RPKI TAL: Resource Public Key Infrastructure (RPKI) Trust Anchor Locator

Internet Engineering Task Force (IETF)                         G. Huston
Request for Comments: 8630                                         APNIC
Obsoletes: 7730                                                S. Weiler
Category: Standards Track                                        W3C/MIT
ISSN: 2070-1721                                            G. Michaelson
                                                                   APNIC
                                                                 S. Kent
                                                            Unaffiliated
                                                          T. Bruijnzeels
                                                              NLnet Labs
                                                             August 2019


     Resource Public Key Infrastructure (RPKI) Trust Anchor Locator

Abstract

   This document defines a Trust Anchor Locator (TAL) for the Resource
   Public Key Infrastructure (RPKI).  The TAL allows Relying Parties in
   the RPKI to download the current Trust Anchor (TA) Certification
   Authority (CA) certificate from one or more locations and verify that
   the key of this self-signed certificate matches the key on the TAL.
   Thus, Relying Parties can be configured with TA keys but can allow
   these TAs to change the content of their CA certificate.  In
   particular, it allows TAs to change the set of IP Address Delegations
   and/or Autonomous System Identifier Delegations included in the
   extension(s) (RFC 3779) of their certificate.

   This document obsoletes the previous definition of the TAL as
   provided in RFC 7730 by adding support for Uniform Resource
   Identifiers (URIs) (RFC 3986) that use HTTP over TLS (HTTPS) (RFC
   7230) as the scheme.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in 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/rfc8630.

Copyright Notice

   Copyright (c) 2019 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
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   publication of this document.  Please review these documents
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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
      1.1. Terminology ................................................3
      1.2. Changes from RFC 7730 ......................................3
   2. Trust Anchor Locator ............................................3
      2.1. Trust Anchor Locator Motivation ............................3
      2.2. Trust Anchor Locator File Format ...........................4
      2.3. TAL and TA Certificate Considerations ......................4
      2.4. Example ....................................................6
   3. Relying Party Use ...............................................6
   4. URI Scheme Considerations .......................................7
   5. Security Considerations .........................................8
   6. IANA Considerations .............................................8
   7. References ......................................................8
      7.1. Normative References .......................................8
      7.2. Informative References ....................................10
   Acknowledgements ..................................................10
   Authors' Addresses ................................................11

1.  Introduction

   This document defines a Trust Anchor Locator (TAL) for the Resource
   Public Key Infrastructure (RPKI) [RFC6480].  This format may be used
   to distribute Trust Anchor (TA) material using a mix of out-of-band
   and online means.  Procedures used by Relying Parties (RPs) to verify
   RPKI signed objects SHOULD support this format to facilitate
   interoperability between creators of TA material and RPs.  This
   document obsoletes [RFC7730] by adding support for Uniform Resource
   Identifiers (URIs) [RFC3986] that use HTTP over TLS (HTTPS) [RFC7230]
   as the scheme.

1.1.  Terminology

   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.

1.2.  Changes from RFC 7730

   The TAL format defined in this document differs from the definition
   in [RFC7730] in that:

   o  it allows for the use of the HTTPS scheme in URIs [RFC7230], and

   o  it allows for the inclusion of an optional comment section.

   Note that current RPs may not support this new format yet.
   Therefore, it is RECOMMENDED that a TA operator maintain a TAL file
   as defined in [RFC7730] for a time as well, until they are satisfied
   that RP tooling has been updated.

2.  Trust Anchor Locator

2.1.  Trust Anchor Locator Motivation

   This document does not propose a new format for TA material.  A TA in
   the RPKI is represented by a self-signed X.509 Certification
   Authority (CA) certificate, a format commonly used in PKIs and widely
   supported by RP software.  This document specifies a format for data
   used to retrieve and verify the authenticity of a TA in a very simple
   fashion.  That data is referred to as the TAL.

   The motivation for defining the TAL is to enable selected data in the
   TA to change, without needing to redistribute the TA per se.

   In the RPKI, certificates contain one or more extensions [RFC3779]
   that can contain a set of IP Address Delegations and/or Autonomous
   System Identifier Delegations.  In this document, we refer to these
   delegations as the Internet Number Resources (INRs) contained in an
   RPKI certificate.

   The set of INRs associated with an entity acting as a TA is likely to
   change over time.  Thus, if one were to use the common PKI convention
   of distributing a TA to RPs in a secure fashion, then this procedure
   would need to be repeated whenever the INR set for the entity acting
   as a TA changed.  By distributing the TAL (in a secure fashion)
   instead of distributing the TA, this problem is avoided, i.e., the
   TAL is constant so long as the TA's public key and its location do
   not change.

   The TAL is analogous to the TrustAnchorInfo data structure specified
   in [RFC5914], which is on the Standards Track.  That specification
   could be used to represent the TAL, if one defined an rsync or HTTPS
   URI extension for that data structure.  However, the TAL format was
   adopted by RPKI implementors prior to the PKIX TA work, and the RPKI
   implementor community has elected to utilize the TAL format rather
   than define the requisite extension.  The community also prefers the
   simplicity of the ASCII encoding of the TAL, versus the binary
   (ASN.1) encoding for TrustAnchorInfo.

2.2.  Trust Anchor Locator File Format

   In this document, we define a TA URI as a URI that can be used to
   retrieve a current TA certificate.  This URI MUST be either an rsync
   URI [RFC5781] or an HTTPS URI [RFC7230].

   The TAL is an ordered sequence of:

   1.  an optional comment section consisting of one or more lines each
       starting with the "#" character, followed by human-readable
       informational UTF-8 text, conforming to the restrictions defined
       in Section 2 of [RFC5198], and ending with a line break,

   2.  a URI section that is comprised of one or more ordered lines,
       each containing a TA URI, and ending with a line break,

   3.  a line break, and

   4.  a subjectPublicKeyInfo [RFC5280] in DER format [X.509], encoded
       in base64 (see Section 4 of [RFC4648]).  To avoid long lines,
       line breaks MAY be inserted into the base64-encoded string.

   Note that line breaks in this file can use either "<CRLF>" or "<LF>".

2.3.  TAL and TA Certificate Considerations

   Each TA URI in the TAL MUST reference a single object.  It MUST NOT
   reference a directory or any other form of collection of objects.
   The referenced object MUST be a self-signed CA certificate that
   conforms to the RPKI certificate profile [RFC6487].  This certificate
   is the TA in certification path discovery [RFC4158] and validation
   [RFC5280] [RFC3779].

   The validity interval of this TA is chosen such that (1) the
   "notBefore" time predates the moment that this certificate is
   published and (2) the "notAfter" time is after the planned time of
   reissuance of this certificate.

   The INR extension(s) of this TA MUST contain a non-empty set of
   number resources.  It MUST NOT use the "inherit" form of the INR
   extension(s).  The INR set described in this certificate is the set
   of number resources for which the issuing entity is offering itself
   as a putative TA in the RPKI [RFC6480].

   The public key used to verify the TA MUST be the same as the
   subjectPublicKeyInfo in the CA certificate and in the TAL.

   The TA MUST contain a stable key that does not change when the
   certificate is reissued due to changes in the INR extension(s), when
   the certificate is renewed prior to expiration.

   Because the public key in the TAL and the TA MUST be stable, this
   motivates operation of that CA in an offline mode.  In that case, a
   subordinate CA certificate containing the same INRs, or, in theory,
   any subset of INRs, can be issued for online operations.  This allows
   the entity that issues the TA to keep the corresponding private key
   of this certificate offline, while issuing all relevant child
   certificates under the immediate subordinate CA.  This measure also
   allows the Certificate Revocation List (CRL) issued by that entity to
   be used to revoke the subordinate CA certificate in the event of
   suspected key compromise of this online operational key pair that is
   potentially more vulnerable.

   The TA MUST be published at a stable URI.  When the TA is reissued
   for any reason, the replacement CA certificate MUST be accessible
   using the same URI.

   Because the TA is a self-signed certificate, there is no
   corresponding CRL that can be used to revoke it, nor is there a
   manifest [RFC6486] that lists this certificate.

   If an entity wishes to withdraw a self-signed CA certificate as a
   putative TA, for any reason, including key rollover, the entity MUST
   remove the object from the location referenced in the TAL.

   Where the TAL contains two or more TA URIs, the same self-signed
   CA certificate MUST be found at each referenced location.  In order
   to increase operational resilience, it is RECOMMENDED that
   (1) the domain name parts of each of these URIs resolve to distinct
   IP addresses that are used by a diverse set of repository publication
   points and (2) these IP addresses be included in distinct Route
   Origin Authorization (ROA) objects signed by different CAs.

2.4.  Example

         # This TAL is intended for documentation purposes only.
         # Do not attempt to use this in a production setting.
         rsync://rpki.example.org/rpki/hedgehog/root.cer
         https://rpki.example.org/rpki/hedgehog/root.cer

         MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAovWQL2lh6knDx
         GUG5hbtCXvvh4AOzjhDkSHlj22gn/1oiM9IeDATIwP44vhQ6L/xvuk7W6
         Kfa5ygmqQ+xOZOwTWPcrUbqaQyPNxokuivzyvqVZVDecOEqs78q58mSp9
         nbtxmLRW7B67SJCBSzfa5XpVyXYEgYAjkk3fpmefU+AcxtxvvHB5OVPIa
         BfPcs80ICMgHQX+fphvute9XLxjfJKJWkhZqZ0v7pZm2uhkcPx1PMGcrG
         ee0WSDC3fr3erLueagpiLsFjwwpX6F+Ms8vqz45H+DKmYKvPSstZjCCq9
         aJ0qANT9OtnfSDOS+aLRPjZryCNyvvBHxZXqj5YCGKtwIDAQAB

3.  Relying Party Use

   In order to use the TAL to retrieve and validate a (putative) TA, an
   RP SHOULD:

   1.  Retrieve the object referenced by (one of) the TA URI(s)
       contained in the TAL.

   2.  Confirm that the retrieved object is a current, self-signed RPKI
       CA certificate that conforms to the profile as specified in
       [RFC6487].

   3.  Confirm that the public key in the TAL matches the public key in
       the retrieved object.

   4.  Perform other checks, as deemed appropriate (locally), to ensure
       that the RP is willing to accept the entity publishing this
       self-signed CA certificate to be a TA.  These tests apply to the
       validity of attestations made in the context of the RPKI relating
       to all resources described in the INR extension(s) of this
       certificate.

   An RP SHOULD perform these functions for each instance of a TAL that
   it is holding for this purpose every time the RP performs a
   resynchronization across the local repository cache.  In any case, an
   RP also SHOULD perform these functions prior to the expiration of the
   locally cached copy of the retrieved TA referenced by the TAL.

   In the case where a TAL contains multiple TA URIs, an RP MAY use a
   locally defined preference rule to select the URI to retrieve the
   self-signed RPKI CA certificate that is to be used as a TA.  Some
   examples are:

   o  Using the order provided in the TAL

   o  Selecting the TA URI randomly from the available list

   o  Creating a prioritized list of URIs based on RP-specific
      parameters, such as connection establishment delay

   If the connection to the preferred URI fails or the retrieved CA
   certificate public key does not match the TAL public key, the RP
   SHOULD retrieve the CA certificate from the next URI, according to
   the local preference ranking of URIs.

4.  URI Scheme Considerations

   Please note that the RSYNC protocol provides neither transport
   security nor any means by which the RP can validate that they are
   connected to the proper host.  Therefore, it is RECOMMENDED that
   HTTPS be used as the preferred scheme.

   Note that, although a Man in the Middle (MITM) cannot produce a CA
   certificate that would be considered valid according to the process
   described in Section 3, this type of attack can prevent the RP from
   learning about an updated CA certificate.

   RPs MUST do TLS certificate and host name validation when they fetch
   a CA certificate using an HTTPS URI on a TAL.  RPs SHOULD log any TLS
   certificate or host name validation issues found so that an operator
   can investigate the cause.

   It is RECOMMENDED that RPs and Repository Servers follow the Best
   Current Practices outlined in [RFC7525] on the use of HTTPS
   [RFC7230].  RPs SHOULD do TLS certificate and host name validation
   using subjectAltName dNSName identities as described in [RFC6125].
   The rules and guidelines defined in [RFC6125] apply here, with the
   following considerations:

   o  RPs and Repository Servers SHOULD support the DNS-ID identifier
      type.  The DNS-ID identifier type SHOULD be present in Repository
      Server certificates.

   o  DNS names in Repository Server certificates SHOULD NOT contain the
      wildcard character "*".

   o  This protocol does not require the use of SRV-IDs.

   o  This protocol does not require the use of URI-IDs.

5.  Security Considerations

   Compromise of a TA private key permits unauthorized parties to
   masquerade as a TA, with potentially severe consequences.  Reliance
   on an inappropriate or incorrect TA has similar potentially severe
   consequences.

   This TAL does not directly provide a list of resources covered by the
   referenced self-signed CA certificate.  Instead, the RP is referred
   to the TA itself and the INR extension(s) within this certificate.
   This provides necessary operational flexibility, but it also allows
   the certificate issuer to claim to be authoritative for any resource.
   RPs should either (1) have great confidence in the issuers of such
   certificates that they are configuring as TAs or (2) issue their own
   self-signed certificate as a TA and, in doing so, impose constraints
   on the subordinate certificates.

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [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>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
              March 2011, <https://www.rfc-editor.org/info/rfc6125>.

   [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>.

   [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>.

   [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>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
              May 2015, <https://www.rfc-editor.org/info/rfc7525>.

   [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>.

   [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>.

   [X.509]    ITU-T, "Information technology - Open Systems
              Interconnection - The Directory: Public-key and attribute
              certificate frameworks", ITU-T Recommendation X.509,
              October 2016, <https://www.itu.int/rec/T-REC-X.509>.

7.2.  Informative References

   [RFC4158]  Cooper, M., Dzambasow, Y., Hesse, P., Joseph, S., and R.
              Nicholas, "Internet X.509 Public Key Infrastructure:
              Certification Path Building", RFC 4158,
              DOI 10.17487/RFC4158, September 2005,
              <https://www.rfc-editor.org/info/rfc4158>.

   [RFC5914]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Format", RFC 5914, DOI 10.17487/RFC5914, June 2010,
              <https://www.rfc-editor.org/info/rfc5914>.

   [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>.

Acknowledgements

   This approach to TA material was originally described by Robert
   Kisteleki.

   The authors acknowledge the contributions of Rob Austein and Randy
   Bush, who assisted with drafting this document and with helpful
   review comments.

   The authors acknowledge the work of Roque Gagliano, Terry Manderson,
   and Carlos Martinez-Cagnazzo in developing the ideas behind the
   inclusion of multiple URIs in the TAL.

   The authors acknowledge Job Snijders for suggesting the inclusion of
   comments at the start of the TAL.

Authors' Addresses

   Geoff Huston
   APNIC

   Email: gih@apnic.net
   URI:   https://www.apnic.net


   Samuel Weiler
   W3C/MIT

   Email: weiler@csail.mit.edu


   George Michaelson
   APNIC

   Email: ggm@apnic.net
   URI:   https://www.apnic.net


   Stephen Kent
   Unaffiliated

   Email: kent@alum.mit.edu


   Tim Bruijnzeels
   NLnet Labs

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