RPKI Certificate Tree Validation by a Relying Party Tool
draft-ietf-sidr-rpki-tree-validation-02

SIDR                                                        O. Muravskiy
Internet-Draft                                            T. Bruijnzeels
Intended status: Informational                                  RIPE NCC
Expires: January 21, 2017                                  July 20, 2016

        RPKI Certificate Tree Validation by a Relying Party Tool
                draft-ietf-sidr-rpki-tree-validation-02

Abstract

   This document describes the approach to validate the content of the
   RPKI certificate tree, as used by the RIPE NCC RPKI Validator.  This
   approach is independent of a particular object retrieval mechanism.
   This allows it to be used with repositories available over the rsync
   protocol, the RPKI Repository Delta Protocol, and repositories that
   use a mix of both.

Status of This Memo

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  General Considerations  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Hash comparisons  . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Discovery of RPKI objects issued by a CA  . . . . . . . .   3
     2.3.  Manifest entries versus repository content  . . . . . . .   4
   3.  Top-down Validation of a Single Trust Anchor Certificate Tree   4
     3.1.  Fetching the Trust Anchor Certificate Using the Trust
           Anchor Locator  . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  CA Certificate Validation . . . . . . . . . . . . . . . .   5
       3.2.1.  Finding the most recent valid manifest and CRL  . . .   6
       3.2.2.  Manifest entries validation . . . . . . . . . . . . .   7
     3.3.  Object Store Cleanup  . . . . . . . . . . . . . . . . . .   8
   4.  Remote Objects Fetcher  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Fetcher Operations  . . . . . . . . . . . . . . . . . . .   8
       4.1.1.  Fetch repository objects  . . . . . . . . . . . . . .   9
       4.1.2.  Fetch single repository object  . . . . . . . . . . .   9
   5.  Local Object Store  . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Store Operations  . . . . . . . . . . . . . . . . . . . .  10
       5.1.1.  Store Repository Object . . . . . . . . . . . . . . .  10
       5.1.2.  Get objects by hash . . . . . . . . . . . . . . . . .  10
       5.1.3.  Get certificate objects by URI  . . . . . . . . . . .  10
       5.1.4.  Get manifest objects by AKI . . . . . . . . . . . . .  10
       5.1.5.  Delete objects for a URI  . . . . . . . . . . . . . .  10
       5.1.6.  Delete outdated objects . . . . . . . . . . . . . . .  10
       5.1.7.  Update object's validation time . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   In order to use information published in RPKI repositories, Relying
   Parties (RP) need to retrieve and validate the content of
   certificates, CRLs, and other RPKI signed objects.  To validate a
   particular object, one must ensure that all certificates in the
   certificate chain up to the Trust Anchor (TA) are valid.  Therefore
   the validation of a certificate tree is performed top-down, starting
   from the TA certificate and descending down the certificate chain,
   validating every encountered certificate and its products.  The

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   result of this process is a list of all encountered RPKI objects with
   a validity status attached to each of them.  These results may later
   be used by a Relying Party in taking routing decisions, etc.

   Traditionally RPKI data is made available to RPs through the
   repositories [RFC6481] accessible over rsync protocol.  Relying
   parties are advised to keep a local copy of repository data, and
   perform regular updates of this copy from the repository (Section 5
   of [RFC6481]).  The RPKI Repository Delta Protocol
   [I-D.ietf-sidr-delta-protocol] introduces another method to fetch
   repository data and keep the local copy up to date with the
   repository.

   This document describes how the RIPE NCC RPKI Validator discovers
   RPKI objects to download, builds certificate paths, and validates
   RPKI objects, independently from what repository access protocol is
   used.  To achieve this, it puts downloaded RPKI objects in an object
   store, where each RPKI object can be found by its URI, the hash of
   its content, value of its Authority Key Identifier (AKI) extension,
   or a combination of these.  It also keeps track of the download and
   the validation time for every object, to perform cleanups of the
   local copy.

2.  General Considerations

2.1.  Hash comparisons

   This algorithm relies on the properties of the file hash algorithm
   (defined in [RFC6485]) to compute the hash of repository objects.  It
   assumes that any two objects for which the hash value is the same,
   are identical.

   The hash comparison is used when matching objects in the repository
   with entries on the manifest, and when looking up objects in the
   object store (Section 5).

2.2.  Discovery of RPKI objects issued by a CA

   There are several possible ways of discovering products of a CA
   certificate: one could use all objects located in a repository
   directory designated as a publication point for a CA, or only objects
   mentioned on the manifest located at that publication point (see
   Section 6 of [RFC6486]), or use all objects whose AKI extension
   matches the Subject Key Identifier (SKI) extension (Section 4.2.1 of
   [RFC5280]) of a CA certificate.

   For publication points whose content is consistent with the manifest
   and issuing certificate all of these approaches should produce the

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   same result.  For inconsistent publication points the results might
   be different.  Section 6 of [RFC6486] leaves the decision on how to
   deal with inconsistencies to a local policy.

   The implementation described here does not rely on content of
   repository directories, but uses the Authority Key Identifier (AKI)
   extension of a manifest and a certificate revocation list (CRL) to
   find in an object store (Section 5) a manifest and a CRL issued by a
   particular Certification Authority (CA) (see Section 3.2.1).  It
   further uses the hashes of manifest's fileList entries (Section 4.2.1
   of [RFC6486]) to find other objects issued by the CA, as described in
   Section 3.2.2.

2.3.  Manifest entries versus repository content

   Since the current set of RPKI standards requires use of the manifest
   [RFC6486] to describe the content of a publication point, this
   implementation requires strict consistency between the publication
   point content and manifest content.  (This is a more stringent
   requirement than established in [RFC6486].)  Therefore it will not
   process objects that are found in the publication point but do not
   match any of the entries of that publication point's manifest (see
   Section 3.2.2).  It will also issue warnings for all found
   mismatches, so that the responsible operators could be made aware of
   inconsistencies and fix them.

3.  Top-down Validation of a Single Trust Anchor Certificate Tree

   1.  The validation of a Trust Anchor (TA) certificate tree starts
       from its TA certificate.  To retrieve the TA certificate, a Trust
       Anchor Locator (TAL) object is used, as described in Section 3.1.

   2.  If the TA certificate is retrieved, it is validated according to
       Section 7 of [RFC6487] and Section 2.2 of [RFC7730].  Otherwise
       the validation of certificate tree is aborted and an error is
       issued.

   3.  If the TA certificate is valid, then all its subordinate objects
       are validated as described in Section 3.2.  Otherwise the
       validation of certificate tree is aborted and an error is issued.

   4.  For each repository object that was validated during this
       validation run, its validation timestamp is updated in the object
       store (see Section 5.1.7).

   5.  Outdated objects are removed from the store as described in
       Section 3.3.  This completes the validation of the TA certificate
       tree.

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3.1.  Fetching the Trust Anchor Certificate Using the Trust Anchor
      Locator

   The following steps are performed in order to fetch a Trust Anchor
   Certificate:

   1.  (Optional) If the Trust Anchor Locator contains a "prefetch.uris"
       field, pass the URIs contained in that field to the fetcher (see
       Section 4.1.1).  (This field is a non-standard addition to the
       TAL format.  It helps fetching non-hierarchical rsync
       repositories more efficiently.)

   2.  Extract the TA certificate URI from the TAL's URI section (see
       Section 2.1 of [RFC7730]) and pass it to the object fetcher
       (Section 4.1.2).

   3.  Retrieve from the object store (see Section 5.1.3) all
       certificate objects, for which the URI matches the URI extracted
       from the TAL in the previous step, and the public key matches the
       subjectPublicKeyInfo extension of the TAL (see Section 2.1 of
       [RFC7730]).

   4.  If no, or more than one such objects are found, issue an error
       and abort certificate tree validation process with an error.
       Otherwise, use the single found object as the Trust Anchor
       certificate.

3.2.  CA Certificate Validation

   The following steps describe the validation of a single CA Resource
   certificate:

   1.  If both the caRepository (Section 4.8.8.1 of [RFC6487]), and the
       id-ad-rpkiNotify (Section 3.2 of [I-D.ietf-sidr-delta-protocol])
       SIA pointers are present in the CA certificate, use a local
       policy to determine which pointer to use.  Extract the URI from
       the selected pointer and pass it to the object fetcher (see
       Section 4.1.1).

   2.  For the CA certificate, find the current manifest and certificate
       revocation list (CRL), using the procedure described in
       Section 3.2.1.  If no such manifest and CRL could be found, stop
       validation of this certificate, consider it invalid, and issue an
       error.

   3.  Compare the URI found in the id-ad-rpkiManifest field
       (Section 4.8.8.1 of [RFC6487]) of the SIA extension of the

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       certificate with the URI of the manifest found in the previous
       step.  If they are different, issue a warning.

   4.  Perform manifest entries discovery and validation as described in
       Section 3.2.2.

   5.  Validate all resource certificate objects found on the manifest,
       using the CRL object found on the manifest, according to
       Section 7 of [RFC6487].

   6.  Validate all ROA objects found on the manifest, using the CRL
       object found on the manifest, according to Section 4 of
       [RFC6482].

   7.  Validate all Ghostbusters Record objects found on the manifest,
       using the CRL object found on the manifest, according to
       Section 7 of [RFC6493].

   8.  For every valid CA certificate object found on the manifest,
       apply the procedure described in this section (Section 3.2),
       recursively, provided that this CA certificate (identified by its
       SKI) has not yet been validated during current tree validation
       run.

3.2.1.  Finding the most recent valid manifest and CRL

   1.  Fetch from the store (see Section 5.1.4) all objects of type
       manifest, whose certificate's AKI extension matches the SKI of
       the current CA certificate.  If no such objects are found, stop
       processing the current CA certificate and issue an error.

   2.  Find among found objects the manifest object with the highest
       manifestNumber field (Section 4.2.1 of [RFC6486]), for which all
       following conditions are met:

       *  There is only one entry in the manifest for which the store
          contains exactly one object of type CRL, the hash of which
          matches the hash of the entry.

       *  The manifest's certificate AKI equals the above CRL's AKI.

       *  The above CRL is a valid object according to Section 6.3 of
          [RFC5280].

       *  The manifest is a valid object according to Section 4.4 of
          [RFC6486], and its EE certificates is not in the CRL found
          above.

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   3.  If there is an object that matches above criteria, consider this
       object to be the valid manifest, and the CRL found at the
       previous step - the valid CRL for the current CA certificate's
       publication point.

   4.  Report an error for every other manifest with a number higher
       than the number of the valid manifest.

3.2.2.  Manifest entries validation

   For every entry in the manifest object:

   1.  Construct an entry's URI by appending the entry name to the
       current CA's publication point URI.

   2.  Get all objects from the store whose hash attribute equals
       entry's hash (see Section 5.1.2).

   3.  If no such objects are found, issue an error for this manifest
       entry and progress to the next entry.  This case indicates that
       the repository does not have an object at the location listed in
       the manifest, or that the object's hash does not match the hash
       listed in the manifest.

   4.  For every found object, compare its URI with the URI of the
       manifest entry.

       *  For every object with a non-matching URI issue a warning.
          This case indicates that the object from the manifest entry is
          (also) found at a different location in a (possibly different)
          repository.

       *  If no objects with a matching URI are found, issue a warning.
          This case indicates that there is no object found in the
          repository at the location listed in the manifest entry (but
          there is at least one matching object found at a different
          location).

   5.  Use all found objects for further validation as per Section 3.2.

   Please note that the above steps will not reject objects whose hash
   matches the hash listed in the manifest, but the URI does not.  The
   warning is generated in this case.  It indicates that there is an
   inconsistency in a repository between the content of the publication
   point and its manifest.  The choice has been made in favour of the
   manifest, because the manifest, and the object it refers to by the
   hash are both RPKI signed objects, while the repository's directory
   listing is not.

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3.3.  Object Store Cleanup

   At the end of every TA tree validation some objects are removed from
   the store using the following steps:

   1.  Given all objects that were encountered during the current
       validation run, remove from the store (Section 5.1.6) all objects
       whose URI attribute matches the URI of one of the encountered
       objects, but the content's hash is different.  This removes from
       the store objects that were replaced in the repository by their
       newer versions with the same URIs.

   2.  Remove from the store all objects that were last encountered
       during validation a long time ago (as specified by the local
       policy).  This removes objects that do not appear on any valid
       manifest anymore (but possibly are still published in a
       repository).

   3.  Remove from the store all objects that were downloaded recently
       (as specified by the local policy), but have never been used in
       the validation process.  This removes objects that have never
       appeared on any valid manifest.

   Shortening the time interval used in step 2 will free disk space used
   by the store, at the expense of downloading removed objects again if
   they are still published in the repository.

   Extending the time interval used in step 3 will prevent repeated
   downloads of repository objects, with the risk that such objects, if
   created massively by mistake or by an adversary, will fill up local
   disk space, if they are not cleaned up promptly.

4.  Remote Objects Fetcher

   The fetcher is responsible for downloading objects from remote
   repositories (described in Section 3 of [RFC6481]) using rsync
   protocol ([rsync]), or RPKI Repository Delta Protocol (RRDP)
   ([I-D.ietf-sidr-delta-protocol]).

4.1.  Fetcher Operations

   For every visited URI the fetcher keeps track of the last time a
   successful fetch occurred.

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4.1.1.  Fetch repository objects

   This operation receives one parameter - a URI.  For an rsync
   repository this URI points to a directory.  For an RRDP repository it
   points to the repository's notification file.

   The fetcher performs following steps:

   1.  If data associated with the URI has been downloaded recently (as
       specified by the local policy), skip following steps.

   2.  Download remote objects using the URI provided (for an rsync
       repository use recursive mode).

   3.  Perform syntactic verification of fetched objects.  The type of
       every object (certificate, manifest, CRL, ROA, or Ghostbusters
       record), is determined based on the object's filename extension
       (.cer, .mft, .crl, .roa, and .gbr, respectively).  The syntax of
       the object is described in Section 4 of [RFC6487] for resource
       certificates, step 1 of Section 3 of [RFC6488] for signed
       objects, and specifically, Section 4 of [RFC6486] for manifests,
       [RFC5280] for CRLs, Section 3 of [RFC6482] for ROAs, and
       Section 5 of [RFC6493] for Ghostbusters records.

   4.  Put every downloaded and syntactically correct object in the
       object store (Section 5.1.1).

   The time interval used in the step 1 should be chosen based on the
   acceptable delay in receiving repository updates.

4.1.2.  Fetch single repository object

   This operation receives one parameter - a URI that points to an
   object in a repository.

   The fetcher performs following operations:

   1.  If data associated with the URI has been downloaded recently (as
       specified by the local policy), skip all following steps.

   2.  Download the remote object using the URI provided.

   3.  Perform syntactic verification of fetched object.  The type of
       object (certificate, manifest, CRL, ROA, or Ghostbusters record),
       is determined based on the object's filename extension (.cer,
       .mft, .crl, .roa, and .gbr, respectively).  The syntax of the
       object is described in Section 4 of [RFC6487] for resource
       certificates, step 1 of Section 3 of [RFC6488] for signed

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       objects, and specifically, Section 4 of [RFC6486] for manifests,
       [RFC5280] for CRLs, Section 3 of [RFC6482] for ROAs, and
       Section 5 of [RFC6493] for Ghostbusters records.

   4.  If the downloaded object is not syntactically correct, issue an
       error and skip further steps.

   5.  Delete all objects from the object store (Section 5.1.5) whose
       URI matches the URI given.

   6.  Put the downloaded object in the object store (Section 5.1.1).

5.  Local Object Store

5.1.  Store Operations

5.1.1.  Store Repository Object

   Put given object in the store, along with its type, URI, hash, and
   AKI, if there is no record with the same hash and URI fields.  Note
   that in the (unlikely) event of hash collision the given object will
   not replace the object in the store.

5.1.2.  Get objects by hash

   Retrieve all objects from the store whose hash attribute matches the
   given hash.

5.1.3.  Get certificate objects by URI

   Retrieve from the store all objects of type certificate, whose URI
   attribute matches the given URI.

5.1.4.  Get manifest objects by AKI

   Retrieve from the store all objects of type manifest, whose AKI
   attribute matches the given AKI.

5.1.5.  Delete objects for a URI

   For a given URI, delete all objects in the store with matching URI
   attribute.

5.1.6.  Delete outdated objects

   For a given URI and a list of hashes, delete all objects in the store
   with matching URI, whose hash attribute is not in the given list of
   hashes.

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5.1.7.  Update object's validation time

   For all objects in the store whose hash attribute matches the given
   hash, set the last validation time attribute to the given timestamp.

6.  Acknowledgements

   This document describes the algorithm as it is implemented by the
   software development team at the RIPE NCC.  The authors would also
   like to acknowledge contributions by Carlos Martinez, Andy Newton,
   Rob Austein, and Stephen Kent.

7.  IANA Considerations

   This document has no actions for IANA.

8.  Security Considerations

   This implementation will not detect possible hash collisions in the
   hashes of repository objects (calculated using the file hash
   algorithm specified in [RFC6485]).  It considers objects with same
   hash values as identical.

   This algorithm uses the content of a manifest object to discover
   other objects issued by a specified CA.  It verifies that the
   manifest is located in the publication point designated in the CA
   Certificate.  However, if there are other (not listed in the
   manifest) objects located in that publication point directory, they
   will be ignored, even if their content is correct and they are issued
   by the same CA as the manifest.  (This behavior is allowed, but not
   required, by [RFC6486].)

   In contrast, objects whose content hash matches the hash listed in
   the manifest, but that are not located in the publication directory
   listed in their CA certificate, will be used in the validation
   process (although a warning will be issued in that case), as
   explained in Section 3.2.2.

   The store cleanup procedure described in Section 3.3 tries to
   minimise removal and subsequent re-fetch of objects that are
   published in a repository, but not used in the validation.  Once such
   objects are removed from the remote repository, they will be
   discarded from the local object store after a period of time
   specified by a local policy.  By generating an excessive amount of
   syntactically valid RPKI objects, a man-in-the-middle attack between
   a validating tool and a repository could force an implementation to
   fetch and store those objects in the object store before they are

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   validated and discarded, leading to an out-of-memory or out-of-disk-
   space conditions, and, subsequently, a denial of service.

9.  References

9.1.  Normative References

   [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,
              <http://www.rfc-editor.org/info/rfc5280>.

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

   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482,
              DOI 10.17487/RFC6482, February 2012,
              <http://www.rfc-editor.org/info/rfc6482>.

   [RFC6485]  Huston, G., "The Profile for Algorithms and Key Sizes for
              Use in the Resource Public Key Infrastructure (RPKI)",
              RFC 6485, DOI 10.17487/RFC6485, February 2012,
              <http://www.rfc-editor.org/info/rfc6485>.

   [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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6488>.

   [RFC6493]  Bush, R., "The Resource Public Key Infrastructure (RPKI)
              Ghostbusters Record", RFC 6493, DOI 10.17487/RFC6493,
              February 2012, <http://www.rfc-editor.org/info/rfc6493>.

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   [RFC7730]  Huston, G., Weiler, S., Michaelson, G., and S. Kent,
              "Resource Public Key Infrastructure (RPKI) Trust Anchor
              Locator", RFC 7730, DOI 10.17487/RFC7730, January 2016,
              <http://www.rfc-editor.org/info/rfc7730>.

9.2.  Informative References

   [I-D.ietf-sidr-delta-protocol]
              Bruijnzeels, T., Muravskiy, O., Weber, B., Austein, R.,
              and D. Mandelberg, "RPKI Repository Delta Protocol",
              draft-ietf-sidr-delta-protocol-02 (work in progress),
              March 2016.

   [rsync]    "Rsync home page", <https://rsync.samba.org>.

Authors' Addresses

   Oleg Muravskiy
   RIPE NCC

   Email: oleg@ripe.net

   Tim Bruijnzeels
   RIPE NCC

   Email: tim@ripe.net

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