Algorithm Agility Procedure for the Resource Public Key Infrastructure (RPKI)
RFC 6916

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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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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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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   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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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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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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   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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   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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   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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   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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   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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   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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   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.

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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: rogaglia@cisco.com

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

   EMail: kent@bbn.com

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

   EMail: turners@ieca.com

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