Network Working Group B. Weis
Internet-Draft R. Gagliano
Intended status: Standards Track Cisco Systems
Expires: February 15, 2018 K. Patel
Arrcus, Inc.
August 14, 2017
BGPsec Router Certificate Rollover
draft-ietf-sidrops-bgpsec-rollover-01
Abstract
Certificate Authorities (CAs) managing CA certificates and End-Entity
(EE) certificates within the Resource Public Key Infrastructure
(RPKI) will also manage BGPsec router certificates. But the rollover
of CA and EE certificates BGPsec router certificates have additional
considerations for Normal and emergency rollover processes. The
rollover must be carefully managed in order to synchronize
distribution of router public keys and BGPsec routers creating BGPsec
Update messages verified with those router public keys. This
document provides general recommendations for the rollover process,
as well as describing reasons why the rollover of BGPsec router
certificates might be necessary. When this rollover process is
followed the rollover should be accomplished without routing
information being lost.
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 February 15, 2018.
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Copyright Notice
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Key rollover in BGPsec . . . . . . . . . . . . . . . . . . . 4
3.1. A proposed process for BGPsec router key rollover . . . . 4
4. BGPsec router key rollover as a measure against replay
attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. BGP UPDATE window of exposure requirement . . . . . . . . 6
4.2. BGPsec key rollover as a mechanism to protect against
replay attacks . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. 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
[RFC2119].
2. Introduction
In BGPsec, a key rollover (or re-key) is the process of changing a
router's 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 the local policies of a network.
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This document provides general recommendations for that process.
Certificate Practice Statements (CPS) documents MAY reference these
recommendations. This memo only addresses changing of a router's key
pair within the RPKI. Refer to [RFC6489] for a procedure to rollover
RPKI Certificate 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 updates [I-D.ietf-sidr-bgpsec-protocol] that are originated or
that transit through the BGP speaker. Additionally, the BGP speaker
MUST 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, and thus any BGPsec Update that
includes 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. It is
therefore extremely important that new BGPsec router certificates
have been distributed throughout the RPKI before the router begin
signing BGPsec updates with a new private key.
It is also important for an AS to minimize the BGPsec router key
rollover interval (i.e., in between the time 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 updates signed with a new
private key in order to invalidate BGPsec updates signed with the old
private key. In particular, if the AS suspects that a stale BGPsec
updates is being distributed instead of the most recently signed
attribute it can cause the stale BGPsec updates 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.
The Security Requirements for BGP Path Validation [RFC7353] also
describes the need for protecting against suppression of BGP WITHDRAW
messages 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 [I-D.ietf-sidr-rtr-keying], the "operator-driven" method is
introduced, in which a key pair can be shared among multiple BGP
speakers. In this scenario, the roll-over of the correspondent
BGPsec router certificate will impact all the BGP speakers sharing
the same private key.
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3. Key rollover in BGPsec
An BGPsec router certificate SHOULD be replaced when the following
events occur, and 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 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 withdrawel supporession and replay attacks: An AS
may determine withdrawn BGPsec updates are being propagated
instead of the most recently propagated BGPsec updates.
Changing the BGPsec router signing key, distributing a new
BGPsec router certificate, and revoking the old BGPsec router
certificate will invalidate the replayed BGPsec updates.
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 Updates 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 Updates. 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 updates are being distributed.
3.1. A proposed process for BGPsec router key rollover
The key rollover process will be dependent on the key provisioning
mechanisms adopted by an AS [I-D.ietf-sidr-rtr-keying]. An automatic
provisioning mechanism such as EST will allow router key management
procedures to include automatic re-keying methods with minimum
development cost.
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If we work under the assumption that an automatic mechanism will
exist to rollover a BGPsec router certificate, a RECOMMENDED process
is as follows.
1. New Certificate Publication: The first step in the rollover
mechanism is to publish the new public key in a new certificate.
In order to accomplish this goal, the new key pair and
certificate will need to be generated and the certificate
published at the appropriate RPKI repository publication point.
The details of this process will vary as they depend on whether
the keys are assigned per-BGPsec speaker or shared among multiple
BGPsec speakers, whether the keys are generated on each BGPsec
speaker or in a central location, and 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 RPKI global 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 then a staging period
may not be possible to implement during emergency key rollover.
If there is no staging period, routing information may be lost.
3. Twilight: At this moment, the BGPsec speaker holding the rolled-
over private key will stop using the OLD key for signing and
start using the NEW key. Also, the router will generate
appropriate refreshed BGPsec updates just as in the typical
operation of refreshing out-bound BGP polices. This operation
may generate a great number of BGPsec updates. A BGPsec speaker
may vary the Twilight moment 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 SHOULD be
taken when the goal is to invalidate updates signed with the OLD
key. Reasons to invalidate OLD updates 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 updates 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 CRL.
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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
[I-D.ietf-sidr-rpki-rtr-rfc6810-bis]). 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 WITHDRAWALs (either implicit or explicit).
The proposed rollover mechanism will depend on the existence of an
automatic provisioning process for BGPsec router certificates. It
will require a staging mechanism based on the RPKI propagation time
(typically a 24 hour period at the time this document was published),
and it will require re-signing all originated and transited BGPsec
updates 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 roll-over 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 provide either measure. The
timestamp approach was originally proposed for BGPsec
[I-D.sriram-replay-protection-design-discussion] but later dropped in
favor of the key rollover approach. This section discusses the use
of using a key roll-over 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
[RFC7353] Section 4.3. 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
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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 because the
signed information is still valid.
The BGPsec Operational Considerations document
[I-D.ietf-sidr-bgpsec-ops] 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 [I-D.ietf-sidr-bgpsec-ops]) 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 before, 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 updates when received.
For a transit AS that also originates BGPsec updates for its own
prefixes, the key rollover process may generate a large number of
UPDATE messages (even the complete Default Free Zone or DFZ). For
this reason, it is recommended that routers in a transit AS that also
originate BGPsec updates be provisioned with two certificates: one to
sign BGPsec updates in transit and a second one to sign BGPsec
updates for prefixes originated from its AS. Only the second
certificate (for originating prefixes) should be rolled-over
frequently as a means of limiting replay attack windows. The router
certificate used for signing updates in transit is expected to live
longer than the one used for signing origination updates.
Advantages to re-keying as 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
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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 [I-D.ietf-sidr-bgpsec-ops].
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 RPKI
and RPs are well deployed, as well as the propagation time for
objects in the RPKI is better understood.
3. Re-keying increases the dynamics and size of the RPKI repository.
5. IANA Considerations
There are no IANA considerations. This section may be removed upon
publication.
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 rollover their signing
keys as part of conventional certificate management processes.
However, because rolling over signing keys will also have an effect
of invalidating BGPsec updates signatures, the rollover process must
be carefully orchestrated to ensure that valid BGPsec updates 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.
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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 updates
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 provisioning 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
[I-D.sriram-replay-protection-design-discussion].
7. Acknowledgements
Randy Bush, Kotikalapudi Sriram, Stephen Kent and Sandy Murphy each
provided valuable suggestions resulting in an improved document.
8. References
8.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
8.2. Informative References
[I-D.ietf-sidr-bgpsec-ops]
Bush, R., "BGPsec Operational Considerations", draft-ietf-
sidr-bgpsec-ops-16 (work in progress), January 2017.
[I-D.ietf-sidr-bgpsec-protocol]
Lepinski, M. and K. Sriram, "BGPsec Protocol
Specification", draft-ietf-sidr-bgpsec-protocol-23 (work
in progress), April 2017.
[I-D.ietf-sidr-rpki-rtr-rfc6810-bis]
Bush, R. and R. Austein, "The Resource Public Key
Infrastructure (RPKI) to Router Protocol, Version 1",
draft-ietf-sidr-rpki-rtr-rfc6810-bis-09 (work in
progress), February 2017.
[I-D.ietf-sidr-rtr-keying]
Bush, R., Turner, S., and K. Patel, "Router Keying for
BGPsec", draft-ietf-sidr-rtr-keying-13 (work in progress),
April 2017.
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[I-D.sriram-replay-protection-design-discussion]
Sriram, K. and D. Montgomery, "Design Discussion and
Comparison of Protection Mechanisms for Replay Attack and
Withdrawal Suppression in BGPsec", draft-sriram-replay-
protection-design-discussion-08 (work in progress), April
2017.
[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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc7353>.
Authors' Addresses
Brian Weis
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
US
Email: bew@cisco.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
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