Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Standards Track S. Turner
Expires: March 13, 2017 sn3rd
September 9, 2016
Secure Telephone Identity Credentials: Certificates
draft-ietf-stir-certificates-08.txt
Abstract
In order to prevent the impersonation of telephone numbers on the
Internet, some kind of credential system needs to exist that
cryptographically asserts authority over telephone numbers. This
document describes the use of certificates in establishing authority
over telephone numbers, as a component of a broader architecture for
managing telephone numbers as identities in protocols like SIP.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on March 13, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Authority for Telephone Numbers in Certificates . . . . . . . 3
4. Certificate Usage with STIR . . . . . . . . . . . . . . . . . 5
5. Enrollment and Authorization using the TN Authorization List 6
5.1. Levels Of Assurance . . . . . . . . . . . . . . . . . . . 7
5.2. Certificate Extension Scope and Structure . . . . . . . . 8
6. Provisioning Private Keying Material . . . . . . . . . . . . 8
7. Acquiring Credentials to Verify Signatures . . . . . . . . . 9
8. TN Authorization List Syntax . . . . . . . . . . . . . . . . 10
9. Certificate Freshness and Revocation . . . . . . . . . . . . 12
9.1. Choosing a Verification Method . . . . . . . . . . . . . 12
9.2. Using OCSP with TN Auth List . . . . . . . . . . . . . . 13
9.2.1. OCSP Extension Specification . . . . . . . . . . . . 14
9.3. Acquiring TN Lists By Reference . . . . . . . . . . . . . 16
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The STIR problem statement [RFC7340] identifies the primary enabler
of robocalling, vishing, swatting and related attacks as the
capability to impersonate a calling party number. The starkest
examples of these attacks are cases where automated callees on the
PSTN rely on the calling number as a security measure, for example to
access a voicemail system. Robocallers use impersonation as a means
of obscuring identity; while robocallers can, in the ordinary PSTN,
block (that is, withhold) their caller identity, callees are less
likely to pick up calls from blocked identities, and therefore
appearing to calling from some number, any number, is preferable.
Robocallers however prefer not to call from a number that can trace
back to the robocaller, and therefore they impersonate numbers that
are not assigned to them.
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. With these credentials,
parties can assert that they are in fact authorized to use telephony
numbers, and thus distinguish themselves from impersonators unable to
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present such credentials. For that reason the STIR threat model
[RFC7375] stipulates, "The design of the credential system envisioned
as a solution to these threats must, for example, limit the scope of
the credentials issued to carriers or national authorities to those
numbers that fall under their purview." This document describes
credential systems for telephone numbers based on X.509 version 3
certificates in accordance with [RFC5280]. While telephone numbers
have long been part of the X.509 standard (X.509 supports arbitrary
naming attributes to be included in a certificate; the
telephoneNumber attribute was defined in the 1988 [X.520]
specification) this document provides ways to determine authority
more aligned with telephone network requirements, including extending
X.509 with a Telephone Number Authorization List certificate
extension which binds certificates to asserted authority for
particular telephone numbers, or potentially telephone number blocks
or ranges.
In the STIR in-band architecture specified in
[I-D.ietf-stir-rfc4474bis], two basic types of entities need access
to these credentials: authentication services, and verification
services (or verifiers). An authentication service must be operated
by an entity enrolled with the certification authority (CA, see
Section 5), whereas a verifier need only trust the trust anchor of
the authority, and have a means to access and validate the public
keys associated with these certificates. Although the guidance in
this document is written with the STIR in-band architecture in mind,
the credential system described in this document could be useful for
other protocols that want to make use of certificates to assert
authority over telephone numbers on the Internet.
This document specifies only the credential syntax and semantics
necessary to support this architecture. It does not assume any
particular CA or deployment environment. We anticipate that some
deployment experience will be necessary to determine optimal
operational models.
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 RFC
2119 [RFC2119].
3. Authority for Telephone Numbers in Certificates
At a high level, this specification details two non-exclusive
approaches that can be employed to determine authority over telephone
numbers with certificates.
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The first approach is to leverage the existing subject of the
certificate to ascertain that the holder of the certificate is
authorized to claim authority over a telephone number. The subject
might be represented as a domain name in the SubjectAltName, such as
an "example.net" where that domain is known to relying parties as a
carrier, or represented with other identifiers related to the
operation of the telephone network including Service Provider
Identifiers (SPIDs) could serve as a subject as well. A relying
party could then employ an external data set or service that
determines whether or not a specific telephone number is under the
authority of the carrier identified as the subject of the
certificate, and use that to ascertain whether or not the carrier
should have authority over a telephone number. Potentially, a
certificate extension to convey the URI of such an information
service trusted by the issuer of the certificate could be developed
(though this specification does not propose one). Alternatively,
some relying parties could form bilateral or multilateral trust
relationships with peer carriers, trusting one another's assertions
just as telephone carriers in the SS7 network today rely on
transitive trust when displaying the calling party telephone number
received through SS7 signaling.
The second approach is to extend the syntax of certificates to
include a new attribute, defined here as TN Authorization List, which
contains a list of telephone numbers defining the scope of authority
of the certificate. Relying parties, if they trust the issuer of the
certificate as a source of authoritative information on telephone
numbers, could therefore use the TN Authorization List instead of the
subject of the certificate to make a decision about whether or not
the signer has authority over a particular telephone number. The TN
Authorization List could be provided in one of two ways: as a literal
value in the certificate, or as a network service that allows relying
parties to query in real time to determine that a telephone number is
in the scope of a certificate. Using the TN Authorization list
rather than the certificate subject makes sense when, for example,
for privacy reasons, the certificate owner would prefer not to be
identified, or in cases where the holder of the certificate does not
participate in the sort of traditional carrier infrastructure that
the first approach assumes.
The first approach requires little change to existing Public Key
Infrastructure (PKI) certificates; for the second approach, we must
define an appropriate enrollment and authorization process. For the
purposes of STIR, the over-the-wire format specified in
[I-D.ietf-stir-rfc4474bis] accommodates either of these approaches:
the methods for canonicalizing, signing, for identifying and
accessing the certificate and so on remain the same; it is only the
verifier behavior and authorization decision that will change
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depending on the approach to telephone number authority taken by the
certificate. For that reason, the two approaches are not mutually
exclusive, and in fact a certificate issued to a traditional
telephone network service provider could contain a TN Authorization
List or not, were it supported by the CA issuing the credential.
Regardless of which approach is used, certificates that assert
authority over telephone numbers are subject to the ordinary
operational procedures that govern certificate use per [RFC5280].
This means that verification services must be mindful of the need to
ensure that they trust the trust anchor that issued the certificate,
and that they have some means to determine the freshness of the
certificate (see Section 9).
4. Certificate Usage with STIR
[I-D.ietf-stir-rfc4474bis] Section 7.4 requires that all credential
systems used by STIR explain how they address the requirements
enumerated below. Certificates as described in this document address
the STIR requirements as follows:
1. The URI schemes permitted in the SIP Identity header "info"
parameter, as well as any special procedures required to
dereference the URIs. While normative text is given below in
Section 7, this mechanism permits the HTTP, CID and SIP URI
schemes to appear in the "info" parameter.
2. Procedures required to extract keying material from the resources
designated by the URI. Implementations perform no special
procedures beyond dereferencing the "info" URI. See Section 7.
3. Procedures used by the verification service to determine the
scope of the credential. This specification effectively proposes
two methods, as outlined in Section 3: one where the subject (or
more properly subjectAltName) of the certificate indicates the
scope of authority through a domain name, and relying parties
either trust the subject entirely or have some direct means of
determining whether or not a number falls under a subject's
authority; and another where an extension to the certificate as
described in Section 8 identifies the scope of authority of the
certificate.
4. The cryptographic algorithms required to validate the
credentials. For this specification, that means the signature
algorithms used to sign certificates. This specification
REQUIRES that implementations support both ECDSA with the P-256
curve (see [RFC4754]) and RSA PKCS#1 v1.5 (see [RFC3447]
Section 8.2) for certificate signatures. Implementers are
advised that RS256 is mandated only as a transitional mechanism,
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due to its widespread use in existing PKI, but we anticipate that
this mechanism will eventually be deprecated.
5. Finally, note that all certificates compliant with this
specification:
* MUST provide cryptographic keying material sufficient to
generate the ECDSA using P-256 and SHA-256 signatures
necessary to support the ES256 hashed signatures required by
PASSporT [I-D.ietf-stir-passport], which in turn follows JSON
Web Token (JWT) [RFC7519].
* MUST support both ECDSA with P-256 and RSA PKCS#1 v1.5 for
certificate signature verification.
This document also includes additional certificate-related
requirements:
o See Section 5.1 for requirements related to the certificate
policies extension.
o See Section 7 for requirements related to the TN Query certificate
extension.
o See Section 9.2 and Section 9.3 for requirements related to the
Authority Information Access (AIA) certificate extension.
o See Section 9.2.1 for requirements related to the authority key
identifier and subject key identifier certificate extensions.
5. Enrollment and Authorization using the TN Authorization List
This document covers three models for enrollment when using the TN
Authorization List extension.
The first enrollment model is one where the CA acts in concert with
national numbering authorities to issue credentials to those parties
to whom numbers are assigned. In the United States, for example,
telephone number blocks are assigned to Local Exchange Carriers
(LECs) by the North American Numbering Plan Administrator (NANPA),
who is in turn directed by the national regulator. LECs may also
receive numbers in smaller allocations, through number pooling, or
via an individual assignment through number portability. LECs assign
numbers to customers, who may be private individuals or organizations
- and organizations take responsibility for assigning numbers within
their own enterprise. This model requires top-down adoption of the
model from regulators through to carriers. Assignees of E.164
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numbering resources participating in this enrollment model should
take appropriate steps to establish trust anchors.
The second enrollment model is a bottom-up approach where a CA
requires that an entity prove control by means of some sort of test,
which, as with certification authorities for web PKI, might either be
automated or a manual administrative process. As an example of an
automated process, an authority might send a text message to a
telephone number containing a URL (which might be dereferenced by the
recipient) as a means of verifying that a user has control of
terminal corresponding to that number. Checks of this form are
frequently used in commercial systems today to validate telephone
numbers provided by users. This is comparable to existing enrollment
systems used by some certificate authorities for issuing S/MIME
credentials for email by verifying that the party applying for a
credential receives mail at the email address in question.
The third enrollment model is delegation: that is, the holder of a
certificate (assigned by either of the two methods above) might
delegate some or all of their authority to another party. In some
cases, multiple levels of delegation could occur: a LEC, for example,
might delegate authority to a customer organization for a block of
100 numbers used by an IP PBX, and the organization might in turn
delegate authority for a particular number to an individual employee.
This is analogous to delegation of organizational identities in
traditional hierarchical PKIs who use the name constraints extension
[RFC5280]; the root CA delegates names in sales to the sales
department CA, names in development to the development CA, etc. As
lengthy certificate delegation chains are brittle, however, and can
cause delays in the verification process, this document considers
optimizations to reduce the complexity of verification.
Future work might explore methods of partial delegation, where
certificate holders delegate only part of their authority. For
example, individual assignees may want to delegate to a service
authority for text messages associated with their telephone number,
but not for other functions.
5.1. Levels Of Assurance
This specification supports different level of assurance (LOA). The
LOA indications, which are Object Identifiers (OIDs) included in the
certificate's certificate policies extension [RFC5280], allow CAs to
differentiate those enrolled from proof-of-possession versus
delegation. A Certification Policy and a Certification Practice
Statement [RFC3647] are produced as part of the normal PKI
bootstrapping process (i.e., the CP is written first and then the CAs
say how they conform to the CP in the CPS). OIDs are used to
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reference the CP and if the CA wishes it can also include a reference
to the CPS with the certificate policies extension. CAs that wish to
express different LOAs MUST include the certificate policies
extension in issued certificates. See Section 11 for additional
information about the LOA registry.
5.2. Certificate Extension Scope and Structure
This specification places no limits on the number of telephone
numbers that can be associated with any given certificate. Some
service providers may be assigned millions of numbers, and may wish
to have a single certificate that can be applied to signing for any
one of those numbers. Others may wish to compartmentalize authority
over subsets of the numbers they control.
Moreover, service providers may wish to have multiple certificates
with the same scope of authority. For example, a service provider
with several regional gateway systems may want each system to be
capable of signing for each of their numbers, but not want to have
each system share the same private key.
The set of telephone numbers for which a particular certificate is
valid is expressed in the certificate through a certificate
extension; the certificate's extensibility mechanism is defined in
[RFC5280] but the TN Authorization List extension is specified in
this document.
The subjects of certificates containing the TN Authorization List
extension are typically the administrative entities to whom numbers
are assigned or delegated. For example, a LEC might hold a
certificate for a range of telephone numbers. In some cases, the
organization or individual issued such a certificate may not want to
associate themselves with a certificate; for example, a private
individual with a certificate for a single telephone number might not
want to distribute that certificate publicly if every verifier
immediately knew their name. The certification authorities issuing
certificates with the TN Authorization List extensions may, in
accordance with their policies, obscure the identity of the subject,
though mechanisms for doing so are outside the scope of this
document.
6. Provisioning Private Keying Material
In order for authentication services to sign calls via the procedures
described in [I-D.ietf-stir-rfc4474bis], they must hold a private key
corresponding to a certificate with authority over the calling
number. [I-D.ietf-stir-rfc4474bis] does not require that any
particular entity in a SIP deployment architecture sign requests,
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only that it be an entity with an appropriate private key; the
authentication service role may be instantiated by any entity in a
SIP network. For a certificate granting authority only over a
particular number which has been issued to an end user, for example,
an end user device might hold the private key and generate the
signature. In the case of a service provider with authority over
large blocks of numbers, an intermediary might hold the private key
and sign calls.
The specification recommends distribution of private keys through
PKCS#8 objects signed by a trusted entity, for example through the
CMS package specified in [RFC5958].
7. Acquiring Credentials to Verify Signatures
This specification documents multiple ways that a verifier can gain
access to the credentials needed to verify a request. As the
validity of certificates does not depend on the method of their
acquisition, there is no need to standardize any single mechanism for
this purpose. All entities that comply with
[I-D.ietf-stir-rfc4474bis] necessarily support SIP, and consequently
SIP itself can serve as a way to deliver certificates.
[I-D.ietf-stir-rfc4474bis] provides an "info" parameter of the
Identity header which contains a URI for the credential used to
generate the Identity header; [I-D.ietf-stir-rfc4474bis] also
requires documents which define credential systems list the URI
schemes that may be present in the "info" parameter. For
implementations compliant with this specification, three URI schemes
are REQUIRED: the CID URI, the SIP URI, and the HTTP URI.
The simplest way for a verifier to acquire the certificate needed to
verify a signature is for the certificate be conveyed in a SIP
request along with the signature itself. In SIP, for example, a
certificate could be carried in a multipart MIME body [RFC2046], and
the URI in the Identity header "info" parameter could specify that
body with a CID URI [RFC2392]. However, in many environments this is
not feasible due to message size restrictions or lack of necessary
support for multipart MIME.
The Identity header "info" parameter in a SIP request may contain a
URI that the verifier dereferences. Implementations of this
specification are required to support the use of SIP for this
function (via the SUBSCRIBE/NOTIFY mechanism), as well as HTTP, via
the Enrollment over Secure Transport mechanisms described in RFC 7030
[RFC7030].
Note well that as an optimization, a verifier may have access to a
service, a cache or other local store that grants access to
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certificates for a particular telephone number. However, there may
be multiple valid certificates that can sign a call setup request for
a telephone number, and as a consequence, there needs to be some
discriminator that the signer uses to identify their credentials.
The Identity header "info" parameter itself can serve as such a
discriminator, provided implementations use that parameter as a key
when accessing certificates from caches or other sources.
8. TN Authorization List Syntax
The subjects of certificates containing the TN Authorization List
extension are the administrative entities to whom numbers are
assigned or delegated. When a verifier is validating a caller's
identity, local policy always determines the circumstances under
which any particular subject may be trusted, but the purpose of the
TN Authorization List extension in particular is to allow a verifier
to ascertain when the CA has designated that the subject has
authority over a particular telephone number or number range. The
Telephony Number (TN) Authorization List certificate extension is
included in the Certificate's extension field [RFC5280]. The
extension is defined with ASN.1, defined in [X.680] through [X.683].
What follows is the syntax and semantics of the extension.
The Telephony Number (TN) Authorization List certificate extension is
identified by the following object identifier (OID), which is defined
under the id-ce OID arc defined in [RFC5280] and managed by IANA (see
Section 10):
id-ce-TNAuthList OBJECT IDENTIFIER ::= { id-ce TBD }
The TN Authorization List certificate extension has the following
syntax:
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TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization
TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry
TNEntry ::= CHOICE {
spid [0] ServiceProviderIdentifierList,
range [1] TelephoneNumberRange,
one E164Number }
ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF
OCTET STRING
-- When all three are present: SPID, Alt SPID, and Last Alt SPID
TelephoneNumberRange ::= SEQUENCE {
start E164Number,
count INTEGER }
E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789"))
The TN Authorization List certificate extension indicates the
authorized phone numbers for the call setup signer. It indicates one
or more blocks of telephone number entries that have been authorized
for use by the call setup signer. There are three ways to identify
the block:
1. A Service Provider Identifier (SPID, also known as an Operating
Company Number (OCN) or Carrier Identification Code (CIC), as
specified in [ATIS-0300050]) can be used to indirectly name all
of the telephone numbers associated with that service provider,
2. Telephone numbers can be listed in a range (in the
TelephoneNumberRange format), or
3. A single telephone number can be listed (as an E164Number).
Note that because large-scale service providers may want to associate
many numbers, possibly millions of numbers, with a particular
certificate, optimizations are required for those cases to prevent
certificate size from becoming unmanageable. In these cases, the TN
Authorization List may be given by reference rather than by value,
through the presence of a separate certificate extension that permits
verifiers to either securely download the list of numbers associated
with a certificate, or to verify that a single number is under the
authority of this certificate. This optimization is left for future
work.
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9. Certificate Freshness and Revocation
Regardless of which of the approaches in Section 3 is followed for
using certificates, a certificate verification mechanism is required.
However, the traditional problem of certificate freshness gains a new
wrinkle when using the TN Authorization List extension, because
verifiers must establish not only that a certificate remains valid,
but also that the certificate's scope contains the telephone number
that the verifier is validating. Dynamic changes to number
assignments can occur due to number portability, for example. So
even if a verifier has a valid cached certificate for a telephone
number (or a range containing the number), the verifier must
determine that the entity that signed is still a proper authority for
that number.
To verify the status of the certificate, the verifier needs to
acquire the certificate if necessary (via the methods described in
Section 7), and then would need to either:
(a) Rely on short-lived certificates and not check the certificate's
status, or
(b) Rely on status information from the authority (e.g. OCSP, see
Section 9.2)
The tradeoff between short lived certificates and using status
information is that the former's burden is on the front end (i.e.,
enrollment) and the latter's burden is on the back end (i.e.,
verification). Both impact call setup time, but it is assumed that
generating a short-lived certificate for each all, and consequently
performing enrollment for each call, is more of an impact than
acquiring status information. This document therefore recommends
relying on status information.
9.1. Choosing a Verification Method
There are three common certificate verification mechanisms employed
by CAs:
1. Certificate Revocation Lists (CRLs) [RFC5280]
2. Online Certificate Status Protocol (OCSP) [RFC6960], and
3. Server-based Certificate Validation Protocol (SCVP) [RFC5055].
When relying on status information, the verifier needs to obtain the
status information - but before that can happen, the verifier needs
to know where to locate it. Placing the location of the status
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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 obviously attractive solution because they are supported
by every CA. 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 RP have all of the CRL partitions).
CAs in the STIR architecture will likely all create CRLs for audit
purposes, but it NOT RECOMMENDED that they be relied upon for status
information. Instead, one of the two "online" options is preferred.
Between the two, 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 certificates, based on
[RFC6960], incorporating some (but not all) of the optimizations of
[RFC5019]. CRLs MUST be signed with the same algorithm as the
certificate.
9.2. Using OCSP with TN Auth List
Certificates compliant with this specification therefore 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 [RFC5280]. 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.
At a high level, there are two ways that a client might pose this
authorization question:
For this certificate, is the following number currently in its
scope of validity?
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What are all the telephone numbers associated with this
certificate, or this certificate subject?
Only the former lends itself to piggybacking on the OCSP status
mechanism; since the verifier is already asking an authority about
the certificate's status, why not reuse that mechanism, instead of
creating a new service that requires additional round trips? Like
most PKIX-developed protocols, OCSP is extensible; OCSP supports
request extensions (including sending multiple requests at once) and
per-request extensions. It seems unlikely that the verifier will be
requesting authorization checks on multiple telephone numbers in one
request, so a per-request extension is what is needed.
The requirement to consult OCSP in real time results in a network
round-trip time of day, 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.
9.2.1. 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 may also appear in the response's
singleExtensions. When an OCSP server includes a 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.
id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD }
TNQuery ::= E164Number
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The 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:
o 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] and the values are
truncated to 160-bits as specified Option 1 in Section 2 of
[RFC7093].
o Clients MUST include the OCSP TNQuery extension in requests'
singleRequestExtensions.
o Servers MUST include the OCSP TNQuery extension in responses'
singleExtensions.
o Servers SHOULD return responses that would otherwise have been
"unknown" as "not good" (i.e., return only "good" and "not good"
responses).
o Clients MUST treat returned "unknown" responses as "not good".
o If the server uses ResponderID, it MUST generate the KeyHash using
SHA-256 and truncate the value to 160-bits as specified in Option
1 in Section 2 of [RFC7093].
o Implementations MUST support ECDSA using P-256 and SHA-256. Note
that [RFC6960] requires RSA with SHA-256 be supported.
o There is no 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 and truncate the value to 160-bits as specified in Option 1
in Section 2 of [RFC7093].
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
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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.
Strategies for stapling OCSP [RFC6961] have become common in some web
PKI environments as an optimization which allows web servers to send
up-to-date certificate status information acquired from OCSP to
clients as TLS is negotiated. A similar mechanism could be
implemented for SIP requests, in which the authentication service
adds status information for its certificate to the SIP request, which
would save the verifier the trouble of performing the OCSP dip
itself. Especially for high-volume authentication and verification
services, this could result in significant performance improvements.
This is left as an optimization for future work.
9.3. Acquiring TN Lists By Reference
Acquiring a list of the telephone numbers associated with a
certificate or its subject lends itself to an application-layer
query/response interaction outside of OCSP, one which could be
initiated through a separate URI included in the certificate. The
AIA extension (see [RFC5280]) supports such a mechanism: it
designates an OID to identify the accessMethod and an accessLocation,
which would most likely be a URI. A verifier would then follow the
URI to ascertain whether the list of TNs are authorized for use by
the caller.
HTTPS is the most obvious candidate for a protocol to be used for
fetching the list of telephone numbers associated with a particular
certificate. This document defines a new AIA accessMethod, called
"id-ad-stir-tn", which uses the following AIA OID:
id-ad-stir-tn OBJECT IDENTIFIER ::= { id-ad TBD }
When the "id-ad-stir-tn" accessMethod is used, the accessLocation
MUST be an HTTPS URI. The document returned by dereferencing that
URI will contain the complete TN Authorization List (see Section 8)
for the certificate.
Delivering the entire list of telephone numbers associated with a
particular certificate will divulge to STIR verifiers information
about telephone numbers other than the one associated with the
particular call that the verifier is checking. In some environments,
where STIR verifiers handle a high volume of calls, maintaining an
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up-to-date and complete cache for the numbers associated with crucial
certificate holders could give an important boost to performance.
10. IANA Considerations
This document makes use of object identifiers for the TN Certificate
Extension defined in Section 8, TN-HVE OCSP extension in
Section 9.2.1, the TN by reference AIA access descriptor defined in
Section 9.3, and the ASN.1 module identifier defined in Appendix A.
It therefore requests that the IANA make the following assignments:
o TN Certificate Extension in the SMI Security for PKIX Certificate
Extension registry: http://www.iana.org/assignments/smi-numbers/
smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.1
o TN-HVE OCSP extension in the SMI Security for PKIX Online
Certificate Status Protocol (OCSP) registry:
http://www.iana.org/assignments/smi-numbers/smi-numbers.xhtml#smi-
numbers-1.3.6.1.5.5.7.48.1
o TNS by reference access descriptor in the SMI Security for PKIX
Access Descriptor registry: http://www.iana.org/assignments/smi-
numbers/smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.48
o The TN ASN.1 module in SMI Security for PKIX Module Identifier
registry: http://www.iana.org/assignments/smi-numbers/smi-
numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.0
This document also makes use of the Level of Assurance (LoA) Profiles
registry defined in [RFC6711] because as is stated in RFC 6711: "Use
of the registry by protocols other than SAML is encouraged." IANA is
requested to creae the STIR Levels of Assurance (LOA) sub-registry in
the Level of Assurance (LoA) Profile registry. Instead of
registering a SAML Context Class, the Certificate Policy's Object
Identifier representing the LOA is included in the registry. An
example registration is as follows:
To: loa-profiles-experts@icann.org
From: jrandom@example.com
1. Name of requester: J. Random User
2. Email address of requester: jrandom@example.com
3. Organization of requester: Example Trust Frameworks LLP
4. Requested registration:
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URI http://foo.example.com/assurance/loa-1
Name foo-loa-1
Informational URL https://foo.example.com/assurance/
Certificate Policy Object Identifier: 0.0.0.0
NOTE: Do not register this example. The OID is purposely invalid.
Experts are expected to ensure the reference CP includes the OID
being registered.
11. 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]
12. Acknowledgments
Russ Housley, Brian Rosen, Cullen Jennings, Dave Crocker, Tony
Rutkowski, John Braunberger, and Eric Rescorla provided key input to
the discussions leading to this document.
13. References
[ATIS-0300050]
ATIS Recommendation 0300050, "Carrier Identification Code
(CIC) Assignment Guidelines", 2012.
[I-D.ietf-stir-passport]
Wendt, C. and J. Peterson, "Persona Assertion Token",
draft-ietf-stir-passport-07 (work in progress), September
2016.
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-11
(work in progress), August 2016.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<http://www.rfc-editor.org/info/rfc2046>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
<http://www.rfc-editor.org/info/rfc2392>.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
2003, <http://www.rfc-editor.org/info/rfc3447>.
[RFC3647] Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
Wu, "Internet X.509 Public Key Infrastructure Certificate
Policy and Certification Practices Framework", RFC 3647,
DOI 10.17487/RFC3647, November 2003,
<http://www.rfc-editor.org/info/rfc3647>.
[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,
<http://www.rfc-editor.org/info/rfc4055>.
[RFC4754] Fu, D. and J. Solinas, "IKE and IKEv2 Authentication Using
the Elliptic Curve Digital Signature Algorithm (ECDSA)",
RFC 4754, DOI 10.17487/RFC4754, January 2007,
<http://www.rfc-editor.org/info/rfc4754>.
[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, <http://www.rfc-editor.org/info/rfc5019>.
[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,
<http://www.rfc-editor.org/info/rfc5055>.
[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,
<http://www.rfc-editor.org/info/rfc5280>.
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[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,
<http://www.rfc-editor.org/info/rfc5912>.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958,
DOI 10.17487/RFC5958, August 2010,
<http://www.rfc-editor.org/info/rfc5958>.
[RFC6711] Johansson, L., "An IANA Registry for Level of Assurance
(LoA) Profiles", RFC 6711, DOI 10.17487/RFC6711, August
2012, <http://www.rfc-editor.org/info/rfc6711>.
[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,
<http://www.rfc-editor.org/info/rfc6960>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013,
<http://www.rfc-editor.org/info/rfc6961>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<http://www.rfc-editor.org/info/rfc7030>.
[RFC7093] Turner, S., Kent, S., and J. Manger, "Additional Methods
for Generating Key Identifiers Values", RFC 7093,
DOI 10.17487/RFC7093, December 2013,
<http://www.rfc-editor.org/info/rfc7093>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014,
<http://www.rfc-editor.org/info/rfc7340>.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014,
<http://www.rfc-editor.org/info/rfc7375>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.
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[X.509] ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-8,
"Information technology - Open Systems Interconnection -
The Directory: Public-key and attribute certificate
frameworks", 2012.
[X.520] ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-6,
"Information technology - Open Systems Interconnection -
The Directory: Selected Attribute Types", 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".
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].
TN-Module {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-tn-module(TBD) }
DEFINITIONS EXPLICIT TAGS ::= BEGIN
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IMPORTS
id-ad, id-ad-ocsp -- From [RFC5912]
FROM PKIX1Explicit-2009 {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) }
id-ce -- From [RFC5912]
FROM PKIX1Implicit-2009 {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59) }
EXTENSION -- From [RFC5912]
FROM PKIX-CommonTypes-2009 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57) }
;
-- TN Entry Certificate Extension
ext-tnAuthList EXTENSION ::= {
SYNTAX TNAuthorizationList IDENTIFIED BY id-ce-TNAuthList }
TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization
TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry
TNEntry ::= CHOICE {
spid [0] ServiceProviderIdentifierList,
range [1] TelephoneNumberRange,
one E164Number }
ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF
OCTET STRING
-- When all three are present: SPID, Alt SPID, and Last Alt SPID
TelephoneNumberRange ::= SEQUENCE {
start E164Number,
count INTEGER }
E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789"))
-- TN OCSP Extension
re-ocsp-tn-query EXTENSION ::= {
SYNTAX TNQuery IDENTIFIED BY id-pkix-ocsp-stir-tn }
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TNQuery ::= E164Number
-- TN Access Descriptor
id-ad-stir-tn OBJECT IDENTIFIER ::= { id-ad TBD }
--
-- Object Identifiers
--
id-pkix-ocsp OBJECT IDENTIFIER ::= id-ad-ocsp
id-ce-TNAuthList OBJECT IDENTIFIER ::= { id-ce TBD }
id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD }
END
Authors' Addresses
Jon Peterson
Neustar, Inc.
Email: jon.peterson@neustar.biz
Sean Turner
sn3rd
Email: sean@sn3rd.com
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