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Internet Engineering Task Force (IETF) J. Peterson
Request for Comments: 8226 Neustar
Category: Standards Track S. Turner
ISSN: 2070-1721 sn3rd
February 2018
Secure Telephone Identity Credentials: Certificates
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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8226.
Copyright Notice
Copyright (c) 2018 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
(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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Peterson & Turner Standards Track [Page 1]
RFC 8226 STIR Certs February 2018
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Authority for Telephone Numbers in Certificates .................4
4. Certificate Usage with STIR .....................................5
5. Enrollment and Authorization Using the TN Authorization List ....6
5.1. Constraints on Signing PASSporTs ...........................8
5.2. Certificate Extension Scope and Structure ..................8
6. Provisioning Private Keying Material ............................9
7. Acquiring Credentials to Verify Signatures ......................9
8. JWT Claim Constraints Syntax ...................................10
9. TN Authorization List Syntax ...................................12
10. Certificate Freshness and Revocation ..........................14
10.1. Acquiring the TN List by Reference .......................15
11. IANA Considerations ...........................................16
11.1. ASN.1 Registrations ......................................16
11.2. Media Type Registrations .................................16
12. Security Considerations .......................................17
13. References ....................................................18
13.1. Normative References .....................................18
13.2. Informative References ...................................20
Appendix A. ASN.1 Module ..........................................21
Acknowledgments ...................................................24
Authors' Addresses ................................................24
1. Introduction
The Secure Telephone Identity Revisited (STIR) problem statement
[RFC7340] identifies the primary enabler of robocalling, vishing
(voicemail hacking), 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 Public
Switched Telephone Network (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; therefore, appearing to call 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 that identify the
parties who control telephone numbers. With these credentials,
parties can assert that they are in fact authorized to use telephony
numbers (TNs), and thus they distinguish themselves from
Peterson & Turner Standards Track [Page 2]
RFC 8226 STIR Certs February 2018
impersonators unable to present such credentials. For that reason,
the STIR threat model [RFC7375] stipulates that "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 Telephony 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 [RFC8224], 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 also 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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Peterson & Turner Standards Track [Page 3]
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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.
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 Codes
(SPCs) such as Operating Company Numbers (OCNs) or Service Provider
Identifiers (SPIDs) via the TN Authorization List specified in this
document. 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 Signaling System 7 (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 the 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.
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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 [RFC8224]
accommodates either of these approaches: the methods for
canonicalizing, for 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, 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 10).
4. Certificate Usage with STIR
[RFC8224], 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 [RFC3986] 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 [RFC7230], CID
(Content-ID) [RFC2392], 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 9 identifies the scope of authority of the
certificate.
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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 the Elliptic Curve
Digital Signature Algorithm (ECDSA) with the P-256 curve (see
[DSS]) and RSA PKCS #1 v1.5 ("PKCS" stands for "Public-Key
Cryptography Standards") (see [RFC8017], Section 8.2) for
certificate signatures. Implementers are advised that the latter
algorithm is mandated only as a transitional mechanism, due to
its widespread use in existing PKIs, 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 [RFC8225], which in turn follows the 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 JWT Claim
Constraints certificate extension.
o See Section 7 for requirements related to relying parties
acquiring credentials.
o See Sections 10 and 10.1 for requirements related to certificate
freshness and the Authority Information Access (AIA) certificate
extension.
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 Administration (NANPA),
who is in turn directed by the national regulator. LECs may also
Peterson & Turner Standards Track [Page 6]
RFC 8226 STIR Certs February 2018
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
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
that, as with certification authorities for web PKI, might either be
(1) automated or (2) 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 a
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.
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5.1. Constraints on Signing PASSporTs
The public key in the certificate is used to validate the signature
on a JWT [RFC7519] that conforms to the conventions specified in
PASSporT [RFC8225]. This specification supports constraints on the
JWT claims, thereby allowing the CA to grant different permissions to
certificate holders -- for example, those enrolled from
proof-of-possession versus delegation. A Certificate Policy (CP) and
a Certification Practice Statement (CPS) [RFC3647] are produced as
part of the normal PKI bootstrapping process (i.e., the CP is written
first, and then the CA says how it conforms to the CP in the CPS). A
CA that wishes to place constraints on the JWT claims MUST include
the JWT Claim Constraints certificate extension in issued
certificates. See Section 8 for information about the certificate
extension.
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
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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 [RFC8224], they must hold a private key corresponding to
a certificate with authority over the calling number. [RFC8224]
does not require that any particular entity in a SIP deployment
architecture sign requests, 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 that 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 Cryptographic Message Syntax (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 [RFC8224] necessarily
support SIP, and consequently SIP itself can serve as a way to
deliver certificates. [RFC8224] provides an "info" parameter of the
Identity header; this parameter contains a URI for the credential
used to generate the Identity header. [RFC8224] also requires that
documents that 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 to 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.
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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 and
HTTPS.
Note well that as an optimization, a verifier may have access to a
service, a cache, or other local store that grants access to
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. JWT Claim Constraints Syntax
Certificate subjects are limited to specific values for PASSporT
claims with the JWT Claim Constraints certificate extension; issuers
permit all claims by omitting the JWT Claim Constraints certificate
extension from the certificate's extension field [RFC5280]. The
extension is non-critical, applicable only to end-entity
certificates, and defined with ASN.1 [X.680] [X.681] [X.682] [X.683]
later in this section. The syntax of the claims is given in
PASSporT; specifying new claims follows the procedures in [RFC8225],
Section 8.3.
This certificate extension is optional, but if present, it constrains
the claims that authentication services may include in the PASSporT
objects they sign. Constraints are applied by issuers and enforced
by verifiers when validating PASSporT claims as follows:
1. mustInclude indicates claims that MUST appear in the PASSporT in
addition to iat, orig, and dest. The baseline claims of PASSporT
("iat", "orig", and "dest") are considered to be permitted by
default and SHOULD NOT be included. If mustInclude is absent,
iat, orig, and dest MUST appear in the PASSporT.
2. permittedValues indicates that if the claim name is present, the
claim MUST contain one of the listed values.
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Consider two examples with a PASSporT claim called "confidence" with
values "low", "medium", and "high":
o If a CA issues to an authentication service a certificate that
contains the mustInclude JWTClaimName "confidence", then an
authentication service MUST include the "confidence" claim in all
PASSporTs it generates; a verification service will treat as
invalid any PASSporT it receives with a PASSporT claim that
does not include the "confidence" claim.
o If a CA issues to an authentication service a certificate that
contains the permittedValues JWTClaimName "confidence" and a
permitted "high" value, then an authentication service will treat
as invalid any PASSporT it receives with a PASSporT claim that
does not include the "confidence" claim with a "high" value.
The JWT Claim Constraints certificate extension is identified by the
following object identifier (OID), which is defined under the id-pe
OID arc defined in [RFC5280] and managed by IANA (see Section 11):
id-pe-JWTClaimConstraints OBJECT IDENTIFIER ::= { id-pe 27 }
The JWT Claim Constraints certificate extension has the following
syntax:
JWTClaimConstraints ::= SEQUENCE {
mustInclude [0] JWTClaimNames OPTIONAL,
-- The listed claim names MUST appear in the PASSporT
-- in addition to iat, orig, and dest. If absent, iat, orig,
-- and dest MUST appear in the PASSporT.
permittedValues [1] JWTClaimPermittedValuesList OPTIONAL }
-- If the claim name is present, the claim MUST contain one of
-- the listed values.
( WITH COMPONENTS { ..., mustInclude PRESENT } |
WITH COMPONENTS { ..., permittedValues PRESENT } )
JWTClaimPermittedValuesList ::= SEQUENCE SIZE (1..MAX) OF
JWTClaimPermittedValues
JWTClaimPermittedValues ::= SEQUENCE {
claim JWTClaimName,
permitted SEQUENCE SIZE (1..MAX) OF UTF8String }
JWTClaimNames ::= SEQUENCE SIZE (1..MAX) OF JWTClaimName
JWTClaimName ::= IA5String
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9. 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
non-critical TN Authorization List certificate extension is included
in the certificate's extension field [RFC5280]. The extension is
defined with ASN.1 [X.680] [X.681] [X.682] [X.683]. The syntax and
semantics of the extension are as follows.
The subjects of certificates containing the TN Authorization List
extension are the administrative entities to whom numbers are
assigned or delegated. In an end-entity certificate, the TN
Authorization List indicates the TNs that it has authorized. In a CA
certificate, the TN Authorization List limits the set of TNs for
certification paths that include this certificate.
The TN Authorization List certificate extension is identified by the
following object identifier (OID), which is defined under the id-pe
OID arc defined in [RFC5280] and managed by IANA (see Section 11):
id-pe-TNAuthList OBJECT IDENTIFIER ::= { id-pe 26 }
The TN Authorization List certificate extension has the following
syntax:
TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNEntry
TNEntry ::= CHOICE {
spc [0] ServiceProviderCode,
range [1] TelephoneNumberRange,
one [2] TelephoneNumber
}
ServiceProviderCode ::= IA5String
-- SPCs may be OCNs, various SPIDs, or other SP identifiers
-- from the telephone network.
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TelephoneNumberRange ::= SEQUENCE {
start TelephoneNumber,
count INTEGER (2..MAX),
...
}
TelephoneNumber ::= 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. SPCs as described in this document are a generic term for the
identifiers used to designate service providers in telephone
networks today. In North American context, these would include
OCNs as specified in [ATIS-0300251], related SPIDs, or other
similar identifiers for service providers. SPCs can be used to
indirectly name all of the telephone numbers associated with that
identifier for a service provider.
2. Telephone numbers can be listed in a range (in the
TelephoneNumberRange format), which consists of a starting
telephone number and then an integer count of numbers within the
range, where the valid boundaries of ranges may vary according to
national policies. The count field is only applicable to start
fields whose values do not include "*" or "#" (i.e., a
TelephoneNumber that does not include "*" or "#"). count
MUST NOT make the number increase in length (i.e., a
TelephoneNumberRange with TelephoneNumber=10 and count=91 is
invalid); formally, given the inputs count and TelephoneNumber of
length D, TelephoneNumber + count MUST be less than 10^D.
3. A single telephone number can be listed (as a TelephoneNumber).
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
the 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 (1) securely download the list of numbers
associated with a certificate or (2) verify that a single number is
under the authority of this certificate. For more on this
optimization, see Section 10.1.
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10. 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 with telephone
numbers or number ranges (as opposed to SPCs), 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 created the PASSporT, which includes a digital signature, is
still a proper authority for that number.
To verify the status of such a 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., the Online
Certificate Status Protocol (OCSP)).
The trade-off 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 some approaches to
generating a short-lived certificate, like requiring one for each
call, would incur a greater operational cost than acquiring status
information. This document makes no particular recommendation for a
means of determining certificate freshness for STIR, as this requires
further study and implementation experience. Acquiring online status
information for certificates has the potential to disclose private
information [RFC7258] if proper precautions are not taken. Future
specifications that define certificate freshness mechanisms for STIR
MUST note any such risks and provide countermeasures where possible.
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10.1. Acquiring the TN List by Reference
One alternative to checking certificate status for a particular
telephone number is simply acquiring the TN Authorization List by
reference, that is, through dereferencing a URL in the certificate,
rather than including the value of the TN Authorization List in the
certificate itself.
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 certificate status, one that
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 TNs in the list are
authorized for use by the caller. As with the certificate extension
defined in Section 9, a URI dereferenced from an end-entity
certificate will indicate the TNs that the caller has been
authorized. Verifiers MUST support the AIA extension, and the
dereferenced URI from a CA certificate limits the set of TNs for
certification paths that include this certificate.
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-stirTNList", which uses the following AIA OID:
id-ad-stirTNList OBJECT IDENTIFIER ::= { id-ad 14 }
When the "id-ad-stirTNList" accessMethod is used, the accessLocation
MUST be an HTTPS URI. Dereferencing the URI will return the complete
DER-encoded TN Authorization List (see Section 9) for the certificate
with a Content-Type of application/tnauthlist (see Section 11.2).
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
up-to-date and complete cache for the numbers associated with crucial
certificate holders could give an important boost to performance.
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11. IANA Considerations
11.1. ASN.1 Registrations
This document makes use of object identifiers for the TN certificate
extension defined in Section 9, the "TN List by reference" AIA access
descriptor defined in Section 10.1, and the ASN.1 module identifier
defined in Appendix A. Therefore, per this document, IANA has made
the following assignments, as shown on
<https://www.iana.org/assignments/smi-numbers>:
o TN Authorization List certificate extension in the "SMI Security
for PKIX Certificate Extension" (1.3.6.1.5.5.7.1) registry:
26 id-pe-TNAuthList
o JWT Claim Constraints certificate extension in the "SMI Security
for PKIX Certificate Extension" (1.3.6.1.5.5.7.1) registry:
27 id-pe-JWTClaimConstraints
o TN List by reference access descriptor in the "SMI Security for
PKIX Access Descriptor" (1.3.6.1.5.5.7.48) registry:
14 id-ad-stirTNList
o The TN ASN.1 module in the "SMI Security for PKIX Module
Identifier" (1.3.6.1.5.5.7.0) registry:
89 id-mod-tn-module
11.2. Media Type Registrations
Type name: application
Subtype name: tnauthlist
Required parameters: None
Optional parameters: None
Encoding considerations: Binary
Security considerations: See Section 12 of RFC 8226
Interoperability considerations:
The TN Authorization List inside this media type MUST be
DER-encoded TNAuthorizationList.
Published specification: RFC 8226
Applications that use this media type:
Issuers and relying parties of secure telephone identity
certificates, to limit the subject's authority to a
particular telephone number or telephone number range.
Fragment identifier considerations: None
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Additional information:
Deprecated alias names for this type: None
Magic number(s): None
File extension(s): None
Macintosh File Type Code(s): None
Person & email address to contact for further information:
Jon Peterson <jon.peterson@team.neustar>
Intended usage: COMMON
Restrictions on usage: None
Author: Sean Turner <sean@sn3rd.com>
Change controller: The IESG <iesg@ietf.org>
12. Security Considerations
This document is entirely about security. For further information on
certificate security and practices, see [RFC5280], in particular its
Security Considerations section.
If a certification authority issues a certificate attesting authority
over many telephone numbers, the TNAuthList element can divulge to
relying parties extraneous telephone numbers associated with the
certificate that have no bearing on any given call in progress. The
potential privacy risk can be exacerbated by the use of AIA, as
described in Section 10.1, to link many thousands of numbers to a
single certificate. Even an SPC in a certificate can be used to link
a certificate to a particular carrier and, with access to industry
databases, potentially the set of numbers associated with that SPC.
While these practices may not cause concern in some environments, in
other scenarios alternative approaches could minimize the data
revealed to relying parties. For example, a service provider with
authority over a large block of numbers could generate short-lived
certificates for individual TNs that are not so easily linked to the
service provider or any other numbers that the service provider
controls. Optimizations to facilitate acquiring short-lived
certificates are a potential area of future work for STIR.
The TN Authorization List returned through a dereferenced URI is
served over HTTPS; the TN Authorization List is therefore protected
in transit. But, the TN Authorization List served is not a signed
object and therefore the server is trusted to faithfully return the
TN Authorization List provided to it by the list generator.
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13. References
13.1. Normative References
[ATIS-0300251]
ATIS Recommendation 0300251, "Codes for Identification of
Service Providers for Information Exchange", 2007.
[DSS] National Institute of Standards and Technology, U.S.
Department of Commerce, "Digital Signature Standard
(DSS)", NIST FIPS PUB 186-4, DOI 10.6028/NIST.FIPS.186-4,
July 2013, <http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.186-4.pdf>.
[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>.
[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
<https://www.rfc-editor.org/info/rfc2392>.
[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>.
[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>.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958,
DOI 10.17487/RFC5958, August 2010,
<https://www.rfc-editor.org/info/rfc5958>.
[RFC7230] Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
Peterson & Turner Standards Track [Page 18]
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[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258,
May 2014, <https://www.rfc-editor.org/info/rfc7258>.
[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>.
[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
"PKCS #1: RSA Cryptography Specifications Version 2.2",
RFC 8017, DOI 10.17487/RFC8017, November 2016,
<https://www.rfc-editor.org/info/rfc8017>.
[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>.
[X.509] International Telecommunication Union, "Information
technology - Open Systems Interconnection - The Directory:
Public-key and attribute certificate frameworks", ITU-T
Recommendation X.509, ISO/IEC 9594-8, October 2016,
<https://www.itu.int/rec/T-REC-X.509>.
[X.680] International Telecommunication Union, "Information
Technology - Abstract Syntax Notation One (ASN.1):
Specification of basic notation", ITU-T Recommendation
X.680, ISO/IEC 8824-1, August 2015,
<https://www.itu.int/rec/T-REC-X.680>.
[X.681] International Telecommunication Union, "Information
Technology - Abstract Syntax Notation One (ASN.1):
Information object specification", ITU-T Recommendation
X.681, ISO/IEC 8824-2, August 2015,
<https://www.itu.int/rec/T-REC-X.681>.
Peterson & Turner Standards Track [Page 19]
RFC 8226 STIR Certs February 2018
[X.682] International Telecommunication Union, "Information
Technology - Abstract Syntax Notation One (ASN.1):
Constraint specification", ITU-T Recommendation
X.682, ISO/IEC 8824-3, August 2015,
<https://www.itu.int/rec/T-REC-X.682>.
[X.683] International Telecommunication Union, "Information
Technology - Abstract Syntax Notation One (ASN.1):
Parameterization of ASN.1 specifications", ITU-T
Recommendation X.683, ISO/IEC 8824-4, August 2015,
<https://www.itu.int/rec/T-REC-X.683>.
13.2. Informative References
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[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,
<https://www.rfc-editor.org/info/rfc3647>.
[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>.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014,
<https://www.rfc-editor.org/info/rfc7375>.
[X.520] International Telecommunication Union, "Information
technology - Open Systems Interconnection - The Directory:
Selected attribute types", ITU-T Recommendation
X.520, ISO/IEC 9594-6, October 2016,
<https://www.itu.int/rec/T-REC-X.520>.
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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], [X.681], [X.682], and [X.683].
The modules defined in this document are compatible with the most
current ASN.1 specifications published in 2015 (see [X.680], [X.681],
[X.682], and [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-2016
{ iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-tn-module(89) }
DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
id-ad, id-pe
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) }
;
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--
-- JWT Claim Constraints Certificate Extension
--
ext-jwtClaimConstraints EXTENSION ::= {
SYNTAX JWTClaimConstraints IDENTIFIED BY id-pe-JWTClaimConstraints
}
id-pe-JWTClaimConstraints OBJECT IDENTIFIER ::= { id-pe 27 }
JWTClaimConstraints ::= SEQUENCE {
mustInclude [0] JWTClaimNames OPTIONAL,
-- The listed claim names MUST appear in the PASSporT
-- in addition to iat, orig, and dest. If absent, iat, orig,
-- and dest MUST appear in the PASSporT.
permittedValues [1] JWTClaimPermittedValuesList OPTIONAL }
-- If the claim name is present, the claim MUST contain one of
-- the listed values.
( WITH COMPONENTS { ..., mustInclude PRESENT } |
WITH COMPONENTS { ..., permittedValues PRESENT } )
JWTClaimPermittedValuesList ::= SEQUENCE SIZE (1..MAX) Of
JWTClaimPermittedValues
JWTClaimPermittedValues ::= SEQUENCE {
claim JWTClaimName,
permitted SEQUENCE SIZE (1..MAX) OF UTF8String }
JWTClaimNames ::= SEQUENCE SIZE (1..MAX) OF JWTClaimName
JWTClaimName ::= IA5String
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--
-- Telephony Number Authorization List Certificate Extension
--
ext-tnAuthList EXTENSION ::= {
SYNTAX TNAuthorizationList IDENTIFIED BY id-pe-TNAuthList
}
id-pe-TNAuthList OBJECT IDENTIFIER ::= { id-pe 26 }
TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNEntry
TNEntry ::= CHOICE {
spc [0] ServiceProviderCode,
range [1] TelephoneNumberRange,
one [2] TelephoneNumber
}
ServiceProviderCode ::= IA5String
-- SPCs may be OCNs, various SPIDs, or other SP identifiers
-- from the telephone network.
TelephoneNumberRange ::= SEQUENCE {
start TelephoneNumber,
count INTEGER (2..MAX),
...
}
TelephoneNumber ::= IA5String (SIZE (1..15)) (FROM ("0123456789#*"))
-- TN Access Descriptor
id-ad-stirTNList OBJECT IDENTIFIER ::= { id-ad 14 }
END
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Acknowledgments
Anders Kristensen, Russ Housley, Brian Rosen, Cullen Jennings, Dave
Crocker, Tony Rutkowski, John Braunberger, Eric Rescorla, and Martin
Thomson provided key input to the discussions leading to this
document. Russ Housley provided some direct assistance and text
surrounding the ASN.1 module.
Authors' Addresses
Jon Peterson
Neustar, Inc.
Email: jon.peterson@neustar.biz
Sean Turner
sn3rd
Email: sean@sn3rd.com
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