Internet DRAFT - draft-peterson-stir-cert-delegation
draft-peterson-stir-cert-delegation
Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Standards Track March 11, 2019
Expires: September 12, 2019
STIR Certificate Delegation
draft-peterson-stir-cert-delegation-00.txt
Abstract
The Secure Telephone Identity Revisited (STIR) certificate profile
provides a way to attest authority over telephone nunbers and related
identifiers for the purpose of preventing telephone number spoofing.
This specification details how that authority can be delegated from a
parent certificate to a subordinate certificate, in cases where
service providers grant credentials to enterprises or other customers
capable of signing calls with STIR.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 12, 2019.
Copyright Notice
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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. Delegation of STIR Certificates . . . . . . . . . . . . . . . 3
3.1. Authentication Services Signing with Delegate
Certificates . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Verification Service Behavior for Delegate Certificate
Signatures . . . . . . . . . . . . . . . . . . . . . . . 4
4. Acquiring Certificate Chains in STIR . . . . . . . . . . . . 5
5. ACME and Delegation . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The STIR problem statement [RFC7340] reviews the difficulties facing
the telephone network that are enabled by impersonation, including
various forms of robocalling, voicemail hacking, and swatting. One
of the most important components of a system to prevent impersonation
is the implementation of credentials which identify the parties who
control telephone numbers. The STIR certificates [RFC8226]
specification describes a credential system based on [X.509] version
3 certificates in accordance with [RFC5280] for that purpose. Those
credentials can then be used by STIR authentication services
[RFC8224] to sign PASSporT objects [RFC8225] carried in SIP [RFC3261]
requests.
[RFC8226] specifies an extension to X.509 that defines a Telephony
Number (TN) Authorization List that may be included by certificate
authorities in certificates. This extension provides additional
information that relying parties can use when validating transactions
with the certificate. When a SIP request, for example, arrives at a
terminating administrative domain, the calling number attested by the
SIP request can be compared to the TN Authorization List of the
certificate that signed the PASSporT to determine if the caller is
authorized to use that calling number.
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Initial deployment of [RFC8226] has focused on the use of Service
Provider Codes (SPCs) to attest the scope of authority of a
certificate. Typically, these codes are internal telephone network
identifiers such as the Operating Company Numbers (OCNs) assigned to
carriers in the United States. Allocations at finer levels of
granularity, to blocks of telephone numbers or even to individual
numbers, are also desirable for enterprise use cases. [RFC8226] gave
an overview of a certificate enrollment model based on "delegation,"
whereby the holder of certificate might allocate a subset of that
certificates authority to another party. This specification details
how delegation of authority works for STIR certificates.
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. Delegation of STIR Certificates
STIR delegate certificates are certificates containing a TNAuthList
object that have been signed with the private key of a parent
certificate that itself contains a TNAuthList object. The parent
certificate needs to have its CA boolean set to "true", indicating
that that it can sign certificates. Every STIR delegate certificate
identifies its parent certificate with a standard [RFC5280] Authority
Key Identifier.
The authority bestowed on the holder of the delegate certificate by
the parent certificate is recorded in the delegate certificate's
TNAuthList. Because STIR certificates use the TNAuthList object
rather than the Subject Name for indicating the scope of their
authority, traditional [RFC5280] name constraints are not directly
applicable to STIR. In a manner similar to the RPKI [RFC6480]
"encompassing" semantics, each delegate certificate must have a
TNAuthList scope that is equal to or a subset of its parent
certificate's scope: it must be "encompassed." For example, a parent
certificate with a TNAuthList that attested authority for the
numbering range +1-212-555-1000 through 1999 could issue a
certificate to one delegate attesting authority for the range
+1-212-555-1500 through 1599, and to another delegate a certificate
for the individual number +1-212-555-1824.
Delegate certificates may themselves be issued with the CA boolean
set to "true" so that they can serve as parent certificates to
further delegates; effectively, this delegate certificate is a cross-
certificate, as its issuer is not the same as its subject. In the
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STIR ecosystem, certification authority certificates may be used to
sign PASSporTs; this removes the need for creating a redundant end-
entity certificate with an identical TNAuthList to its parent, though
if for operational or security reasons certificate holders wish to do
so, they may.
Parent certificates may have a TNAuthList containing one or more
SPCs, one or more telephone number ranges, or both. Delegations from
a parent certificate that contains only SPCs to a delegate
certificate containing a telephone number or number range are
permitted. Ascertaining whether or not a given telephone number
belongs to the service provider identified by an SPC requires access
to industry numbering databases that are outside the scope of this
specification; entities that are constructing a certificate path who
have access to those resources can validate those delegations.
3.1. Authentication Services Signing with Delegate Certificates
Authentication service behavior for delegate certificates is little
changed from baseline STIR behavior. The same checks are performed
by the authentication service, comparing the calling party number
attested in call signaling with the scope of the authority of the
signing certificate. Authentication services SHOULD NOT use a
delegate certificate without validating that its scope of authority
is encompassed by that of its parent certificate, and if that
certificate in turn has its own parent, the entire certificate path
should be validated.
Note that authentication services creating a PASSporT for a call
signed with a delegate certificate MUST provide an "x5u" link
corresponding to the entire certificate chain, rather than just the
delegate certificate used to sign the call, as described in
Section 4.
3.2. Verification Service Behavior for Delegate Certificate Signatures
The responsibility of a verification service validating PASSporTs
signed with delegate certificates, while largely following baseline
[RFC8224] and [RFC8225], requires some additional procedures. When
the verification service dereferences the "x5u" parameter, it will
acquire a certificate list rather than a single certificate. It MUST
then validate all of the credentials in the list, identifying the
parent certificate for each delegate through its AKID object.
While ordinarily, relying parties have significant latitude in path
construction when validating a certificate chain, STIR assumes a more
rigid hierarchical subordination model, rather than one where relying
parties may want to derive their own chains to particular trust
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anchors. If the certificate chain acquired from the "x5u" element of
a PASSporT does not lead to an anchor that the verification service
trusts, it treats the validation no differently than it would when a
non-delegated certificate was issued by an untrust root; in SIP, it
MAY return a 437 "Unsupported Credential" response if the call should
be failed for lack of a valid Identity header.
4. Acquiring Certificate Chains in STIR
PASSporT [RFC8225] uses the "x5u" element to convey the URL where
verification services can acquire the certificate used to sign a
PASSporT. This value is mirrored by the "info" parameter of the
Identity header when a PASSporT is conveyed via SIP. Commonly, this
is an HTTPS URI.
When a STIR delegate certificate is used to sign a PASSporT, the
"x5u" element in the PASSporT will contain a URI indicating where a
certificate list is available. That list will be a concatenation of
PEM encoded certificates of the type "application/pem-certificate-
chain" defined in [I-D.ietf-acme-acme]. The list begins with the
certificate used to sign the PASSporT, followed by its parent, and
then any subsequent grandparents, great-grandparents, and so on. The
ordering MUST conform to the AKID/SKID order chain encoded in the
certs themselves. Note that ACME requires the first element in a
pem-certificate-chain to be an end-entity certificate; STIR relaxes
this requirement, as CA certificates are permitted to sign PASSporTs,
so the first element in a pem-certificate-chain used for STIR MAY be
a CA certificate.
5. ACME and Delegation
STIR deployments commonly use ACME [I-D.ietf-acme-acme] for
certificate acquisition, and it is anticipated that delegate
certificates as well will be acquired through an ACME interface. An
entity that wishes to acquire a certificate from a particular CA will
request an Authority Token [I-D.ietf-acme-authority-token] from the
parent with the desired TNAuthList
[I-D.ietf-acme-authority-token-tnauthlist] object. Note that if the
client wishes to do further subdelegation of its own, it should
request a token with the "ca" Authority Token flag set.
The entity then presents that Authority Token to a certificate
authority to acquire a STIR delegate certificate. ACME returns an
"application/pem-certificate-chain" object with suitable for
publishing as an HTTPS resource for retrieval with the PASSporT "x5u"
mechanism as discussed in Section 4. If the CSR presented to the
ACME server is for a certificate with the CA boolean set to "true",
then the ACME server makes a policy decision to determine whether or
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not it is appropriate to issue that certificate to the requesting
entity. In most ACME cases, that policy decision will be made based
on the "ca" flag in the Authority Token.
6. IANA Considerations
This document contains no actions for the IANA.
7. Privacy Considerations
[TBD.]
8. Security Considerations
This document is entirely about security. For further information on
certificate security and practices, see [RFC5280], in particular its
Security Considerations.
9. Acknowledgments
We would like to thank Richard Barnes, Chris Wendt, Dave Hancock,
Russ Housley, and Sean Turner for key input to the discussions
leading to this document.
10. References
10.1. Normative References
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", draft-ietf-acme-acme-18 (work in progress),
December 2018.
[I-D.ietf-acme-authority-token]
Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
Challenges Using an Authority Token", draft-ietf-acme-
authority-token-01 (work in progress), October 2018.
[I-D.ietf-acme-authority-token-tnauthlist]
Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
"TNAuthList profile of ACME Authority Token", draft-ietf-
acme-authority-token-tnauthlist-01 (work in progress),
October 2018.
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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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC7044] Barnes, M., Audet, F., Schubert, S., van Elburg, J., and
C. Holmberg, "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC 7044,
DOI 10.17487/RFC7044, February 2014,
<https://www.rfc-editor.org/info/rfc7044>.
[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>.
[RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224,
DOI 10.17487/RFC8224, February 2018,
<https://www.rfc-editor.org/info/rfc8224>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
10.2. Informative 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 | NIST FIPS PUB 186-4, "Digital
Signature Standard, version 4", 2013.
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[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>.
[RFC5806] Levy, S. and M. Mohali, Ed., "Diversion Indication in
SIP", RFC 5806, DOI 10.17487/RFC5806, March 2010,
<https://www.rfc-editor.org/info/rfc5806>.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
February 2012, <https://www.rfc-editor.org/info/rfc6480>.
[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>.
[X.509] ITU-T Recommendation X.509 (10/2012) | ISO/IEC 9594-8,
"Information technology - Open Systems Interconnection -
The Directory: Public-key and attribute certificate
frameworks", 2012.
[X.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".
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Author's Address
Jon Peterson
Neustar, Inc.
Email: jon.peterson@team.neustar
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