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 © 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 Provisions Relating to IETF Documents (https://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 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.  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