On the use of the CMS signing-time attribute in RPKI Signed Objects
draft-ietf-sidrops-cms-signing-time-07

SIDROPS                                                      J. Snijders
Internet-Draft                                                    Fastly
Updates: 6488 (if approved)                                  T. Harrison
Intended status: Standards Track                                   APNIC
Expires: 18 October 2024                                   16 April 2024

  On the use of the CMS signing-time attribute in RPKI Signed Objects
                 draft-ietf-sidrops-cms-signing-time-07

Abstract

   In the Resource Public Key Infrastructure (RPKI), Signed Objects are
   defined as Cryptographic Message Syntax (CMS) protected content
   types.  Signed Objects contain a signing-time attribute, representing
   the purported time at which the object was signed by its issuer.
   RPKI repositories are accessible using the rsync and RPKI Repository
   Delta protocols, allowing Relying Parties (RPs) to synchronize a
   local copy of the RPKI repository used for validation with the remote
   repositories.  This document describes how the CMS signing-time
   attribute can be used to avoid needless retransfers of data when
   switching between different synchronization protocols.  This document
   updates RFC 6488 by mandating the presence of the CMS signing-time
   attribute and disallowing the use of the binary-signing-time
   attribute.

Requirements Language

   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.

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."

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   This Internet-Draft will expire on 18 October 2024.

Copyright Notice

   Copyright (c) 2024 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 (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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Optimized switchover from RRDP to rsync . . . . . . . . . . .   3
     2.1.  Guidance for Repository Operators . . . . . . . . . . . .   4
     2.2.  Guidance for Relying Parties  . . . . . . . . . . . . . .   4
   3.  Presence of the CMS signing-time attribute in public
           repositories  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Update to RFC 6488  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Considerations and Alternative Approaches  . . . . .   9
   Appendix B.  Implementation status  . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   In the Resource Public Key Infrastructure (RPKI) [RFC6480], Signed
   Objects are defined as Cryptographic Message Syntax (CMS) [RFC5652]
   [RFC6268] protected content types by way of a standard template
   [RFC6488].  That template includes an optional CMS signing-time
   attribute, representing the time at which the object was signed by
   its issuer.  At the time when the standard template was defined,
   rsync was the only distribution mechanism for RPKI repositories.

   Since the publication of the standard template, a new, additional
   protocol for distribution of RPKI repositories has been developed:
   the RPKI Repository Delta Protocol (RRDP) [RFC8182].  While RPKI

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   repository operators must provide rsync service, RRDP is typically
   deployed alongside it as well, and preferred by default by most RP
   implementations.  However, RP implementations also support fallback
   to rsync in the event of problems with the RRDP service.  As
   deployment experience with RRDP has increased, the usefulness of
   optimizing switchovers by RPs from one mechanism to the other has
   become apparent.

   This document describes how Repository Operators [RFC6481] and RPs
   can use the CMS signing-time attribute to minimize the burden of
   switching over from RRDP to rsync.  Additionally, this document
   updates [RFC6488] by mandating the presence of the CMS signing-time
   attribute and disallowing the use of the binary-signing-time
   attribute.

2.  Optimized switchover from RRDP to rsync

   To avoid needless re-transfers of unchanged files in consecutive
   rsync synchronizations, [I-D.timbru-sidrops-publication-server-bcp]
   recommends the use of so-called 'deterministic' (normalized)
   timestamps for files.  When the content of a file is unchanged,
   Repository Operators SHOULD ensure that the last modification
   timestamp of the file remains unchanged as well.

   This document advances the aforementioned concept by describing a
   synchronization strategy through which needless transfers are also
   avoided upon first use of rsync, by leveraging data previously
   fetched via RRDP.

   At the time of writing, all commonly used RP implementations will
   first attempt synchronization via RRDP, as described in
   [I-D.ietf-sidrops-prefer-rrdp].  If synchronization via RRDP fails
   for some reason (e.g. malformed XML, expired TLS certificate, HTTP
   connection timeout), the RP will attempt to synchronize via rsync
   instead.

   In the rsync synchronization protocol, a file's last modification
   timestamp (from here on 'mod-time') and filesize are used to
   determine whether the general-purpose rsync synchronization algorithm
   needs to be executed for the file.  This is the default mode for both
   the original rsync implementation [rsync] and the OpenBSD
   implementation [openrsync].  If the sender's copy of the file and the
   receiver's copy of the file both have the same mod-time and filesize,
   the files are assumed to contain the same content, and will be
   omitted from the list of files to be transferred.  Ensuring
   consistency with respect to mod-time for both senders and receivers
   helps to reduce the burden of rsync synchronization in terms of
   network bandwidth, disk I/O operations, and CPU usage.

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   In order to reduce the burden of the rsync synchronization (following
   an RRDP failure), Repository Operators and RPs SHOULD adhere to the
   following guidelines.

2.1.  Guidance for Repository Operators

   When serializing RPKI Signed Objects to a filesystem hierarchy for
   publication via rsync, the mod-time of the file containing the Signed
   Object SHOULD be set to the value of the CMS signing-time attribute
   contained within the Signed Object.

2.2.  Guidance for Relying Parties

   When serializing RPKI Signed Objects retrieved via RRDP to a
   filesystem hierarchy, the mod-time of the file containing the Signed
   Object SHOULD be set to the value of the CMS signing-time attribute
   contained within the Signed Object.

   If an RP uses RRDP to synthesize a filesystem hierarchy for the
   repository, then synchronizing to the corresponding directory
   directly is an option.  Alternatively, the RP can synchronize to a
   new (empty) directory using the _--compare-dest=DIR_ rsync feature,
   in order to avoid retrieving files that are already available by way
   of the synthesized filesystem hierarchy stemming from previous RRDP
   fetches.  The _DIR_ component is to be substituted with the name of
   the directory containing previously fetched and validated RPKI data
   (in its original DER-encoded form, to ensure the filesize parameter
   matches).

   From the [rsync] man page for _--compare-dest=DIR_:

      This option instructs rsync to use _DIR_ on the destination
      machine as an additional hierarchy to compare destination files
      against doing transfers (if the files are missing in the
      destination directory).  If a file is found in _DIR_ that is
      identical to the sender's file, the file will NOT be transferred
      to the destination directory.  This is useful for creating a
      sparse backup of just files that have changed from an earlier
      backup.

   From the [openrsync] man page for _--compare-dest=directory_:

      Use _directory_ as an alternate base directory to compare files
      against on the destination machine.  If file in _directory_ is
      found and identical to the sender's file, the file will not be
      transferred.

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3.  Presence of the CMS signing-time attribute in public repositories

   Analysing the [rpkiviews] archives containing millions of RPKI Signed
   Objects discovered via the five Regional Internet Registry (RIR)
   Trust Anchors (TAs) from June 6th, 2022 until January 29th, 2024,
   each Signed Object contained a CMS signing-time attribute.

   The above means that all of the commonly-used TAs and their
   subordinate Certification Authorities (CAs) produce Signed Objects
   that contain a CMS signing-time attribute.  This means that making
   the CMS signing-time attribute mandatory would not cause any existing
   commonly-used TA or CA to become non-compliant.

   As of January 29th, 2024, for 83.8% of Signed Objects the CMS
   signing-time timestamp matches the file's mod-time observed via
   rsync.  This means that it is already the case that RPs would see a
   significant reduction in the amount of processing required in rsync
   if they adopted the strategy outlined in Section 2.2.

   In the above-mentioned period of time, no Signed Objects were
   discovered with a CMS binary-signing-time [RFC6019] attribute in the
   specified repositories.  Therefore, disallowing the use of the CMS
   binary-signing-time attribute would not cause any existing commonly-
   used TA or CA to become non-compliant.

4.  Update to RFC 6488

   This section updates [RFC6488] to make the CMS signing-time attribute
   mandatory and to disallow the presence of the CMS binary-signing-time
   attribute.

   In section 2.1.6.4, the paragraph starting with "The signedAttrs
   element MUST be present and ..." and ending in "Other signed
   attributes MUST NOT be included." is replaced with the following
   text:

      The signedAttrs element MUST be present and MUST include the
      content-type, message-digest, and signing-time attributes
      [RFC5652].  Other signed attributes MUST NOT be included.

   In section 2.1.6.4.3, the first sentence "The signing-time attribute
   MAY be present." is replaced with the following text:

      The signing-time attribute MUST be present.

   In section 2.1.6.4.3, the sentence "Note that the presence or absence
   of the signing-time attribute MUST NOT affect the validity of the
   signed object (as specified in Section 3)." is removed.

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   Section 2.1.6.4.4 is removed in its entirety.

   In section 3, the paragraph starting with "The signedAttrs field in
   the SignerInfo object is present ..." (1.f) is replaced with the
   following text:

      The signedAttrs field in the SignerInfo object is present and
      contains the content-type attribute (OID 1.2.840.113549.1.9.3),
      the message-digest attribute (OID 1.2.840.113549.1.9.4), and the
      signing-time attribute (1.2.840.113549.1.9.5).

   In section 3, the paragraph starting with "The signedAttrs field in
   the SignerInfo object does not ..." (1.g) is replaced with the
   following text:

      The signedAttrs field in the SignerInfo object does not contain
      any attributes other than the following three: the content-type
      attribute (OID 1.2.840.113549.1.9.3), the message-digest attribute
      (OID 1.2.840.113549.1.9.4), and the signing-time attribute (OID
      1.2.840.113549.1.9.5).

   In section 9 ("Informative References"), [RFC6019] is removed from
   the list.

5.  Security Considerations

   No requirement is imposed concerning the correctness of the signing
   time attribute.  It does not provide reliable information on the time
   the signature was produced and it bears no relevance for seamless
   switchover between RRDP and rsync.

   While the Security Considerations in [RFC6019] mandate that the
   signing-time and binary-signing-time attributes, if both present,
   MUST provide the same date and time; a potential for ambiguity is
   removed by restricting the RPKI Signed Object profile to have only
   one field to store the purported signing time.

6.  IANA Considerations

   This document has no IANA actions.

7.  Acknowledgements

   The authors would like to thank Ties de Kock, Niels Bakker, Mikael
   Abrahamsson, Russ Housley, Zaheduzzaman Sarker, Éric Vyncke, Mahesh
   Jethanandani, and Roman Danyliw, for their helpful review of this
   document.

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8.  References

8.1.  Normative References

   [openrsync]
              Jeker, C., Obser, F., and K. Dzonsons, "openrsync", 2023,
              <https://www.openrsync.org/>.

   [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>.

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

   [RFC6268]  Schaad, J. and S. Turner, "Additional New ASN.1 Modules
              for the Cryptographic Message Syntax (CMS) and the Public
              Key Infrastructure Using X.509 (PKIX)", RFC 6268,
              DOI 10.17487/RFC6268, July 2011,
              <https://www.rfc-editor.org/info/rfc6268>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/info/rfc6480>.

   [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>.

   [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>.

   [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>.

   [RFC8182]  Bruijnzeels, T., Muravskiy, O., Weber, B., and R. Austein,
              "The RPKI Repository Delta Protocol (RRDP)", RFC 8182,
              DOI 10.17487/RFC8182, July 2017,
              <https://www.rfc-editor.org/info/rfc8182>.

   [rsync]    Tridgell, A., Mackerras, P., and W. Davison, "rsync",
              2022, <https://rsync.samba.org/>.

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8.2.  Informative References

   [apnicrepository]
              APNIC, "APNIC Repository", 2023,
              <https://rpki.apnic.net/>.

   [I-D.ietf-sidrops-prefer-rrdp]
              Bruijnzeels, T., Bush, R., and G. G. Michaelson, "Resource
              Public Key Infrastructure (RPKI) Repository Requirements",
              Work in Progress, Internet-Draft, draft-ietf-sidrops-
              prefer-rrdp-02, 23 December 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
              prefer-rrdp-02>.

   [I-D.timbru-sidrops-publication-server-bcp]
              Bruijnzeels, T., de Kock, T., Hill, F., and T. Harrison,
              "RPKI Publication Server Best Current Practices", Work in
              Progress, Internet-Draft, draft-timbru-sidrops-
              publication-server-bcp-02, 18 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-timbru-
              sidrops-publication-server-bcp-02>.

   [krill-sync]
              NLNet Labs, "krill-sync - 0.3.0 development branch",
              December 2023, <https://github.com/NLnetLabs/krill-sync/
              commit/1df59eac3112384e11b44c2da3010f63925ec50e>.

   [ls]       IEEE and The Open Group, "ls - The Open Group Base
              Specifications Issue 7", 2018,
              <https://pubs.opengroup.org/onlinepubs/9699919799/
              utilities/ls.html>.

   [PAM23]    Fontugne, R., Phokeer, A., Pelsser, C., Vermeulen, K., and
              R. Bush, "RPKI Time-of-Flight: Tracking Delays in the
              Management, Control, and Data Planes", February 2023,
              <https://www.iijlab.net/en/members/romain/pdf/
              romain_pam23.pdf>.

   [RFC6019]  Housley, R., "BinaryTime: An Alternate Format for
              Representing Date and Time in ASN.1", RFC 6019,
              DOI 10.17487/RFC6019, September 2010,
              <https://www.rfc-editor.org/info/rfc6019>.

   [RFC9286]  Austein, R., Huston, G., Kent, S., and M. Lepinski,
              "Manifests for the Resource Public Key Infrastructure
              (RPKI)", RFC 9286, DOI 10.17487/RFC9286, June 2022,
              <https://www.rfc-editor.org/info/rfc9286>.

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   [rpki-client]
              Jeker, C., Snijders, J., Dzonsons, K., and T. Buehler,
              "rpki-client", June 2023, <https://www.rpki-client.org/>.

   [rpki-rrdp-tools-py]
              Kock, T. D., "rpki-rrdp-tools-py", November 2023,
              <https://github.com/ties/rpki-rrdp-tools-py/>.

   [rpkitouch]
              Snijders, J., "rpkitouch", June 2023,
              <https://github.com/job/rpkitouch>.

   [rpkiviews]
              Snijders, J., "rpkiviews", June 2023,
              <http://www.rpkiviews.org/>.

   [rsyncit]  RIPE NCC, "rsyncit", November 2023,
              <https://github.com/RIPE-NCC/rsyncit/>.

Appendix A.  Considerations and Alternative Approaches

   This section is to be removed before publishing as an RFC.

   A slightly different approach that has been suggested is to normalize
   file mod-times based on the Signed Object's embedded End-Entity (EE)
   X.509 notBefore timestamp value.  A downside of this approach is that
   objects from CAs not using one-time use EE certificates, per section
   5.1.1 of [RFC9286] would result in multiple objects signed at
   different points in time with the same mod-times.

   Additionally, CAs might backdate the notBefore timestamp to increase
   the validity window of the Signed Object, which in turn decreases
   insight for RPKI operators as to when exactly the Signed Object
   purportedly came into existence.  Along similar lines, the notBefore
   timestamp may be set in the future for contractual reasons.  Setting
   the mod-time of a file to a future date may be unintuitive for users,
   and some programs (e.g.  GNU make) will warn on encountering files
   with such mod-times.

   There is also an increased chance of two distinct objects published
   to the same path having the same mod-time and filesize under this
   approach, due to CAs setting the notBefore timestamp to some stable
   value for a given object and reissuance often not changing the file
   size (e.g. where a prefix or a max-length value is changed in a ROA).
   In such a situation, if the receiver has the first copy of a file,
   rsync retrieval will skip the second copy of the file, and the
   synchronization operation for the associated repository will result
   in a "failed fetch", per section 6.6 of [RFC9286], due to an

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   inconsistency between the file's hash and the hash listed in the
   associated manifest.  That in turn necessitates further retrieval
   operations on the part of the receiver.  The chance of two distinct
   objects being issued with the same mod-time and filesize when CMS
   signing-time is used to set the mod-time is much smaller, since it
   requires that those distinct objects be issued in very close
   succession.

   Another downside of using notBefore is that Repository operators
   would need to deserialize both the CMS envelope and the X.509 EE
   certificate contained therein to extract a timestamp, instead of
   merely parsing the CMS envelope.

   Ensuring the mod-time is set to the CMS signing-time gives RPKI
   operators a headstart when using tools like [ls], in the sense that
   the mod-time aligns with the purported time of object issuance.

   The CMS signing-time attribute has proven useful in researching and
   tracking delays in various layers of the RPKI [PAM23].  Mandating the
   CMS signing-time to be present might aid future researchers studying
   the RPKI ecosystem.

   The _--checksum_ option to rsync disables the mod-time and filesize
   comparison check in favour of a check based on a whole-file checksum.
   This check is slower than the mod-time and filesize check, but (in
   instances where the file content has not changed) faster than the
   general-purpose rsync synchronization algorithm.  Since ensuring
   consistency between the mod-time and filesize on both sides of the
   transaction is straightforward, there is no particular reason to
   pursue an approach based on rsync's _--checksum_ feature.

Appendix B.  Implementation status

   This section is to be removed before publishing 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 RFC 7942.
   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.

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   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".

   *  [rpkitouch] - a timestamp setter utility for both rsync servers
      and RRDP clients by Job Snijders in C.

   *  [rpki-client] - a Relying Party implementation by OpenBSD in C, a
      client side implementation.

   *  [rsyncit] - a RRDP-to-rsync sync tool by RIPE NCC in Java, run on
      the server side.

   *  [apnicrepository] - the public APNIC RPKI repository - the APNIC
      rsync server normalizes timestamps.

   *  [rpki-rrdp-tools-py] - a number of client-side RRDP utilities by
      Ties de Kock in Python.

   *  [krill-sync] - a RRDP-to-rsync sync tool by NLNet Labs in Rust,
      run on the server side.

Authors' Addresses

   Job Snijders
   Fastly
   Amsterdam
   Netherlands
   Email: job@fastly.com

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

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