdrawal suppression and replay attacks:  An AS
         may determine that withdrawn BGPsec UPDATE messages are being
         propagated instead of the most recently propagated BGPsec
         UPDATE messages.  Changing the BGPsec router signing key,
         distributing a new BGPsec router certificate, and revoking the
         old BGPsec router certificate will invalidate the replayed
         BGPsec UPDATE messages.

   In some of these cases, it is possible to generate a new certificate
   without changing the key pair.  This practice simplifies the rollover
   process as the BGP speakers receiving BGPsec UPDATE messages do not
   even need to be aware of the change of certificate.  However, not
   replacing the certificate key for a long period of time increases the
   risk that a compromised router private key may be used by an attacker
   to deliver unauthorized or false BGPsec UPDATE messages.
   Distributing the old public key in a new certificate is NOT
   RECOMMENDED when the rollover event is due to a compromised key or
   when it is suspected that withdrawn BGPsec UPDATE messages are being
   distributed.

3.1.  Rollover Process

   The key-rollover process is dependent on the key provisioning
   mechanisms adopted by an AS [RFC8635].  An automatic provisioning
   mechanism such as EST will allow procedures for router key management
   to include automatic re-keying methods with minimum development cost.

   A safe BGPsec router key-rollover process is as follows.

   1.  New Certificate Publication: The first step in the rollover
       mechanism is to publish the new certificate.  If required, a new
       key pair will be generated for the BGPsec router.  A new
       certificate will be generated and the certificate will be
       published at the appropriate RPKI repository publication point.

       The details of this process will vary as they depend on 1)
       whether the keys are assigned per-BGPsec speaker or shared among
       multiple BGPsec speakers, 2) whether the keys are generated on
       each BGPsec speaker or in a central location, and 3) whether the
       RPKI repository is locally or externally hosted.

   2.  Staging Period: A staging period will be required from the time a
       new certificate is published in the global RPKI repository until
       the time it is fetched by RPKI caches around the globe.  The
       exact minimum staging time will be dictated by the conventional
       interval chosen between repository fetches.  If rollovers will be
       done more frequently, an administrator can provision two
       certificates for every router concurrently with different valid
       start times.  In this case, when the rollover operation is
       needed, the relying parties around the globe would already have
       the new router public keys.  However, if an administrator has not
       previously provisioned the next certificate, implementing a
       staging period may not be possible during emergency key rollover.
       If there is no staging period, routing may be disrupted due to
       the inability of a BGPsec router to validate BGPsec UPDATE
       messages signed with a new private key.

   3.  Twilight: In this step, the BGPsec speaker holding the rolled-
       over private key will stop using the old key for signing and will
       start using the new key.  Also, the router will generate
       appropriate refreshed BGPsec UPDATE messages, just as in the
       typical operation of refreshing outbound BGP polices.  This
       operation may generate a great number of BGPsec UPDATE messages.
       A BGPsec speaker may vary the distribution of BGPsec UPDATE
       messages in this step for every peer in order to distribute the
       system load (e.g., skewing the rollover for different peers by a
       few minutes each would be sufficient and effective).

   4.  Certificate Revocation: This is an optional step, but it SHOULD
       be taken when the goal is to invalidate BGPsec UPDATE messages
       signed with the old key.  Reasons to invalidate old BGPsec UPDATE
       messages include (a) the AS has reason to believe that the router
       signing key has been compromised, and (b) the AS needs to
       invalidate already-propagated BGPsec UPDATE messages signed with
       the old key.  As part of the rollover process, a CA MAY decide to
       revoke the old certificate by publishing its serial number on the
       CA's Certificate Revocation List (CRL).  Alternatively, the CA
       will just let the old certificate expire and not revoke it.  This
       choice will depend on the reasons that motivated the rollover
       process.

   5.  RPKI-Router Protocol Withdrawals: At the expiration of the old
       certificate's validation, the RPKI relying parties around the
       globe will need to communicate to their router peers that the old
       certificate's public key is no longer valid (e.g., using the
       RPKI-Router Protocol described in [RFC8210]).  A router's
       reaction to a message indicating withdrawal of a router key in
       the RPKI-Router Protocol SHOULD include the removal of any RIB
       entries (i.e., BGPsec updates) signed with that key and the
       generation of the corresponding BGP UPDATE message with Withdrawn
       Routes (either implicit or explicit).

   This rollover mechanism depends on the existence of an automatic
   provisioning process for BGPsec router certificates.  It requires a
   staging mechanism based on the RPKI propagation time (at the time of
   writing, this is typically a 24-hour period), and an AS is REQUIRED
   to re-sign all originated and transited BGPsec UPDATE messages that
   were previously signed with the old key.

   The first two steps (New Certificate Publication and Staging Period)
   may happen in advance of the rest of the process.  This will allow a
   network operator to perform its subsequent key rollover in an
   efficient and timely manner.

   When a new BGPsec router certificate is generated without changing
   its key, steps 3 (Twilight) and 5 (RPKI-Router Protocol Withdrawals)
   SHOULD NOT be executed.

4.  BGPsec Router Key Rollover as a Measure against Replay Attacks

   There are two typical generic measures to mitigate replay attacks in
   any protocol: the addition of a timestamp or the addition of a serial
   number.  However, neither BGP nor BGPsec provides these measures.
   The timestamp approach was originally proposed for BGPsec
   [PROTECTION-DESIGN-DISCUSSION] but was later dropped in favor of the
   key-rollover approach.  This section discusses the use of key
   rollover as a measure to mitigate replay attacks.

4.1.  BGP UPDATE Window of Exposure Requirement

   The need to limit the vulnerability to replay attacks is described in
   Section 4.3 of [RFC7353].  One important comment is that during a
   window of exposure, a replay attack is effective only in very
   specific circumstances: there is a downstream topology change that
   makes the signed AS path no longer current, and the topology change
   makes the replayed route preferable to the route associated with the
   new update.  In particular, if there is no topology change at all,
   then no security threat comes from a replay of a BGPsec UPDATE
   message because the signed information is still valid.

   "BGPsec Operational Considerations" [RFC8207] gives some idea of
   requirements for the size of the window of exposure to replay
   attacks.  It states that the requirement will be in the order of a
   day or longer.

4.2.  BGPsec Key Rollover as a Mechanism to Protect against Replay
      Attacks

   Since the window requirement is on the order of a day (as documented
   in [RFC8207]) and the BGP speaker performing re-keying is the edge
   router of the origin AS, it is feasible to use key rollover to
   mitigate replays.  In this case, it is important to complete the full
   process (i.e., the old and new certificates do not share the same
   key).  By re-keying, an AS is letting the BGPsec router certificate
   validation time be a type of "timestamp" to mitigate replay attacks.
   However, the use of frequent key rollovers comes with an additional
   administrative cost and risks if the process fails.  As documented in
   [RFC8207], re-keying should be supported by automatic tools, and for
   the great majority of the Internet, it will be done with good lead
   time to ensure that the public key corresponding to the new router
   certificate will be available to validate the corresponding BGPsec
   UPDATE messages when received.

   If a transit AS also originates BGPsec UPDATE messages for its own
   prefixes and it wishes to mitigate replay attacks on those prefixes,
   then the transit AS SHOULD be provisioned with two unique key pairs
   and certificates.  One of the key pairs is used to sign BGPsec UPDATE
   messages for prefixes originated from the transit AS, and it can have
   a replay protection policy applied to it.  The other key pair is used
   to sign BGPsec UPDATE messages in transit and SHOULD NOT have a
   replay protection policy applied to it.  Because the transit AS is
   not likely to know or care about the policy of origin ASes elsewhere,
   there is no value gained by the transit AS performing key rollovers
   to mitigate replay attacks against prefixes originated elsewhere.  If
   the transit AS were instead to perform replay protection for all
   updates that it signs, its process for key rollovers would generate a
   large number of BGPsec UPDATE messages, even in the complete Default-
   Free Zone (DFZ).  Therefore, it is best to let each AS independently
   manage the replay attack vulnerability window for the prefixes it
   originates.

   Advantages to re-keying as a replay attack protection mechanism are
   as follows:

   1.  All expiration policies are maintained in the RPKI.

   2.  Much of the additional administrative cost is paid by the
       provider that wants to protect its infrastructure, as it bears
       the cost of creating and initiating distribution of new router
       key pairs and BGPsec router certificates.  (It is true that the
       cost of relying parties will be affected by the new objects, but
       their responses should be completely automated or otherwise
       routine.)

   3.  The re-keying can be implemented in coordination with planned
       topology changes by either origin ASes or transit ASes (e.g., if
       an AS changes providers, it completes a key rollover).

   Disadvantages to re-keying as replay attack protection mechanism are
   as follows:

   1.  Frequent rollovers add administrative and BGP processing loads,
       although the required frequency is not clear.  Some initial ideas
       are found in [RFC8207].

   2.  The minimum replay vulnerability is bounded by the propagation
       time for RPKI caches to obtain the new certificate and CRL (2x
       propagation time because first the new certificate and then the
       CRL need to propagate through the RPKI system).  If provisioning
       is done ahead of time, the minimum replay vulnerability window
       size is reduced to 1x propagation time (i.e., propagation of the
       CRL).  However, these bounds will be better understood when the
       RPKI and RPKI relying party software are well deployed; this will
       also contribute to the propagation time for objects in the RPKI
       being better understood.

   3.  Re-keying increases the dynamics and size of the RPKI repository.

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   This document does not contain a protocol update to either the RPKI
   or BGPsec.  It describes a process for managing BGPsec router
   certificates within the RPKI.

   Routers participating in BGPsec will need to roll over their signing
   keys as part of conventional processing of certificate management.
   However, because rolling over signing keys will also have the effect
   of invalidating BGPsec UPDATE message signatures, the rollover
   process must be carefully orchestrated to ensure that valid BGPsec
   UPDATE messages are not treated as invalid.  This situation could
   affect Internet routing.  This document describes a safe method for
   rolling over BGPsec router certificates.  It takes into account both
   normal and emergency key-rollover requirements.

   Additionally, the key-rollover method described in this document can
   be used as a measure to mitigate BGP UPDATE replay attacks, in which
   an entity in the routing system is suppressing current BGPsec UPDATE
   messages and replaying withdrawn updates.  When the key used to sign
   the withdrawn updates has been rolled over, the withdrawn updates
   will be considered invalid.  When certificates containing a new
   public key are provisioned ahead of time, the minimum replay
   vulnerability window size is reduced to the propagation time of a CRL
   invalidating the certificate containing an old public key.  For a
   discussion of the difficulties deploying a more effectual replay
   protection mechanism for BGPSEC, see [PROTECTION-DESIGN-DISCUSSION].

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8635]  Bush, R., Turner, S., and K. Patel, "Router Keying for
              BGPsec", RFC 8635, DOI 10.17487/RFC8635, August 2019,
              <https://www.rfc-editor.org/info/rfc8635>.

7.2.  Informative References

   [PROTECTION-DESIGN-DISCUSSION]
              Sriram, K. and D. Montgomery, "Design Discussion and
              Comparison of Protection Mechanisms for Replay Attack and
              Withdrawal Suppression in BGPsec", Work in Progress,
              draft-sriram-replay-protection-design-discussion-12, April
              2019.

   [RFC6489]  Huston, G., Michaelson, G., and S. Kent, "Certification
              Authority (CA) Key Rollover in the Resource Public Key
              Infrastructure (RPKI)", BCP 174, RFC 6489,
              DOI 10.17487/RFC6489, February 2012,
              <https://www.rfc-editor.org/info/rfc6489>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [RFC7353]  Bellovin, S., Bush, R., and D. Ward, "Security
              Requirements for BGP Path Validation", RFC 7353,
              DOI 10.17487/RFC7353, August 2014,
              <https://www.rfc-editor.org/info/rfc7353>.

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <https://www.rfc-editor.org/info/rfc8205>.

   [RFC8207]  Bush, R., "BGPsec Operational Considerations", BCP 211,
              RFC 8207, DOI 10.17487/RFC8207, September 2017,
              <https://www.rfc-editor.org/info/rfc8207>.

   [RFC8210]  Bush, R. and R. Austein, "The Resource Public Key
              Infrastructure (RPKI) to Router Protocol, Version 1",
              RFC 8210, DOI 10.17487/RFC8210, September 2017,
              <https://www.rfc-editor.org/info/rfc8210>.

Acknowledgments

   Randy Bush, Kotikalapudi Sriram, Stephen Kent, and Sandy Murphy each
   provided valuable suggestions resulting in an improved document.
   Kotikalapudi Sriram contributed valuable guidance regarding the use
   of key rollovers to mitigate BGP UPDATE replay attacks.

Authors' Addresses

   Brian Weis
   Independent

   Email: bew.stds@gmail.com


   Roque Gagliano
   Cisco Systems
   Avenue des Uttins 5
   Rolle, VD  1180
   Switzerland

   Email: rogaglia@cisco.com


   Keyur Patel
   Arrcus, Inc.

   Email: keyur@arrcus.com