Network Working Group B. Weis
Internet-Draft R. Gagliano
Intended status: Standards Track Cisco Systems
Expires: September 9, 2017 K. Patel
Arrcus, Inc.
March 8, 2017
BGPsec Router Certificate Rollover
draft-ietf-sidrops-bgpsec-rollover-00
Abstract
BGPsec will need to address the impact from regular and emergency
rollover processes for the BGPsec end-entity (EE) certificates that
will be performed by Certificate Authorities (CAs) participating at
the Resource Public Key Infrastructure (RPKI). Rollovers of BGPsec
EE certificates must be carefully managed in order to synchronize
distribution of router public keys and the usage of those public keys
by BGPsec routers. This memo provides general recommendations for
that process, as well as describing reasons why the rollover of
BGPsec EE certificates might be necessary.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 9, 2017.
Copyright Notice
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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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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Key rollover in BGPsec . . . . . . . . . . . . . . . . . . . 3
3.1. A proposed process for BGPsec key rollover . . . . . . . 4
4. BGPsec key rollover as a measure against replays attacks in
BGPsec . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. BGPsec Replay attack window 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 . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 end-
entity (EE) 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.
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
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BGPsec_Path attributes [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 replaced key).
When the rollover process finishes, the old BGPsec certificate (and
its key) will not longer be valid, and thus any BGPsec Update that
includes a BGPsec_Path attribute with a signature performed by the
old key will be invalid. Consequently, if the router does not
refresh its outbound BGPsec Update messages, routing information may
be treated as unauthenticated after the rollover process is finished.
It is therefore extremely important that the BGPsec router key
rollover be performed in such a way that the probability of new
router EE certificates have been distributed throughout the RPKI
before the router begin signing BGPsec_Path attributes 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 an EE
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_Path attributes signed with a new private
key in order to invalidate BGPsec_Path attributes signed with the old
private key. In particular, if the AS suspects that a stale
BGPsec_Path attribute is being distributed instead of the most
recently signed attribute it can cause the stale BGPsec_Path
attribute to be invalidated by completing a key rollover procedure.
The BGPsec 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 the replay of BGP UPDATE
messages, such as controlling BGPsec's window of exposure to replay
attacks. The BGPsec 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 different BGP
Speakers. In this scenario, the roll-over of the correspondent
BGPsec certificate will impact all the BGP Speakers sharing the same
private key.
3. Key rollover in BGPsec
An BGPsec EE 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 EE certificate.
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BGPsec scheduled rollover: BGPsec certificates have an expiration
date (NotValidAfter) that requires a frequent rollover process.
The validity period for these certificates is typically
expressed at the CA's CPS document.
BGPsec certificate fields changes: Information contained in a BGPsec
certificate (such as the ASN or the Subject) may need to be
changed.
BGPsec emergency rollover Some special circumstances (such as a
compromised key) may require the replacement of a BGPsec
certificate.
BGPsec signature anti-replay protection An AS may determine stale
BGPsec_Path attributes signed by the AS are being propagated
instead of the most recently signed BGPsec_Path attributes.
Changing the BGPsec router signing key, distributing a new
BGPsec EE certificate for the router, and revoking the old
BGPsec EE certificate will invalidate the replayed BGPsec_Path
attributes.
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 corresponding BGP speakers do not even need to be
aware of the changes to its correspondent 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 BGPsec Updates. Distributing the OLD public
key in a new certificate is NOT RECOMMENDED when the rollover event
is due to the key being compromised, or when stale BGPsec_Path
attribute signatures are being distributed.
3.1. A proposed process for BGPsec key rollover
The BGPsec key rollover process will be dependent on the key
provisioning mechanisms that are adopted by an AS. The key
provisioning mechanisms for BGPsec are not yet fully documented (see
[I-D.ietf-sidr-rtr-keying] as a work in progress document). An
automatic provisioning mechanism such as EST will allow BGPsec code
to include automatic re-keying scripts with minimum development cost.
If we work under the assumption that an automatic mechanism will
exist to rollover a BGPsec 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
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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-BGP speaker or shared, whether the keys
are generated on each BGP 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. A staging period may not be possible to
implement during emergency key rollover, in which case routing
information may be lost.
3. Twilight: At this moment, the BGP speaker that holds the private
key that has been rolled-over will stop using the OLD key for
signing and start using the NEW key. Also, the router will
generate appropriate BGPsec_Path attributes just as in the
typical operation of refreshing out-bound BGP polices. This
operation may generate a great number of BGPsec_Path attributes
(due to the need to refresh BGP outbound policies). In any given
BGP SPEAKER, the Twilight moment may be different for every peer
in order to distribute the system load (probably in the order of
minutes to avoid reaching any expiration time).
4. Certificate Revocation: This is an optional step, but SHOULD be
taken when the goal is to invalidate signatures used with the OLD
key. Reasons to invalidate OLD signatures include: when the AS
has reason to believe that the router signing key has been
compromised, and when the AS needs to invalidate BGPsec_Path
attribute signatures used with this 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. Alternatively, the
CA will just let the OLD certificate to 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 not longer valid (e.g., using the
RPKI-Router Protocol described in [RFC6810]). A router's
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reaction to a message indicating withdrawal of a prefix in the
RPKI-Router Protocol SHOULD include the removal of any RIB entry
that includes a BGPsec attribute signed with that key and the
generation of the correspondent BGP WITHDRAWALs (either implicit
or explicit).
The proposed rollover mechanism will depend on the existence of an
automatic provisioning process for BGPsec certificates. It will
require a staging mechanism based on the RPKI propagation time of
around 24 hours, and it will generate BGPsec_Path attributes for all
prefixes in the router been re-keyed.
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 accelerate its subsequent key roll-over.
When a new BGPsec certificate is generated without changing its key,
steps 3 (Twilight) and 5 (RPKI-Router Protocol Withdrawals) SHOULD
NOT be executed.
4. BGPsec key rollover as a measure against replays attacks in BGPsec
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. This
section discusses the use of BGPsec Rollover as a measure to mitigate
replay attacks.
4.1. BGPsec Replay attack window requirement
In [RFC7353] Section 4.3, the need to limit the vulnerability to
replay attacks is described. 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 have been no topology change at
all, then no security threat comes from a replay of a BGPsec_Path
attribute because the signed information is still valid.
The BGPsec Ops document [I-D.ietf-sidr-bgpsec-ops] gives some idea of
requirements for the size of the BGPsec windows of exposure to replay
attacks. It states that the requirement will be in the order of a
day or longer.
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4.2. BGPsec key rollover as a mechanism to protect against replay
attacks
Since the window requirement is in the order of a day (as documented
in [I-D.ietf-sidr-bgpsec-ops]) and the BGP speaker re-keying is the
edge router of the origin AS, it is feasible for a BGPsec Rollover to
mitigate replays. In this case it is important to complete the full
process (i.e. the OLD and NEW certificate do not share the same key).
By re-keying an AS is letting the BGPsec certificate validation time
be a type of "timestamp" against 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 correct any risk.
For a transit AS that also originates BGPsec_Path attributes 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_Path attributes be provisioned with two
certificates: one to sign BGPsec_Path attributes in transit and a
second one to sign an BGPsec_Path attribute for prefixes originated
in its AS. Only the second certificate (for prefixes originated in
its AS) should be rolled-over frequently as a means of limiting
replay attack windows. The transit BGPsec certificate is expected to
live longer than the origin BGPsec certificate.
Advantage of Re-keying as replay attack protection mechanism:
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 human cost of creating and initiating distribution of new
router key pairs and router EE 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 BGP Rollover)
Disadvantage of Re-keying as replay attack protection mechanism:
1. There is more administrative load due to frequent rollover,
although how frequent is still not clear. Some initial ideas are
found in [I-D.ietf-sidr-bgpsec-ops]
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2. The minimum window size is bounded by the propagation time for
RPKI caches to obtain the new certificate and CRL (2x propagation
time). If provisioning is done ahead of time, the minimum window
size is reduced to 1x propagation time for the CRL. However,
these bounds will be better understood when RPKI and RPs are well
deployed.
3. Re-keying increases the dynamics and size of RPKI repository.
5. IANA Considerations
There are no IANA considerations. This section may be removed upon
publication.
6. Security Considerations
Several possible reasons can cause routers participating in BGPsec to
rollover their signing keys, which also has the effect of
invalidating BGPsec_Path attributes signatures using the current
signature verification key. Conventional key management operations
my dicate a re-key (e.g.,key exposure, change of certificate
attributes, rollover policy). BGPsec routers also may need to change
their signing keys and associated certificate as an anti-replay
protection.
The BGPsec Rollover method allows for an expedient rollover process
when router certificates are distributed through the RPKI, but
without causing routing failures due to a receiving router not being
able to validate a BGPsec_Path attribute created by a router that is
the subject of the rollover.
7. Acknowledgements
We would like to acknowledge Randy Bush, Sriram Kotikalapudi, Stephen
Kent and Sandy Murphy.
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>.
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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-22 (work
in progress), January 2017.
[I-D.ietf-sidr-rtr-keying]
Bush, R., Turner, S., and K. Patel, "Router Keying for
BGPsec", draft-ietf-sidr-rtr-keying-12 (work in progress),
June 2016.
[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>.
[RFC6810] Bush, R. and R. Austein, "The Resource Public Key
Infrastructure (RPKI) to Router Protocol", RFC 6810,
DOI 10.17487/RFC6810, January 2013,
<http://www.rfc-editor.org/info/rfc6810>.
[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
CA
Email: bew@cisco.com
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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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