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RPKI Signed Object for Trust Anchor Key
draft-ietf-sidrops-signed-tal-14

Document Type Active Internet-Draft (sidrops WG)
Authors Carlos M. Martínez , George G. Michaelson , Tom Harrison , Tim Bruijnzeels , Rob Austein
Last updated 2023-09-05
Replaces draft-tbruijnzeels-sidrops-signed-tal
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draft-ietf-sidrops-signed-tal-14
Network Working Group                                        C. Martinez
Internet-Draft                                                    LACNIC
Intended status: Standards Track                           G. Michaelson
Expires: 8 March 2024                                        T. Harrison
                                                                   APNIC
                                                          T. Bruijnzeels
                                                              NLnet Labs
                                                              R. Austein
                                                    Dragon Research Labs
                                                        5 September 2023

                RPKI Signed Object for Trust Anchor Key
                    draft-ietf-sidrops-signed-tal-14

Abstract

   A Trust Anchor Locator (TAL) is used by Relying Parties (RPs) in the
   Resource Public Key Infrastructure (RPKI) to locate and validate a
   Trust Anchor (TA) Certification Authority (CA) certificate used in
   RPKI validation.  This document defines an RPKI signed object for a
   Trust Anchor Key (TAK), that can be used by a TA to signal the
   location(s) of the accompanying CA certificate for the current key to
   RPs, as well as the successor key and the location(s) of its CA
   certificate.  This object helps to support planned key rolls without
   impacting RPKI validation.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 March 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  TAK Object Definition . . . . . . . . . . . . . . . . . . . .   4
     3.1.  The TAK Object Content Type . . . . . . . . . . . . . . .   4
     3.2.  The TAK Object eContent . . . . . . . . . . . . . . . . .   4
       3.2.1.  TAKey . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.2.2.  TAK . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  TAK Object Validation . . . . . . . . . . . . . . . . . .   5
   4.  TAK Object Generation and Publication . . . . . . . . . . . .   6
   5.  Relying Party Use . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Manual update of TA key details . . . . . . . . . . . . .   9
   6.  Maintaining Multiple TA Keys  . . . . . . . . . . . . . . . .   9
   7.  Performing TA Key Rolls . . . . . . . . . . . . . . . . . . .  11
     7.1.  Phase 1: Add a TAK for Key 'A'  . . . . . . . . . . . . .  11
     7.2.  Phase 2: Add a Key 'B'  . . . . . . . . . . . . . . . . .  11
     7.3.  Phase 3: Update TAL to point to 'B' . . . . . . . . . . .  11
     7.4.  Phase 4: Remove Key 'A' . . . . . . . . . . . . . . . . .  12
   8.  Using TAK objects to distribute TAL data  . . . . . . . . . .  12
   9.  Deployment Considerations . . . . . . . . . . . . . . . . . .  13
     9.1.  Relying Party Support . . . . . . . . . . . . . . . . . .  13
     9.2.  Alternate Transition Models . . . . . . . . . . . . . . .  13
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  14
     10.1.  Acceptance Timers  . . . . . . . . . . . . . . . . . . .  14
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  14
     11.1.  Previous Keys  . . . . . . . . . . . . . . . . . . . . .  14
     11.2.  TA Compromise  . . . . . . . . . . . . . . . . . . . . .  15
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     12.1.  Content Type . . . . . . . . . . . . . . . . . . . . . .  15
     12.2.  Signed Object  . . . . . . . . . . . . . . . . . . . . .  15
     12.3.  File Extension . . . . . . . . . . . . . . . . . . . . .  15
     12.4.  Module Identifier  . . . . . . . . . . . . . . . . . . .  16
     12.5.  Registration of Media Type application/
            rpki-signed-tal  . . . . . . . . . . . . . . . . . . . .  16
   13. Implementation Status . . . . . . . . . . . . . . . . . . . .  17
     13.1.  APNIC  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     13.2.  rpki-client  . . . . . . . . . . . . . . . . . . . . . .  18
   14. Revision History  . . . . . . . . . . . . . . . . . . . . . .  18

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   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     16.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Overview

   A TAL [RFC8630] is used by an RP in the RPKI to locate and validate
   TA CA certificates used in RPKI validation.  However, until now there
   has been no in-band way of notifying RPs of updates to a TAL.  In-
   band notification means that TAs can be more confident of RPs being
   aware of key roll operations.

   This document defines a new RPKI signed object that can be used to
   document the location(s) of the TA CA certificate for the current TA
   key, as well as the value of the successor key and the location(s) of
   its TA CA certificate.  This allows RPs to be notified automatically
   of such changes, and enables TAs to stage a successor key so that
   planned key rolls can be performed without risking the invalidation
   of the RPKI tree under the TA.  We call this object the Trust Anchor
   Key (TAK) object.

   When RPs are first bootstrapped, they use a TAL to discover the key
   and location(s) of the CA certificate for a TA.  The RP can then
   retrieve and validate the CA certificate, and subsequently validate
   the manifest [RFC6486] and CRL published by that TA (section 5 of
   [RFC6487]).  However, before processing any other objects it will
   first validate the TAK object, if present.  If the TAK object lists
   only the current key, then the RP continues processing as per normal.
   If the TAK object includes a successor key, the RP starts an
   acceptance timer, and then continues processing as per normal.  If,
   during the following validation runs up until the expiry of the
   acceptance timer, the RP has not observed any changes to the keys and
   certificate URLs listed in the TAK object, then the RP will fetch the
   successor key, update its local state with that key and its
   associated certification location(s), and continue processing using
   that key.

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   The primary motivation for this work is being able to migrate from a
   Hardware Security Module (HSM) produced by one vendor to one produced
   by another, where the first vendor does not support exporting keys
   for use by the second.  There may be other scenarios in which key
   rollover is useful, though.

3.  TAK Object Definition

   The TAK object makes use of the template for RPKI digitally signed
   objects [RFC6488], which defines a Cryptographic Message Syntax (CMS)
   [RFC5652] wrapper for the content as well as a generic validation
   procedure for RPKI signed objects.  Therefore, to complete the
   specification of the TAK object (see Section 4 of [RFC6488]), this
   document defines:

   *  The OID (in Section 3.1) that identifies the signed object as
      being a TAK.  (This OID appears within the eContentType in the
      encapContentInfo object, as well as the content-type signed
      attribute in the signerInfo object.)

   *  The ASN.1 syntax for the TAK eContent, in Section 3.2.

   *  The additional steps required to validate a TAK, in Section 3.3.

3.1.  The TAK Object Content Type

   This document requests an OID for the TAK object as follows:

      id-ct-signedTAL OBJECT IDENTIFIER ::=
         { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
           smime(16) ct(1) 50 }

   This OID MUST appear both within the eContentType in the
   encapContentInfo object, as well as the content-type signed attribute
   in the signerInfo object (see [RFC6488]).

3.2.  The TAK Object eContent

   The content of a TAK object is ASN.1 encoded using the Distinguished
   Encoding Rules (DER) [X.690], and is defined per the module in
   Appendix A.

3.2.1.  TAKey

   This structure defines a TA key, similarly to [RFC8630].  It contains
   a sequence of zero or more comments, one or more certificate URIs,
   and a SubjectPublicKeyInfo.

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3.2.1.1.  comments

   This field is equivalent to the comment section defined in section
   2.2 of [RFC8630].  Each comment is human-readable informational UTF-8
   text, conforming to the restrictions defined in Section 2 of
   [RFC5198].  The leading "#" character is omitted.

3.2.1.2.  certificateURIs

   This field is equivalent to the URI section defined in section 2.2 of
   [RFC8630].  It MUST contain at least one CertificateURI element.
   Each CertificateURI element contains the IA5String representation of
   either an rsync URI [RFC5781], or an HTTPS URI [RFC7230].

3.2.1.3.  subjectPublicKeyInfo

   This field contains a SubjectPublicKeyInfo (section 4.1.2.7 of
   [RFC5280]) in DER format [X.690].

3.2.2.  TAK

3.2.2.1.  version

   The version number of the TAK object MUST be 0.

3.2.2.2.  current

   This field contains the TA key of the repository in which the TAK
   object is published.

3.2.2.3.  predecessor

   This field contains the TA key that was in use for this TA
   immediately prior to the current TA key, if applicable.

3.2.2.4.  successor

   This field contains the TA key to be used in place of the current
   key, after expiry of the relevant acceptance timer.

3.3.  TAK Object Validation

   To determine whether a TAK object is valid, the RP MUST perform the
   following checks in addition to those specified in [RFC6488]:

   *  The eContentType OID matches the OID described in Section 3.1.

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   *  The TAK object appears as the product of a TA CA certificate (i.e.
      the TA CA certificate is itself the issuer of the EE certificate
      of the TAK object).

   *  The TA CA has published only one TAK object in its repository for
      this key, and this object appears on the manifest as the only
      entry using the ".tak" extension (see [RFC6481]).

   *  The EE certificate of this TAK object describes its Internet
      Number Resources (INRs) using the "inherit" attribute.

   *  The decoded TAK content conforms to the format defined in
      Section 3.2.

   *  The SubjectPublicKeyInfo value of the current TA key in the TAK
      object matches that of the TA CA certificate used to issue the EE
      certificate of the TAK object.

   If any of these checks does not succeed, the RP MUST ignore the TAK
   object, and proceed as though it were not listed on the manifest.

   The RP is not required to compare its current set of certificateURIs
   for the current key with those listed in the TAK object.  The RP MAY
   alert the user that these sets of certificateURIs do not match, with
   a view to the user manually updating the set of certificateURIs in
   their configuration.  The RP MUST NOT automatically update its
   configuration to use these certificateURIs in the event of
   inconsistency, though, because migration of users to new
   certificateURIs should happen by way of the successor key process.

4.  TAK Object Generation and Publication

   A TA MAY choose to use TAK objects to communicate its current,
   predecessor, and successor keys.  If a TA chooses to use TAK objects,
   then it SHOULD generate and publish TAK objects under each of its
   keys.

   A non-normative guideline for naming this object is that the filename
   chosen for the TAK object in the publication repository be a value
   derived from the public key part of the entity's key pair, using the
   algorithm described for CRLs in section 2.2 of [RFC6481] for
   generation of filenames.  The filename extension of ".tak" MUST be
   used to denote the object as a TAK.

   In order to generate a TAK object, the TA MUST perform the following
   actions:

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   *  The TA MUST generate a key pair for a "one-time-use" EE
      certificate to use for the TAK.

   *  The TA MUST generate a one-time-use EE certificate for the TAK.

   *  This EE certificate MUST have an SIA extension access description
      field with an accessMethod OID value of id-ad-signedObject, where
      the associated accessLocation references the publication point of
      the TAK as an object URL.

   *  As described in [RFC6487], an [RFC3779] extension is required in
      the EE certificate used for this object.  However, because the
      resource set is irrelevant to this object type, this certificate
      MUST describe its Internet Number Resources (INRs) using the
      "inherit" attribute, rather than explicit description of a
      resource set.

   *  This EE certificate MUST have a "notBefore" time that matches or
      predates the moment that the TAK will be published.

   *  This EE certificate MUST have a "notAfter" time that reflects the
      intended duration for which this TAK will be published.  If the EE
      certificate for a TAK object is expired, it MUST no longer be
      published, but it MAY be replaced by a newly generated TAK object
      with equivalent content and an updated "notAfter" time.

   *  The current TA key for the TAK MUST match that of the TA CA
      certificate under which the TAK was issued.

5.  Relying Party Use

   Relying Parties MUST keep a record of the current key for each
   configured TA, as well as the URI(s) where the CA certificate for
   this key may be retrieved.  This record is typically bootstrapped by
   the use of a pre-configured (and unsigned) TAL file [RFC8630].

   When performing top-down validation, RPs MUST first validate and
   process the TAK object for its current known key, by performing the
   following steps:

   *  A CA certificate is retrieved and validated from the known URIs as
      described in sections 3 and 4 of [RFC8630].

   *  The manifest and CRL for this certificate are then validated as
      described in [RFC6487] and [RFC6486].

   *  The TAK object, if present, is validated as described in
      Section 3.3.

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   If the TAK object includes a successor key, then the RP must verify
   the successor key by doing the following:

   *  performing top-down validation using the successor key, in order
      to validate the TAK object for the successor TA;

   *  ensuring that a valid TAK object exists for the successor TA;

   *  ensuring that the successor TAK object's current key matches the
      initial TAK object's successor key; and

   *  ensuring that the successor TAK object's predecessor key matches
      the initial TAK object's current key.

   If any of these steps fails, then the successor key has failed
   verification.

   If the successor key passes verification, and the RP has not seen
   that successor key on the previous successful validation run for this
   TA, then the RP:

   *  sets an acceptance timer of 30 days for this successor key for
      this TA;

   *  cancels the existing acceptance timer for this TA (if applicable);
      and

   *  continues standard top-down validation as described in [RFC6487]
      using the current key.

   If the successor key passes verification, and the RP has seen that
   successor key on the previous successful validation run for this TA:

   *  if the relevant acceptance timer has not expired, the RP continues
      standard top-down validation using the current key;

   *  otherwise, the RP updates its current known key details for this
      TA to be those of the successor key, and then begins top-down
      validation again using the successor key.

   If the successor key does not pass verification, or if the TAK object
   does not include a successor key, the RP cancels the existing
   acceptance timer for this TA (if applicable).

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   An RP MUST NOT use a successor key for top-down validation outside of
   the process described above, except for the purpose of testing that
   the new key is working correctly.  This allows a TA to publish a
   successor key for a period of time, allowing RPs to test it, while
   still being able to rely on RPs using the current key for their
   production RPKI operations.

   A successor key may have the same SubjectPublicKeyInfo value as the
   current key: this will be the case where a TA is updating the
   certificateURIs for that key.

5.1.  Manual update of TA key details

   A Relying Party may opt not to support the automatic transition of TA
   key data, as defined in the previous section.  An alternative
   approach is for the Relying Party to alert the user when a new
   successor key is seen, and also when the relevant acceptance timer
   has expired.  The user can then manually transition to the new TA key
   data.  This process ensures that the benefits of the acceptance timer
   period are still realised, as compared with TA key update based on a
   TAL distributed out-of-band by a TA.

6.  Maintaining Multiple TA Keys

   Although an RP that can process TAK objects will only ever use one
   key for validation (either the current key, or the successor key,
   once the relevant acceptance timer has expired), an RP that cannot
   process TAK objects will continue to use the key details per its TAL
   (or equivalent manual configuration) indefinitely.  As a result, even
   when a TA is using a TAK object in order to migrate clients to a new
   key, the TA may have to maintain the previous key for a period of
   time alongside the new key in order to ensure continuity of service
   for older clients.

   For each TA key that a TA is maintaining, the signed material for
   these keys MUST be published under different directories in the
   context of the 'id-ad-caRepository' and 'id-ad-rpkiManifest' Subject
   Information Access descriptions contained on the CA certificates
   [RFC6487].  Publishing objects under the same directory is
   potentially confusing for RPs, and could lead to object invalidity in
   the event of file name collisions.

   Also, the CA certificates for each maintained key, and the contents
   published by each key, MUST be equivalent (except for the TAK
   object).  In other words, for the purposes of RPKI validation, it
   MUST NOT make a difference which of the keys is used as a starting
   point.

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   This means that the IP and AS resources contained on all current CA
   certificates for the maintained TA keys MUST be the same.
   Furthermore, for any delegation of IP and AS resources to a child,
   the TA MUST have an equivalent CA certificate published under each of
   its keys.  Any updates in delegations MUST be reflected under each of
   its keys.  A TA SHOULD NOT publish any other objects besides a CRL, a
   Manifest, a single TAK object, and any number of CA certificates for
   delegation to child CAs.

   If a TA uses a single remote publication server for its keys, per
   [RFC8181], then it MUST include all <publish/> and <withdraw/> PDUs
   for the products of each of its keys in a single query, in order to
   ensure that they will reflect the same content at all times.

   If a TA uses multiple publication servers, then it is by definition
   inevitable that the content of different keys will be out of sync at
   times.  In such cases, the TA SHOULD ensure that the duration of
   these moments are limited to the shortest possible time.
   Furthermore, the following should be observed:

   *  In cases where a CA certificate is revoked completely, or replaced
      by a certificate with a reduced set of resources, these changes
      will not take effect fully until all the relevant repository
      publication points have been updated.  Given that TA key
      operations are normally performed infrequently, this is unlikely
      to be a problem: if the revocation or shrinking of an issued CA
      certificate is staged for days/weeks, then experiencing a delay of
      several minutes for the repository publication points to be
      updated is fairly insignificant.

   *  In cases where a CA certificate is replaced by a certificate with
      an extended set of resources, the TA MUST inform the receiving CA
      only after all of its repository publication points have been
      updated.  This ensures that the receiving CA will not issue any
      products that could be invalid if an RP uses a TA key just before
      the CA certificate was due to be updated.

   Finally, note that the publication locations of CA certificates for
   delegations to child CAs under each key will be different, and
   therefore the Authority Information Access 'id-ad-caIssuers' values
   (section 4.8.7 of [RFC6487]) on certificates issued by the child CAs
   may not be as expected when performing top-down validation, depending
   on the TA key that is used.  However, these values are not critical
   to top-down validation, so RPs performing such validation MUST NOT
   reject a certificate simply because this value is not as expected.

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7.  Performing TA Key Rolls

   In this section we will describe how present-day RPKI TAs that use
   only one key pair, and that do not use TAK objects, can use a TAK
   object to perform a planned key roll.

7.1.  Phase 1: Add a TAK for Key 'A'

   Before adding a successor key, a TA may want to confirm that it can
   maintain a TAK object for its current key only.  We will refer to
   this key as key 'A' throughout this section.

7.2.  Phase 2: Add a Key 'B'

   The TA can now generate a new key pair for key 'B'.  This key MUST
   now be used to create a new CA certificate for this key, and to issue
   equivalent CA certificates for delegations to child CAs, as described
   in Section 6.

   At this point, the TA can also construct a new TAL file [RFC8630] for
   key 'B', and test locally that the validation outcome for the new key
   is equivalent to that of the other current key(s).

   When the TA is certain that both keys are equivalent, and wants to
   initiate the migration from 'A' to 'B', it issues a new TAK object
   under key 'A', with key 'A' as the current key for that object, key
   'B' as the successor key, and no predecessor key.  It also issues a
   TAK object under key 'B', with key 'B' as the current key for that
   object, key 'A' as the predecessor key, and no successor key.

   Once this has happened, RP clients will start seeing the new key and
   setting acceptance timers accordingly.

7.3.  Phase 3: Update TAL to point to 'B'

   At about the time that the TA expects clients to start setting key
   'B' as the current key, the TA must release a new TAL file for key
   'B'.  It SHOULD use a different set of URIs in the TAL compared to
   the TAK file, so that the TA can learn the proportion of RPs that can
   successfully validate and use the updated TAK objects.

   To support RPs that do not take account of TAK objects, the TA should
   continue operating key 'A' for a period of time after the expected
   migration of clients to 'B'.  The length of that period of time is a
   local policy matter for that TA: it might operate the key until no
   clients are attempting to validate using it, for example.

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7.4.  Phase 4: Remove Key 'A'

   The TA SHOULD now remove all content from the repository used by key
   'A', and destroy the private key for key 'A'.  RPs attempting to rely
   on a TAL for key 'A' from this point will not be able to perform RPKI
   validation for the TA, and will have to update their local state
   manually, by way of a new TAL file.

8.  Using TAK objects to distribute TAL data

   Relying Parties must be configured with RPKI Trust Anchor data in
   order to function correctly.  This Trust Anchor data is typically
   distributed in the Trust Anchor Locator (TAL) format defined in RFC
   8630.  A TAK object can also serve as a format for distribution of
   this data, though, because the TAKey data stored in the TAK object
   contains the same data that would appear in a TAL for the associated
   Trust Anchor.

   Relying Parties may support conversion of TAK objects into TAL files.
   Relying Parties that support conversion MUST validate the TAK object
   using the process from section 3.3.  One exception to the standard
   validation process in this context is that a Relying Party MAY treat
   a TAK object as valid, even though it is associated with a Trust
   Anchor that the Relying Party is not currently configured to trust.
   If the Relying Party is relying on this exception when converting a
   given TAK object, the Relying Party MUST communicate that fact to the
   user.

   When converting a TAK object, a Relying Party MUST default to
   producing a TAL file based on the 'current' TAKey in the TAK object,
   though it MAY optionally support producing TAL files based on the
   'predecessor' and 'successor' TAKeys.

   When converting a TAK object, a Relying Party MUST include in the TAL
   file any comments from the corresponding TAKey.

   If TAK object validation fails, then the Relying Party MUST NOT
   produce a TAL file based on the TAK object.

   Users should be aware that TAK objects distributed out-of-band have
   similar security properties to TAL files (i.e. there is no
   authentication).  In particular, TAK objects that are not signed by
   TAs with which the Relying Party is currently configured should only
   be used if the source that distributes them is one the user trusts to
   distribute TAL files.

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   If a Relying Party is not transitioning to new Trust Anchor data
   using the automatic process described in section 5 or the partially-
   manual process described in section 5.1, then the user will have to
   rely on an out-of-band mechanism for validating and updating the
   Trust Anchor data for the Relying Party.  Users in this situation
   should take similar care when updating a trust anchor using a TAK
   object file as when using a TAL file to update TA data.

9.  Deployment Considerations

9.1.  Relying Party Support

   Publishing TAK objects while RPs do not support this standard will
   result in those RPs rejecting these objects.  It is not expected that
   this will result in the invalidation of any other object under a
   Trust Anchor.

   Some RPs may purposefully not support this mechanism: for example,
   they may be implemented or configured such that they are unable to
   update local current key data.  TAs should take this into
   consideration when planning key rollover.  However, these RPs would
   ideally still notify their operators of planned key rollovers, so
   that the operator could update the relevant configuration manually.

9.2.  Alternate Transition Models

   Alternate models of TAL update exist and are complementary to this
   mechanism.  For example, TAs can liaise directly with RP software
   developers to include updated and reissued TAL files in new code
   releases, and use existing code update mechanisms in the RP community
   to distribute the changes.

   Additionally, these non-TA channels for distributing TAL data may
   themselves rely on monitoring for TAK objects and then updating the
   TAL data in their distributions or packages accordingly.  In this
   way, TAK objects may be useful even for RPs that don't implement in-
   band support for the protocol.

   Non-TA channels for distributing TAL data should ensure so far as is
   possible that their update mechanisms take account of any changes
   that a user has made to their local TA key configuration.  For
   example, if a new key is published for a TA, but the non-TA channel's
   mechanism is able to detect that a user had removed the TA's previous
   key from their local TA key configuration such that the user no
   longer relies on it, then the mechanism should not by default add the
   new key to the user's TA key configuration.

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10.  Operational Considerations

10.1.  Acceptance Timers

   Acceptance timers are used in TAK objects in order to permit RPs to
   test that the new key is working correctly.  This in turn means that
   the TA will be able to gain confidence in the correct functioning of
   the new key before RPs are relying on that in their production RPKI
   operations.  If a successor key is not working correctly, a TA may
   remove that key from the current TAK object.

   A TA that removes a successor key from a TAK object SHOULD NOT add
   the same successor key back into the TAK object for that TA.  This is
   because there may be an RP that has fetched the TAK object while the
   successor key was listed in it, and has started an acceptance timer
   accordingly, but has not fetched the TAK object during the period
   when the successor key was not listed in it.  If the unchanged
   successor key is added back in to the TA, such an RP will transition
   to using new the TA key more quickly than other RPs, which may in
   turn make debugging and similar more complicated.  A simple way of
   addressing this problem in a situation where the TA doesn't want to
   reissue the SubjectPublicKeyInfo content for the successor key that
   was withdrawn is to update the URL set for the successor key, since
   RPs must take that URL set into account for the purposes of
   initiating and cancelling acceptance timers.

11.  Security Considerations

11.1.  Previous Keys

   A TA needs to consider the length of time for which it will maintain
   previously-current keys and their associated repositories.  An RP
   that is seeded with old TAL data will run for 30 days using the
   previous key before migrating to the next key, due to the acceptance
   timer requirements, and this 30-day delay applies to each new key
   that has been issued since the old TAL data was initially published.
   It may be better in these instances to have the old publication URLs
   simply fail to resolve, so that the RP reports an error to its
   operator and the operator seeds it with up-to-date TAL data
   immediately.

   Once a TA has decided not to maintain a previously-current key and
   its associated repository, the TA SHOULD destroy that key.  The TA
   SHOULD also reuse the TA CA certificate URLs from the previous TAL
   data for the next TAL that it generates.  These measures will help to
   mitigate the risk of an adversary gaining access to the key and its
   associated publication points in order to send invalid/incorrect data
   to RPs seeded with the TAL data for that key.

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11.2.  TA Compromise

   TAK objects do not offer protection against compromise of the current
   TA key or the successor TA key.  TA key compromise in general is out
   of scope for this document.

12.  IANA Considerations

12.1.  Content Type

   IANA is asked to register an object identifier for one content type
   in the "SMI Security for S/MIME CMS Content Type
   (1.2.840.113549.1.9.16.1)" registry as follows:

          Decimal | Description                    | References
          --------+--------------------------------+---------------
          50      | id-ct-signedTAL                | [section 3.1]

   *  Description: id-ct-signedTAL

   *  OID: 1.2.840.113549.1.9.16.1.50

   *  Specification: [section 3.1]

12.2.  Signed Object

   IANA is asked to add the following to the "RPKI Signed Objects"
   registry:

        Name             | OID                        | Reference
        -----------------+----------------------------+---------------
        Trust Anchor Key | 1.2.840.113549.1.9.16.1.50 | [section 3.1]

   IANA is also asked to add the following note to the "RPKI Signed
   Objects" registry:

   |  Objects of the types listed in this registry, as well as RPKI
   |  resource certificates and CRLs, are expected to be validated using
   |  the RPKI.

12.3.  File Extension

   IANA is asked to add an item for the Signed TAL file extension to the
   "RPKI Repository Name Scheme" created by [RFC6481] as follows:

       Filename Extension  | RPKI Object              | Reference
       --------------------+--------------------------+----------------
        .tak               | Trust Anchor Key         | [this document]

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12.4.  Module Identifier

   IANA is asked to register an object identifier for one module
   identifier in the "SMI Security for S/MIME Module Identifier
   (1.2.840.113549.1.9.16.0)" registry as follows:

          Decimal | Description                    | References
          --------+--------------------------------+---------------
          74      | RPKISignedTrustAnchorList-2021 | [this document]

   *  Description: RPKISignedTrustAnchorList-2021

   *  OID: 1.2.840.113549.1.9.16.0.74

   *  Specification: [this document]

12.5.  Registration of Media Type application/rpki-signed-tal

   IANA is asked to register the media type "application/rpki-signed-
   tal" in the "Media Types" registry as follows:

   Type name:  application

   Subtype name:  rpki-signed-tal

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  Carries an RPKI Signed TAL.  This media
      type contains no active content.  See the Security Considerations
      section of RFC XXXX for further information.

   Interoperability considerations:  N/A

   Published specification:  RFC XXXX

   Applications that use this media type:  RPKI operators

   Fragment identifier considerations:  N/A

   Additional information:  Content:  This media type is for a signed
         object, as defined in RFC 6488, which contains trust anchor key
         material as defined in RFC XXXX.

                            Magic number(s):  N/A

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                            File extension(s):  .tak

                            Macintosh file type code(s):  N/A

   Person & email address to contact for further information:
   iesg@ietf.org

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  sidrops WG

   Change controller:  IESG

13.  Implementation Status

   NOTE: Please remove this section and the reference to RFC 7942 prior
   to publication as an RFC.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

13.1.  APNIC

   *  Responsible Organization: Asia-Pacific Network Information Centre

   *  Location: https://github.com/APNIC-net/rpki-signed-tal-demo

   *  Description: A proof-of-concept for relying party TAK usage.

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   *  Level of Maturity: This is a proof-of-concept implementation.

   *  Coverage: This implementation includes all of the features
      described in version 13 of this specification, except for writing
      TAL files based on TAK data.  The repository includes a link to
      various test TALs that can be used for testing TAK scenarios, too.

   *  Contact Information: Tom Harrison, tomh@apnic.net

13.2.  rpki-client

   *  Responsible Organization: Job Snijders

   *  Location: https://marc.info/?l=openbsd-tech&m=166635746808783&w=2

   *  Description: A relying party implementation which can validate
      TAKs.

   *  Level of Maturity: Mature.  Trust Anchor operators are encouraged
      to use rpki-client as part of smoke testing to help ensure high
      levels of standards compliance when introducing changes, and use
      rpki-client in a continuous monitoring fashion to help maintain
      high levels of operational excellence.

   *  Coverage: This implementation includes all features except TAK
      acceptance timers.

   *  Contact information: Job Snijders, job@fastly.com

14.  Revision History

   03 - Last draft under Tim's authorship.

   04 - First draft with George's authorship.  No substantive revisions.

   05 - First draft with Tom's authorship.  No substantive revisions.

   06 - Rob Kisteleki's critique.

   07 - Switch to two-key model.

   08 - Keepalive.

   09 - Acceptance timers, predecessor keys, no long-lived CRL/MFT.

   10 - Using TAK objects for distribution of TAL data.

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   11 - Manual update guidance, additional security considerations,
   identifier updates.

   12 - TAK object comments.

   13 - Removal of compromise text, extra RP support text, key
   destruction text, media type registration, signed object registry
   note.

   14 - Keepalive.

15.  Acknowledgments

   The authors wish to thank Martin Hoffmann for a thorough review of
   the document, Russ Housley for multiple reviews of the ASN.1
   definitions and for providing a new module for the TAK object, Job
   Snijders for the extensive suggestions around TAK object structure/
   distribution and rpki-client implementation work, and Ties de Kock
   for text/suggestions around TAK/TAL distribution and general security
   considerations.

16.  References

16.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779,
              DOI 10.17487/RFC3779, June 2004,
              <https://www.rfc-editor.org/info/rfc3779>.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
              <https://www.rfc-editor.org/info/rfc5198>.

   [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, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5781]  Weiler, S., Ward, D., and R. Housley, "The rsync URI
              Scheme", RFC 5781, DOI 10.17487/RFC5781, February 2010,
              <https://www.rfc-editor.org/info/rfc5781>.

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   [RFC6481]  Huston, G., Loomans, R., and G. Michaelson, "A Profile for
              Resource Certificate Repository Structure", RFC 6481,
              DOI 10.17487/RFC6481, February 2012,
              <https://www.rfc-editor.org/info/rfc6481>.

   [RFC6486]  Austein, R., Huston, G., Kent, S., and M. Lepinski,
              "Manifests for the Resource Public Key Infrastructure
              (RPKI)", RFC 6486, DOI 10.17487/RFC6486, February 2012,
              <https://www.rfc-editor.org/info/rfc6486>.

   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              DOI 10.17487/RFC6487, February 2012,
              <https://www.rfc-editor.org/info/rfc6487>.

   [RFC6488]  Lepinski, M., Chi, A., and S. Kent, "Signed Object
              Template for the Resource Public Key Infrastructure
              (RPKI)", RFC 6488, DOI 10.17487/RFC6488, February 2012,
              <https://www.rfc-editor.org/info/rfc6488>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8181]  Weiler, S., Sonalker, A., and R. Austein, "A Publication
              Protocol for the Resource Public Key Infrastructure
              (RPKI)", RFC 8181, DOI 10.17487/RFC8181, July 2017,
              <https://www.rfc-editor.org/info/rfc8181>.

   [RFC8630]  Huston, G., Weiler, S., Michaelson, G., Kent, S., and T.
              Bruijnzeels, "Resource Public Key Infrastructure (RPKI)
              Trust Anchor Locator", RFC 8630, DOI 10.17487/RFC8630,
              August 2019, <https://www.rfc-editor.org/info/rfc8630>.

   [X.690]    ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
              "Information technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", 2002.

16.2.  Informative References

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   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

Appendix A.  ASN.1 Module

   This appendix includes the ASN.1 module for the TAK object.

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   <CODE BEGINS>
   RPKISignedTrustAnchorList-2021
       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs9(9) smime(16) mod(0) 74 }

   DEFINITIONS EXPLICIT TAGS ::=
   BEGIN

   IMPORTS

   CONTENT-TYPE
       FROM CryptographicMessageSyntax-2009 -- in [RFC5911]
       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
         pkcs-9(9) smime(16) modules(0) id-mod-cms-2004-02(41) }

   SubjectPublicKeyInfo
       FROM PKIX1Explicit-2009 -- in [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-explicit-02(51) } ;

   ct-signedTAL CONTENT-TYPE ::=
       { TYPE TAK IDENTIFIED BY
         id-ct-signedTAL }

   id-ct-signedTAL OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 50 }

   CertificateURI ::= IA5String

   TAKey ::= SEQUENCE {
       comments  SEQUENCE SIZE (0..MAX) OF UTF8String,
       certificateURIs  SEQUENCE SIZE (1..MAX) OF CertificateURI,
       subjectPublicKeyInfo  SubjectPublicKeyInfo
   }

   TAK ::= SEQUENCE {
       version     INTEGER DEFAULT 0,
       current     TAKey,
       predecessor [0] TAKey OPTIONAL,
       successor   [1] TAKey OPTIONAL
   }

   END
   <CODE ENDS>

Authors' Addresses

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   Carlos Martinez
   LACNIC
   Email: carlos@lacnic.net
   URI:   https://www.lacnic.net/

   George G. Michaelson
   Asia Pacific Network Information Centre
   6 Cordelia St
   South Brisbane
                QLD 4101
   Australia
   Email: ggm@apnic.net

   Tom Harrison
   Asia Pacific Network Information Centre
   6 Cordelia St
   South Brisbane
                QLD 4101
   Australia
   Email: tomh@apnic.net

   Tim Bruijnzeels
   NLnet Labs
   Email: tim@nlnetlabs.nl
   URI:   https://www.nlnetlabs.nl/

   Rob Austein
   Dragon Research Labs
   Email: sra@hactrn.net

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