Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Standards Track July 3, 2017
Expires: January 4, 2018
Short-Lived Certificates for Secure Telephone Identity
draft-peterson-stir-certificates-shortlived-01.txt
Abstract
When certificates are used as credentials to attest the assignment of
ownership of telephone numbers, some mechanism is required to provide
certificate freshness. This document specifies short-lived
certificates as a means of guaranteeing certificate freshness for
secure telephone identity (STIR), in particular relying on the
Automated Certificate Management Environment (ACME) to allow signers
to acquire certifcates as needed.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 4, 2018.
Copyright Notice
Copyright (c) 2017 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
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
Peterson Expires January 4, 2018 [Page 1]
Internet-Draft STIR Certs July 2017
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. Short-lived certificates for STIR . . . . . . . . . . . . . . 3
4. Certificate Acquisition with ACME . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The STIR problem statement [RFC7340] discusses many attacks on the
telephone network that are enabled by impersonation, including
various forms of robocalling, voicemail hacking, and swatting. One
of the most important components of a system to prevent impersonation
is the implementation of credentials which identify the parties who
control telephone numbers. The STIR certificates
[I-D.ietf-stir-certificates] 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 [I-D.ietf-stir-rfc4474bis] to sign
PASSporT objects [I-D.ietf-stir-passport] carried in a SIP [RFC3261]
request.
The STIR certificates document specifies an extension to X.509 that
defines a Telephony Number (TN) Authorization List that may be
included by certificate authorities in certificates. This extension
provides additional information that relying parties can use when
validating transactions with the certificate. When a SIP request,
for example, arrives at a terminating administrative domain, the
calling number attested by the SIP request can be compared to the TN
Authorization List of the certificate that signed the request to
determine if the caller is authorized to use that calling number in
SIP.
No specific recommendation is made in the STIR certificates document
for a means of determining the freshness of certificates with a TN
Authorization List. This document explores how short-lived
certificates could be used as a means of preserving that freshness.
Peterson Expires January 4, 2018 [Page 2]
Internet-Draft STIR Certs July 2017
Short-lived certificates also have a number of other desirable
properties that fulfill important operational requirements for
network operators. The use of the Automated Certificate Management
Environment (ACME) [I-D.ietf-acme-acme] to manage these short-lived
certificates is the focus of the architecture specified, in
particular adapting the Short-Term Automatically Renewed (STAR)
[I-D.ietf-acme-star] mechanism to STIR. The interaction of STIR with
ACME has already been explored in [I-D.peterson-acme-telephone].
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. Short-lived certificates for STIR
While there is no easy definition of what constitutes a "short-lived"
certificate, the term typically refers to certificates that are valid
only for days or even hours, as opposed to the months or years common
in traditional public key infrastructures. When the private keying
material associated with that has an expiry of months or years is
compromised by an adversary, the issuing authority must revoke the
certificate, which requires relying parties to review certificate
revocation lists or to access real-time status information with
protocols such as OCSP. Sha certificateort-lived certificates offer
an alternative where, if compromised, certificates will shortly
expire anyway, and rather than revoking and reissuing the certificate
in response to a crisis, certificates routinely roll-over and cannot
be cached for a long term by relying parties, minimizing their value
to attackers.
One of the additional benefits of using short-lived certificates is
that they do not require relying parties to perform any certificate
freshness check. The trade-off is that the signer must acquire new
certificates frequently, so the cost of round-trip times to the
certificate authority is paid on the signer's side rather than the
verifier's side; however, in environments where many parties may rely
on a single certificate, or at least where a single certificate will
be used to sign many transactions during its short lifetime, the
overall architecture will incur fewer round-trip times to the
certificate authority and thus less processing delay.
In the STIR context, the TN Authorization List defined in
[I-D.ietf-stir-certificates] adds a new wrinkle to the behavior of
short-lived certificates. Because a subject may have authority over
multiple telephone numbers, a certificate issued to that subject
Peterson Expires January 4, 2018 [Page 3]
Internet-Draft STIR Certs July 2017
could attest the authority over all, some, or just one of those
telephone numbers. If an authentication service wanted to acquire a
new certificate on a per-call basis, for example, they could acquire
a certificate that can only sign for the calling party number of the
call in question. At the other end of the spectrum, a large service
provider could acquire a certificate valid for millions of numbers,
but expire the certificate after a very short duration - on the order
of hours - to reduce the risk that the certificate would be
compromised.
This inherent flexibility in the short-lived certificate architecture
would also permit authentication services to implement very narrow
policies for certificate usage. A large service provider who wanted
to avoid revealing which phone numbers they controlled, for example,
could provide no information in the certificate that signs a call
other than just the single telephone number that corresponds to the
calling party's number. How frequently the service provider feels
that they need to expire that certificate and acquire a new one is
entirely a matter of policy to them. This makes it much harder for
entities monitoring signatures over calls to guess who owns which
numbers, and provides a much more complicated threat surface for
attackers trying to compromise the service.
In order to reduce the burden on verification services, an
authentication service could also piggyback a short-lived certificate
onto the signed SIP request, so that no network lookup and consequent
round-trip delay would be required on the terminating side to acquire
the new certificate. [I-D.ietf-stir-rfc4474bis] already provides a
way of pointing to a certificate in a MIME body associated with the
SIP request. Future work could specify other means of carrying
certificates within SIP requests via a header rather than a body, to
optimize for intermediaries adding and extracting these certificates.
4. Certificate Acquisition with ACME
One of the primary challenges facing short-lived certificates is
building an operational system that allows signers to acquire new
certificates and put them to immediate use. ACME
[I-D.ietf-acme-acme] is designed for exactly this purpose. After a
client registers with an ACME server, and the authority of the client
for the names in question is established (through means such as
[I-D.peterson-acme-telephone]), the client can at any time apply for
a certificate to be issued by sending an appropriate JSON request to
the server. That request will contain a CSR [RFC2986] indicating the
intended scope of authority as well the validity interval of the
certificate in question. Ultimately, this will enable the client to
download the certificate from a certificate URL designated by the
server.
Peterson Expires January 4, 2018 [Page 4]
Internet-Draft STIR Certs July 2017
ACME is based on the concept that clients establish accounts at an
ACME server, and that through challenges, the server learns which
identifiers it will issue for certificates requested for an account.
Any given certificate issued for an account can be for just one of
those identifiers, or potentially for more: this is determined by the
CSR that an ACME client creates for a particular order. Thus, a
service provider with authority for millions of identifiers - that
is, millions of telephone numbers - could create a CSR for an ACME
order that requests a certificate only associated with one of those
telephone numbers if it so desired. The same would be true of
certificates based on Service Provider Codes (SPCs) as described in
[I-D.ietf-stir-certificates]: a service provider might have just one
SPC or perhaps many. ACME thus puts needed flexibility into the
hands of the clients requesting certificates to determine how much of
their authority they want to invest in any given certificate.
ACME also provides a mechanism that allows the assignee of a number
to delegate temporary authority for it to a user. ACME Short-Term
Automatically-Renewed (STAR [I-D.ietf-acme-star]) certificates
provide a property of automatical renewal for ACME orders, one that
assumes that certificates issuance is based on a hierarchical
delegation. A short-term certificate attesting authority for a
particular identifier might be issued for an interval of 72 hours,
for example, by the owner of the identifer to a delegate. In the
STAR model, the interface used by the owner of the identifier and the
delegate is out of the scope of ACME, as it would be for an
adaptation of STAR to telephone numbers (likely it would be an
interface similar to MODERN [I-D.ietf-modern-problem-framework]).
STAR permits the delegate to acquire new certificates directly from
the ACME server at each renewal interval. Because the owner of the
identifier in STAR actually fulfills the ACME challenge and retrieves
the Order ID for the certificate, the owner may at any time send a
certificate termination request to the ACME server, which will
prevent the certificate from being renewed by the delegate at the
next renewal interval.
[TBD: More on adapting STAR]
[TBD: What needs to fixed for the TN Authorization List extension,
including both TN and SPC cases>]
5. IANA Considerations
This document contains no actions for the IANA.
Peterson Expires January 4, 2018 [Page 5]
Internet-Draft STIR Certs July 2017
6. Privacy Considerations
Short-lived certificates provide attractive privacy properties when
compared to real-time status query protocols like OCSP, which require
relying parties to perform a network dip that can reveal a great deal
about the source and destination of communications. For STIR, these
problems are compounded by the presence of the TN Authorization List
extension to certificates. Short-lived certificates can minimize the
data that needs to appear in the TN Authorization List, and
consequently reduce the amount of information about the caller leaked
by certificate usage to an amount equal to what is leaked by the call
signaling itself.
[More TBD]
7. Security Considerations
This document is entirely about security. For further information on
certificate security and practices, see [RFC5280], in particular its
Security Considerations.
8. Acknowledgments
Stephen Farrell provided key input to the discussions leading to this
document.
9. References
9.1. 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 | NIST FIPS PUB 186-4, "Digital
Signature Standard, version 4", 2013.
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-07 (work in progress), June 2017.
[I-D.ietf-acme-star]
Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
Fossati, "Use of Short-Term, Automatically-Renewed (STAR)
Certificates to Delegate Authority over Web Sites", draft-
ietf-acme-star-00 (work in progress), June 2017.
Peterson Expires January 4, 2018 [Page 6]
Internet-Draft STIR Certs July 2017
[I-D.ietf-stir-certificates]
Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", draft-ietf-stir-
certificates-14 (work in progress), May 2017.
[I-D.ietf-stir-passport]
Wendt, C. and J. Peterson, "Personal Assertion Token
(PASSporT)", draft-ietf-stir-passport-11 (work in
progress), February 2017.
[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-16
(work in progress), February 2017.
[I-D.peterson-acme-telephone]
Peterson, J. and R. Barnes, "ACME Identifiers and
Challenges for Telephone Numbers", draft-peterson-acme-
telephone-00 (work in progress), October 2016.
[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>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
<http://www.rfc-editor.org/info/rfc2986>.
[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,
<http://www.rfc-editor.org/info/rfc3261>.
Peterson Expires January 4, 2018 [Page 7]
Internet-Draft STIR Certs July 2017
[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>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
[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>.
[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>.
[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>.
[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>.
[RFC6818] Yee, P., "Updates to the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
2013, <http://www.rfc-editor.org/info/rfc6818>.
[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>.
Peterson Expires January 4, 2018 [Page 8]
Internet-Draft STIR Certs July 2017
[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>.
[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,
<http://www.rfc-editor.org/info/rfc7230>.
[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>.
[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.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".
9.2. Informative References
[I-D.ietf-modern-problem-framework]
Peterson, J. and T. McGarry, "Modern Problem Statement,
Use Cases, and Framework", draft-ietf-modern-problem-
framework-02 (work in progress), March 2017.
Peterson Expires January 4, 2018 [Page 9]
Internet-Draft STIR Certs July 2017
[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>.
[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>.
[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>.
[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>.
[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>.
[X.520] ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-6,
"Information technology - Open Systems Interconnection -
The Directory: Selected Attribute Types", 2012.
Author's Address
Jon Peterson
Neustar, Inc.
Email: jon.peterson@neustar.biz
Peterson Expires January 4, 2018 [Page 10]