Short-Lived Certificates for Secure Telephone Identity
draft-peterson-stir-certificates-shortlived-05

Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                         9 November 2023
Expires: 12 May 2024

         Short-Lived Certificates for Secure Telephone Identity
             draft-peterson-stir-certificates-shortlived-05

Abstract

   When certificates are used as credentials to attest the assignment of
   ownership of telephone numbers, some mechanism is required to provide
   certificate freshness.  This document specifies short-lived
   certificates as a means of guaranteeing certificate freshness for
   secure telephone identity (STIR), potentially relying on the
   Automated Certificate Management Environment (ACME) or similar
   mechanisms to allow signers to acquire certificates as needed.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 12 May 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Short-lived certificates for STIR . . . . . . . . . . . . . .   3
   4.  Certificate conveyance with 'x5c' . . . . . . . . . . . . . .   4
   5.  Certificate Acquisition with ACME . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The STIR problem statement [RFC7340] discusses many attacks on the
   telephone network that are enabled by impersonation, including
   various forms of robocalling, voicemail hacking, and swatting.  One
   of the most important components of a system to prevent impersonation
   is the implementation of credentials which identify the parties who
   control telephone numbers.  The STIR certificates [RFC8226]
   specification describes a credential system based on [X.509] version
   3 certificates in accordance with [RFC5280] for that purpose.  Those
   credentials can then be used by STIR authentication services
   [RFC8224] to sign PASSporT objects [RFC8225] carried in a SIP
   [RFC3261] request.

   The STIR certificates document specifies an extension to X.509 that
   defines a Telephony Number (TN) Authorization List that may be
   included by certificate authorities in certificates.  This extension
   provides additional information that relying parties can use when
   validating transactions with the certificate: either in the form of
   Service Provider Codes (SPCs) or telephone numbers.  Telephone
   numbers or number ranges are used in delegate STIR certificates
   [RFC9060].  When a SIP request arrives at a terminating
   administrative domain, for example, the calling number attested by

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   the SIP request can be compared to the TN Authorization List of the
   delegate certificate that signed the request to determine if the
   caller is authorized to use that calling number in SIP.

   No specific recommendation is made in the STIR certificates document
   for a means of determining the freshness of certificates with a TN
   Authorization List.  This document explores how short-lived
   certificates could be used as a means of preserving that freshness.
   Short-lived certificates also have a number of other desirable
   properties that fulfill important operational requirements for
   network operators.  A mechanism such as the Automated Certificate
   Management Environment (ACME) [RFC8555] could be leveraged to manage
   these short-lived certificates, as well as various web-based
   interfaces or other out-of-band mechanisms.  The interaction of STIR
   with ACME has already been explored in [RFC9448], so it provides a
   potentially attractive way of delivering short-lived certificates.

2.  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.

3.  Short-lived certificates for STIR

   While there is no easy definition of what constitutes a "short-lived"
   certificate, the term typically refers to certificates that are valid
   only for days or even hours, as opposed to the months or years common
   in traditional public key infrastructures.  When the private keying
   material associated with a certificate with an expiry of months or
   years is compromised by an adversary, the issuing authority must
   revoke the certificate, which requires relying parties to review
   certificate revocation lists or to access real-time status
   information with protocols such as OCSP.  Short-lived certificates
   offer an alternative where, if compromised, certificates will shortly
   expire anyway, and rather than revoking and reissuing the certificate
   in response to a crisis, certificates routinely roll-over and cannot
   be cached for a long term by relying parties, minimizing their value
   to attackers.

   One of the additional benefits of using short-lived certificates is
   that they do not require relying parties to perform any certificate
   freshness check.  The trade-off is that the signer must acquire new
   certificates frequently, so the cost of round-trip times to the
   certificate authority is paid on the signer's side rather than the
   verifier's side; however, in environments where many parties may rely

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   on a single certificate, or at least where a single certificate will
   be used to sign many transactions during its short lifetime, the
   overall architecture will incur fewer round-trip times to the
   certificate authority and thus less processing delay.

   In the STIR context, the TN Authorization List defined in [RFC8226]
   adds a new wrinkle to the behavior of short-lived certificates,
   especially when the List is populated with telephone numbers or
   number ranges instead of Service Provider Codes (SPCs).  A subject
   may have authority over multiple telephone numbers, but a particular
   short-lived certificate issued to that subject could attest the
   authority over all, some, or just one of those telephone numbers.
   Short-lived certificates permit a more on-demand certification
   process, where subjects acquire certificates as needed, potentially
   in reaction to calls being placed.  A STIR authentication service
   could even acquire a new certificate on a per-call basis that can
   only sign for the calling party number of the call in question, as it
   would expire immediately thereafter.  At the other end of the
   spectrum, a large enterprise service provider could acquire a
   certificate valid for millions of numbers, but expire the certificate
   after a very short duration - on the order of hours - to reduce the
   risk that the certificate would be compromised.

   This inherent flexibility in the short-lived certificate architecture
   would also permit authentication services to implement very narrow
   policies for certificate usage.  A large service provider who wanted
   to avoid revealing which phone numbers they controlled, for example,
   could provide no information in the certificate that signs a call
   other than just the single telephone number that corresponds to the
   calling party's number.  How frequently the service provider feels
   that they need to expire that certificate and acquire a new one is
   entirely a matter of local policy.  This makes it much harder for
   entities monitoring signatures over calls to guess who owns which
   numbers, and provides a much more complicated threat surface for
   attackers trying to compromise the service.

4.  Certificate conveyance with 'x5c'

   In order to reduce the burden on verification services, an
   authentication service could also piggyback a short-lived certificate
   onto the PASSporT, so that no network lookup and consequent round-
   trip delay would be required on the terminating side to acquire the
   new certificate.  In particular, the poor cacheability of short-lived
   certificates may require frequent fetches of certificates via the
   "x5u" PASSporT header element when relying parties validate
   PASSporTs.

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   As an optimization, this specification permits the conveyance of the
   certificate chain for a short-lived certificate via the "x5c" JWS
   header element ([RFC7515] Section 4.1.6).  The "x5c" element contains
   a base64 encoded DER representation of the certificate chain.  STIR
   Verification service implementations compliant with this
   specification MUST support the "x5c" element; authentication services
   SHOULD use the "x5c" format for PASSporTs signed by certificates with
   an expiry shorter than one week.  The presence of x5y creates
   PASSporT objects that are considerable larger than typical RFC8225
   tokens, and the longer the certificate chain, the larger the PASSporT
   header will be.  But provided the certificate chain leads to a
   trusted certification authority, "x5u" precludes the need for a
   round-trip time before validation at the STIR verification service.

   An example PASSporT header with an "x5c" element with three
   certificates in its chain might look as follows:

    { "typ":"passport",
      "ppt":"div",
      "alg":"ES256",
      "x5c":
      [ "MIIE3jCCA8agAwIBAgICAwEwDQYJKoZIhvcNAQEFBQAwYzELMAkGA1UEBhMCVVM
         xITAfBgNVBAoTGFRoZSBHbyBEYWRkeSBHcm91cCwgSW5jLjExMC8GA1UECxMoR2
         8gRGFkZHkgQ2xhc3MgMiBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eTAeFw0wNjExM
         TYwMTU0MzdaFw0yNjExMTYwMTU0MzdaMIHKMQswCQYDVQQGEwJVUzEQMA4GA1UE
         CBMHQXJpem9uYTETMBEGA1UEBxMKU2NvdHRzZGFsZTEaMBgGA1UEChMRR29EYWR
         keS5jb20sIEluYy4xMzAxBgNVBAsTKmh0dHA6Ly9jZXJ0aWZpY2F0ZXMuZ29kYW
         RkeS5jb20vcmVwb3NpdG9yeTEwMC4GA1UEAxMnR28gRGFkZHkgU2VjdXJlIENlc
         nRpZmljYXRpb24gQXV0aG9yaXR5MREwDwYDVQQFEwgwNzk2OTI4NzCCASIwDQYJ
         KoZIhvcNAQEBBQADggEPADCCAQoCggEBAMQt1RWMnCZM7DI161+4WQFapmGBWTt
         wY6vj3D3HKrjJM9N55DrtPDAjhI6zMBS2sofDPZVUBJ7fmd0LJR4h3mUpfjWoqV
         Tr9vcyOdQmVZWt7/v+WIbXnvQAjYwqDL1CBM6nPwT27oDyqu9SoWlm2r4arV3aL
         GbqGmu75RpRSgAvSMeYddi5Kcju+GZtCpyz8/x4fKL4o/K1w/O5epHBp+YlLpyo
         7RJlbmr2EkRTcDCVw5wrWCs9CHRK8r5RsL+H0EwnWGu1NcWdrxcx+AuP7q2BNgW
         JCJjPOq8lh8BJ6qf9Z/dFjpfMFDniNoW1fho3/Rb2cRGadDAW/hOUoz+EDU8CAw
         EAAaOCATIwggEuMB0GA1UdDgQWBBT9rGEyk2xF1uLuhV+auud2mWjM5zAfBgNVH
         SMEGDAWgBTSxLDSkdRMEXGzYcs9of7dqGrU4zASBgNVHRMBAf8ECDAGAQH/AgEA
         MDMGCCsGAQUFBwEBBCcwJTAjBggrBgEFBQcwAYYXaHR0cDovL29jc3AuZ29kYWR
         keS5jb20wRgYDVR0fBD8wPTA7oDmgN4Y1aHR0cDovL2NlcnRpZmljYXRlcy5nb2
         RhZGR5LmNvbS9yZXBvc2l0b3J5L2dkcm9vdC5jcmwwSwYDVR0gBEQwQjBABgRVH
         SAAMDgwNgYIKwYBBQUHAgEWKmh0dHA6Ly9jZXJ0aWZpY2F0ZXMuZ29kYWRkeS5j
         b20vcmVwb3NpdG9yeTAOBgNVHQ8BAf8EBAMCAQYwDQYJKoZIhvcNAQEFBQADggE
         BANKGwOy9+aG2Z+5mC6IGOgRQjhVyrEp0lVPLN8tESe8HkGsz2ZbwlFalEzAFPI
         UyIXvJxwqoJKSQ3kbTJSMUA2fCENZvD117esyfxVgqwcSeIaha86ykRvOe5GPLL
         5CkKSkB2XIsKd83ASe8T+5o0yGPwLPk9Qnt0hCqU7S+8MxZC9Y7lhyVJEnfzuz9
         p0iRFEUOOjZv2kWzRaJBydTXRE4+uXR21aITVSzGh6O1mawGhId/dQb8vxRMDsx
         uxN89txJx9OjxUUAiKEngHUuHqDTMBqLdElrRhjZkAzVvb3du6/KFUJheqwNTrZ
         EjYx8WnM25sgVjOuH0aBsXBTWVU+4=",

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        "MIIE+zCCBGSgAwIBAgICAQ0wDQYJKoZIhvcNAQEFBQAwgbsxJDAiBgNVBAcTG1Z
         hbGlDZXJ0IFZhbGlkYXRpb24gTmV0d29yazEXMBUGA1UEChMOVmFsaUNlcnQsIE
         luYy4xNTAzBgNVBAsTLFZhbGlDZXJ0IENsYXNzIDIgUG9saWN5IFZhbGlkYXRpb
         24gQXV0aG9yaXR5MSEwHwYDVQQDExhodHRwOi8vd3d3LnZhbGljZXJ0LmNvbS8x
         IDAeBgkqhkiG9w0BCQEWEWluZm9AdmFsaWNlcnQuY29tMB4XDTA0MDYyOTE3MDY
         yMFoXDTI0MDYyOTE3MDYyMFowYzELMAkGA1UEBhMCVVMxITAfBgNVBAoTGFRoZS
         BHbyBEYWRkeSBHcm91cCwgSW5jLjExMC8GA1UECxMoR28gRGFkZHkgQ2xhc3MgM
         iBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eTCCASAwDQYJKoZIhvcNAQEBBQADggEN
         ADCCAQgCggEBAN6d1+pXGEmhW+vXX0iG6r7d/+TvZxz0ZWizV3GgXne77ZtJ6XC
         APVYYYwhv2vLM0D9/AlQiVBDYsoHUwHU9S3/Hd8M+eKsaA7Ugay9qK7HFiH7Eux
         6wwdhFJ2+qN1j3hybX2C32qRe3H3I2TqYXP2WYktsqbl2i/ojgC95/5Y0V4evLO
         tXiEqITLdiOr18SPaAIBQi2XKVlOARFmR6jYGB0xUGlcmIbYsUfb18aQr4CUWWo
         riMYavx4A6lNf4DD+qta/KFApMoZFv6yyO9ecw3ud72a9nmYvLEHZ6IVDd2gWMZ
         Eewo+YihfukEHU1jPEX44dMX4/7VpkI+EdOqXG68CAQOjggHhMIIB3TAdBgNVHQ
         4EFgQU0sSw0pHUTBFxs2HLPaH+3ahq1OMwgdIGA1UdIwSByjCBx6GBwaSBvjCBu
         zEkMCIGA1UEBxMbVmFsaUNlcnQgVmFsaWRhdGlvbiBOZXR3b3JrMRcwFQYDVQQK
         Ew5WYWxpQ2VydCwgSW5jLjE1MDMGA1UECxMsVmFsaUNlcnQgQ2xhc3MgMiBQb2x
         pY3kgVmFsaWRhdGlvbiBBdXRob3JpdHkxITAfBgNVBAMTGGh0dHA6Ly93d3cudm
         FsaWNlcnQuY29tLzEgMB4GCSqGSIb3DQEJARYRaW5mb0B2YWxpY2VydC5jb22CA
         QEwDwYDVR0TAQH/BAUwAwEB/zAzBggrBgEFBQcBAQQnMCUwIwYIKwYBBQUHMAGG
         F2h0dHA6Ly9vY3NwLmdvZGFkZHkuY29tMEQGA1UdHwQ9MDswOaA3oDWGM2h0dHA
         6Ly9jZXJ0aWZpY2F0ZXMuZ29kYWRkeS5jb20vcmVwb3NpdG9yeS9yb290LmNybD
         BLBgNVHSAERDBCMEAGBFUdIAAwODA2BggrBgEFBQcCARYqaHR0cDovL2NlcnRpZ
         mljYXRlcy5nb2RhZGR5LmNvbS9yZXBvc2l0b3J5MA4GA1UdDwEB/wQEAwIBBjAN
         BgkqhkiG9w0BAQUFAAOBgQC1QPmnHfbq/qQaQlpE9xXUhUaJwL6e4+PrxeNYiY+
         Sn1eocSxI0YGyeR+sBjUZsE4OWBsUs5iB0QQeyAfJg594RAoYC5jcdnplDQ1tgM
         QLARzLrUc+cb53S8wGd9D0VmsfSxOaFIqII6hR8INMqzW/Rn453HWkrugp++85j
         09VZw==",
        "MIIC5zCCAlACAQEwDQYJKoZIhvcNAQEFBQAwgbsxJDAiBgNVBAcTG1ZhbGlDZXJ
         0IFZhbGlkYXRpb24gTmV0d29yazEXMBUGA1UEChMOVmFsaUNlcnQsIEluYy4xNT
         AzBgNVBAsTLFZhbGlDZXJ0IENsYXNzIDIgUG9saWN5IFZhbGlkYXRpb24gQXV0a
         G9yaXR5MSEwHwYDVQQDExhodHRwOi8vd3d3LnZhbGljZXJ0LmNvbS8xIDAeBgkq
         hkiG9w0BCQEWEWluZm9AdmFsaWNlcnQuY29tMB4XDTk5MDYyNjAwMTk1NFoXDTE
         5MDYyNjAwMTk1NFowgbsxJDAiBgNVBAcTG1ZhbGlDZXJ0IFZhbGlkYXRpb24gTm
         V0d29yazEXMBUGA1UEChMOVmFsaUNlcnQsIEluYy4xNTAzBgNVBAsTLFZhbGlDZ
         XJ0IENsYXNzIDIgUG9saWN5IFZhbGlkYXRpb24gQXV0aG9yaXR5MSEwHwYDVQQD
         ExhodHRwOi8vd3d3LnZhbGljZXJ0LmNvbS8xIDAeBgkqhkiG9w0BCQEWEWluZm9
         AdmFsaWNlcnQuY29tMIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDOOnHK5a
         vIWZJV16vYdA757tn2VUdZZUcOBVXc65g2PFxTXdMwzzjsvUGJ7SVCCSRrCl6zf
         N1SLUzm1NZ9WlmpZdRJEy0kTRxQb7XBhVQ7/nHk01xC+YDgkRoKWzk2Z/M/VXwb
         P7RfZHM047QSv4dk+NoS/zcnwbNDu+97bi5p9wIDAQABMA0GCSqGSIb3DQEBBQU
         AA4GBADt/UG9vUJSZSWI4OB9L+KXIPqeCgfYrx+jFzug6EILLGACOTb2oWH+heQ
         C1u+mNr0HZDzTuIYEZoDJJKPTEjlbVUjP9UNV+mWwD5MlM/Mtsq2azSiGM5bUMM
         j4QssxsodyamEwCW/POuZ6lcg5Ktz885hZo+L7tdEy8W9ViH0Pd"]
   }

   [TBD - certificate above is the example from RFC7515]

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   A potential alternative approach would be that [RFC8224] already
   provides a way of pointing to a certificate in a MIME body associated
   with the SIP request.  For out-of-band uses of STIR, however, having
   the certificate embedded in the PASSporT itself is a superior option.

5.  Certificate Acquisition with ACME

   One of the primary challenges facing short-lived certificates is
   building an operational system that allows signers to acquire new
   certificates and put them to immediate use.  ACME [RFC8555] is
   designed for exactly this purpose.  After a client registers with an
   ACME server, and the authority of the client for the names in
   question is established (through means such as [RFC9448]), the client
   can at any time apply for a certificate to be issued by sending an
   appropriate JSON request to the server.  That request will contain a
   CSR [RFC2986] indicating the intended scope of authority as well the
   validity interval of the certificate in question.  Ultimately, this
   will enable the client to download the certificate from a certificate
   URL designated by the server.

   ACME is based on the concept that clients establish accounts at an
   ACME server, and that through challenges, the server learns which
   identifiers it will issue for certificates requested for an account.
   Any given certificate issued for an account can be for just one of
   those identifiers, or potentially for more: this is determined by the
   CSR that an ACME client creates for a particular order.  Thus, a
   service provider with authority for millions of identifiers - that
   is, millions of telephone numbers - could create a CSR for an ACME
   order that requests a certificate only associated with one of those
   telephone numbers if it so desired.  The same would be true of
   certificates based on Service Provider Codes (SPCs) as described in
   [RFC8226]: a service provider might have just one SPC or perhaps
   many.  ACME thus puts needed flexibility into the hands of the
   clients requesting certificates to determine how much of their
   authority they want to invest in any given certificate.

   [RFC9448] uses the ATC framework of [RFC9447] to generate tokens that
   are provided to the CA in response to ACME challenges.  For a usage
   with short-term certificates, it may make sense for the ATC tokens to
   have a relatively long expiry, so that the ACME client does not have
   to constantly return to the Token Authority for new tokens.  This
   could potentially be used with the ACME STAR [RFC8739] mechanism as
   well.

6.  IANA Considerations

   This document contains no actions for the IANA.

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7.  Privacy Considerations

   Short-lived certificates provide attractive privacy properties when
   compared to real-time status query protocols like OCSP, which require
   relying parties to perform a network dip that can reveal a great deal
   about the source and destination of communications.  For STIR, these
   problems are compounded by the presence of the TN Authorization List
   extension to certificates.  Short-lived certificates can minimize the
   data that needs to appear in the TN Authorization List, and
   consequently reduce the amount of information about the caller leaked
   by certificate usage to an amount equal to what is leaked by the call
   signaling itself.

8.  Security Considerations

   This document is entirely about security.  For further information on
   certificate security and practices, see [RFC5280], in particular its
   Security Considerations.  The Security Considerations of [RFC8555]
   are relevant to the use of ACME to acquire short-lived certificates.

9.  Acknowledgments

   Stephen Farrell, Jack Richard and Chris Wendt provided key input to
   the discussions leading to this document.

10.  References

10.1.  Normative References

   [ATIS-0300251]
              ATIS Recommendation 0300251, "Codes for Identification of
              Service Providers for Information Exchange", 2007.

   [DSS]      National Institute of Standards and Technology, U.S.
              Department of Commerce | NIST FIPS PUB 186-4, "Digital
              Signature Standard, version 4", 2013.

   [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>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [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>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [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>.

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,
              <https://www.rfc-editor.org/info/rfc8224>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [RFC8739]  Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
              Perales, A., and T. Fossati, "Support for Short-Term,
              Automatically Renewed (STAR) Certificates in the Automated
              Certificate Management Environment (ACME)", RFC 8739,
              DOI 10.17487/RFC8739, March 2020,
              <https://www.rfc-editor.org/info/rfc8739>.

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   [RFC9060]  Peterson, J., "Secure Telephone Identity Revisited (STIR)
              Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
              September 2021, <https://www.rfc-editor.org/info/rfc9060>.

   [RFC9447]  Peterson, J., Barnes, M., Hancock, D., and C. Wendt,
              "Automated Certificate Management Environment (ACME)
              Challenges Using an Authority Token", RFC 9447,
              DOI 10.17487/RFC9447, September 2023,
              <https://www.rfc-editor.org/info/rfc9447>.

   [RFC9448]  Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
              "TNAuthList Profile of Automated Certificate Management
              Environment (ACME) Authority Token", RFC 9448,
              DOI 10.17487/RFC9448, September 2023,
              <https://www.rfc-editor.org/info/rfc9448>.

   [X.509]    ITU-T Recommendation X.509 (10/2012) | ISO/IEC 9594-8,
              "Information technology - Open Systems Interconnection -
              The Directory: Public-key and attribute certificate
              frameworks", 2012.

   [X.680]    ITU-T Recommendation X.680 (08/2015) | ISO/IEC 8824-1,
              "Information Technology - Abstract Syntax Notation One:
              Specification of basic notation".

   [X.681]    ITU-T Recommendation X.681 (08/2015) | ISO/IEC 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Information Object Specification".

   [X.682]    ITU-T Recommendation X.682 (08/2015) | ISO/IEC 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Constraint Specification".

   [X.683]    ITU-T Recommendation X.683 (08/2015) | ISO/IEC 8824-3,
              "Information Technology - Abstract Syntax Notation One:
              Parameterization of ASN.1 Specifications".

10.2.  Informative References

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <https://www.rfc-editor.org/info/rfc7340>.

   [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
              RFC 7375, DOI 10.17487/RFC7375, October 2014,
              <https://www.rfc-editor.org/info/rfc7375>.

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   [X.520]    ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-6,
              "Information technology - Open Systems Interconnection -
              The Directory: Selected Attribute Types", 2012.

Author's Address

   Jon Peterson
   Neustar, Inc.
   Email: jon.peterson@team.neustar

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