    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.

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    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
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include 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 ....................................................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.

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    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)

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       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].

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       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

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       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.

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       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 "*".

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       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>.

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    RFC 8630                        HTTPS TAL                    August 2019

       [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>.

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       [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.

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    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

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