Internet Engineering Task Force (IETF) R. Gagliano

Internet Engineering Task Force (IETF) R. Gagliano
Request for Comments: 6916 Cisco Systems
BCP: 182 S. Kent
Category: Best Current Practice BBN Technologies
ISSN: 2070-1721 S. Turner
IECA, Inc.
April 2013

Algorithm Agility Procedure
for the Resource Public Key Infrastructure (RPKI)

Abstract

This document specifies the process that Certification Authorities
(CAs) and Relying Parties (RPs) participating in the Resource Public
Key Infrastructure (RPKI) will need to follow to transition to a new
(and probably cryptographically stronger) algorithm set. The process
is expected to be completed over a timescale of several years.
Consequently, no emergency transition is specified. The transition
procedure defined in this document supports only a top-down migration
(parent migrates before children).

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 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6916.

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RFC 6916 RPKI Algorithm Agility April 2013

Copyright Notice

Copyright (c) 2013 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
(http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Key Rollover Steps for Algorithm Migration . . . . . . . . . . 6
4.1. Milestones Definition . . . . . . . . . . . . . . . . . . 6
4.2. Process Overview . . . . . . . . . . . . . . . . . . . . . 7
4.3. Phase 0 . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3.1. Milestone 1 . . . . . . . . . . . . . . . . . . . . . 9
4.4. Phase 1 . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5. Phase 2 . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.6. Phase 3 . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.7. Phase 4 . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.8. Return to Phase 0 . . . . . . . . . . . . . . . . . . . . 14
5. Support for Multiple Algorithms in the RPKI Provisioning
Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Validation of Multiple Instances of Signed Products . . . . . 15
7. Revocation . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Key Rollover . . . . . . . . . . . . . . . . . . . . . . . . . 17
9. Repository Structure . . . . . . . . . . . . . . . . . . . . . 17
10. Deprecating an Algorithm Suite . . . . . . . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
13. Normative References . . . . . . . . . . . . . . . . . . . . . 19

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RFC 6916 RPKI Algorithm Agility April 2013

1. Introduction

The Resource Public Key Infrastructure (RPKI) must accommodate
transitions between the public keys used by Certification Authorities
(CAs). Transitions of this sort are usually termed "key rollover".
Planned key rollover will occur regularly throughout the life of the
RPKI, as each CA changes its public keys, in a non-coordinated
fashion. (By non-coordinated we mean that the time at which each CA
elects to change its keys is locally determined, not coordinated
across the RPKI.) Moreover, because a key change might be
necessitated by suspected private key compromise, one can never
assume coordination of these events among all of the CAs in the RPKI.
In an emergency key rollover, the old certificate is revoked and a
new certificate with a new key is issued. The mechanisms to perform
a key rollover in RPKI (either planned or in an emergency), while
maintaining the same algorithm suite, are covered in [RFC6489].

This document describes the mechanism to perform a key rollover in
the RPKI due to the migration to a new signature algorithm suite. It
specifies the process that CAs and Relying Parties (RPs)
participating in the RPKI will need to follow to transition to a new
(and probably cryptographically stronger) algorithm set. The process
is expected to be completed over a timescale of months or years.
Consequently, no emergency transition is specified. The transition
procedure defined in this document supports only a top-down migration
(parent migrates before children).

A signature-algorithm suite encompasses both a signature algorithm
(with a specified key size range) and a one-way hash algorithm. It
is anticipated that the RPKI will require the adoption of updated key
sizes and/or different algorithm suites over time. This document
treats the adoption of a new hash algorithm while retaining the
current signature algorithm as equivalent to an algorithm migration,
and requires the CA to change its key. Migration to a new algorithm
suite will be required in order to maintain an acceptable level of
cryptographic security and protect the integrity of certificates,
Certificate Revocation Lists (CRLs), and signed objects in the RPKI.
All of the data structures in the RPKI explicitly identify the
signature and hash algorithms being used. However, experience has
demonstrated that the ability to represent algorithm IDs is not
sufficient to enable migration to new algorithm suites (algorithm
agility). One also must ensure that protocols, infrastructure
elements, and operational procedures also accommodate the migration
from one algorithm suite to another. Algorithm migration is expected
to be very infrequent, and it will require the support of a "current"
and "next" suite for a prolonged interval, probably several years.

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RFC 6916 RPKI Algorithm Agility April 2013

This document defines how entities in the RPKI execute a planned CA
key rollover when the algorithm suite changes. The description
covers actions by CAs, repository operators, and RPs. It describes
the behavior required of both CAs and RPs to make such key changes
work in the RPKI context, including how the RPKI repository system is
used to support key rollover.

This document does not specify any algorithm suite per se. The RPKI
Certificate Policy (CP) [RFC6484] mandates the use of the algorithms
defined in [RFC6485] by CAs and RPs. When an algorithm transition is
initiated, [RFC6485] MUST be updated (as defined in Section 4.1 of
this document) to redefine the required algorithms for compliant RPKI
CAs and RPs under the CP. The CP will not change as a side effect of
algorithm transition, and thus the policy OID in RPKI certificates
will not change.

For each algorithm transition, an additional document (the algorithm
transition timetable) MUST be published (as a BCP) to define the
dates for each milestone defined in this document. It will define
dates for the phase transitions consistent with the descriptions
provided in Section 4. It also will describe how the RPKI community
will measure the readiness of CAs and RPs to transition to each
phase. CAs publish certificates, CRLs, and other signed objects
under the new algorithm suite as the transition progresses. This
provides visibility into the deployment of the new algorithm suite,
enabling the community to evaluate deployment progress. The
transition procedure allows CAs to remove old certificates, CRLs, and
signed products after the twilight date, which provides the ability
to observe and measure the withdrawal of the old algorithm suite.
Thus, the phases defined in this document enable the community to
evaluate the progress of the transition. The timetable document will
also describe procedures to amend the timetable if problems arise in
implementing later phases of the transition. It is RECOMMENDED that
the timetable document be developed by representatives of the RPKI
community, e.g., IANA, Internet Registries, and network operators.

2. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "NOT RECOMMENDED" and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].

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3. Terminology

This document assumes that the reader is familiar with the terms and
concepts described in "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile" [RFC5280],
"X.509 Extensions for IP Addresses and AS Identifiers" [RFC3779], and
"A Profile for Resource Certificate Repository Structure" [RFC6481].
Additional terms and conventions used in examples are provided below.

Algorithm migration: A planned transition from one signature and
hash algorithm to a new signature and hash algorithm.

Algorithm Suite A: The "current" algorithm suite used for hashing
and signing; used in examples in this document.

Algorithm Suite B: The "next" algorithm suite used for hashing and
signing; used in examples in this document.

CA X: The CA that issued CA Y's certificate (i.e., CA Y's
parent); used in examples in this document.

CA Y: The non-leaf CA; used in examples in this document.

CA Z: A CA that is a "child" of CA Y; used in examples in this
document.

Correspond: Two certificates issued under different algorithm suites
correspond to one another if they are issued to the same
entity by the same CA and bind identical Internet Number
Resources (INRs) to that entity. Two CRLs correspond if
they are issued by the same CA and enumerate
corresponding certificates. Two signed objects (other
than manifests) correspond if they are verified using
corresponding end-entity (EE) certificates and they
contain the same encapsulated Context Info field. Two
manifests correspond if they encompass corresponding
certificates, Route Origination Authorizations (ROAs),
CRLs, and other signed objects. (The term "equivalent"
is used synonymously when referring to such RPKI signed
products.)

Leaf CA: A CA that issues only EE certificates.

Non-Leaf CA: A CA that issues certificates to other CAs.

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PoP (proof of possession): Execution of a protocol that demonstrates
to an issuer that a subject requesting a certificate
possesses the private key corresponding to the public key
in the certificate request submitted by the subject.

ROA: Route Origination Authorization, as defined in [RFC6482].

Signed product set (also called set or product set): A collection of
certificates, signed objects, a CRL and a manifest that
are associated by virtue of being verifiable under the
same parent CA certificate

4. Key Rollover Steps for Algorithm Migration

The "current" RPKI algorithm suite (Suite A) is defined in the RPKI
CP document, by reference to [RFC6485]. When a migration of the RPKI
algorithm suite is needed, the first step MUST be an update of
[RFC6485] to define the new algorithm suite. The algorithm
transition timeline document MUST also be published (as a BCP) to
inform the community of the dates selected for milestones in the
transition process, as described in Section 4.1.

4.1. Milestones Definition

CA Ready Algorithm B Date: After this date, all non-leaf CAs MUST be
ready to process a request from a child CA to issue a
certificate under the Algorithm Suite B. All CAs
publishing an [RFC6490] Trust Anchor Locator (TAL) for
Algorithm Suite A MUST also publish the correspondent TAL
for Algorithm Suite B.

CA Go Algorithm B Date: After this date, all CAs MUST have reissued
all their signed product sets under Algorithm Suite B.

RP Ready Algorithm B Date: After this date, all RPs MUST be prepared
to process signed material issued under Algorithm Suite
B.

Twilight Date: After this date, a CA MAY cease issuing signed
products under Algorithm Suite A. Also, after this date,
an RP MAY cease to validate signed materials issued under
Algorithm Suite A.

End-Of-Life (EOL) Date: After this date, Algorithm Suite A MUST be
deprecated using the process in Section 10, and all
Algorithm Suite A TALs MUST be removed from their
publication points.

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RFC 6916 RPKI Algorithm Agility April 2013

4.2. Process Overview

The migration process described in this document involves a series of
steps that MUST be executed in chronological order by CAs and RPs.
The only milestone at which both CAs and RPs take action at the same
time is the EOL Date. Due to the decentralized nature of the RPKI
infrastructure, it is expected that an algorithm transition will span
several years.

In order to facilitate the transition, CAs will start issuing
certificates using Algorithm B in a hierarchical, top-down fashion.
In our example, CA Y will issue certificates using Algorithm Suite B
only after CA X has started to do so (CA Y Ready Algorithm B Date >
CA X Ready Algorithm B Date). This ordered transition avoids the
issuance of "mixed" suite CA certificates, e.g., a CA certificate
signed using Suite A that contains a key from Suite B. In the RPKI,
a CA MUST NOT sign a CA certificate carrying a subject key that
corresponds to an algorithm suite that differs from the one used to
sign the certificate. (X.509 accommodates such mixed algorithm
certificates, but this process avoids using that capability.) A non-
top-down transition approach would require the use of such mixed-mode
certificates and would lead to exponential growth of the RPKI
repository. Also, because the RPKI CP mandates PoP for certificate
requests, it is not possible for a CA to request a certificate for
Algorithm Suite B until its parent CA supports that suite. (See
Section 5 for more details.)

The algorithm agility model described here does not prohibit a CA
from issuing an EE certificate with a subject public key from a
different algorithm suite, if that certificate is not used to verify
repository objects. This exception to the mixed algorithm suite
certificate rule is allowed because an EE certificate that is not
used to verify repository objects does not interfere with the ability
of RPs to download and verify repository content. As noted above,
every CA in the RPKI is required to perform a PoP check for the
subject public key when issuing a certificate. In general, a subject
cannot assume that a CA is capable of supporting a different
algorithm. However, if the subject is closely affiliated with the
CA, it is reasonable to assume that there are ways for the subject to
know whether the CA can support a request to issue an EE certificate
containing a specific, different public key algorithm. This document
does not specify how a subject can determine whether a CA is capable
of issuing a mixed suite EE certificate, because it anticipates that
such certificates will be issued only in contexts where the subject
and CA are sufficiently closely affiliated (for example, an ISP
issuing certificates to devices that it manages).

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The following figure gives an overview of the process:

Process for RPKI CAs:

Phase 0 Phase 1 Phase 2 Phase 4 Phase 0
--x--------x---------x-------------------x--------x---------
^ ^ ^ ^ ^
| | | | |
(1) (2) (3) (5) (6)

Process for RPKI RPs:

Phase 0 Phase 3 Phase 4 Phase 0
-------------------------------x---------x--------x---------
^ ^ ^ ^
| | | |
(1) (4) (5) (6)

(1) RPKI algorithm document is updated, and the algorithm
transition timeline document is issued
(2) CA Ready Algorithm B Date
(3) CA Go Algorithm B Date
(4) RP Ready Algorithm B Date
(5) Twilight Date
(6) End-Of-Life (EOL) Date

Each of these milestones is discussed in the next section when each
phase of the transition process is described.

Two situations have been identified that motivate pausing or rolling
back the transition process. The first situation arises if the RPKI
community is not ready to make the transition. For example, many CAs
might not be prepared to issue signed products under Suite B, or many
RPs might not be ready to process Suite B products. Under these
circumstances, the timetable MUST be reissued, postponing the date
for the phase in question and pushing back the dates for later
phases. The other situation arises if, during the transition,
serious concerns arise about the security of the Suite B algorithms.
Such concerns would motivate terminating the transition and rolling
back signed products, i.e., reverting to Suite A. In this case, the
timetable MUST be republished, and the RPKI algorithm document MUST
be superseded. The phase descriptions below allude to these two
situations, as appropriate.

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RFC 6916 RPKI Algorithm Agility April 2013

4.3. Phase 0

Phase 0 is the steady-state phase of the process; throughout this
phase, Algorithm Suite A is the only supported algorithm suite in the
RPKI. Phase 0 is also the steady state for the RPKI.

During Phase 0, CAs X, Y, and Z are required to generate signed
product sets using only Algorithm Suite A. Also, RPs are required to
validate signed product sets issued using only Algorithm Suite A.

The following figure shows an example of the structure of signed
objects in the repository, indicating the algorithm suites in use and
showing the relationships between three CAs (X, Y, and Z) that form a
certification chain. Vertical alignment in the figure indicates
objects signed by the same CA using the same private key. The
differences in horizontal indentation also represent the use of
different publication points for objects signed by different CAs.
The characters "|->" are used for visualization purposes for both the
signing relationship and the publication point change. For example,
the objects CA-Y-Certificate-Algorithm-Suite-A, CA-X-CRL-Algorithm-
Suite-A, and CA-X-Signed-Objects-Algorithm-Suite-A are all signed
using the private key corresponding to CA-X-Certificate-Algorithm-
Suite-A and published at CA X's corresponding publication point.

CA-X-Certificate-Algorithm-Suite-A (Cert-XA)
|-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA)
|-> CA-Z-Certificate-Algorithm-Suite-A (Cert-ZA)
|-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZA)
|-> CA-Z-Signed-Objects-Algorithm-Suite-A
|-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA)
|-> CA-Y-Signed-Objects-Algorithm-Suite-A
|-> CA-X-CRL-Algorithm-Suite-A (CRL-XA)
|-> CA-X-Signed-Objects-Algorithm-Suite-A

Note: Cert-XA represents the certificate for CA X, which is signed
using Algorithm Suite A.

4.3.1. Milestone 1

The first milestone initiates the migration process. It updates
[RFC6485] with the following definitions for the RPKI:

o Algorithm Suite A

o Algorithm Suite B

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Additionally, the new algorithm transition timeline document MUST be
published with the following information:

o CA Ready Algorithm B Date

o CA Go Algorithm B Date

o RP Ready Algorithm B Date

o Twilight Date

o EOL Date

o Readiness metrics for CAs and RPs in each phase

Each date specified here is assumed to be at one minute after
midnight, UTC. No finer granularity time specification is required
or supported.

4.4. Phase 1

Phase 1 starts at the CA Ready Algorithm B Date. During Phase 1, all
non-leaf CAs MUST be ready to process a request from a child CA to
issue or revoke a certificate using Algorithm Suite B. If it is
determined that a substantial number of CAs are not ready, the
algorithm transition timeline document MUST be reissued, as noted in
Section 4.2. However, CAs that are capable of issuing Suite B
certificates may continue to do so, if requested by their child CAs.
As this phase does not require any RPs to process signed objects
under Suite B, and since Suite B product sets SHOULD be stored at
independent publication points, there is no adverse impact on RPs.
If the Suite B algorithm is deemed unsuitable, the algorithm
transition timeline and the algorithm specification documents MUST be
replaced, and Algorithm Suite B MUST be deprecated using the process
described in Section 10.

Because the transition will happen using a hierarchical, top-down
model, a child CA will be able to issue certificates using Algorithm
Suite B only after its parent CA has issued its own. The RPKI
provisioning protocol can identify if a parent CA is capable of
issuing certificates using Algorithm Suite B and can identify the
corresponding algorithm suite in each Certificate Signing Request
(CSR; see Section 5). During much of this phase, the Suite B product
tree will be incomplete, i.e., not all CAs will have issued products
under Suite B. Thus, for production purposes, RPs MUST fetch and
validate only Suite A products. Suite B products should be fetched
and processed only for testing purposes.

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RFC 6916 RPKI Algorithm Agility April 2013

The following figure shows the status of repository entries for the
three example CAs during this phase. Two distinct certificate chains
are maintained, and CA Z has not yet requested any material using
Algorithm Suite B.

CA-X-Certificate-Algorithm-Suite-A (Cert-XA)
|-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA)
|-> CA-Z-Certificate-Algorithm-Suite-A (Cert-ZA)
|-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZA)
|-> CA-Z-Signed-Objects-Algorithm-Suite-A
|-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA)
|-> CA-Y-Signed-Objects-Algorithm-Suite-A
|-> CA-X-CRL-Algorithm-Suite-A (CRL-XA)
|-> CA-X-Signed-Objects-Algorithm-Suite-A

CA-X-Certificate-Algorithm-Suite-B (Cert-XB)
|-> CA-Y-Certificate-Algorithm-Suite-B (Cert-YB)
|-> CA-Y-CRL-Algorithm-Suite-B (CRL-YB)
|-> CA-Y-Signed-Objects-Algorithm-Suite-B
|-> CA-X-CRL-Algorithm-Suite-B (CRL-XB)
|-> CA-X-Signed-Objects-Algorithm-Suite-B

4.5. Phase 2

Phase 2 starts at the CA Go Algorithm B Date. At the start of this
phase, each signed product set MUST be available using both Algorithm
Suite A and Algorithm Suite B. Thus, prior to the start of this
phase, every CA MUST ensure that there is a Suite B product
corresponding to each Suite A product that the CA has issued.
Throughout this phase, each CA MUST maintain this correspondence.
During this phase, RPs MUST be prepared to validate sets issued using
Algorithm Suite A and MAY be prepared to validate sets issued using
the Algorithm Suite B.

If it is determined that a substantial number of CAs are not ready,
the algorithm transition timeline document MUST be reissued, as noted
in Section 4.2. (Since the processing requirement for RPs here is a
MAY, if RPs have problems with Suite B products, this does not
require pushing back the Phase 2 milestone, but it does motivate
delaying the start of Phase 3.) CAs that are capable of publishing
products under Suite B MAY continue to do so. Phase 2, like Phase 1,
does not require any RPs to process signed objects under Suite B.
Also, Suite B products SHOULD be stored at independent publication
points so that there is no adverse impact on RPs that are not
prepared to process Suite B products. (See Section 9 for additional
details.) If the Suite B algorithm is deemed unsuitable, the

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RFC 6916 RPKI Algorithm Agility April 2013

algorithm transition timeline and the algorithm specification
documents MUST be replaced, and Algorithm Suite B MUST be deprecated
using the process described in Section 10.

It is RECOMMENDED that RPs that can process Algorithm Suite B fetch
and validate Suite B products. RPs that are not ready to process
Suite B products MUST continue to make use of Suite A products. An
RP that elects to validate signed product sets using both Algorithm
Suite A and Algorithm Suite B should expect the same results. If
there are discrepancies when evaluating corresponding signed product
sets, successful validation of either product set is acceptable. A
detailed analysis of the validation of multiple instances of signed
objects is included in Section 6.

The following figure shows the status of the repository entries for
the three example CAs throughout this phase, where all signed objects
are available using both algorithm suites.

CA-X-Certificate-Algorithm-Suite-A (Cert-XA)
|-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA)
|-> CA-Z-Certificate-Algorithm-Suite-A (Cert-ZA)
|-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZA)
|-> CA-Z-Signed-Objects-Algorithm-Suite-A
|-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA)
|-> CA-Y-Signed-Objects-Algorithm-Suite-A
|-> CA-X-CRL-Algorithm-Suite-A (CRL-XA)
|-> CA-X-Signed-Objects-Algorithm-Suite-A

CA-X-Certificate-Algorithm-Suite-B (Cert-XB)
|-> CA-Y-Certificate-Algorithm-Suite-B (Cert-YB)
|-> CA-Z-Certificate-Algorithm-Suite-B (Cert-ZB)
|-> CA-Z-CRL-Algorithm-Suite-B (CRL-ZB)
|-> CA-Z-Signed-Objects-Algorithm-Suite-B
|-> CA-Y-CRL-Algorithm-Suite-B (CRL-YB)
|-> CA-Y-Signed-Objects-Algorithm-Suite-B
|-> CA-X-CRL-Algorithm-Suite-B (CRL-XB)
|-> CA-X-Signed-Objects-Algorithm-Suite-B

4.6. Phase 3

Phase 3 starts at the RP Ready Algorithm B Date. During this phase,
all signed product sets are available using both algorithm suites,
and all RPs MUST be able to validate them. (The correspondence
between Suite A and Suite B products was required for Phase 2 and was
maintained throughout that phase. The same requirements apply
throughout this phase.) It is RECOMMENDED that, in preparation for
Phase 4, RPs retrieve and process Suite B product sets first and

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RFC 6916 RPKI Algorithm Agility April 2013

treat them as the preferred product sets for validation throughout
this phase. Thus, an RP SHOULD try to validate the sets of signed
products retrieved from the Algorithm Suite B repository first.

If a substantial number of RPs are unable to process product sets
signed with Suite B, the algorithm transition timeline document MUST
be reissued, pushing back the date for this and later milestones, as
discussed in Section 4.2. Since the Suite B products SHOULD be
published at distinct publication points, RPs that cannot process
Suite B products can be expected to revert to the Suite A products
that still exist. If the Suite B algorithm is deemed unsuitable, the
algorithm transition timeline and the algorithm specification
documents MUST be replaced and Algorithm Suite B MUST be deprecated
using the process described in Section 10.

There are no changes to the CA behavior throughout this phase.

4.7. Phase 4

Phase 4 starts at the Twilight Date. At that date, Algorithm A is
labeled as "old" and the Algorithm B is labeled as "current".

During this phase, all signed product sets MUST be issued using
Algorithm Suite B and MAY be issued using Algorithm Suite A. All
signed products sets issued using Suite B MUST be published at their
corresponding publication points. Signed products sets issued using
Suite A might not be available at their corresponding publication
points. Every RP MUST validate signed product sets using Suite B.
RPs MAY validate signed product sets using Suite A. However, RPs
SHOULD NOT assume that the collection of Suite A product sets is
complete. Thus, RPs SHOULD make use of only Suite B products sets.
(See Section 6 for further details.)

If it is determined that many RPs are not capable of processing the
new algorithm suite, the algorithm transition timeline document MUST
be reissued, pushing back the date for this and the next milestone.
The document MUST require the CA not to remove Suite A product sets
if this phase is delayed. If Algorithm Suite B is deemed unsuitable,
the algorithm transition timeline and the algorithm specification
documents MUST be replaced, Algorithm Suite B MUST be deprecated
using the process described in Section 10, and CAs MUST NOT remove
Suite A product sets. At this stage, RPs are still capable of
processing Suite A signed products, so the RPKI is still viable.

The following figure describes a possible status for the repositories
of the example CAs.

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RFC 6916 RPKI Algorithm Agility April 2013

CA-X-Certificate-Algorithm-Suite-A (Cert-XA)
|-> CA-Y-Certificate-Algorithm-Suite-A (Cert-YA)
|-> CA-Y-CRL-Algorithm-Suite-A (CRL-YA)
|-> CA-Y-Signed-Objects-Algorithm-Suite-A
|-> CA-X-CRL-Algorithm-Suite-A (CRL-XA)
|-> CA-X-Signed-Objects-Algorithm-Suite-A

CA-X-Certificate-Algorithm-Suite-B (Cert-XB)
|-> CA-Y-Certificate-Algorithm-Suite-B (Cert-YB)
|-> CA-Z-Certificate-Algorithm-Suite-B (Cert-ZB)
|-> CA-Z-CRL-Algorithm-Suite-A (CRL-ZB)
|-> CA-Z-Signed-Objects-Algorithm-Suite-B
|-> CA-Y-CRL-Algorithm-Suite-A (CRL-YB)
|-> CA-Y-Signed-Objects-Algorithm-Suite-B
|-> CA-X-CRL-Algorithm-Suite-A (CRL-XB)
|-> CA-X-Signed-Objects-Algorithm-Suite-B

4.8. Return to Phase 0

The EOL Date triggers the return to Phase 0 (steady state). At this
point, the old algorithm suite, Algorithm Suite A, MUST be deprecated
using the process described in Section 10.

This phase closes the loop, as the new algorithm suite (Algorithm
Suite B) is now the only required algorithm suite in RPKI. From this
point forward, this suite is referred to as Algorithm Suite A.

If it is determined that many RPs are not capable of processing the
new algorithm suite, the algorithm transition timeline document MUST
be reissued, pushing back the date for this milestone.

5. Support for Multiple Algorithms in the RPKI Provisioning Protocol

The migration described in this document is a top-down process where
two synchronization issues need to be solved between child and parent
CAs:

o A child CA needs to identify which algorithm suites are supported
by its parent CA.

o A child CA needs to signal which algorithm suite should be used by
its parent CA to sign a CSR.

The RPKI provisioning protocol [RFC6492] supports multiple algorithms
suites by implementing different resource classes for each suite.
Several different resource classes also may use the same algorithm
suite for different resource sets.

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A child CA that wants to identify which algorithm suites are
supported by its parent CA MUST perform the following tasks:

1. Establish a provisioning protocol session with its parent CA.

2. Perform a "list" command as described in Section 3.3.1 of
[RFC6492].

3. From the Payload in the "list response" resource class, extract
the "issuer's certificate" for each class. The algorithm suite
for each class will match the algorithm suite used to issue the
corresponding "issuer's certificate" (as specified in the
SubjectPublicKeyInfo field of that certificate).

A child CA that wants to specify an algorithm suite to its parent CA
(e.g., in a certificate request) MUST perform the following tasks:

1. Perform the tasks described above to identify the algorithm
suites supported by its parent CA and the resource class
corresponding to each suite.

2. Identify the corresponding resource class in the appropriate
provisioning protocol command (e.g., "issue" or "revoke").

Upon receipt of a certificate request from a child CA, a parent CA
will verify the PoP of the private key. If a child CA requests the
issuing of a certificate using an algorithm suite that does not match
a resource class, the PoP validation will fail and the request will
not be performed.

6. Validation of Multiple Instances of Signed Products

During Phases 1, 2, 3, and 4, two algorithm suites will be valid
simultaneously in RPKI. In this section, we describe the RP behavior
when validating corresponding signed products using different
algorithm suites.

During Phase 1, two corresponding instances MAY be available for each
signed product, one signed under Algorithm Suite A and one under
Algorithm Suite B. As noted in Section 4.4, in this phase there is a
preference for Suite A product sets. All products are available
under Suite A, while only some products may be available under Suite
B. For production purposes, an RP MAY fetch and validate only Suite
A products. Suite B products SHOULD be fetched and validated only
for test purposes. When product sets exist under both suites, they
should yield equivalent results, to facilitate testing. (It is not
possible to directly compare Suite A and Suite B product sets,
because certificates, CRLs, and manifests will appear syntactically

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RFC 6916 RPKI Algorithm Agility April 2013

different. However, the output of the process, i.e., the ROA
payloads -- Autonomous System number and address prefix data --
SHOULD match, modulo timing issues.)

During Phases 2 and 3 of this process, two corresponding instances of
all signed products MUST be available to RPs. As noted in
Section 4.5, it is RECOMMENDED that Suite B capable RPs fetch and
validate Suite B products sets during Phase 2. If an RP encounters
validation problems with the Suite B products, it SHOULD revert to
using Suite A products. RPs that are Suite B capable MAY fetch both
product sets and compare the results (e.g., ROA outputs) for testing.

In Phase 3, all RPs MUST be Suite B capable and MUST fetch Suite B
product sets. If an RP encounters problems with Suite B product
sets, it can revert to Suite A products. RPs encountering such
problems SHOULD contact the relevant repository maintainers (e.g.,
using the mechanism defined in [RFC6493] to report problems.)

During Phase 4, only Suite B product sets are required to be present
for all RPKI entities, as per Section 4.7. Thus, RPs SHOULD retrieve
and validate only these product sets. Retrieval of Suite A products
sets may yield an incomplete set of signed products and is NOT
RECOMMENDED.

7. Revocation

The algorithm migration process mandates the maintenance of two
parallel but equivalent certification hierarchies during Phases 2 and
3 of the process. During these phases, a CA MUST revoke and request
revocation of certificates consistently under both algorithm suites.
When not performing a key rollover operation (as described in
Section 8), a CA requesting the revocation of its certificate during
these two phases MUST perform that request for both algorithm suites
(A and B). A non-leaf CA SHOULD NOT verify that its child CAs comply
with this requirement. Note that a CA MUST request revocation of its
certificate relative to a specific algorithm suite using the
mechanism described in Section 5

During Phase 1, a CA that revokes a certificate under Suite A SHOULD
revoke the corresponding certificate under Suite B if that
certificate exists. During Phase 4, a CA that revokes a certificate
under Suite B SHOULD revoke the corresponding certificate under Suite
A if that certificate exists.

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During Phase 1, a CA may revoke certificates under Suite B without
revoking them under Suite A, since the Suite B products are for test
purposes. During Phase 4, a CA may revoke certificates issued under
Suite A without revoking them under Suite B, since Suite A products
are being deprecated.

8. Key Rollover

Key rollover (without algorithm changes) is effected independently
for each algorithm suite and MUST follow the process described in
[RFC6489].

9. Repository Structure

The two parallel hierarchies that will exist during the transition
process SHOULD have independent publications points. The repository
structures for each algorithm suite are described in [RFC6481].

10. Deprecating an Algorithm Suite

To deprecate an algorithm suite, the following process MUST be
executed by every CA in the RPKI:

1. Each CA MUST cease issuing certificates under the suite. This
means that any request for a CA certificate from a child will be
rejected, e.g., sending an "error_response" message with error
code "request - no such resource class", as defined in [RFC6492].

2. Each CA MUST cease generating signed products, except the CRL and
manifest, under the deprecated algorithm suite.

3. Each CA MUST revoke the EE certificates for all signed products
that it has issued under the deprecated algorithm suite. The CA
SHOULD delete these products from its publication point to avoid
burdening RPs with the need to download and process these
products.

4. Each CA MUST revoke all CA certificates that it has issued under
the deprecated algorithm suite.

5. Each CA SHOULD remove all CA certificates that it has issued
under the deprecated algorithm suite.

6. Each CA that publishes a TAL under the deprecated algorithm suite
MUST removed it from the TAL's publication point.

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RFC 6916 RPKI Algorithm Agility April 2013

7. Each CA SHOULD continue to maintain the publication point for the
deprecated algorithm suite at least until the CRL nextUpdate.
This publication point MUST contain only the CRL and a manifest
for that publication point. This behavior provides a window in
which RPs may be able to become aware of the revoked status of
the signed products that have been deleted.

8. Each RP MUST remove any TALs that is has published under the
deprecated algorithm suite.

CAs in the RPKI hierarchy may become aware of the deprecation of the
algorithm suite at different times and may execute the procedure
above asynchronously relative to one another. Thus, for example, a
CA may request revocation of its CA certificate, only to learn that
the certificate has already been revoked by the issuing CA. The
revocation of a CA certificate makes the CRL and manifest issued
under it incapable of validation. The asynchronous execution of this
procedure likely will result in transient "inconsistencies" among the
publication points associated with the deprecated algorithm suite.
However, these inconsistencies should yield "fail-safe" results,
i.e., the objects signed under the deprecated suite should be
rejected by RPs.

11. Security Considerations

An algorithm transition in RPKI should be a very infrequent event,
and it requires wide community consensus. The events that may lead
to an algorithm transition may be related to a weakness of the
cryptographic strength of the algorithm suite in use by RPKI, which
is normal to happen over time. The procedures described in this
document mean that it will take years to complete an algorithm
transition. During that time, the RPKI system will be vulnerable to
any cryptographic weakness that may have triggered this procedure
(e.g., a downgrade attack).

This document does not describe an emergency mechanism for algorithm
migration. Due to the distributed nature of RPKI and the very large
number of CAs and RPs, the authors do not believe it is feasible to
effect an emergency algorithm migration procedure.

If a CA does not complete its migration to the new algorithm suite as
described in this document (after the EOL of the "old" algorithm
suite), its signed product set will no longer be valid.
Consequently, the RPKI may, at the end of Phase 4, have a smaller
number of valid signed products than before starting the process.
Conversely, an RP that does not follow this process will lose the
ability to validate signed products issued under the new algorithm

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RFC 6916 RPKI Algorithm Agility April 2013

suite. The resulting incomplete view of routing information from the
RPKI (as a result of a failure by CAs or RPs to complete the
transition) could degrade routing in the public Internet.

12. Acknowledgements

The authors would like to acknowledge the work of the SIDR working
group co-chairs (Sandra Murphy, Chris Morrow, and Alexey Melnikov) as
well as the contributions given by Geoff Huston, Arturo Servin, Brian
Weis, Terry Manderson, Brian Dickson, David Black, and Danny
McPherson.

13. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779, June 2004.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.

[RFC6481] Huston, G., Loomans, R., and G. Michaelson, "A Profile for
Resource Certificate Repository Structure", RFC 6481,
February 2012.

[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
Origin Authorizations (ROAs)", RFC 6482, February 2012.

[RFC6484] Kent, S., Kong, D., Seo, K., and R. Watro, "Certificate
Policy (CP) for the Resource Public Key Infrastructure
(RPKI)", BCP 173, RFC 6484, February 2012.

[RFC6485] Huston, G., "The Profile for Algorithms and Key Sizes for
Use in the Resource Public Key Infrastructure (RPKI)",
RFC 6485, February 2012.

[RFC6489] Huston, G., Michaelson, G., and S. Kent, "Certification
Authority (CA) Key Rollover in the Resource Public Key
Infrastructure (RPKI)", BCP 174, RFC 6489, February 2012.

[RFC6490] Huston, G., Weiler, S., Michaelson, G., and S. Kent,
"Resource Public Key Infrastructure (RPKI) Trust Anchor
Locator", RFC 6490, February 2012.

Gagliano, et al. Best Current Practice [Page 19]

RFC 6916 RPKI Algorithm Agility April 2013

[RFC6492] Huston, G., Loomans, R., Ellacott, B., and R. Austein, "A
Protocol for Provisioning Resource Certificates",
RFC 6492, February 2012.

[RFC6493] Bush, R., "The Resource Public Key Infrastructure (RPKI)
Ghostbusters Record", RFC 6493, February 2012.

Authors' Addresses

Roque Gagliano
Cisco Systems
Avenue des Uttins 5
Rolle 1180
Switzerland

EMail: [email protected]

Stephen Kent
BBN Technologies
10 Moulton St.
Cambridge, MA 02138
USA

EMail: [email protected]

Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA

EMail: [email protected]

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