OCSP Usage for Secure Telephone Identity Certificates

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Table of Contents

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])¶

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. 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, but CRLs do support a variety of mechanisms to scope the size of the CRLs: 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 (i.e., does the relying party have 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 is validating 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.¶

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. If possible, both of these OCSP concepts should be adopted for use with STIR.¶

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

  TNQuery ::= TelephoneNumber

MIGHMIGEMEEwPzA9MAkGBSsOAwIaBQAEFLdmsxX0LkOSjTdofXdwRl6mmDfCBBSS
pHUspJ6+gUTrefyKxZWl6xB1cwIENd70z6I/MD0wHwYJKwYBBQUHMAECBBIEEGN0
k6Ihb0QokYQs01/+t0AwGgYJKwYBBQUHMAEKBA0WCzEyMDI1NTUxMjEy
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5. Approaches to OCSP Stapling

At a high level, there are a number of potential solutions 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.peterson-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.¶

Thus, the solution proposed here is that the authentication service instead 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 effectively incur the same delay as an OCSP dip from the verification service.¶

One alternative design would be to carry an OCSP staple at the SIP layer, in a body or header. But the 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.¶

Finally, 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.¶

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

   { "orig":{"tn":"12155551212"},
     "dest":{"tn":["12155551214"]},
     "iat":1443208345
         "stpl":"MIIE2QoBAKCCBNIwggTOBgkrBgEFBQcwAQEEggS/MIIEuzCCASuhgYAwfjELMAkG
     A1UEBhMCQVUxEzARBgNVBAgTClNvbWUtU3RhdGUxITAfBgNVBAoTGEludGVybmV0
     IFdpZGdpdHMgUHR5IEx0ZDEVMBMGA1UEAxMMc25tcGxhYnMuY29tMSAwHgYJKoZI
     hvcNAQkBFhFpbmZvQHNubXBsYWJzLmNvbRgPMjAyNDAzMTcwNTA5MDBaMFQwUjA9
     MAkGBSsOAwIaBQAEFLdmsxX0LkOSjTdofXdwRl6mmDfCBBSSpHUspJ6+gUTrefyK
     xZWl6xB1cwIENd70z4IAGA8yMDEyMDQxMTE0MDkyMlqhPzA9MB8GCSsGAQUFBzAB
     AgQSBBBjdJOiIW9EKJGELNNf/rdAMBoGCSsGAQUFBzABCgQNFgsxMjAyNTU1MTIx
     MjANBgkqhkiG9w0BAQUFAAOBgQA5O6EYgsuHsNbtDedkC0RaVvrXW9DX5Fdl8rvh
     woSok04WT6/WV2pSIJCdcNwQJ84WwdCV/86uz3/MhM/zq0OBhh+x8g91YD5DLvie
     iNwNgJ/m1EKPfQJgm2ef7Uh7Q2EDELd4jW79X5NMrw5oe1HSrl1DUsiXR3oNu3TD
     cuJPAKCCAvUwggLxMIIC7TCCAlagAwIBAgIBATANBgkqhkiG9w0BAQUFADB+MQsw
     CQYDVQQGEwJBVTETMBEGA1UECBMKU29tZS1TdGF0ZTEhMB8GA1UEChMYSW50ZXJu
     ZXQgV2lkZ2l0cyBQdHkgTHRkMRUwEwYDVQQDEwxzbm1wbGFicy5jb20xIDAeBgkq
     hkiG9w0BCQEWEWluZm9Ac25tcGxhYnMuY29tMB4XDTEyMDQxMTEzMjUzNVoXDTEz
     MDQxMTEzMjUzNVowfjELMAkGA1UEBhMCQVUxEzARBgNVBAgTClNvbWUtU3RhdGUx
     ITAfBgNVBAoTGEludGVybmV0IFdpZGdpdHMgUHR5IEx0ZDEVMBMGA1UEAxMMc25t
     cGxhYnMuY29tMSAwHgYJKoZIhvcNAQkBFhFpbmZvQHNubXBsYWJzLmNvbTCBnzAN
     BgkqhkiG9w0BAQEFAAOBjQAwgYkCgYEAww1ORzpzVfCNgqI8QfIpSFkR2ELmgI54
     6xEzDqa6LgxxV58FqkKPyN5tG12JqHK4fZA3n2/nIHO/niSrwLwaq6l0Z1N/A5kF
     P84cqQn7Rhnz/MY7gWdZ9t5Ud4aZTdcmANCdl0oAWgIOnvDrCn9b3F/BLNPaw6PJ
     kKbeBts0eesCAwEAAaN7MHkwCQYDVR0TBAIwADAsBglghkgBhvhCAQ0EHxYdT3Bl
     blNTTCBHZW5lcmF0ZWQgQ2VydGlmaWNhdGUwHQYDVR0OBBYEFPGLNnaSRSzB5cmO
     ew+ATZpapxHGMB8GA1UdIwQYMBaAFPGLNnaSRSzB5cmOew+ATZpapxHGMA0GCSqG
     SIb3DQEBBQUAA4GBAFkdLhSVZUCHeoVaVG4FxU6csLTYrTVxYmGJEUb++zHEiaiw
     mv3NcJ7i5qnBXLkVCtKDevGSQz9hwwynvDAmfPrMfgheeHjPFQoDfbkPV8hO8fV6
     1w3d1MPUSVWlkiHs5DSjXgRNJQzNo1IwuBwBEnX+53m89cLagDlxNY1hf8vI"
         }

6. IANA Considerations

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 potentailly 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.¶

Preventing eavesdropping reduces on 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. For further information on certificate security and practices, see [RFC5280], in particular its Security Considerations. For OCSP-related security considerations see [RFC6960] and [RFC5019].¶

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.¶

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].¶

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

Authors' Addresses