



Network Working Group                                        J. Peterson
Internet-Draft                                                        TU
Intended status: Standards Track                               S. Turner
Expires: 11 October 2026                                           sn3rd
                                                            9 April 2026


         OCSP Usage for Secure Telephone Identity Certificates
                  draft-ietf-stir-certificates-ocsp-13

Abstract

   When certificates are used as credentials to attest the assignment or
   ownership of telephone numbers, some mechanism is required to convey
   certificate freshness to relying parties.  Certificate Revocation
   Lists (CRLs) are commonly used for this purpose, but for certain
   classes of certificates, including delegate certificates conveying
   their scope of authority by-reference in Secure Telephone Identity
   Revisited (STIR) systems, they may not be aligned with the needs of
   relying parties.  This document specifies the use of the Online
   Certificate Status Protocol (OCSP) as a means of retrieving real-time
   status information about such certificates, defining new extensions
   to compensate for the dynamism of telephone number assignments.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview of Certificate Verification Methods  . . . . . . . .   3
   4.  Using OCSP with TN Authorization Lists  . . . . . . . . . . .   5
     4.1.  OCSP Extension Specification  . . . . . . . . . . . . . .   5
     4.2.  Example OCSP Request  . . . . . . . . . . . . . . . . . .   7
     4.3.  Example OCSP Response . . . . . . . . . . . . . . . . . .   7
     4.4.  STIR Certification Authorities and OCSP . . . . . . . . .   8
   5.  OCSP Stapling . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  TN-HVE OCSP Extension . . . . . . . . . . . . . . . . . .  10
     6.2.  'stpl' JSON Web Token Claim . . . . . . . . . . . . . . .  10
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  14
   Appendix B.  OCSP Request and Response Breakdown  . . . . . . . .  15
   Appendix C.  Alternative OCSP Staple Design Considerations  . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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.




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   [RFC8226] 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.  When a SIP request, for example, arrives at a
   terminating administrative domain, the calling number attested by the
   SIP request can be compared to the TN Authorization List of the
   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 [RFC8226] for a means of
   determining the freshness of certificates with a TN Authorization
   List.  Moreover, there is significant dynamism in telephone number
   assignment, and due to practices like number portability, information
   about number assignment can suddenly become stale.  This problem is
   especially pronounced when a TN Authorization List extension
   associates a large block of telephone numbers with a certificate, as
   relying parties need a way to learn if any one of those telephone
   numbers has been ported to a different administrative entity.  To
   facilitate this, [RFC8226] Section 10.1 specifies a way that the TN
   Authorization List can be shared by-reference in a certificate, via a
   URL in the Authority Information Access extension, so that a more
   dynamic list can be maintained without continually reissuing the
   certificate.  For very large and/or complex TN Authorization Lists,
   however, this could require relying parties to redownload the entire
   list virtually every time they process a call.  Moreover, some
   certificate holders may be reluctant to share the entire list of
   telephone numbers associated with a certificate in cases where a
   relying party only needs to know, effectively, whether a single
   number (the calling party number for a particular call) is in the
   scope of authority for a certificate or not.  This document explores
   approaches to real-time status information for such certificates, and
   recommends an approach.

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.  Overview of Certificate Verification Methods

   For traditional certificate status information, there are three
   common certificate verification mechanisms employed by CAs:

   1.  Certificate Revocation Lists (CRLs) [RFC5280] (and [RFC6818])



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   2.  Online Certificate Status Protocol (OCSP) [RFC6960], and

   3.  Server-based Certificate Validation Protocol (SCVP) [RFC5055].

   Verifiers relying on status information need a way to obtain it -
   that is, where to locate it.  Placing the location of the status
   information in the certificate makes the certificate larger, but it
   eases the client workload.  The CRL Distribution Point certificate
   extension includes the location of the CRL and the Authority
   Information Access certificate extension includes the location of
   OCSP and/or SCVP servers; both of these extensions are defined in
   [RFC5280].  In all cases, the status information location is provided
   in the form of an URI.

   CRLs are an attractive solution because they are supported by
   traditional web PKI environments.  That said, CRLs have a reputation
   of being quite large (10s of MBytes), because CAs maintain and issue
   one monolithic CRL with all of their revoked certificates.  CRLs do
   support a variety of scoping mechanisms to reduce their size: based
   on revocation reasons (e.g., key compromise vs CA compromise), user
   certificates only, and CA certificates only as well as just
   operationally deciding to keep the CRLs small.  However, scoping the
   CRL introduces other issues, as it can result in multiple sections of
   CRLs called partitions, and consequently the relying parties would
   need access to all of the CRL partitions.  In practice, CRLs are
   widely used in STIR environments, often through a federated approach
   where a community of trusted CAs pool their CRLs for distribution
   from a central point.

   CAs in the STIR architecture thus have already implemented CRLs,
   largely for audit purposes rather than real-time status information.
   The need for these CRLs is not likely to go away, especially for the
   case of service providers whose certificates are based on Service
   Provider Codes (SPCs).  For delegate STIR certificates ([RFC9060]),
   however, especially those with TN Authorization Lists based on
   telephone numbers, OCSP may provide important optimizations.  Between
   the OCSP and SCVP, OCSP is much more widely deployed and in this
   document it is therefore RECOMMENDED to use OCSP in high-volume
   environments (HVE) for validating the freshness of telephone-number
   based certificates, based on [RFC6960], incorporating some (but not
   all) of the optimizations of [RFC5019].

   Like most PKIX-developed protocols, OCSP is extensible; OCSP supports
   request extensions (including sending multiple requests at once) and
   per-request extensions.  As the relying party in STIR validates a
   PASSporT associated with a telephone call, it is unlikely that the
   verifier will request authorization checks on multiple telephone
   numbers in one request, so a per-request extension is what is needed.



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   OCSP requires an additional round-trip request and response from the
   verification service to the OCSP responder, and the telephony
   applications are delay sensitive.  Thus, this document also specifies
   a means to incorporate an OCSP staple into the PASSporT object below
   (in Section 5).

4.  Using OCSP with TN Authorization Lists

   Certificates compliant with this specification SHOULD include a URL
   [RFC3986] pointing to an OCSP service in the Authority Information
   Access (AIA) certificate extension, via the "id-ad-ocsp" accessMethod
   specified in [RFC5280].  This can appear in addition to, or as an
   alternative to, the "id-ad-stirTNList" accessMethod specified in
   [RFC8226].  It is RECOMMENDED that entities that issue certificates
   with the Telephone Number Authorization List certificate extension
   run an OCSP server for this purpose.  Baseline OCSP however supports
   only three possible response values: good, revoked, or unknown.
   Without some extension, OCSP would not indicate whether the
   certificate is authorized for a particular telephone number that the
   verifier is validating.

   Consulting OCSP in real time results in a network round-trip delay,
   which is something to consider because it will add to the call setup
   time.  OCSP server implementations commonly pre-generate responses,
   and to speed up HTTPS connections, servers often provide OCSP
   responses for each certificate in their hierarchy.  Such techniques
   can also be applied to optimizing OCSP for STIR.

4.1.  OCSP Extension Specification

   The extension mechanism for OCSP follows X.509 v3 certificate
   extensions, and thus requires an OID, a criticality flag, and ASN.1
   syntax as defined by the OID.  The criticality specified here is
   optional: per [RFC6960] Section 4.4, support for all OCSP extensions
   is optional.  If the OCSP server does not understand the requested
   extension, it will still provide the baseline validation of the
   certificate itself.  Moreover, in practical STIR deployments, the
   issuer of the certificate will set the accessLocation for the OCSP
   AIA extension to point to an OCSP service that supports this
   extension, so the risk of interoperability failure due to lack of
   support for this extension is minimal.

   The TNQuery OCSP extension is included as one of the request's
   singleRequestExtensions; it carries the telephone number for which
   the query is being performed, typically the telephone number in the
   "orig" field of a PASSporT being validated.  The TNQuery extension
   may also appear in the response's singleExtensions; when an OCSP
   server includes a telephone number in the response's



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   singleExtensions, this informs the client that the certificate is
   still valid for the number that appears in the TNQuery extension
   field.  If the TNQuery is absent from a response to a query
   containing a TNQuery in its singleRequestExtension, then the server
   is not able to validate that the number is still in the scope of
   authority of the certificate.

                    id-pkix-ocsp-stir-tn  OBJECT IDENTIFIER ::= { id-pkix-ocsp 10 }

                    TNQuery ::= TelephoneNumber

   The High-Volume Environment (HVE) OCSP profile [RFC5019] prohibits
   the use of per-request extensions.  As it is anticipated that STIR
   will use OCSP in a high-volume environment, many of the optimizations
   recommended by HVE are desirable for the STIR environment.  This
   document therefore uses the HVE optimizations augmented as follows:

   *  Implementations MUST use SHA-256 as the hashing algorithm for the
      CertID.issuerNameHash and the CertID.issuerKeyHash values.  That
      is CertID.hashAlgorithm is id-sha256 [RFC4055].

   *  Clients MUST include the OCSP TNQuery extension in requests'
      singleRequestExtensions.

   *  Servers MUST include the OCSP TNQuery extension in responses'
      singleExtensions.

   *  Servers SHOULD return responses that would otherwise have been
      "unknown" as "not good" (i.e., return only "good" and "not good"
      responses).

   *  Clients MUST treat returned "unknown" responses as "not good".

   *  If the server uses ResponderID, it MUST generate the KeyHash using
      SHA-256.

   *  Implementations MUST support ECDSA using P-256 and SHA-256.  Note
      that [RFC6960] requires RSA with SHA-256 be supported.

   *  The ECDSA support above removes the requirement to support SHA-1,
      RSA with SHA-1, or DSA with SHA-1.

   OCSP responses MUST be signed using the same algorithm as the
   certificate being checked.







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   To facilitate matching the authority key identifier values found in
   CA certificates with the KeyHash used in the OCSP response,
   certificates compliant with this specification MUST generate
   authority key identifiers and subject key identifiers using the SHA-
   256.

   Ideally, once a certificate has been acquired by a verifier, some
   sort of asynchronous mechanism could notify and update the verifier
   if the scope of the certificate changes so that verifiers could
   implement a cache.  While not all possible categories of verifiers
   could implement such behavior, some sort of event-driven notification
   of certificate status is another potential subject of future work.
   One potential direction is that a future SIP SUBSCRIBE/NOTIFY-based
   accessMethod for AIA might be defined (which would also be applicable
   to the method described in the following section) by some future
   specification.

4.2.  Example OCSP Request

   OCSP Request: PEM:

   MIGjMIGgMF0wWzBZMA0GCWCGSAFlAwQCAQUABCCdRGd1m8TsykXHpoWP+cRdO4E2
   6WxG1ImeNnW+W+QcUgQg1OQi1Ss3Hf9J6kAZpKnfmm77CUVHUbmhh7NioCytJW4C
   BDXe9M+iPzA9MB8GCSsGAQUFBzABAgQSBBBjdJOiIW9EKJGELNNf/rdAMBoGCSsG
   AQUFBzABCgQNFgsxMjAyNTU1MTIxMg==


4.3.  Example OCSP Response

   OCSP Response: PEM:





















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   MIIDbQoBAKCCA2YwggNiBgkrBgEFBQcwAQEEggNTMIIDTzCB9aIWBBQ8T5f++IIw
   3WyQrjVceyyHwm0EuxgPMjAyNDA2MTgwNTA5MDBaMIGkMIGhMFkwDQYJYIZIAWUD
   BAIBBQAEIJ1EZ3WbxOzKRcemhY/5xF07gTbpbEbUiZ42db5b5BxSBCDU5CLVKzcd
   /0nqQBmkqd+abvsJRUdRuaGHs2KgLK0lbgIENd70z4IAGA8yMDI0MDYxODA4MDAw
   MFqgERgPMjAyNDA2MjAwODAwMDBaoR4wHDAaBgkrBgEFBQcwAQoEDRYLMTIwMjU1
   NTEyMTKhIzAhMB8GCSsGAQUFBzABAgQSBBBjdJOiIW9EKJGELNNf/rdAMAoGCCqG
   SM49BAMCA0gAMEUCIQCsV623IDsTwo1upzfTgR0D4HPI3906jgZYSKbq1dR18wIg
   IX6iW1BOReyZqx5R3NE1MDag6aJeMILyxwxrgm127BWgggH9MIIB+TCCAfUwggGb
   oAMCAQICFH90o/wDbOIUeFxZYU5vjfJMR6hzMAoGCCqGSM49BAMCMDcxCzAJBgNV
   BAYTAlVTMRMwEQYDVQQKEwpFeGFtcGxlIENBMRMwEQYDVQQDEwpjYS5leGFtcGxl
   MB4XDTI0MDMyNDA3Mzg1NFoXDTI1MDMyNDA3Mzg1NFowNzELMAkGA1UEBhMCVVMx
   EzARBgNVBAoTCkV4YW1wbGUgQ0ExEzARBgNVBAMTCmNhLmV4YW1wbGUwWTATBgcq
   hkjOPQIBBggqhkjOPQMBBwNCAASYMgUfqSdYEO6RxLSaas3wbW5Qpul5g5gqOuwP
   ZoKsWqCyHiiYE9ZNY5N0jYgqFiDvpmJgSbMWAITWLcAdUUp8o4GEMIGBMAwGA1Ud
   EwEB/wQCMAAwCwYDVR0PBAQDAgeAMB0GA1UdDgQWBBQ8T5f++IIw3WyQrjVceyyH
   wm0EuzAfBgNVHSMEGDAWgBQuz95oyHOEmDVVEJcTmKw3WepikDATBgNVHSUEDDAK
   BggrBgEFBQcDCTAPBgkrBgEFBQcwAQUEAgUAMAoGCCqGSM49BAMCA0gAMEUCIGGq
   VQ+e5BXYumtngE6GWVM6LN35PW2cKCcWQWB0lf7zAiEA8YXYz4joTWKfLwmL/G9q
   IH59+KaeWOQ7DDfDgGQi2vI=

4.4.  STIR Certification Authorities and OCSP

   In a STIR deployment, certification authorities will typically be the
   entities that operate OCSP servers.  Ultimately, the OCSP response
   MUST be signed by a CA in the certification chain of the end entitiy
   certificate that signed the PASSporT being verified.  In the case of
   multilevel certificate delegation (i.e. [RFC9060]), this means the
   OCSP response may be signed by any of the parent "encompassing"
   certificates of the end entity delegate certificate in question.

5.  OCSP Stapling

   In order to eliminate the need for verification services to query
   OCSP servers and thus incur a round trip time, this document defines
   OCSP stapling for STIR.  The approach to OCSP stapling specified here
   is that the authentication service inserts a new PASSporT payload
   element, "stpl", which has as its value an OCSP staple compliant with
   the TNQuery OCSP extension defined in Section 4.1.  Such staples can
   either be pre-generated ([RFC6960] Section 2.5) and published
   regularly to the authentication service, or the authentication
   service can query for a staple on a per-call basis.  Note that OCSP
   for STIR does furnish a response concerning only a single telephone
   number, and thus if a certificate can sign for a large number range,
   one pre-generated staple would need to be furnished to the
   authentication service for each telephone number that could
   potentially originate a call.  Generating OCSP staples on the fly may
   however cause a round-trip time delay of its own, which depending on
   how the authentication service and the certificate authority are



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   connected, could effectively incur the same delay as an OCSP dip from
   the verification service.  Some stapling-related design
   considerations are given in Appendix C.

   The header of a PASSporT with an OCSP staple follows baseline
   [RFC8225]; no new PASSporT Type is required for transmission of
   staples.

                           { "typ":"passport",
                           "alg":"ES256",
                           "x5u":"https://www.example.com/cert.cer" }

   The payload of the PASSporT contains a new payload claim for "stpl".
   This is a base64 encoded representation of an OCSP response that the
   STIR authentication service receives from a CA, either asynchronously
   (prefetched) or synchronously after querying the CA when a call
   signed by the certificate in the "x5u" value specified in the header
   has arrived.

   { "orig":{"tn":"12155551212"},
     "dest":{"tn":["12155551214"]},
     "iat":1443208345,
         "stpl":"MIIDbQoBAKCCA2YwggNiBgkrBgEFBQcwAQEEggNTMIIDTzCB9aIWBBQ8T5f++IIw
        3WyQrjVceyyHwm0EuxgPMjAyNDA2MTgwNTA5MDBaMIGkMIGhMFkwDQYJYIZIAWUD
        BAIBBQAEIJ1EZ3WbxOzKRcemhY/5xF07gTbpbEbUiZ42db5b5BxSBCDU5CLVKzcd
        /0nqQBmkqd+abvsJRUdRuaGHs2KgLK0lbgIENd70z4IAGA8yMDI0MDYxODA4MDAw
        MFqgERgPMjAyNDA2MjAwODAwMDBaoR4wHDAaBgkrBgEFBQcwAQoEDRYLMTIwMjU1
        NTEyMTKhIzAhMB8GCSsGAQUFBzABAgQSBBBjdJOiIW9EKJGELNNf/rdAMAoGCCqG
        SM49BAMCA0gAMEUCIQCsV623IDsTwo1upzfTgR0D4HPI3906jgZYSKbq1dR18wIg
        IX6iW1BOReyZqx5R3NE1MDag6aJeMILyxwxrgm127BWgggH9MIIB+TCCAfUwggGb
        oAMCAQICFH90o/wDbOIUeFxZYU5vjfJMR6hzMAoGCCqGSM49BAMCMDcxCzAJBgNV
        BAYTAlVTMRMwEQYDVQQKEwpFeGFtcGxlIENBMRMwEQYDVQQDEwpjYS5leGFtcGxl
        MB4XDTI0MDMyNDA3Mzg1NFoXDTI1MDMyNDA3Mzg1NFowNzELMAkGA1UEBhMCVVMx
        EzARBgNVBAoTCkV4YW1wbGUgQ0ExEzARBgNVBAMTCmNhLmV4YW1wbGUwWTATBgcq
        hkjOPQIBBggqhkjOPQMBBwNCAASYMgUfqSdYEO6RxLSaas3wbW5Qpul5g5gqOuwP
        ZoKsWqCyHiiYE9ZNY5N0jYgqFiDvpmJgSbMWAITWLcAdUUp8o4GEMIGBMAwGA1Ud
        EwEB/wQCMAAwCwYDVR0PBAQDAgeAMB0GA1UdDgQWBBQ8T5f++IIw3WyQrjVceyyH
        wm0EuzAfBgNVHSMEGDAWgBQuz95oyHOEmDVVEJcTmKw3WepikDATBgNVHSUEDDAK
        BggrBgEFBQcDCTAPBgkrBgEFBQcwAQUEAgUAMAoGCCqGSM49BAMCA0gAMEUCIGGq
        VQ+e5BXYumtngE6GWVM6LN35PW2cKCcWQWB0lf7zAiEA8YXYz4joTWKfLwmL/G9q
        IH59+KaeWOQ7DDfDgGQi2vI="
         }

6.  IANA Considerations







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6.1.  TN-HVE OCSP Extension

   This document makes use of object identifiers for the TNQuery OCSP
   extension in Section 4.1 and the ASN.1 module identifier defined in
   Appendix A.  It therefore requests that the IANA make the following
   assignments:

   TN-OCSP-Module-2026 OID in the SMI Security for PKIX Module
   Identifier registry: https://www.iana.org/assignments/smi-numbers/
   smi- numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.0

   TNQuery OCSP extension in the SMI Security for PKIX Online
   Certificate Status Protocol (OCSP) registry: 1.3.6.1.5.5.7.48.1.10.

6.2.  'stpl' JSON Web Token Claim

   This specification requests that the IANA add one new claim to the
   JSON Web Token Claims registry as defined in [RFC7519].

   Claim Name: "stpl"

   Claim Description: OCSP Staple

   Change Controller: IETF

   Specification Document(s): [RFCThis]

7.  Privacy Considerations

   Querying for real-time status information about certificates can
   allow parties monitoring communications to gather information about
   relying parties and the originators of communications.
   Unfortunately, the TNQuery extension adds a new field that could
   potentially be monitored by OCSP eavesdroppers: the calling telephone
   number provides a specific piece of additional data about the
   originator of communications.  Using OCSP over TLS is one potential
   countermeasure to this threat, as described in [RFC6960]
   Appendix A.1.













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   Preventing eavesdropping reduces one potential privacy leak, though
   of course using OCSP reveals to the OCSP service (likely acting for
   the certification authority) the verification service where calls
   from a given telephone number are terminating.  Bear in mind that
   STIR assumes that verification services use HTTPS to acquire
   certificates (by referencing the "x5u" field of the PASSporT)
   already, so some connection between the verification service and a
   certificate repository (likely acting for the certification authority
   or authentication service) is unavoidable.  This OCSP extension
   further reveals the calling telephone number as it arrives at the
   verification service to the OCSP service.

   One way to mitigate leaking information about relying parties is to
   use OCSP stapling (see Section 5).

8.  Security Considerations

   This document is entirely about security.  See the Privacy
   Considerations (Section 7) for guidance for OCSP service operators on
   preventing leakage of information relating to certificates and
   subscriber activities.  For further information on certificate
   security and practices, see [RFC5280], in particular its Security
   Considerations.  For OCSP-related security considerations see
   [RFC6960] and [RFC5019].

   Any ecosystem dependent on real-time certificate status information
   will be susceptible to denial-of-service attacks aimed at OCSP
   service operators.  OCSP stapling helps to mitigate this
   vulnerability, as staples need not be acquired during call
   processing, but all the same the lack of availability of the service
   could still obstruct call processing in the longer term.

9.  Acknowledgments

   Stephen Farrell provided key input to the discussions leading to this
   document.  Russ Housley provided some direct assistance and text
   surrounding the ASN.1 module, and with the OCSP request and staple
   example.

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>.




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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>.

   [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>.

   [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
              Algorithms and Identifiers for RSA Cryptography for use in
              the Internet X.509 Public Key Infrastructure Certificate
              and Certificate Revocation List (CRL) Profile", RFC 4055,
              DOI 10.17487/RFC4055, June 2005,
              <https://www.rfc-editor.org/info/rfc4055>.

   [RFC5019]  Deacon, A. and R. Hurst, "The Lightweight Online
              Certificate Status Protocol (OCSP) Profile for High-Volume
              Environments", RFC 5019, DOI 10.17487/RFC5019, September
              2007, <https://www.rfc-editor.org/info/rfc5019>.

   [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>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC6818]  Yee, P., "Updates to the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
              2013, <https://www.rfc-editor.org/info/rfc6818>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.



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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>.

   [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>.

   [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>.

   [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






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   [I-D.ietf-stir-certificates-shortlived]
              Peterson, J., "Short-Lived Certificates for Secure
              Telephone Identity", Work in Progress, Internet-Draft,
              draft-ietf-stir-certificates-shortlived-04, 4 November
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              stir-certificates-shortlived-04>.

   [I-D.ietf-tls-rfc8446bis]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, Internet-Draft, draft-
              ietf-tls-rfc8446bis-14, 13 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              rfc8446bis-14>.

   [RFC5055]  Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
              Polk, "Server-Based Certificate Validation Protocol
              (SCVP)", RFC 5055, DOI 10.17487/RFC5055, December 2007,
              <https://www.rfc-editor.org/info/rfc5055>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <https://www.rfc-editor.org/info/rfc6961>.

   [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>.

Appendix A.  ASN.1 Module

   This appendix provides the normative ASN.1 [X.680] definitions for
   the structures described in this specification using ASN.1, as
   defined in [X.680] through [X.683].

   The modules defined in this document are compatible with the most
   current ASN.1 specification published in 2015 (see [X.680], [X.681],
   [X.682], [X.683]).  None of the newly defined tokens in the 2008
   ASN.1 (DATE, DATE-TIME, DURATION, NOT-A-NUMBER, OID-IRI, RELATIVE-
   OID-IRI, TIME, TIME-OF-DAY)) are currently used in any of the ASN.1
   specifications referred to here.

   This ASN.1 module imports ASN.1 from [RFC5912] and [RFC8226].








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 TN-OCSP-Module-2026
    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-tn-ocsp-module-2026(TBD) }

  DEFINITIONS EXPLICIT TAGS ::= BEGIN

  IMPORTS

  id-ad-ocsp
    FROM PKIX1Explicit-2009  -- From RFC 5912
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) }

  EXTENSION
    FROM PKIX-CommonTypes-2009  -- From RFC 5912
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57) }

  TelephoneNumber
    FROM TN-Module-2016  -- From RFC 8226
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-tn-module(89) }
  ;

  id-pkix-ocsp OBJECT IDENTIFIER ::= id-ad-ocsp

  --
  -- TNQuery OCSP extension
  --

  ext-ocsp-tn-query  EXTENSION ::= {
    SYNTAX TNQuery IDENTIFIED BY id-pkix-ocsp-stir-tn }

  TNQuery ::= TelephoneNumber

  id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp 10 }

  END


Appendix B.  OCSP Request and Response Breakdown

   The following two examples show the OCSP Request and Response with
   and without nonces.  In cases where OCSP Responses are pre-generated
   for staples, nonces are not used.

   First, an example with the nonce.



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  OCSP Request:
  MIGjMIGgMF0wWzBZMA0GCWCGSAFlAwQCAQUABCCdRGd1m8TsykXHpoWP+cRdO4E2
  6WxG1ImeNnW+W+QcUgQg1OQi1Ss3Hf9J6kAZpKnfmm77CUVHUbmhh7NioCytJW4C
  BDXe9M+iPzA9MB8GCSsGAQUFBzABAgQSBBBjdJOiIW9EKJGELNNf/rdAMBoGCSsG
  AQUFBzABCgQNFgsxMjAyNTU1MTIxMg==

    0 163: SEQUENCE {
    3 160:   SEQUENCE {
    6  93:     SEQUENCE {
    8  91:       SEQUENCE {
   10  89:         SEQUENCE {
   12  13:           SEQUENCE {
   14   9:             OBJECT IDENTIFIER
         :               sha-256 (2 16 840 1 101 3 4 2 1)
   25   0:             NULL
         :             }
   27  32:           OCTET STRING
         :             9D 44 67 75 9B C4 EC CA 45 C7 A6 85 8F F9 C4 5D
         :             3B 81 36 E9 6C 46 D4 89 9E 36 75 BE 5B E4 1C 52
   61  32:           OCTET STRING
         :             D4 E4 22 D5 2B 37 1D FF 49 EA 40 19 A4 A9 DF 9A
         :             6E FB 09 45 47 51 B9 A1 87 B3 62 A0 2C AD 25 6E
   95   4:           INTEGER 903804111
         :           }
         :         }
         :       }
  101  63:     [2] {
  103  61:       SEQUENCE {
  105  31:         SEQUENCE {
  107   9:           OBJECT IDENTIFIER ocspNonce (1 3 6 1 5 5 7 48 1 2)
  118  18:           OCTET STRING, encapsulates {
  120  16:             OCTET STRING
         :             63 74 93 A2 21 6F 44 28 91 84 2C D3 5F FE B7 40
         :             }
         :           }
  138  26:         SEQUENCE {
  140   9:           OBJECT IDENTIFIER
         :             ocspStirTN (1 3 6 1 5 5 7 48 1 10)
  151  13:           OCTET STRING, encapsulates {
  153  11:             IA5String '12025551212'
         :             }
         :           }
         :         }
         :       }
         :     }
         :   }





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  OCSP Response:
  MIIDbQoBAKCCA2YwggNiBgkrBgEFBQcwAQEEggNTMIIDTzCB9aIWBBQ8T5f++IIw
  3WyQrjVceyyHwm0EuxgPMjAyNDA2MTgwNTA5MDBaMIGkMIGhMFkwDQYJYIZIAWUD
  BAIBBQAEIJ1EZ3WbxOzKRcemhY/5xF07gTbpbEbUiZ42db5b5BxSBCDU5CLVKzcd
  /0nqQBmkqd+abvsJRUdRuaGHs2KgLK0lbgIENd70z4IAGA8yMDI0MDYxODA4MDAw
  MFqgERgPMjAyNDA2MjAwODAwMDBaoR4wHDAaBgkrBgEFBQcwAQoEDRYLMTIwMjU1
  NTEyMTKhIzAhMB8GCSsGAQUFBzABAgQSBBBjdJOiIW9EKJGELNNf/rdAMAoGCCqG
  SM49BAMCA0gAMEUCIQCsV623IDsTwo1upzfTgR0D4HPI3906jgZYSKbq1dR18wIg
  IX6iW1BOReyZqx5R3NE1MDag6aJeMILyxwxrgm127BWgggH9MIIB+TCCAfUwggGb
  oAMCAQICFH90o/wDbOIUeFxZYU5vjfJMR6hzMAoGCCqGSM49BAMCMDcxCzAJBgNV
  BAYTAlVTMRMwEQYDVQQKEwpFeGFtcGxlIENBMRMwEQYDVQQDEwpjYS5leGFtcGxl
  MB4XDTI0MDMyNDA3Mzg1NFoXDTI1MDMyNDA3Mzg1NFowNzELMAkGA1UEBhMCVVMx
  EzARBgNVBAoTCkV4YW1wbGUgQ0ExEzARBgNVBAMTCmNhLmV4YW1wbGUwWTATBgcq
  hkjOPQIBBggqhkjOPQMBBwNCAASYMgUfqSdYEO6RxLSaas3wbW5Qpul5g5gqOuwP
  ZoKsWqCyHiiYE9ZNY5N0jYgqFiDvpmJgSbMWAITWLcAdUUp8o4GEMIGBMAwGA1Ud
  EwEB/wQCMAAwCwYDVR0PBAQDAgeAMB0GA1UdDgQWBBQ8T5f++IIw3WyQrjVceyyH
  wm0EuzAfBgNVHSMEGDAWgBQuz95oyHOEmDVVEJcTmKw3WepikDATBgNVHSUEDDAK
  BggrBgEFBQcDCTAPBgkrBgEFBQcwAQUEAgUAMAoGCCqGSM49BAMCA0gAMEUCIGGq
  VQ+e5BXYumtngE6GWVM6LN35PW2cKCcWQWB0lf7zAiEA8YXYz4joTWKfLwmL/G9q
  IH59+KaeWOQ7DDfDgGQi2vI=

    0 877: SEQUENCE {
    4   1:   ENUMERATED 0
    7 870:   [0] {
   11 866:     SEQUENCE {
   15   9:       OBJECT IDENTIFIER ocspBasic (1 3 6 1 5 5 7 48 1 1)
   26 851:       OCTET STRING, encapsulates {
   30 847:         SEQUENCE {
   34 245:           SEQUENCE {
   37  22:             [2] {
   39  20:               OCTET STRING
         :             3C 4F 97 FE F8 82 30 DD 6C 90 AE 35 5C 7B 2C 87
         :             C2 6D 04 BB
         :               }
   61  15:             GeneralizedTime 18/06/2024 05:09:00 GMT
   78 164:             SEQUENCE {
   81 161:               SEQUENCE {
   84  89:                 SEQUENCE {
   86  13:                   SEQUENCE {
   88   9:                     OBJECT IDENTIFIER
         :                       sha-256 (2 16 840 1 101 3 4 2 1)
   99   0:                     NULL
         :                     }
  101  32:                   OCTET STRING
         :             9D 44 67 75 9B C4 EC CA 45 C7 A6 85 8F F9 C4 5D
         :             3B 81 36 E9 6C 46 D4 89 9E 36 75 BE 5B E4 1C 52
  135  32:                   OCTET STRING
         :             D4 E4 22 D5 2B 37 1D FF 49 EA 40 19 A4 A9 DF 9A



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         :             6E FB 09 45 47 51 B9 A1 87 B3 62 A0 2C AD 25 6E
  169   4:                   INTEGER 903804111
         :                   }
  175   0:                 [2]
  177  15:                 GeneralizedTime 18/06/2024 08:00:00 GMT
  194  17:                 [0] {
  196  15:                   GeneralizedTime 20/06/2024 08:00:00 GMT
         :                   }
  213  30:                 [1] {
  215  28:                   SEQUENCE {
  217  26:                     SEQUENCE {
  219   9:                       OBJECT IDENTIFIER
         :                         ocspStirTN (1 3 6 1 5 5 7 48 1 10)
  230  13:                       OCTET STRING, encapsulates {
  232  11:                         IA5String '12025551212'
         :                         }
         :                       }
         :                     }
         :                   }
         :                 }
         :               }
  245  35:             [1] {
  247  33:               SEQUENCE {
  249  31:                 SEQUENCE {
  251   9:                   OBJECT IDENTIFIER
         :                     ocspNonce (1 3 6 1 5 5 7 48 1 2)
  262  18:                   OCTET STRING, encapsulates {
  264  16:                     OCTET STRING
         :             63 74 93 A2 21 6F 44 28 91 84 2C D3 5F FE B7 40
         :                     }
         :                   }
         :                 }
         :               }
         :             }
  282  10:           SEQUENCE {
  284   8:             OBJECT IDENTIFIER
         :               ecdsaWithSHA256 (1 2 840 10045 4 3 2)
         :             }
  294  72:           BIT STRING, encapsulates {
  297  69:             SEQUENCE {
  299  33:               INTEGER
         :             00 AC 57 AD B7 20 3B 13 C2 8D 6E A7 37 D3 81 1D
         :             03 E0 73 C8 DF DD 3A 8E 06 58 48 A6 EA D5 D4 75
         :             F3
  334  32:               INTEGER
         :             21 7E A2 5B 50 4E 45 EC 99 AB 1E 51 DC D1 35 30
         :             36 A0 E9 A2 5E 30 82 F2 C7 0C 6B 82 6D 76 EC 15
         :               }



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         :             }
  368 509:           [0] {
  372 505:             SEQUENCE {
  376 501:               SEQUENCE {
  380 411:                 SEQUENCE {
  384   3:                   [0] {
  386   1:                     INTEGER 2
         :                     }
  389  20:                   INTEGER
         :             7F 74 A3 FC 03 6C E2 14 78 5C 59 61 4E 6F 8D F2
         :             4C 47 A8 73
  411  10:                   SEQUENCE {
  413   8:                     OBJECT IDENTIFIER
         :                       ecdsaWithSHA256 (1 2 840 10045 4 3 2)
         :                     }
  423  55:                   SEQUENCE {
  425  11:                     SET {
  427   9:                       SEQUENCE {
  429   3:                         OBJECT IDENTIFIER
         :                           countryName (2 5 4 6)
  434   2:                         PrintableString 'US'
         :                         }
         :                       }
  438  19:                     SET {
  440  17:                       SEQUENCE {
  442   3:                         OBJECT IDENTIFIER
         :                           organizationName (2 5 4 10)
  447  10:                         PrintableString 'Example CA'
         :                         }
         :                       }
  459  19:                     SET {
  461  17:                       SEQUENCE {
  463   3:                         OBJECT IDENTIFIER
         :                           commonName (2 5 4 3)
  468  10:                         PrintableString 'ca.example'
         :                         }
         :                       }
         :                     }
  480  30:                   SEQUENCE {
  482  13:                     UTCTime 24/03/2024 07:38:54 GMT
  497  13:                     UTCTime 24/03/2025 07:38:54 GMT
         :                     }
  512  55:                   SEQUENCE {
  514  11:                     SET {
  516   9:                       SEQUENCE {
  518   3:                         OBJECT IDENTIFIER
         :                           countryName (2 5 4 6)
  523   2:                         PrintableString 'US'



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         :                         }
         :                       }
  527  19:                     SET {
  529  17:                       SEQUENCE {
  531   3:                         OBJECT IDENTIFIER
         :                           organizationName (2 5 4 10)
  536  10:                         PrintableString 'Example CA'
         :                         }
         :                       }
  548  19:                     SET {
  550  17:                       SEQUENCE {
  552   3:                         OBJECT IDENTIFIER
         :                           commonName (2 5 4 3)
  557  10:                         PrintableString 'ca.example'
         :                         }
         :                       }
         :                     }
  569  89:                   SEQUENCE {
  571  19:                     SEQUENCE {
  573   7:                       OBJECT IDENTIFIER
         :                         ecPublicKey (1 2 840 10045 2 1)
  582   8:                       OBJECT IDENTIFIER
         :                         prime256v1 (1 2 840 10045 3 1 7)
         :                       }
  592  66:                     BIT STRING
         :             04 98 32 05 1F A9 27 58 10 EE 91 C4 B4 9A 6A CD
         :             F0 6D 6E 50 A6 E9 79 83 98 2A 3A EC 0F 66 82 AC
         :             5A A0 B2 1E 28 98 13 D6 4D 63 93 74 8D 88 2A 16
         :             20 EF A6 62 60 49 B3 16 00 84 D6 2D C0 1D 51 4A
         :             7C
         :                     }
  660 132:                   [3] {
  663 129:                     SEQUENCE {
  666  12:                       SEQUENCE {
  668   3:                         OBJECT IDENTIFIER
         :                           basicConstraints (2 5 29 19)
  673   1:                         BOOLEAN TRUE
  676   2:                         OCTET STRING, encapsulates {
  678   0:                           SEQUENCE {}
         :                           }
         :                         }
  680  11:                       SEQUENCE {
  682   3:                         OBJECT IDENTIFIER
         :                           keyUsage (2 5 29 15)
  687   4:                         OCTET STRING, encapsulates {
  689   2:                           BIT STRING 7 unused bits
         :                             '1'B (bit 0)
         :                           }



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         :                         }
  693  29:                       SEQUENCE {
  695   3:                         OBJECT IDENTIFIER
         :                           subjectKeyIdentifier (2 5 29 14)
  700  22:                         OCTET STRING, encapsulates {
  702  20:                           OCTET STRING
         :             3C 4F 97 FE F8 82 30 DD 6C 90 AE 35 5C 7B 2C 87
         :             C2 6D 04 BB
         :                           }
         :                         }
  724  31:                       SEQUENCE {
  726   3:                         OBJECT IDENTIFIER
         :                           authorityKeyIdentifier (2 5 29 35)
  731  24:                         OCTET STRING, encapsulates {
  733  22:                           SEQUENCE {
  735  20:                             [0]
         :             2E CF DE 68 C8 73 84 98 35 55 10 97 13 98 AC 37
         :             59 EA 62 90
         :                             }
         :                           }
         :                         }
  757  19:                       SEQUENCE {
  759   3:                         OBJECT IDENTIFIER
         :                           extKeyUsage (2 5 29 37)
  764  12:                         OCTET STRING, encapsulates {
  766  10:                           SEQUENCE {
  768   8:                             OBJECT IDENTIFIER
         :                               ocspSigning (1 3 6 1 5 5 7 3 9)
         :                             }
         :                           }
         :                         }
  778  15:                       SEQUENCE {
  780   9:                         OBJECT IDENTIFIER
         :                           ocspNoCheck (1 3 6 1 5 5 7 48 1 5)
  791   2:                         OCTET STRING, encapsulates {
  793   0:                           NULL
         :                           }
         :                         }
         :                       }
         :                     }
         :                   }
  795  10:                 SEQUENCE {
  797   8:                   OBJECT IDENTIFIER
         :                     ecdsaWithSHA256 (1 2 840 10045 4 3 2)
         :                   }
  807  72:                 BIT STRING, encapsulates {
  810  69:                   SEQUENCE {
  812  32:                     INTEGER



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         :             61 AA 55 0F 9E E4 15 D8 BA 6B 67 80 4E 86 59 53
         :             3A 2C DD F9 3D 6D 9C 28 27 16 41 60 74 95 FE F3
  846  33:                     INTEGER
         :             00 F1 85 D8 CF 88 E8 4D 62 9F 2F 09 8B FC 6F 6A
         :             20 7E 7D F8 A6 9E 58 E4 3B 0C 37 C3 80 64 22 DA
         :             F2
         :                     }
         :                   }
         :                 }
         :               }
         :             }
         :           }
         :         }
         :       }
         :     }
         :   }

   Second, an example without the nonce.

  OCSP Request:
  MIGBMH8wXTBbMFkwDQYJYIZIAWUDBAIBBQAEIJ1EZ3WbxOzKRcemhY/5xF07gTbp
  bEbUiZ42db5b5BxSBCDU5CLVKzcd/0nqQBmkqd+abvsJRUdRuaGHs2KgLK0lbgIE
  Nd70z6IeMBwwGgYJKwYBBQUHMAEKBA0WCzEyMDI1NTUxMjEy

    0 129: SEQUENCE {
    3 127:   SEQUENCE {
    5  93:     SEQUENCE {
    7  91:       SEQUENCE {
    9  89:         SEQUENCE {
   11  13:           SEQUENCE {
   13   9:             OBJECT IDENTIFIER
         :               sha-256 (2 16 840 1 101 3 4 2 1)
   24   0:             NULL
         :             }
   26  32:           OCTET STRING
         :             9D 44 67 75 9B C4 EC CA 45 C7 A6 85 8F F9 C4 5D
         :             3B 81 36 E9 6C 46 D4 89 9E 36 75 BE 5B E4 1C 52
   60  32:           OCTET STRING
         :             D4 E4 22 D5 2B 37 1D FF 49 EA 40 19 A4 A9 DF 9A
         :             6E FB 09 45 47 51 B9 A1 87 B3 62 A0 2C AD 25 6E
   94   4:           INTEGER 903804111
         :           }
         :         }
         :       }
  100  30:     [2] {
  102  28:       SEQUENCE {
  104  26:         SEQUENCE {
  106   9:           OBJECT IDENTIFIER



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         :             ocspStirTN (1 3 6 1 5 5 7 48 1 10)
  117  13:           OCTET STRING, encapsulates {
  119  11:             IA5String '12025551212'
         :             }
         :           }
         :         }
         :       }
         :     }
         :   }


  OCSP Response:
  MIIDSAoBAKCCA0EwggM9BgkrBgEFBQcwAQEEggMuMIIDKjCB0KIWBBQ8T5f++IIw
  3WyQrjVceyyHwm0EuxgPMjAyNDA2MTgwNTA5MDBaMIGkMIGhMFkwDQYJYIZIAWUD
  BAIBBQAEIJ1EZ3WbxOzKRcemhY/5xF07gTbpbEbUiZ42db5b5BxSBCDU5CLVKzcd
  /0nqQBmkqd+abvsJRUdRuaGHs2KgLK0lbgIENd70z4IAGA8yMDI0MDYxODA4MDAw
  MFqgERgPMjAyNDA2MjAwODAwMDBaoR4wHDAaBgkrBgEFBQcwAQoEDRYLMTIwMjU1
  NTEyMTIwCgYIKoZIzj0EAwIDSAAwRQIhAKxXrbcgOxPCjW6nN9OBHQPgc8jf3TqO
  BlhIpurV1HXzAiAhfqJbUE5F7JmrHlHc0TUwNqDpol4wgvLHDGuCbXbsFaCCAf0w
  ggH5MIIB9TCCAZugAwIBAgIUf3Sj/ANs4hR4XFlhTm+N8kxHqHMwCgYIKoZIzj0E
  AwIwNzELMAkGA1UEBhMCVVMxEzARBgNVBAoTCkV4YW1wbGUgQ0ExEzARBgNVBAMT
  CmNhLmV4YW1wbGUwHhcNMjQwMzI0MDczODU0WhcNMjUwMzI0MDczODU0WjA3MQsw
  CQYDVQQGEwJVUzETMBEGA1UEChMKRXhhbXBsZSBDQTETMBEGA1UEAxMKY2EuZXhh
  bXBsZTBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABJgyBR+pJ1gQ7pHEtJpqzfBt
  blCm6XmDmCo67A9mgqxaoLIeKJgT1k1jk3SNiCoWIO+mYmBJsxYAhNYtwB1RSnyj
  gYQwgYEwDAYDVR0TAQH/BAIwADALBgNVHQ8EBAMCB4AwHQYDVR0OBBYEFDxPl/74
  gjDdbJCuNVx7LIfCbQS7MB8GA1UdIwQYMBaAFC7P3mjIc4SYNVUQlxOYrDdZ6mKQ
  MBMGA1UdJQQMMAoGCCsGAQUFBwMJMA8GCSsGAQUFBzABBQQCBQAwCgYIKoZIzj0E
  AwIDSAAwRQIgYapVD57kFdi6a2eAToZZUzos3fk9bZwoJxZBYHSV/vMCIQDxhdjP
  iOhNYp8vCYv8b2ogfn34pp5Y5DsMN8OAZCLa8g==

    0 840: SEQUENCE {
    4   1:   ENUMERATED 0
    7 833:   [0] {
   11 829:     SEQUENCE {
   15   9:       OBJECT IDENTIFIER ocspBasic (1 3 6 1 5 5 7 48 1 1)
   26 814:       OCTET STRING, encapsulates {
   30 810:         SEQUENCE {
   34 208:           SEQUENCE {
   37  22:             [2] {
   39  20:               OCTET STRING
         :             3C 4F 97 FE F8 82 30 DD 6C 90 AE 35 5C 7B 2C 87
         :             C2 6D 04 BB
         :               }
   61  15:             GeneralizedTime 18/06/2024 05:09:00 GMT
   78 164:             SEQUENCE {
   81 161:               SEQUENCE {
   84  89:                 SEQUENCE {



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   86  13:                   SEQUENCE {
   88   9:                     OBJECT IDENTIFIER
         :                       sha-256 (2 16 840 1 101 3 4 2 1)
   99   0:                     NULL
         :                     }
  101  32:                   OCTET STRING
         :             9D 44 67 75 9B C4 EC CA 45 C7 A6 85 8F F9 C4 5D
         :             3B 81 36 E9 6C 46 D4 89 9E 36 75 BE 5B E4 1C 52
  135  32:                   OCTET STRING
         :             D4 E4 22 D5 2B 37 1D FF 49 EA 40 19 A4 A9 DF 9A
         :             6E FB 09 45 47 51 B9 A1 87 B3 62 A0 2C AD 25 6E
  169   4:                   INTEGER 903804111
         :                   }
  175   0:                 [2]
  177  15:                 GeneralizedTime 18/06/2024 08:00:00 GMT
  194  17:                 [0] {
  196  15:                   GeneralizedTime 20/06/2024 08:00:00 GMT
         :                   }
  213  30:                 [1] {
  215  28:                   SEQUENCE {
  217  26:                     SEQUENCE {
  219   9:                       OBJECT IDENTIFIER
         :                         ocspStirTN (1 3 6 1 5 5 7 48 1 10)
  230  13:                       OCTET STRING, encapsulates {
  232  11:                         IA5String '12025551212'
         :                         }
         :                       }
         :                     }
         :                   }
         :                 }
         :               }
         :             }
  245  10:           SEQUENCE {
  247   8:             OBJECT IDENTIFIER
         :               ecdsaWithSHA256 (1 2 840 10045 4 3 2)
         :             }
  257  72:           BIT STRING, encapsulates {
  260  69:             SEQUENCE {
  262  33:               INTEGER
         :             00 AC 57 AD B7 20 3B 13 C2 8D 6E A7 37 D3 81 1D
         :             03 E0 73 C8 DF DD 3A 8E 06 58 48 A6 EA D5 D4 75
         :             F3
  297  32:               INTEGER
         :             21 7E A2 5B 50 4E 45 EC 99 AB 1E 51 DC D1 35 30
         :             36 A0 E9 A2 5E 30 82 F2 C7 0C 6B 82 6D 76 EC 15
         :               }
         :             }
  331 509:           [0] {



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  335 505:             SEQUENCE {
  339 501:               SEQUENCE {
  343 411:                 SEQUENCE {
  347   3:                   [0] {
  349   1:                     INTEGER 2
         :                     }
  352  20:                   INTEGER
         :             7F 74 A3 FC 03 6C E2 14 78 5C 59 61 4E 6F 8D F2
         :             4C 47 A8 73
  374  10:                   SEQUENCE {
  376   8:                     OBJECT IDENTIFIER
         :                       ecdsaWithSHA256 (1 2 840 10045 4 3 2)
         :                     }
  386  55:                   SEQUENCE {
  388  11:                     SET {
  390   9:                       SEQUENCE {
  392   3:                         OBJECT IDENTIFIER
         :                           countryName (2 5 4 6)
  397   2:                         PrintableString 'US'
         :                         }
         :                       }
  401  19:                     SET {
  403  17:                       SEQUENCE {
  405   3:                         OBJECT IDENTIFIER
         :                           organizationName (2 5 4 10)
  410  10:                         PrintableString 'Example CA'
         :                         }
         :                       }
  422  19:                     SET {
  424  17:                       SEQUENCE {
  426   3:                         OBJECT IDENTIFIER
         :                           commonName (2 5 4 3)
  431  10:                         PrintableString 'ca.example'
         :                         }
         :                       }
         :                     }
  443  30:                   SEQUENCE {
  445  13:                     UTCTime 24/03/2024 07:38:54 GMT
  460  13:                     UTCTime 24/03/2025 07:38:54 GMT
         :                     }
  475  55:                   SEQUENCE {
  477  11:                     SET {
  479   9:                       SEQUENCE {
  481   3:                         OBJECT IDENTIFIER
         :                           countryName (2 5 4 6)
  486   2:                         PrintableString 'US'
         :                         }
         :                       }



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  490  19:                     SET {
  492  17:                       SEQUENCE {
  494   3:                         OBJECT IDENTIFIER
         :                           organizationName (2 5 4 10)
  499  10:                         PrintableString 'Example CA'
         :                         }
         :                       }
  511  19:                     SET {
  513  17:                       SEQUENCE {
  515   3:                         OBJECT IDENTIFIER
         :                           commonName (2 5 4 3)
  520  10:                         PrintableString 'ca.example'
         :                         }
         :                       }
         :                     }
  532  89:                   SEQUENCE {
  534  19:                     SEQUENCE {
  536   7:                       OBJECT IDENTIFIER
         :                         ecPublicKey (1 2 840 10045 2 1)
  545   8:                       OBJECT IDENTIFIER
         :                         prime256v1 (1 2 840 10045 3 1 7)
         :                       }
  555  66:                     BIT STRING
         :             04 98 32 05 1F A9 27 58 10 EE 91 C4 B4 9A 6A CD
         :             F0 6D 6E 50 A6 E9 79 83 98 2A 3A EC 0F 66 82 AC
         :             5A A0 B2 1E 28 98 13 D6 4D 63 93 74 8D 88 2A 16
         :             20 EF A6 62 60 49 B3 16 00 84 D6 2D C0 1D 51 4A
         :             7C
         :                     }
  623 132:                   [3] {
  626 129:                     SEQUENCE {
  629  12:                       SEQUENCE {
  631   3:                         OBJECT IDENTIFIER
         :                           basicConstraints (2 5 29 19)
  636   1:                         BOOLEAN TRUE
  639   2:                         OCTET STRING, encapsulates {
  641   0:                           SEQUENCE {}
         :                           }
         :                         }
  643  11:                       SEQUENCE {
  645   3:                         OBJECT IDENTIFIER
         :                           keyUsage (2 5 29 15)
  650   4:                         OCTET STRING, encapsulates {
  652   2:                           BIT STRING 7 unused bits
         :                             '1'B (bit 0)
         :                           }
         :                         }
  656  29:                       SEQUENCE {



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  658   3:                         OBJECT IDENTIFIER
         :                           subjectKeyIdentifier (2 5 29 14)
  663  22:                         OCTET STRING, encapsulates {
  665  20:                           OCTET STRING
         :             3C 4F 97 FE F8 82 30 DD 6C 90 AE 35 5C 7B 2C 87
         :             C2 6D 04 BB
         :                           }
         :                         }
  687  31:                       SEQUENCE {
  689   3:                         OBJECT IDENTIFIER
         :                           authorityKeyIdentifier (2 5 29 35)
  694  24:                         OCTET STRING, encapsulates {
  696  22:                           SEQUENCE {
  698  20:                             [0]
         :             2E CF DE 68 C8 73 84 98 35 55 10 97 13 98 AC 37
         :             59 EA 62 90
         :                             }
         :                           }
         :                         }
  720  19:                       SEQUENCE {
  722   3:                         OBJECT IDENTIFIER
         :                           extKeyUsage (2 5 29 37)
  727  12:                         OCTET STRING, encapsulates {
  729  10:                           SEQUENCE {
  731   8:                             OBJECT IDENTIFIER
         :                               ocspSigning (1 3 6 1 5 5 7 3 9)
         :                             }
         :                           }
         :                         }
  741  15:                       SEQUENCE {
  743   9:                         OBJECT IDENTIFIER
         :                           ocspNoCheck (1 3 6 1 5 5 7 48 1 5)
  754   2:                         OCTET STRING, encapsulates {
  756   0:                           NULL
         :                           }
         :                         }
         :                       }
         :                     }
         :                   }
  758  10:                 SEQUENCE {
  760   8:                   OBJECT IDENTIFIER
         :                     ecdsaWithSHA256 (1 2 840 10045 4 3 2)
         :                   }
  770  72:                 BIT STRING, encapsulates {
  773  69:                   SEQUENCE {
  775  32:                     INTEGER
         :             61 AA 55 0F 9E E4 15 D8 BA 6B 67 80 4E 86 59 53
         :             3A 2C DD F9 3D 6D 9C 28 27 16 41 60 74 95 FE F3



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  809  33:                     INTEGER
         :             00 F1 85 D8 CF 88 E8 4D 62 9F 2F 09 8B FC 6F 6A
         :             20 7E 7D F8 A6 9E 58 E4 3B 0C 37 C3 80 64 22 DA
         :             F2
         :                     }
         :                   }
         :                 }
         :               }
         :             }
         :           }
         :         }
         :       }
         :     }
         :   }

Appendix C.  Alternative OCSP Staple Design Considerations

   At a high level, there are a number of potential approaches to
   stapling that could mitigate the round-trip time incurred on the
   verification service side to perform OCSP validation.

   A verification service validating a PASSporT acquires the certificate
   referenced by its "x5u" header element, if that certificate is not
   cached.  Typically, that acquisition happens by derefencing the URI
   in the value of the "x5u" element.  One could design an system where
   OCSP validation is piggybacked onto that network fetch.  This
   solution is however not optimal for cases where signing certificates
   are long-lived and cached, so that queries will otherwise be very
   infrequent.  Requiring certificate fetches every time a new telephone
   number is seen at the verification service would likely incur roughly
   the same number of round trips as the
   [I-D.ietf-stir-certificates-shortlived] mechanism.

   There are also variants of the "x5u" approach that sidestep OCSP
   entirely, by decorating the "x5u" URI with query parameters that
   incorporate the calling telephone number.  As the authentication
   service necessarily knows the telephone number from the "orig" field,
   and controls the contents of "x5u", it has the means to decorate the
   URI appropriately during PASSporT creation.  The certificate
   repository (i.e. HTTP service) receiving a certificate fetch with a
   decorated URI could then verify that the calling number is currently
   in the scope of the requested certificate - if it is not, the service
   could then fail to return a certificate, preventing the verification
   service from validating.  However, like the approach above, this
   would have implications for certificate fetch frequency similar to
   short-lived certs, as the decorated URIs would be governed by HTTP
   caching mechanics.




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   This specification carries an OCSP staple in a PASSporT payload
   element.  An alternative design would be to carry an OCSP staple in
   the SIP layer, in a body or header field value; see [RFC6961] or
   [I-D.ietf-tls-rfc8446bis].  But because PASSporT can be used in non-
   SIP environments, and the TNQuery OCSP extension is specific to
   certificates that use the TNAuthList extension, embedding the staple
   in the PASSporT is a superior choice.  While encoding and embedding
   an OCSP response will increase the size of the PASSporT, that overall
   increase in SIP message size will ideally be the same as if the
   response had been placed in a separate header field value.

   It could be argued that the round-trip delay incurred at the
   verification service is not actually problematic, as there is a
   fungible delay on the terminating side during which ringing can be
   played to the caller without commencing alerting on the end-user
   called device.  But Section 7 also describes the potential privacy
   implications of revealing to the OCSP responder the verification
   service that has received a call for a particular calling number.  On
   balance, stapling at the authentication service, especially pre-
   generated stapling, seems to offer the best all-around solution for
   using OCSP with STIR.

   Finally, note that the approach in this specification provides OCSP
   responses for a single telephone number - effectively, for the
   calling telephone number.  It does not provide responses for a range
   of numbers specifically because of data minimization concerns, as
   revealing the set of numbers that an OCSP responder is authorized to
   sign for may itself leak private information to relying parties.

Authors' Addresses

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


   Sean Turner
   sn3rd
   Email: sean@sn3rd.com












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