Network Working Group R. Gagliano
Internet-Draft K. Patel
Intended status: Standards Track B. Weis
Expires: September 22, 2016 Cisco Systems
March 21, 2016
BGPsec Router Certificate Rollover
draft-ietf-sidr-bgpsec-rollover-05
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 pubic keys
by BGPsec routers. This document 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.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 22, 2016.
Copyright Notice
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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 . . . . . . . . . . . . . . . . . 8
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-keying) is the process of changing a
router's key pair (or pairs), issuing the corresponding new End-
Entity 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 local policies at each 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 (depending on the
key provisioning mechanism to be selected), this key pair will be
used to sign new BGPsec_Path attributes
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[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 such 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 the router private key may be compromised. Distributing
the OLD public key in a new certificate is NOT RECOMMENDED when the
rollover event is due to the key has been compromised or 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). It is
assumed that an automatic provisioning mechanism such as EST will be
in place as such a provisioning mechanism 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 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
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. On the other side,
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]). It is not
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documented yet what will be a router's reaction to a message with
the withdrawal bit set to 1 in the RPKI-Router Protocol, but it
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 ideas
of requirements for the size of the BGPsec windows of exposure to
replay attacks. At that document, it is stated that for the vast
majority of the prefixes, the requirement will be in the order of
days or weeks.
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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 sort 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 this scenario been
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 attach windows. The transit BGPsec
certificate is expected to be longer living than the origin BGPsec
certificate.
Advantage of Re-keying as replay attack protection mechanism:
1. All expiration policies are maintained in 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. 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. More administrative load due to frequent rollover, although how
frequent is still not clear. Some initial ideas in
[I-D.ietf-sidr-bgpsec-ops]
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2. Minimum window size bounded by RPKI propagation time to RPKI
caches for 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, more
experimentation is needed when RPKI and RPs are more massively
deployed.
3. Increases dynamics and size of RPKI repository.
5. IANA Considerations
No IANA considerations
6. Security Considerations
Several possible reasons can cause routers participating in BGPsec to
replace rollover their signing keys and/or signatures containing
their current signature verification key. Some reasons are due to
the usual key management operations reasons (e.g.,key exposure,
change of certificate attributes, due to policy). However 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>.
8.2. Informative References
[I-D.ietf-sidr-bgpsec-ops]
Bush, R., "BGPsec Operational Considerations", draft-ietf-
sidr-bgpsec-ops-07 (work in progress), December 2015.
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[I-D.ietf-sidr-bgpsec-protocol]
Lepinski, M. and K. Sriram, "BGPsec Protocol
Specification", draft-ietf-sidr-bgpsec-protocol-15 (work
in progress), March 2016.
[I-D.ietf-sidr-rtr-keying]
Bush, R. and K. Patel, "Router Keying for BGPsec", draft-
ietf-sidr-rtr-keying-10 (work in progress), November 2015.
[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
Roque Gagliano
Cisco Systems
Avenue des Uttins 5
Rolle, VD 1180
Switzerland
Email: rogaglia@cisco.com
Keyur Patel
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
CA
Email: keyupate@cisco.com
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Brian Weis
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
CA
Email: bew@cisco.com
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