Internet DRAFT - draft-peterson-stir-certificates
draft-peterson-stir-certificates
Network Working Group J. Peterson
Internet-Draft NeuStar
Intended status: Standards Track S. Turner
Expires: August 18, 2014 IECA
February 14, 2014
Secure Telephone Identity Credentials: Certificates
draft-peterson-stir-certificates-00.txt
Abstract
In order to provide a means of proving ownership of telephone numbers
on the Internet, some kind of public structure needs to exist that
binds cryptographic keys to authority over telephone numbers. This
document describes a certificate-based credential system for
telephone numbers, which could be used as a part of a broader
architecture for managing telephone numbers as identities in
protocols like SIP.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 18, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Enrollment and Authorization . . . . . . . . . . . . . . . . 3
3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4
3.2. Provisioning Private Keying Material . . . . . . . . . . 5
4. Acquiring Credentials to Verify Signatures . . . . . . . . . 5
4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6
4.2. Certificate Freshness and Revocation . . . . . . . . . . 8
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Informative References . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
As is discussed in the STIR problem statement [13], the primary
enabler of robocalling, vishing, swatting and related attacks is 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 an authority responsible for issuing credentials to
parties who control telephone numbers. With these credentials,
parties can prove that they are in fact authorized to use telephony
numbers, and thus distinguish themselves from impersonators unable to
present credentials. This document describes a credential system for
telephone numbers based on X.509 version 3 certificates in accordance
with [7]. While telephone numbers have long been a part of the X.509
standard, the certificates described in this document may contain
telephone number blocks or ranges, and accordingly it uses an
alternate syntax.
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In the STIR in-band architecture, two basic types of entities need
access to these credentials: authentication services, and
verification services (or verifiers); see [15]. An authentication
service must be operated by an entity enrolled with the certificate
authority (see Section 3), whereas a verifier need only trust the
root certificate of the authority, and have a means to acquire and
validate certificates.
The STIR out-of-band architecture is not considered in this document.
[TBD]
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in RFC 2119 [1] and RFC 6919 [2].
3. Enrollment and Authorization
This document assumes a threefold model for certificate enrollment.
The first enrollment model is one where the certificate authority
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.
The second enrollment model is one where a certificate authority
requires that an entity prove control by means of some sort of test.
For example, an authority might send a text message to a telephone
number containing a URL (which might be deferenced 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) may
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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 customer organization for a block of 100
numbers, 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 [3]; 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.
[TBD] Future versions of this specification will also discuss methods
of partial delegation, where certificate holders delegate only part
of their authority. For example, an individual assignee may want to
delegate authority to an entity for text messages associated with
their telephone number, but not for other functions.
3.1. Certificate Scope and Structure
The subjects of telephone number certificates are the administrative
entities to whom numbers are assigned or delegated. For example, a
LEC might hold a certificate for a range of telephone numbers.
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 is capable of 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
RFC 5280 but the telephone number authorization extension is defined
in this document.
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3.2. Provisioning Private Keying Material
In order for authentication services to sign calls via the procedures
described in [15], they must possess a private key corresponding to a
certificate with authority over the calling number. This
specification does not require that any particular entity 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 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 old 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 [8].
4. 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 circumstances of
their acquistion, there is no need to standardize any single
mechanism for this purpose. All entities that comply with [15]
necessarily support SIP, and consequently SIP itself can serve as a
way to acquire certificates. This specific does allow delivery
through alternate means as well.
The simplest way for a verifier to acquire the certificate needed to
verify a signature is for the certificate be conveyed along with the
signature itself. In SIP, for example, a certificate could be
carried in a multipart MIME body [9], and the URI in the Identity-
Info header could specify that body with a CID URI [10]. However, in
many environments this is not feasible due to message size
restrictions or lack of necessary support for multipart MIME.
Alternatively, the Identity-Info header of a SIP request may contain
a URI that the verifier dereferences with a network call.
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 [11].
A verifier can however have access to a service that grants access to
certificates for a particular telephone number. Note however that
there may be multiple valid certificates that can sign a call setup
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request for a telephone number, and that as a consequence, there
needs to be some discriminator that the signer uses to identify their
credentials. The Identity-Info header itself can serve just such a
discriminator.
4.1. Verifying Certificate Scope
The subjects of these certificates 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
for the purpose of validating a caller's identity, this certificate
extension establishes whether or not a signer is authorized to sign
for a particular number.
The TN Authorization List certificate extension is identified by the
following object identifier:
id-ce-TNAuthList OBJECT IDENTIFIER ::= { 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 ServiceProviderIdentifierList,
range 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"))
[TBD- do we really need to do IA5String? The alternative would be
UTF8String, e.g.: UTF8String (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) 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, and
3) a single telephone number can be listed.
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
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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 will be detailed in
future version of this specification.
4.2. Certificate Freshness and Revocation
The problem of certificate freshness gains a new wrinkle in the
telephone number context, 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 the signer
is still a proper authority for that number.
This document therefore recommends the use of OCSP in high-volume
environments for validating the freshness of certificates, per [12].
[TBD - depending on our algorithm choices this profile may need to be
further profiled.]
5. Acknowledgments
Russ Housley, Brian Rosen, Cullen Jennings and Eric Rescorla provided
key input to the discussions leading to this document.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
This document is entirely about security. For further information on
certificate security and practices, see RFC 3280 [5], in particular
its Security Considerations.
8. Informative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
for Use in RFCs to Indicate Requirement Levels", RFC 6919,
April 1 2013.
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[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002.
[5] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[6] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[7] 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, May 2008.
[8] Turner, S., "Asymmetric Key Packages", RFC 5958, August
2010.
[9] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[10] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[11] Pritikin, M., Yee, P., and D. Harkins, "Enrollment over
Secure Transport", RFC 7030, October 2013.
[12] Deacon, A. and R. Hurst, "The Lightweight Online
Certificate Status Protocol (OCSP) Profile for High-Volume
Environments", RFC 5019, September 2007.
[13] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement", draft-ietf-stir-
problem-statement-03 (work in progress), January 2014.
[14] Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.
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[15] Peterson, J., Jennings, C., and E. Rescorla,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-jennings-stir-
rfc4474bis-00 (work in progress), October 2013.
Authors' Addresses
Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@neustar.biz
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Farifax, VA 22031
US
Email: turners@ieca.com
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