]> TransUnion, Inc.

jon.peterson@transunion.com

sn3rd

sean@sn3rd.com

SIP Secure Origin Identification Communication Security Certificates Public Key Infrastructure Real-Time Communication 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.

The STIR problem statement 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 specification describes a credential system based on version 3 certificates in accordance with for that purpose. Those credentials can then be used by STIR authentication services to sign PASSporT objects carried in a SIP request. 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 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, 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.

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 when, and only when, they appear in all capitals, as shown here.

For traditional certificate status information, there are three common certificate verification mechanisms employed by CAs: Certificate Revocation Lists (CRLs) (and ) Online Certificate Status Protocol (OCSP) , and Server-based Certificate Validation Protocol (SCVP) . 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 . 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 (), 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 , incorporating some (but not all) of the optimizations of . 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. 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 ).

Certificates compliant with this specification SHOULD include a URL pointing to an OCSP service in the Authority Information Access (AIA) certificate extension, via the "id-ad-ocsp" accessMethod specified in . This can appear in addition to, or as an alternative to, the "id-ad-stirTNList" accessMethod specified in . 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.

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 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 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.

The High-Volume Environment (HVE) OCSP profile 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 . 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 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. 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.

OCSP Request: PEM:

OCSP Response: PEM:

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. ), this means the OCSP response may be signed by any of the parent "encompassing" certificates of the end entity delegate certificate in question.

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 . Such staples can either be pre-generated ( 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. Some stapling-related design considerations are given in . The header of a PASSporT with an OCSP staple follows baseline ; no new PASSporT Type is required for transmission of staples.

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.

This document makes use of object identifiers for the TNQuery OCSP extension in 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.

This specification requests that the IANA add one new claim to the JSON Web Token Claims registry as defined in . Claim Name: "stpl" Claim Description: OCSP Staple Change Controller: IETF Specification Document(s): [RFCThis]

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 Appendix A.1. 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 ).

This document is entirely about security. See the Privacy Considerations 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 , in particular its Security Considerations. For OCSP-related security considerations see and . 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.

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.

&RFC2119; &RFC8174; &RFC3261; &RFC5280; &RFC5912; &RFC6960; &RFC5019; &RFC4055; &RFC3986; &RFC6818; &RFC8224; &RFC8225; &RFC8226; &RFC9060; &RFC7519; ITU-T Recommendation X.509 (10/2012) | ISO/IEC 9594-8 ITU-T Recommendation X.680 (08/2015) | ISO/IEC 8824-1 ITU-T Recommendation X.681 (08/2015) | ISO/IEC 8824-2 ITU-T Recommendation X.682 (08/2015) | ISO/IEC 8824-2 ITU-T Recommendation X.683 (08/2015) | ISO/IEC 8824-3 &RFC5055; &RFC6961; &RFC7340; &I-D.ietf-stir-certificates-shortlived; &I-D.ietf-tls-rfc8446bis;

This appendix provides the normative ASN.1 definitions for the structures described in this specification using ASN.1, as defined in through . The modules defined in this document are compatible with the most current ASN.1 specification published in 2015 (see , , , ). 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 and .

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.

Second, an example without the nonce.

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 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. 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 or . 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 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.