OCSP Usage for Secure Telephone Identity Certificates
draft-ietf-stir-certificates-ocsp-12
| Document | Type | Active Internet-Draft (stir WG) | |
|---|---|---|---|
| Authors | Jon Peterson , Sean Turner | ||
| Last updated | 2026-02-13 (Latest revision 2025-11-04) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Ben Campbell | ||
| Shepherd write-up | Show Last changed 2025-03-03 | ||
| IESG | IESG state | IESG Evaluation | |
| Action Holder | |||
| Consensus boilerplate | Yes | ||
| Telechat date |
On agenda of 2026-02-19 IESG telechat
Has a DISCUSS. Needs 4 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Orie Steele | ||
| Send notices to | ben@nostrum.com | ||
| IANA | IANA review state | IANA OK - Actions Needed | |
| IANA expert review state | Expert Reviews OK | ||
| IANA expert review comments | The SMI Security for PKIX Module Identifier and JSON Web Token Claims have been approved. |
draft-ietf-stir-certificates-ocsp-12
Network Working Group J. Peterson
Internet-Draft TU
Intended status: Standards Track S. Turner
Expires: 8 May 2026 sn3rd
4 November 2025
OCSP Usage for Secure Telephone Identity Certificates
draft-ietf-stir-certificates-ocsp-12
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. Certififcate 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
working documents as Internet-Drafts. The list of current Internet-
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 8 May 2026.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
Peterson & Turner Expires 8 May 2026 [Page 1]
Internet-Draft STIR Certs OCSP November 2025
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 . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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.
Peterson & Turner Expires 8 May 2026 [Page 2]
Internet-Draft STIR Certs OCSP November 2025
[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])
Peterson & Turner Expires 8 May 2026 [Page 3]
Internet-Draft STIR Certs OCSP November 2025
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, such as whether the relying party has
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 an important optimizations.
Between the OCSP and SCVP, OCSP is much more widely deployed and this
document therefore RECOMMENDS the use of 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.
Peterson & Turner Expires 8 May 2026 [Page 4]
Internet-Draft STIR Certs OCSP November 2025
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 OCSP TNQuery 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
Peterson & Turner Expires 8 May 2026 [Page 5]
Internet-Draft STIR Certs OCSP November 2025
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.
Peterson & Turner Expires 8 May 2026 [Page 6]
Internet-Draft STIR Certs OCSP November 2025
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:
Peterson & Turner Expires 8 May 2026 [Page 7]
Internet-Draft STIR Certs OCSP November 2025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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 STIR 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 connected, could
Peterson & Turner Expires 8 May 2026 [Page 8]
Internet-Draft STIR Certs OCSP November 2025
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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"
}
6. IANA Considerations
Peterson & Turner Expires 8 May 2026 [Page 9]
Internet-Draft STIR Certs OCSP November 2025
6.1. TN-HVE OCSP Extension
This document makes use of object identifiers for the TN-HVE 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-2016 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
TN-HVE 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.
Peterson & Turner Expires 8 May 2026 [Page 10]
Internet-Draft STIR Certs OCSP November 2025
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>.
Peterson & Turner Expires 8 May 2026 [Page 11]
Internet-Draft STIR Certs OCSP November 2025
[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>.
Peterson & Turner Expires 8 May 2026 [Page 12]
Internet-Draft STIR Certs OCSP November 2025
[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
Peterson & Turner Expires 8 May 2026 [Page 13]
Internet-Draft STIR Certs OCSP November 2025
[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-03, 6 July 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-stir-
certificates-shortlived-03>.
[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].
Peterson & Turner Expires 8 May 2026 [Page 14]
Internet-Draft STIR Certs OCSP November 2025
TN-OCSP-Module-2023
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-tn-ocsp-module-2023(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
--
-- Telephone Number Query 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 example
Peterson & Turner Expires 8 May 2026 [Page 15]
Internet-Draft STIR Certs OCSP November 2025
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'
: }
: }
: }
: }
: }
: }
OCSP Response:
MIIDbQoBAKCCA2YwggNiBgkrBgEFBQcwAQEEggNTMIIDTzCB9aIWBBQ8T5f++IIw
3WyQrjVceyyHwm0EuxgPMjAyNDA2MTgwNTA5MDBaMIGkMIGhMFkwDQYJYIZIAWUD
BAIBBQAEIJ1EZ3WbxOzKRcemhY/5xF07gTbpbEbUiZ42db5b5BxSBCDU5CLVKzcd
Peterson & Turner Expires 8 May 2026 [Page 16]
Internet-Draft STIR Certs OCSP November 2025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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
: 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
Peterson & Turner Expires 8 May 2026 [Page 17]
Internet-Draft STIR Certs OCSP November 2025
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
: }
: }
368 509: [0] {
372 505: SEQUENCE {
376 501: SEQUENCE {
380 411: SEQUENCE {
384 3: [0] {
386 1: INTEGER 2
: }
Peterson & Turner Expires 8 May 2026 [Page 18]
Internet-Draft STIR Certs OCSP November 2025
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'
: }
: }
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'
Peterson & Turner Expires 8 May 2026 [Page 19]
Internet-Draft STIR Certs OCSP November 2025
: }
: }
: }
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)
: }
: }
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
Peterson & Turner Expires 8 May 2026 [Page 20]
Internet-Draft STIR Certs OCSP November 2025
: 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
: 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
: }
: }
: }
: }
: }
: }
: }
: }
: }
: }
Peterson & Turner Expires 8 May 2026 [Page 21]
Internet-Draft STIR Certs OCSP November 2025
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 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.
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 this 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
Peterson & Turner Expires 8 May 2026 [Page 22]
Internet-Draft STIR Certs OCSP November 2025
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
Peterson & Turner Expires 8 May 2026 [Page 23]