wdiff ../rfcs-done/rfc8488.txt test8488.v2v3.txt

Internet Engineering Task Force (IETF) O. Muravskiy
Request for Comments: 8488 RIPE NCC
Category: Informational T. Bruijnzeels
ISSN: 2070-1721 NLnet Labs NLNetLabs
December 2018

RIPE NCC's Implementation of Resource Public Key Infrastructure (RPKI)
Certificate Tree Validation

Abstract

This document describes an approach to validating the content of the
Resource Public Key Infrastructure (RPKI) certificate tree, as it is
implemented in the RIPE NCC RPKI Validator. This approach is
independent of a particular object retrieval mechanism, which allows
it to be used with repositories available over the rsync protocol,
the RPKI Repository Delta Protocol (RRDP), and repositories that use
a mix of both.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8488.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info)
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Considerations . . . . . . . . . . . . . . . . . . . 4
2.1. Hash Comparisons . . . . . . . . . . . . . . . . . . . . 4
2.2. Discovery of RPKI Objects Issued by a CA . . . . . . . . 5
2.3. Manifest Entries versus Repository Content . . . . . . . 5
3. Top-Down Validation of a Single Trust Anchor Certificate Tree 6
3.1. Fetching the Trust Anchor Certificate Using the Trust
Anchor Locator . . . . . . . . . . . . . . . . . . . . . 6
3.2. CA Certificate Validation . . . . . . . . . . . . . . . . 7
3.2.1. Finding the Most Recent Valid Manifest and
CRL . . . 8
3.2.2. Validating Manifest Entries . . . . . . . . . . . . . 9
3.3. Object Store Cleanup . . . . . . . . . . . . . . . . . . 10
4. Remote Objects Fetcher . . . . . . . . . . . . . . . . . . . 11
4.1. Fetcher Operations . . . . . . . . . . . . . . . . . . . 11
4.1.1. Fetch Repository Objects . . . . . . . . . . . . . . 12
4.1.2. Fetch Single Repository Object . . . . . . . . . . . 12
5. Local Object Store . . . . . . . . . . . . . . . . . . . . . 12
5.1. Store Operations . . . . . . . . . . . . . . . . . . . . 12
5.1.1. Store Repository Object . . . . . . . . . . . . . . . 12
5.1.2. Get Objects by Hash . . . . . . . . . . . . . . . . . 12
5.1.3. Get Certificate Objects by URI . . . . . . . . . . . 13
5.1.4. Get Manifest Objects by AKI . . . . . . . . . . . . . 13
5.1.5. Delete Objects for a URI . . . . . . . . . . . . . . 13
5.1.6. Delete Outdated Objects . . . . . . . . . . . . . . . 13
5.1.7. Update Object's Validation Time . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Hash Collisions . . . . . . . . . . . . . . . . . . . . . 13
7.2. Algorithm Agility . . . . . . . . . . . . . . . . . . . . 13
7.3. Mismatch between the Expected and Actual Location of
an Object in the Repository . . . . . . . . . . . . . . . . 14
7.4. Manifest Content versus Publication Point Content . . . . 14
7.5. Possible Denial of Service . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . 16
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

This document describes how the RIPE NCC RPKI Validator version 2.25
has been implemented. Source code for this software can be found at
[rpki-validator]. The purpose of this document is to provide
transparency to users of (and contributors to) this software tool.

In order to use information published in RPKI repositories, Relying
Parties (RPs) need to retrieve and validate the content of
certificates, Certificate Revocation Lists (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
the certificate chain, validating every encountered certificate and
its products. The 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 an RP in making routing
decisions, etc.

Traditionally, RPKI data is made available to RPs through the
repositories [RFC6481] accessible over the rsync protocol [rsync].
RPs are advised to keep a local copy of repository data and perform
regular updates of this copy from the repository (see Section 5 of
[RFC6481]). The RRDP [RFC8182] 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 of 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, the value of its Authority Key Identifier (AKI)
extension, or a combination of these. It also keeps track of the
download and validation time for every object, to decide which
locally stored objects are not used in the RPKI tree validation and
could be removed.

2. General Considerations

2.1. Hash Comparisons

This algorithm relies on the collision resistance properties of the
hash algorithm (defined in [RFC7935]) 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 (Section 3.2.2) 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 potential products of
a Certification Authority (CA) certificate: one could 1) use all
objects located in a repository directory designated as a publication
point for a CA, 2) only use objects mentioned on the manifest located
at that publication point (see Section 6 of [RFC6486]), or 3) use all
known repository 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
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 CRL to find in an object store
(Section 5) a manifest and a CRL issued by a particular CA (see
Section 3.2.1). It further uses the hashes of the 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 (see [RFC6481], [RFC6486],
and [RFC6487]) 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

When several Trust Anchors are configured, validation of their
corresponding certificate trees is performed concurrently and
independently from each other. For every configured Trust Anchor,
the following steps are performed:

1. The validation of a 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 the 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 the certificate tree is aborted and an error is
issued.

4. For each repository object that was validated during this
validation run, the 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.

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 TAL 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 with fetching non-hierarchical rsync repositories more
efficiently.)

2. Extract the first 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). If the fetcher returns an error, repeat this
step for every URI in the URI section until no error is
encountered or no more URIs are left.

3. From the object store (see Section 5.1.3), retrieve 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 such objects are found or if more than one such objects are
found, issue an error and abort the certificate tree validation
process with an error. Otherwise, use the single found object as
the TA 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 [RFC8182]) instances of an
accessMethod are present in the Subject Information Access
extension of 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 (that will then fetch all
objects available from that repository; 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
certificate with the URI of the manifest found in the previous
step. If they are different, issue a warning but continue the
validation process using the manifest found in the previous step.
(This warning indicates that there is a mismatch between the
expected and the actual location of an object in a repository.
See Section 7.3 for the explanation of this mismatch and the
decision made.)

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

5. Validate all resource certificate objects found on the manifest
using the CRL object:

* If the strict validation option is enabled by the operator,
the validation is performed according to Section 7 of
[RFC6487].

* Otherwise, the validation is performed according to Section 7
of [RFC6487] but with the exception of the resource
certification path validation, which is performed according to
Section 4.2.4.4 of [RFC8360].

(Note that this implementation uses the operator configuration to
decide which algorithm to use for path validation. It applies
the selected algorithm to all resource certificates, rather than
applying an appropriate algorithm per resource certificate based
on the object identifier (OID) for the Certificate Policy found
in that certificate, as specified in [RFC8360].)

6. Validate all Route Origin Authorization (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, 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

To find the most recent issued manifest and CRL objects of a
particular CA certificate, the following steps are performed:

1. From the store (see Section 5.1.4), fetch 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. Among found objects, find 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 certificate is not in the CRL found
above.

3. If there is an object that matches the above criteria, consider
this object to be the valid manifest, and consider the CRL found
at the previous step to be 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. Validating Manifest Entries

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 the
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 whose URI does not. See
Section 7.3 for additional information.

3.3. Object Store Cleanup

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

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 whose content's hash does not match the hash of any
of the encountered objects. 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 that 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 more 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 unused repository objects. However, it will also extend
the interval at which unused objects are removed. This creates a
risk that such objects will fill up all available disk space if a
large enough amount of such objects is published in the repository
(either by mistake or with a malicious intent).

4. Remote Objects Fetcher

The fetcher is responsible for downloading objects from remote
repositories (described in Section 3 of [RFC6481]) using the rsync
protocol [rsync] or RRDP [RFC8182].

4.1. Fetcher Operations

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

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 follows these steps:

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

2. Download remote objects using the URI provided (for an rsync
repository, use recursive mode). If the URI contains the "https"
schema and download has failed, issue a warning, replace the
"https" schema in the URI with "http", and try to download
objects again using the resulting URI.

3. If remote objects cannot be downloaded, issue an error and skip
the following steps.

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

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

The time interval used in 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 follows these steps:

1. Download a remote object using the URI provided. If the URI
contains the "https" schema and download failed, issue a warning,
replace the "https" schema in the URI with "http", and try to
download the object using the resulting URI.

2. If the remote object cannot be downloaded, issue an error and
skip the following steps.

3. Perform syntactic verification of the 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
objects, 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 the given object in the store 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 a 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 a matching URI whose hash attribute is not in the given list of
hashes.

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. IANA Considerations

This document has no IANA actions.

7. Security Considerations

7.1. Hash Collisions

This implementation will not detect possible hash collisions in the
hashes of repository objects (calculated using the file hash
algorithm specified in [RFC7935]). It considers objects with same
hash values to be identical.

7.2. Algorithm Agility

This implementation only supports hash algorithms and key sizes
specified in [RFC7935]. Algorithm agility described in [RFC6916] is
not supported.

7.3. Mismatch between the Expected and Actual Location of an Object in the
Repository

According to Section 2 of [RFC6481], all objects issued by a
particular CA certificate are expected to be located in one
repository publication point, specified in the SIA extension of that
CA certificate. The manifest object issued by that CA certificate
enumerates all other issued objects, listing their filenames and
content hashes.

However, it is possible that an object whose content hash matches the
hash listed in the manifest either has a different filename or is
located at a different publication point in a repository.

On the other hand, all RPKI objects, either explicitly or within
their embedded EE certificate, have an AKI extension that contains
the key identifier of their issuing CA certificate. Therefore, it is
always possible to perform an RPKI validation of the object whose
expected location does not match its actual location, provided that
the certificate that matches the AKI of the object in question is
known to the system that performs validation.

In the case of a mismatch as described above, this implementation
will not exclude an object from further validation merely because its
actual location or filename does not match the expected location or
filename. This decision was made because the actual location of a
file in a repository is taken from the repository retrieval
mechanism, which, in the case of an rsync repository, does not
provide any cryptographic security, and in the case of an RRDP
repository, provides only a transport-layer security with the
fallback to unsecured transport. On the other hand, the manifest is
an RPKI signed object, and its content could be verified in the
context of the RPKI validation.

7.4. Manifest Content versus Publication Point Content

This algorithm uses the content of a manifest object to determine
other objects issued by a CA certificate. It verifies that the
manifest is located in the publication point designated in the CA
certificate's SIA extension. However, if there are other (not listed
in the manifest) objects located in the same publication point
directory, they are ignored even if they might be valid and issued by
the same CA as the manifest. (This RP behavior is allowed, but not
required, by [RFC6486].)

7.5. Possible Denial of Service

The store cleanup procedure described in Section 3.3 tries to
minimize 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 (see Section 4.1.1)
before they are validated and discarded, leading to out-of-memory or
out-of-disk-space conditions and, subsequently, a denial of service.

8. References

8.1. 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,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc6482>.

[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>.

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

[RFC6916] Gagliano, R., Kent, S., and S. Turner, "Algorithm Agility
Procedure for the Resource Public Key Infrastructure
(RPKI)", BCP 182, RFC 6916, DOI 10.17487/RFC6916, April
2013, <https://www.rfc-editor.org/info/rfc6916>.

[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,
<https://www.rfc-editor.org/info/rfc7730>.

[RFC7935] Huston, G. and G. Michaelson, Ed., "The Profile for
Algorithms and Key Sizes for Use in the Resource Public
Key Infrastructure", RFC 7935, DOI 10.17487/RFC7935,
August 2016, <https://www.rfc-editor.org/info/rfc7935>.

[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>.

[RFC8360] Huston, G., Michaelson, G., Martinez, C., Bruijnzeels, T.,
Newton, A., and D. Shaw, "Resource Public Key
Infrastructure (RPKI) Validation Reconsidered", RFC 8360,
DOI 10.17487/RFC8360, April 2018,
<https://www.rfc-editor.org/info/rfc8360>.

8.2. Informative References

[rpki-validator]
"RIPE-NCC/rpki-validator source code", January 2019,
<https://github.com/RIPE-NCC/rpki-validator>.

[rsync] "rsync", October 2018, <https://rsync.samba.org>.

Acknowledgements

This document describes the algorithm as it is implemented by the
software development team at the RIPE NCC, which, over time, included
Mikhail Puzanov, Erik Rozendaal, Miklos Juhasz, Misja Alma, Thiago da
Cruz Pereira, Yannis Gonianakis, Andrew Snare, Varesh Tapadia, Paolo
Milani, Thies Edeling, Hans Westerbeek, Rudi Angela, and Constantijn
Visinescu. The authors would also like to acknowledge contributions
by Carlos Martinez, Andy Newton, Rob Austein, and Stephen Kent.

Authors' Addresses

Oleg Muravskiy
RIPE NCC

Email: oleg@ripe.net
URI: https://www.ripe.net/

Tim Bruijnzeels
NLnet Labs

Email: tim@nlnetlabs.nl
URI: https://www.nlnetlabs.nl/