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

Network Working Group                                        J. Peterson
Internet-Draft                                                TransUnion
Intended status: Standards Track                            9 April 2026
Expires: 11 October 2026

         Short-Lived Certificates for Secure Telephone Identity
               draft-ietf-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 11 October 2026.

Copyright Notice

   Copyright (c) 2026 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' . . . . . . . . . . . . . .   5
   5.  Certificate Acquisition with ACME . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   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  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 that 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 certification 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 specifies 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.

   As the use of short-lived certificates described below involves a
   certificate chain included by-value in the PASSporT header, this
   document requires the use of "x5c" in the PASSporT header when short-
   lived certificates are present.

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, relying parties must review
   certificate revocation lists (CRLs) or to access real-time status
   information with protocols such as OCSP to learn that the certificate
   has been revoked.  Short-lived certificates offer an alternative
   where, if compromised, certificates will shortly expire anyway, so
   CRLs and OCSP are less meaningful.  Such certificates cannot be
   cached for a long term by relying parties, minimizing their value to
   attackers.

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   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
   certification authority is paid on the signer's side rather than the
   verifier's side; however, in environments where many parties may rely
   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
   certification 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 TN Authorization 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.

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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.

   As an optimization, this specification requires 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 PEM representation of the certificate chain.  STIR
   Verification service implementations compliant with this
   specification MUST support the "x5c" element; authentication services
   MUST use the "x5c" format for PASSporTs signed by certificates with
   an valid lifetime shorter than one week.  The presence of x5c 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, "x5c" precludes the need for a
   round-trip time before validation at the STIR verification service.
   The root cerificate SHOULD NOT be included in the chain, though it
   may be required in some deployment environments.

   An example PASSporT header with an "x5c" element with three
   certificates (including the root) in its chain might look as follows:

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  { "typ":"passport",
    "ppt":"div",
    "alg":"ES256",
    "x5c":
        ["MIICkTCCAjigAwIBAgIUb689vcv/kTP7N6J7Gz9YZU21WT4wCgYIKoZIzj0EAwIw
        SzELMAkGA1UEBhMCVVMxFjAUBgNVBAoMDUFjbWUgQ0EsIEluYy4xJDAiBgNVBAMM
        G0FjbWUgU0hBS0VOIEludGVybWVkaWF0ZSBDQTAeFw0yNDA3MDgxNDI4MjVaFw0y
        NDA3MDkxNDI4MjVaMFUxCzAJBgNVBAYTAlVTMR4wHAYDVQQKDBVBY21lIEVudGVy
        cHJpc2UsIEluYy4xJjAkBgNVBAMMHVNIQUtFTiBUTi1BdXRobi1MaXN0IEJ5IFZh
        bHVlMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEuxmGGgRklHe0GyGbgANc4182
        z3y6LdJ9Xp0r6xWspI/K3msUWD3ijxA5iYEKUBLofBMbwHiDS4CfMldTHUuMFqOB
        7zCB7DAMBgNVHRMBAf8EAjAAMB0GA1UdDgQWBBTbmpeh0wHlpxvtJlVbI6mD+idq
        cjAfBgNVHSMEGDAWgBTNsw0NkoZNWYL1bn2vwna5gwp0szAOBgNVHQ8BAf8EBAMC
        B4AwNAYIKwYBBQUHAQEEKDAmMCQGA1UdH4YdaHR0cHM6Ly9zdGlwYS5hY21lLWNh
        LmNvbS9jcmwwVgYIKwYBBQUHARoESjBIoRMwERYLMTcwMzU1NTIwMDACAgPoog0W
        CzE3MDM1NTUxMjM0oRMwERYLMTU3MTU1NTMwMDACAgfQog0WCzE1NzE1NTUyMzQ1
        MAoGCCqGSM49BAMCA0cAMEQCIFzV0EyHdMXStqzROJkL6CO06+6FuFYZGIod9iMw
        Yyr2AiAmEK1smeSAdoFYG5WTIGfjB13xAWn7EQH0SDkqOKKBwA==",
        "MIICKjCCAdGgAwIBAgIUPV/XdduqC44On5FKIjmcqwWO1JkwCgYIKoZIzj0EAwIw
        QzELMAkGA1UEBhMCVVMxFjAUBgNVBAoMDUFjbWUgQ0EsIEluYy4xHDAaBgNVBAMM
        E0FjbWUgU0hBS0VOIFJvb3QgQ0EwHhcNMjQwNzA4MTQyODI1WhcNMzQwNzA5MTQy
        ODI1WjBLMQswCQYDVQQGEwJVUzEWMBQGA1UECgwNQWNtZSBDQSwgSW5jLjEkMCIG
        A1UEAwwbQWNtZSBTSEFLRU4gSW50ZXJtZWRpYXRlIENBMFkwEwYHKoZIzj0CAQYI
        KoZIzj0DAQcDQgAE0wypYKOwsS8JU0Ahx4niuirUv0M9v01vUDHiSfFzWbeAlbK/
        hWlVYHADeFoRC7cIldDJ3iupafdxVgQggPp+/aOBmjCBlzAPBgNVHRMBAf8EBTAD
        AQH/MB0GA1UdDgQWBBTNsw0NkoZNWYL1bn2vwna5gwp0szAfBgNVHSMEGDAWgBRL
        Uq0usIR/TU27Y5vNN94V7xKnzTAOBgNVHQ8BAf8EBAMCAQYwNAYIKwYBBQUHAQEE
        KDAmMCQGA1UdH4YdaHR0cHM6Ly9zdGlwYS5hY21lLWNhLmNvbS9jcmwwCgYIKoZI
        zj0EAwIDRwAwRAIgUoAbid+kGFQWdQnLpJGwc4c4JoYohenmm3y2duOBN58CIEDm
        nWAQfT1HWvZt2M6q0nOopZLwrt/9LzkBJqomRgzr",
        "MIIB6zCCAZGgAwIBAgIUGEca/xMNdZbTZUP8DsxrnbR246AwCgYIKoZIzj0EAwIw
        QzELMAkGA1UEBhMCVVMxFjAUBgNVBAoMDUFjbWUgQ0EsIEluYy4xHDAaBgNVBAMM
        E0FjbWUgU0hBS0VOIFJvb3QgQ0EwHhcNMjIwNTA3MDQ1ODIyWhcNNDIwNTA3MDQ1
        ODIyWjBDMQswCQYDVQQGEwJVUzEWMBQGA1UECgwNQWNtZSBDQSwgSW5jLjEcMBoG
        A1UEAwwTQWNtZSBTSEFLRU4gUm9vdCBDQTBZMBMGByqGSM49AgEGCCqGSM49AwEH
        A0IABL7UD1CBpsNNq0EzDa90JUQV9etsUe8YOyOXpFB/9A8ahDj6HAJxuZJigSCG
        dG6npehdHMeDHzHsPHMLy22qH6OjYzBhMB0GA1UdDgQWBBRLUq0usIR/TU27Y5vN
        N94V7xKnzTAfBgNVHSMEGDAWgBRLUq0usIR/TU27Y5vNN94V7xKnzTAPBgNVHRMB
        Af8EBTADAQH/MA4GA1UdDwEB/wQEAwIBBjAKBggqhkjOPQQDAgNIADBFAiEA+U9j
        0vQpHOx4nF+jytmr/bpotH001R09zZiywozSydwCIE6hI66JwnmqRKeJ9OeTNyM5
        asC7YWEm3bQqgukw/uEs"]
 }

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   [RFC8224] provides a way of pointing to a certificate in a MIME body
   associated with the SIP request, but for out-of-band uses of STIR,
   having the certificate embedded in the PASSporT itself is a superior
   option.  Note that for backward compatibility reasons,
   implementations MAY include both "x5u" and "x5c" in the payload
   header, but the two URIs MUST indicate the same certification chain.

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 provdes flexibility to clients requesting
   certificates to determine how much of their authority they want to
   invest in any given certificate.

   [RFC9448] uses the Authority Token Challenge (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.

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6.  IANA Considerations

   This document contains no actions for the IANA.

7.  Privacy Considerations

   Short-lived certificates provide attractive privacy properties when
   compared to real-time status protocols like OCSP, which require
   relying parties to perform a network query 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

   For further information on certificate security and practices, see
   [RFC5280], in particular its Security Considerations.
   Implementations should note the Security Considerations of [RFC8226]
   for guidance on STIR certificate usage.  The Security Considerations
   of [RFC8555] are relevant to the use of ACME to acquire short-lived
   certificates, as are those of [RFC8739].

9.  Acknowledgments

   Stephen Farrell, Jack Rickard, Simon Castle, Chris Wendt, Alec
   Feichnel, Eric Rescorla and Ning Zhang provided key input to this
   document.

10.  References

10.1.  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>.

   [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.

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>.

Author's Address

   Jon Peterson
   TransUnion
   Email: jon.peterson@transunion.com

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