wdiff draft-ietf-stir-certificates-01.txt draft-ietf-stir-certificates-02.txt

STIR
Network Working Group J. Peterson
Internet-Draft NeuStar Neustar
Intended status: Standards Track S. Turner
Expires: September 25, 2015 January 6, 2016 IECA
March 24,
July 5, 2015

Secure Telephone Identity Credentials: Certificates
draft-ietf-stir-certificates-01.txt
draft-ietf-stir-certificates-02.txt

Abstract

In order to prove ownership of telephone numbers on the Internet,
some kind of public infrastructure 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

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
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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
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This Internet-Draft will expire on September 22, 2015. January 6, 2016.

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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
4.2.1. Choosing a Verification Method . . . . . . . . . . . 8
4.2.2. Using OCSP with STIR Certificates . . . . . . . . . . 9
4.2.2.1. OCSP Extension Specification . . . . . . . . . . 10
4.2.3. Acquiring TN Lists By Reference . . . . . . . . . 7 . . 11
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 12
8. Informative References . . . . . . . . . . . . . . . . . . . 12
Appendix A. ASN.1 Module . 10 . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 15

1. Introduction

As is discussed in the STIR problem statement [13],
[I-D.ietf-stir-problem-statement], 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
Public Switched Telephone Network (PSTN) 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]. [RFC5280]. 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.

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]. [I-D.ietf-stir-rfc4474bis].
An authentication service must be operated by an entity enrolled with
the certification 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.

This document attempts to specify only the basic elements necessary
for this architecture. Only through deployment experience will it be
possible to decide directions for future work.

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] [RFC2119] and RFC 6919 [2]. [RFC6919].

3. Enrollment and Authorization

This document assumes a threefold model for certificate enrollment.

The first enrollment model is one where the certification authority
(CA)
acts in concert with national numbering authorities to issue
credentials to those parties to whom numbers are assigned. In the
United States, for example, telephone number blocks are assigned to
Local Exchange Carriers (LECs) by the North American Numbering Plan
Administrator (NANPA), who is in turn directed by the national
regulator. LECs may also receive numbers in smaller allocations,
through number pooling, or via an individual assignment through
number portability. LECs assign numbers to customers, who may be
private individuals or organizations - and organizations take
responsibility for assigning numbers within their own enterprise.

The second enrollment model is one where a certification 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 dereferenced by the
recipient) as a means of verifying that a user has control of
terminal corresponding to that number. Checks of this form are
frequently used in commercial systems today to validate telephone
numbers provided by users. This is comparable to existing enrollment
systems used by some certificate authorities for issuing S/MIME
credentials for email by verifying that the party applying for a
credential receives mail at the email address in question.

The third enrollment model is delegation: that is, the holder of a
certificate (assigned by either of the two methods above) may
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
Public Key Infrastructures (PKIs) who use the name constraints
extension [3]; [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.

[TBD] Future versions of this specification may address adding a
level of assurance indication to certificates to differentiate those
enrolled from proof-of-possession versus delegation.

[TBD] Future versions of this specification may also discuss 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.

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.

[TBD - what if the subject is considered a privacy leak?]

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
[7]
[RFC5280] but the telephone number authorization extension is defined
in this document.

3.2. Provisioning Private Keying Material

In order for authentication services to sign calls via the procedures
described in [15], [I-D.ietf-stir-rfc4474bis], 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 hold
the private key and sign calls.

The specification recommends distribution of private keys through
PKCS#8 objects signed by a trusted entity, for example through the
CMS package specified in [8]. [RFC5958].

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, acquisition, there is no need to standardize any single
mechanism for this purpose. All entities that comply with [15]
[I-D.ietf-stir-rfc4474bis] 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], [RFC2046], and the URI in the Identity-
Info
Identity-Info header could specify that body with a CID URI [10].
[RFC2392]. 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]. [RFC7030].

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
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 as 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 telephone number Telephony Number (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:

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

[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
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 signed is
still a proper authority for that number.

To verify the status of the certificate, the verifier needs the
certificate, which is included with the call, and they then would need to:

o to
either:

Rely on short-lived certificates and not check the certificate's
status, or

Rely on status information from the authority; there are three
common mechanisms employed by CAs:

* Certificate Revocation Lists (CRLs) [7],
* Online Certificate Status Protocol (OCSP) [RFC6560], and
* Server-based Certificate Validation Protocol (SCVP) [RFC5055]. authority

The tradeoff between short lived certificates and using status
information is the former's burden is on the front end (i.e.,
enrollment) and the latter's burden is on the back end (i.e.,
verification). Both impact call setup time, but it is assumed that
performing enrollment for each call is more of an impact that using
status information. This document therefore recommends relying on
status information.

4.2.1. Choosing a Verification Method

There are three common certificate verification mechanisms employed
by CAs:

Certificate Revocation Lists (CRLs) [RFC5280]

Online Certificate Status Protocol (OCSP) [RFC6960], and

Server-based Certificate Validation Protocol (SCVP) [RFC5055].

When relying on status information, the verifier needs to obtain the
status information - but before that can happen happen, the verifier needs
to know where to locate it. Placing the location of the status
information in the certificate makes the certificate larger larger, but it
eases the client workload. The CRL Distribution Point certificate
extension includes the location of the CRL and the Authority
Information Access certificate extension includes the location of
OCSP and/or SCVP servers; both of these extensions are defined in
[7].
[RFC5280]. In all cases, the status information location is provided
in the form of an URI.

CRLs are an obviously attractive solution because they are supported
by every CA. CRLs have a reputation of being quite large (10s of
MBytes)
MBytes), because CAs maintain and issue one monolithic CRL with all
of their revoked certificates certificates, but CRLs do support a variety of
mechanisms to scope the size of the CRLs based on revocation reasons
(e.g., key compromise vs CA compromise), user certificates only, and
CA certificates only as well as just operationally deciding to keep
the CRLs small. Scoping However, scoping the CRL though introduces other issues
(i.e., does the RP have all of the CRL partitions).

CAs in this system the STIR architecture will likely all create CRLs for audit purposes
purposes, but it not recommended NOT RECOMMENDED that they be relying upon for status
information. Instead, one of the two "online" options is
recommended. preferred.
Between the two, OCSP is much more widely deployed and this document
therefore recommends the use of OCSP in high-volume environments for
validating the freshness of certificates, based on
[12]. Note that [RFC6960],
incorporating some (but not all) of the optimizations of [RFC5019].

4.2.2. Using OCSP responses have with STIR Certificates

Certificates compliant with this specification therefore SHOULD
include a URL pointing to an OCSP service in the Authority
Information Access (AIA) certificate extension, via the "id-ad-ocsp"
accessMethod specified in [RFC5280]. Baseline OCSP however supports
only three possible response values: good, revoked, or unknown.

[TBD] HVE With
some extension, OCSP requires SHA-1 be used as the hash algorithm, we're
obviously going to change this to be SHA-256.

[TBD] What would happen in the unknown case?

The wrinkle here is that OCSP only provides status information it
does not indicate whether the certificate's certificate is
authorized for the a particular telephone number that the verifier is
validating. There's

[TBD] What would happen in the unknown case? Can we profile OCSP
usage so that unknown is never returned for our extension?

At a high level, there are two ways
to ask the that a client might pose this
authorization question:

For this certificate, is the following number currently in its
scope of validity?

o
What are all the telephone numbers associated with this certificate?

The
certificate, or this certificate subject?

Only the former seems to lend lends itself to piggybacking on the OCSP status
mechanism; since the verifier is already asking an authority about
the certificate's status status, why not use reuse that mechanism mechanism, instead of
creating a new service that requires additional round trips. trips? Like
most PKIX-developed protocols, OCSP is extensible; OCSP supports
request extensions (OCSP supports (including sending multiple requests at once) and
per-request extensions. It seems unlikely that the verifier will be
requesting authorization checks on multiple callers telephone numbers in one request
request, so a per-request extension is what is needed. But, support for any
particular extension is optional and the

[TBD] HVE OCSP profile [12]
prohibits requires SHA-1 be used as the use hash algorithm,
we're6960 obviously going to change this to be SHA-256.

The requirement to consult OCSP in real time results in a network
round-trip time of per-request extensions so there day, which is some
additional work required something to modify existing consider because it
will add to the call setup time. OCSP responders. server implementations
commonly pre-generate responses, and to speed up HTTPS connections,
servers often provide OCSP responses for each certificate in their
hierarchy. If possible, both of these OCSP concepts should be
adopted for use with STIR.

4.2.2.1. OCSP Extension Specification

The extension mechanism itself is fairly straightforward and it's
based on the for OCSP follows X.509 v3 certificate extensions:
extensions, and thus requires an OID, a criticality flag, and ASN.1
syntax as defined by the OID. The OID would be
registered in the IANA PKIX arc, the criticality would likely be set
to critical (i.e., if specified here is
optional: per [RFC6960] Section 4.4, support for all OCSP extensions
is optional. If the OCSP responder doesn't server does not understand the
extension stop processing), and requested
extension, it will still provide the syntax can be anything we desire.
Applying baseline validation of the KISS principle,
certificate itself. Moreover, in practical STIR deployments, the syntax could simply be
issuer of the TN being
asserted by caller. certificate will set the accessLocation for the OCSP
AIA extension to point to an OCSP service that supports this
extension, so the risk of interoperability failure due to lack of
support for this extension is minimal.

The responder could then determine whether OCSP TNQuery extension is included as one of the
TN asserted
requestExtensions in requests. It may also appear in the
responseExtensions. When an OCSP per-request extension server includes a number in the
responseExtensions, this informs the client that the certificate is
still authorized valid for the certificate referred to number that appears in the certificate request field; TNQuery extension
field. If the
reference TNQuery is absent from a tuple of hash algorithm, issuer name hash, issuer key
hash, and serial number.

The second option seems more like response to a query response type of
interaction and could be initiated through
containing a URI included TNQuery in the
certificate. Luckily, the AIA extension supports such a mechanism;
it's an OID to identify the "access method" and an "access location",
which would most most likely be a URI. The verifier would its requestExtensions, then
follow the URI server is not
able to ascertain whether validate that the list number is still in the scope of authority
of TNs authorized for
use by the caller. There are obviously some privacy considerations
with this approach.

The need to check certificate.

id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD }

TNQuery ::= E164Number

Note that HVE OCSP profile [RFC5019] prohibits the authorizations in another round-trip is also
something to consider because use of per-request
extensions. As it is anticipated that STIR will add to the call setup time.
OCSP implementations commonly pre-generate responses and to speed up
HTTPS connections the server provides use OCSP responses for each
certificate in their hierarchy. If possible, both a high-
volume environment, many of the optimizations recommended by HVE are
desirable for the STIR environment. This document therefore uses
these extensions in a baseline OCSP
concepts should be adopted. environment with some HVE
optimizations. [More TBD]

Ideally, once a certificate has been acquired by a verifier, some
sort of asynchronous mechanism could notify and update the verifier
if the scope of the certificate changes. changes so that verifiers could
implement a cache. While not all possible categories of verifiers
could implement such behavior, some sort of event-driven notification
of certificate status is another potential subject of future work.
One potential direction is that a future SIP SUBSCRIBE/NOTIFY-based
accessMethod for AIA might be defined (which would also be applicable
to the method described in the following section) by some future
specification.

4.2.3. Acquiring TN Lists By Reference

Acquiring a list of the telephone numbers associated with a
certificate or its subject lends itself to an application-layer
query/response interaction outside of OCSP, one which could be
initiated through a separate URI included in the certificate. The
AIA extension (see [RFC5280]) supports such a mechanism: it
designates an OID to identify the accessMethod and an accessLocation,
which would most likely be a URI. A verifier would then follow the
URI to ascertain whether the list of TNs authorized for use by the
caller.

HTTPS is the most obvious candidate for a protocol to be used for
fetching the list of telephone number associated with a particular
certificate. This document defines a new AIA accessMethod, called
"id-ad-stir-tn", which uses the following AIA OID:

id-ad-stir-tn OBJECT IDENTIFIER ::= { id-ad TBD }

When the "id-ad-stir-tn" accessMethod is used, the accessLocation
MUST be an HTTPS URI. The document returned by dereferencing that
URI will contain the complete TN Authorization List (see Section 4.1)
for the certificate.

Delivering the entire list of telephone numbers associated with a
particular certificate will divulge to STIR verifiers information
about telephone numbers other than the one associated with the
particular call that the verifier is checking. In some environments,
where STIR verifiers handle a high volume of calls, maintaining an
up-to-date and complete cache for the numbers associated with crucial
certificate holders could give an important boost to performance.

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 at this time. If we define an document makes use of object identifiers for the TN Certificate
Extension defined in Section 4.1, TN-HVE OCSP extension or in
Section 4.2.2.1, and the TN by reference AIA access method then we'll need an OID from descriptor
defined in Section 4.2.3. It therefore requests that the IANA make
the following assignments:

- TN Certificate Extension in the SMI Security for PKIX
Certificate Extension registry: http://www.iana.org/assignments/
smi-numbers/smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.1

- TN-HVE OCSP extension in the SMI Security for PKIX Online
Certificate Status Protocol (OCSP) registry:
http://www.iana.org/assignments/smi-numbers/smi-numbers.xhtml#smi-
numbers-1.3.6.1.5.5.7.48.1

- TNS by reference access descriptor in the
PKIX. SMI Security for PKIX
Access Descriptor registry: http://www.iana.org/assignments/smi-
numbers/smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.48

7. Security Considerations

This document is entirely about security. For further information on
certificate security and practices, see RFC 3280 [5], [RFC3280], in
particular its Security Considerations.

8. Informative References

[1]

[I-D.ietf-stir-problem-statement]
Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
draft-ietf-stir-problem-statement-05 (work in progress),
May 2014.

[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., and E. Rescorla,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-03
(work in progress), March 2015.

[I-D.peterson-sipping-retarget]
Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.

[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[2] Barnes, R., Kent, S.,

[RFC2392] Levinson, E., "Content-ID and E. Message-ID Uniform Resource
Locators", RFC 2392, August 1998.

[RFC2818] Rescorla, "Further Key Words
for Use in RFCs to Indicate Requirement Levels", E., "HTTP Over TLS", RFC 6919,
April 1 2013.

[3] 2818, May 2000.

[RFC3261] 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]

[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263, June
2002.

[5]

[RFC3280] 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",

[RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online
Certificate Status Protocol (OCSP) Profile for High-Volume
Environments", RFC 2818, May 2000.

[7] 5019, September 2007.

[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, December 2007.

[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, May 2008.

[8]

[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, August
2010.

[9] Freed, N.

[RFC6919] Barnes, R., Kent, S., and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", E. Rescorla, "Further Key Words
for Use in RFCs to Indicate Requirement Levels", RFC 2046,
November 1996.

[10] Levinson, E., "Content-ID 6919,
April 2013.

[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and Message-ID Uniform Resource
Locators", C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 2392, August 1998.

[11] 6960, June 2013.

[RFC7030] 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

[RFC7299] Housley, R., "Object Identifier Registry for High-Volume
Environments", the PKIX
Working Group", RFC 5019, September 2007.

[13] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
draft-ietf-stir-problem-statement-05 (work in progress),
May 7299, July 2014.

[14] Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", draft-peterson-sipping-
retarget-00 (work in progress), February 2005.

[15] Peterson, J., Jennings, C., and E. Rescorla,
"Authenticated Identity Management in

[X.680] USC/Information Sciences Institute, "Information
Technology - Abstract Syntax Notation One.", ITU-T X.680,
ISO/IEC 8824-1:2002, 2002.

[X.681] USC/Information Sciences Institute, "Information
Technology - Abstract Syntax Notation One: Information
Object Specification", ITU-T X.681, ISO/IEC 8824-2:2002,
2002.

[X.682] USC/Information Sciences Institute, "Information
Technology - Abstract Syntax Notation One: Constraint
Specification", ITU-T X.682, ISO/IEC 8824-3:2002, 2002.

[X.683] USC/Information Sciences Institute, "Information
Technology - Abstract Syntax Notation One:
Parameterization of ASN.1 Specifications", ITU-T X.683,
ISO/IEC 8824-4:2002, 2002.

Appendix A. ASN.1 Module

This appendix provides the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-02
(work normative ASN.1 [X.680] definitions for
the structures described in progress), October 2014. this specification using ASN.1, as
defined in [X.680] through [X.683].

TBD

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