< draft-ietf-stir-certificates-01.txt		draft-ietf-stir-certificates-02.txt >
	STIR Working Group J. Peterson		Network Working Group J. Peterson
	Internet-Draft NeuStar		Internet-Draft Neustar
	Intended status: Standards Track S. Turner		Intended status: Standards Track S. Turner
	Expires: September 25, 2015 IECA		Expires: January 6, 2016 IECA
	March 24, 2015		July 5, 2015
	Secure Telephone Identity Credentials: Certificates		Secure Telephone Identity Credentials: Certificates
	draft-ietf-stir-certificates-01.txt		draft-ietf-stir-certificates-02.txt
	Abstract		Abstract
	In order to prove ownership of telephone numbers on the Internet,		In order to prove ownership of telephone numbers on the Internet,
	some kind of public infrastructure needs to exist that binds		some kind of public infrastructure needs to exist that binds
	cryptographic keys to authority over telephone numbers. This		cryptographic keys to authority over telephone numbers. This
	document describes a certificate-based credential system for		document describes a certificate-based credential system for
	telephone numbers, which could be used as a part of a broader		telephone numbers, which could be used as a part of a broader
	architecture for managing telephone numbers as identities in		architecture for managing telephone numbers as identities in
	protocols like SIP.		protocols like SIP.
	skipping to change at page 1, line 37 ¶		skipping to change at page 1, line 37 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on September 22, 2015.		This Internet-Draft will expire on January 6, 2016.
	Copyright Notice		Copyright Notice
	Copyright (c) 2015 IETF Trust and the persons identified as the		Copyright (c) 2015 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
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	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	Table of Contents		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
			2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2		3. Enrollment and Authorization . . . . . . . . . . . . . . . . 3
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3		3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4
	3. Enrollment and Authorization . . . . . . . . . . . . . . . . . 3		3.2. Provisioning Private Keying Material . . . . . . . . . . 5
	3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4		4. Acquiring Credentials to Verify Signatures . . . . . . . . . 5
	3.2. Provisioning Private Keying Material . . . . . . . . . . . 5		4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6
	4. Acquiring Credentials to Verify Signatures . . . . . . . . . . 5		4.2. Certificate Freshness and Revocation . . . . . . . . . . 8
	4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6		4.2.1. Choosing a Verification Method . . . . . . . . . . . 8
	4.2. Certificate Freshness and Revocation . . . . . . . . . . . 7		4.2.2. Using OCSP with STIR Certificates . . . . . . . . . . 9
	5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10		4.2.2.1. OCSP Extension Specification . . . . . . . . . . 10
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10		4.2.3. Acquiring TN Lists By Reference . . . . . . . . . . . 11
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 10		5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
	8. Informative References . . . . . . . . . . . . . . . . . . . . 10		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12		7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
			8. Informative References . . . . . . . . . . . . . . . . . . . 12
			Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 14
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
	1. Introduction		1. Introduction
	As is discussed in the STIR problem statement [13], the primary		As is discussed in the STIR problem statement
	enabler of robocalling, vishing, swatting and related attacks is the		[I-D.ietf-stir-problem-statement], the primary enabler of
	capability to impersonate a calling party number. The starkest		robocalling, vishing, swatting and related attacks is the capability
	examples of these attacks are cases where automated callees on the		to impersonate a calling party number. The starkest examples of
	Public Switched Telephone Network (PSTN) rely on the calling number		these attacks are cases where automated callees on the PSTN rely on
	as a security measure, for example to access a voicemail system.		the calling number as a security measure, for example to access a
	Robocallers use impersonation as a means of obscuring identity; while		voicemail system. Robocallers use impersonation as a means of
	robocallers can, in the ordinary PSTN, block (that is, withhold)		obscuring identity; while robocallers can, in the ordinary PSTN,
	their caller identity, callees are less likely to pick up calls from		block (that is, withhold) their caller identity, callees are less
	blocked identities, and therefore appearing to calling from some		likely to pick up calls from blocked identities, and therefore
	number, any number, is preferable. Robocallers however prefer not to		appearing to calling from some number, any number, is preferable.
	call from a number that can trace back to the robocaller, and		Robocallers however prefer not to call from a number that can trace
	therefore they impersonate numbers that are not assigned to them.		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		One of the most important components of a system to prevent
	impersonation is an authority responsible for issuing credentials to		impersonation is an authority responsible for issuing credentials to
	parties who control telephone numbers. With these credentials,		parties who control telephone numbers. With these credentials,
	parties can prove that they are in fact authorized to use telephony		parties can prove that they are in fact authorized to use telephony
	numbers, and thus distinguish themselves from impersonators unable to		numbers, and thus distinguish themselves from impersonators unable to
	present credentials. This document describes a credential system for		present credentials. This document describes a credential system for
	telephone numbers based on X.509 version 3 certificates in accordance		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		with [RFC5280]. While telephone numbers have long been a part of the
	standard, the certificates described in this document may contain		X.509 standard, the certificates described in this document may
	telephone number blocks or ranges, and accordingly it uses an		contain telephone number blocks or ranges, and accordingly it uses an
	alternate syntax.		alternate syntax.
	In the STIR in-band architecture, two basic types of entities need		In the STIR in-band architecture, two basic types of entities need
	access to these credentials: authentication services, and		access to these credentials: authentication services, and
	verification services (or verifiers); see [15]. An authentication		verification services (or verifiers); see [I-D.ietf-stir-rfc4474bis].
	service must be operated by an entity enrolled with the certification		An authentication service must be operated by an entity enrolled with
	authority (see Section 3), whereas a verifier need only trust the		the certification authority (see Section 3), whereas a verifier need
	root certificate of the authority, and have a means to acquire and		only trust the root certificate of the authority, and have a means to
	validate certificates.		acquire and validate certificates.
	This document attempts to specify only the basic elements necessary		This document attempts to specify only the basic elements necessary
	for this architecture. Only through deployment experience will it be		for this architecture. Only through deployment experience will it be
	possible to decide directions for future work.		possible to decide directions for future work.
	2. Terminology		2. Terminology
	In this document, the key words "MUST", "MUST NOT", "REQUIRED",		In this document, the key words "MUST", "MUST NOT", "REQUIRED",
	"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT		"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
	RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as		RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
	described in RFC 2119 [1] and RFC 6919 [2].		described in RFC 2119 [RFC2119] and RFC 6919 [RFC6919].
	3. Enrollment and Authorization		3. Enrollment and Authorization
	This document assumes a threefold model for certificate enrollment.		This document assumes a threefold model for certificate enrollment.
	The first enrollment model is one where the certification authority		The first enrollment model is one where the certification authority
	(CA) acts in concert with national numbering authorities to issue		acts in concert with national numbering authorities to issue
	credentials to those parties to whom numbers are assigned. In the		credentials to those parties to whom numbers are assigned. In the
	United States, for example, telephone number blocks are assigned to		United States, for example, telephone number blocks are assigned to
	Local Exchange Carriers (LECs) by the North American Numbering Plan		Local Exchange Carriers (LECs) by the North American Numbering Plan
	Administrator (NANPA), who is in turn directed by the national		Administrator (NANPA), who is in turn directed by the national
	regulator. LECs may also receive numbers in smaller allocations,		regulator. LECs may also receive numbers in smaller allocations,
	through number pooling, or via an individual assignment through		through number pooling, or via an individual assignment through
	number portability. LECs assign numbers to customers, who may be		number portability. LECs assign numbers to customers, who may be
	private individuals or organizations - and organizations take		private individuals or organizations - and organizations take
	responsibility for assigning numbers within their own enterprise.		responsibility for assigning numbers within their own enterprise.
	The second enrollment model is one where a certification authority		The second enrollment model is one where a certification authority
	requires that an entity prove control by means of some sort of test.		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		For example, an authority might send a text message to a telephone
	number containing a URL (which might be deferenced by the recipient)		number containing a URL (which might be dereferenced by the
	as a means of verifying that a user has control of terminal		recipient) as a means of verifying that a user has control of
	corresponding to that number. Checks of this form are frequently		terminal corresponding to that number. Checks of this form are
	used in commercial systems today to validate telephone numbers		frequently used in commercial systems today to validate telephone
	provided by users. This is comparable to existing enrollment systems		numbers provided by users. This is comparable to existing enrollment
	used by some certificate authorities for issuing S/MIME credentials		systems used by some certificate authorities for issuing S/MIME
	for email by verifying that the party applying for a credential		credentials for email by verifying that the party applying for a
	receives mail at the email address in question.		credential receives mail at the email address in question.
	The third enrollment model is delegation: that is, the holder of a		The third enrollment model is delegation: that is, the holder of a
	certificate (assigned by either of the two methods above) may		certificate (assigned by either of the two methods above) may
	delegate some or all of their authority to another party. In some		delegate some or all of their authority to another party. In some
	cases, multiple levels of delegation could occur: a LEC, for example,		cases, multiple levels of delegation could occur: a LEC, for example,
	might delegate authority to customer organization for a block of 100		might delegate authority to customer organization for a block of 100
	numbers, and the organization might in turn delegate authority for a		numbers, and the organization might in turn delegate authority for a
	particular number to an individual employee. This is analogous to		particular number to an individual employee. This is analogous to
	delegation of organizational identities in traditional hierarchical		delegation of organizational identities in traditional hierarchical
	Public Key Infrastructures (PKIs) who use the name constraints		Public Key Infrastructures (PKIs) who use the name constraints
	extension [3]; the root CA delegates names in sales to the sales		extension [RFC5280]; the root CA delegates names in sales to the
	department CA, names in development to the development CA, etc. As		sales department CA, names in development to the development CA, etc.
	lengthy certificate delegation chains are brittle, however, and can		As lengthy certificate delegation chains are brittle, however, and
	cause delays in the verification process, this document considers		can cause delays in the verification process, this document considers
	optimizations to reduce the complexity of verification.		optimizations to reduce the complexity of verification.
	[TBD] Future versions of this specification may address adding a		[TBD] Future versions of this specification may address adding a
	level of assurance indication to certificates to differentiate those		level of assurance indication to certificates to differentiate those
	enrolled from proof-of-possession versus delegation.		enrolled from proof-of-possession versus delegation.
	[TBD] Future versions of this specification may also discuss methods		[TBD] Future versions of this specification may also discuss methods
	of partial delegation, where certificate holders delegate only part		of partial delegation, where certificate holders delegate only part
	of their authority. For example, individual assignees may want to		of their authority. For example, individual assignees may want to
	delegate to a service authority for text messages associated with		delegate to a service authority for text messages associated with
	their telephone number, but not for other functions.		their telephone number, but not for other functions.
	3.1. Certificate Scope and Structure		3.1. Certificate Scope and Structure
	The subjects of telephone number certificates are the administrative		The subjects of telephone number certificates are the administrative
	entities to whom numbers are assigned or delegated. For example, a		entities to whom numbers are assigned or delegated. For example, a
	LEC might hold a certificate for a range of telephone numbers. [TBD		LEC might hold a certificate for a range of telephone numbers.
	- what if the subject is considered a privacy leak?]
			[TBD - what if the subject is considered a privacy leak?]
	This specification places no limits on the number of telephone		This specification places no limits on the number of telephone
	numbers that can be associated with any given certificate. Some		numbers that can be associated with any given certificate. Some
	service providers may be assigned millions of numbers, and may wish		service providers may be assigned millions of numbers, and may wish
	to have a single certificate that is capable of signing for any one		to have a single certificate that is capable of signing for any one
	of those numbers. Others may wish to compartmentalize authority over		of those numbers. Others may wish to compartmentalize authority over
	subsets of the numbers they control.		subsets of the numbers they control.
	Moreover, service providers may wish to have multiple certificates		Moreover, service providers may wish to have multiple certificates
	with the same scope of authority. For example, a service provider		with the same scope of authority. For example, a service provider
	with several regional gateway systems may want each system to be		with several regional gateway systems may want each system to be
	capable of signing for each of their numbers, but not want to have		capable of signing for each of their numbers, but not want to have
	each system share the same private key.		each system share the same private key.
	The set of telephone numbers for which a particular certificate is		The set of telephone numbers for which a particular certificate is
	valid is expressed in the certificate through a certificate		valid is expressed in the certificate through a certificate
	extension; the certificate's extensibility mechanism is defined in		extension; the certificate's extensibility mechanism is defined in
	[7] but the telephone number authorization extension is defined in		[RFC5280] but the telephone number authorization extension is defined
	this document.		in this document.
	3.2. Provisioning Private Keying Material		3.2. Provisioning Private Keying Material
	In order for authentication services to sign calls via the procedures		In order for authentication services to sign calls via the procedures
	described in [15], they must possess a private key corresponding to a		described in [I-D.ietf-stir-rfc4474bis], they must possess a private
	certificate with authority over the calling number. This		key corresponding to a certificate with authority over the calling
	specification does not require that any particular entity sign		number. This specification does not require that any particular
	requests, only that it be an entity with an appropriate private key;		entity sign requests, only that it be an entity with an appropriate
	the authentication service role may be instantiated by any entity in		private key; the authentication service role may be instantiated by
	a SIP network. For a certificate granting authority only over a		any entity in a SIP network. For a certificate granting authority
	particular number which has been issued to an end user, for example,		only over a particular number which has been issued to an end user,
	an end user device might hold the private key and generate the		for example, an end user device might hold the private key and
	signature. In the case of a service provider with authority over		generate the signature. In the case of a service provider with
	large blocks of numbers, an intermediary might hold the private key		authority over large blocks of numbers, an intermediary might hold
	and sign calls.		the private key and sign calls.
	The specification recommends distribution of private keys through		The specification recommends distribution of private keys through
	PKCS#8 objects signed by a trusted entity, for example through the		PKCS#8 objects signed by a trusted entity, for example through the
	CMS package specified in [8].		CMS package specified in [RFC5958].
	4. Acquiring Credentials to Verify Signatures		4. Acquiring Credentials to Verify Signatures
	This specification documents multiple ways that a verifier can gain		This specification documents multiple ways that a verifier can gain
	access to the credentials needed to verify a request. As the		access to the credentials needed to verify a request. As the
	validity of certificates does not depend on the circumstances of		validity of certificates does not depend on the circumstances of
	their acquistion, there is no need to standardize any single		their acquisition, there is no need to standardize any single
	mechanism for this purpose. All entities that comply with [15]		mechanism for this purpose. All entities that comply with
	necessarily support SIP, and consequently SIP itself can serve as a		[I-D.ietf-stir-rfc4474bis] necessarily support SIP, and consequently
	way to acquire certificates. This specific does allow delivery		SIP itself can serve as a way to acquire certificates. This specific
	through alternate means as well.		does allow delivery through alternate means as well.
	The simplest way for a verifier to acquire the certificate needed to		The simplest way for a verifier to acquire the certificate needed to
	verify a signature is for the certificate be conveyed along with the		verify a signature is for the certificate be conveyed along with the
	signature itself. In SIP, for example, a certificate could be		signature itself. In SIP, for example, a certificate could be
	carried in a multipart MIME body [9], and the URI in the Identity-		carried in a multipart MIME body [RFC2046], and the URI in the
	Info header could specify that body with a CID URI [10]. However, in		Identity-Info header could specify that body with a CID URI
	many environments this is not feasible due to message size		[RFC2392]. However, in many environments this is not feasible due to
	restrictions or lack of necessary support for multipart MIME.		message size restrictions or lack of necessary support for multipart
			MIME.
	Alternatively, the Identity-Info header of a SIP request may contain		Alternatively, the Identity-Info header of a SIP request may contain
	a URI that the verifier dereferences with a network call.		a URI that the verifier dereferences with a network call.
	Implementations of this specification are required to support the use		Implementations of this specification are required to support the use
	of SIP for this function (via the SUBSCRIBE/NOTIFY mechanism), as		of SIP for this function (via the SUBSCRIBE/NOTIFY mechanism), as
	well as HTTP, via the Enrollment over Secure Transport mechanisms		well as HTTP, via the Enrollment over Secure Transport mechanisms
	described in RFC 7030 [11].		described in RFC 7030 [RFC7030].
	A verifier can however have access to a service that grants access to		A verifier can however have access to a service that grants access to
	certificates for a particular telephone number. Note however that		certificates for a particular telephone number. Note however that
	there may be multiple valid certificates that can sign a call setup		there may be multiple valid certificates that can sign a call setup
	request for a telephone number, and that as a consequence, there		request for a telephone number, and that as a consequence, there
	needs to be some discriminator that the signer uses to identify their		needs to be some discriminator that the signer uses to identify their
	credentials. The Identity-Info header itself can serve as such a		credentials. The Identity-Info header itself can serve as such a
	discriminator.		discriminator.
	4.1. Verifying Certificate Scope		4.1. Verifying Certificate Scope
	The subjects of these certificates are the administrative entities to		The subjects of these certificates are the administrative entities to
	whom numbers are assigned or delegated. When a verifier is		whom numbers are assigned or delegated. When a verifier is
	validating a caller's identity, local policy always determines the		validating a caller's identity, local policy always determines the
	circumstances under which any particular subject may be trusted, but		circumstances under which any particular subject may be trusted, but
	for the purpose of validating a caller's identity, this certificate		for the purpose of validating a caller's identity, this certificate
	extension establishes whether or not a signer is authorized to sign		extension establishes whether or not a signer is authorized to sign
	for a particular number.		for a particular number.
	The telephone number (TN) Authorization List certificate extension is		The Telephony Number (TN) Authorization List certificate extension is
	identified by the following object identifier:		identified by the following object identifier:
	id-ce-TNAuthList OBJECT IDENTIFIER ::= { TBD }		id-ce-TNAuthList OBJECT IDENTIFIER ::= { TBD }
	The TN Authorization List certificate extension has the following		The TN Authorization List certificate extension has the following
	syntax:		syntax:
	TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization		TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNAuthorization
	TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry		TNAuthorization ::= SEQUENCE SIZE (1..MAX) OF TNEntry
	TNEntry ::= CHOICE {		TNEntry ::= CHOICE {
	spid ServiceProviderIdentifierList,		spid ServiceProviderIdentifierList,
	range TelephoneNumberRange,		range TelephoneNumberRange,
	one E164Number }		one E164Number }
	ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF		ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF
	OCTET STRING
			OCTET STRING
	-- When all three are present: SPID, Alt SPID, and Last Alt SPID		-- When all three are present: SPID, Alt SPID, and Last Alt SPID
	TelephoneNumberRange ::= SEQUENCE {		TelephoneNumberRange ::= SEQUENCE {
	start E164Number,		start E164Number,
	count INTEGER }		count INTEGER }
	E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789"))		E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789"))
	[TBD] Do we really need to do IA5String? The alternative would be		[TBD- do we really need to do IA5String? The alternative would be
	UTF8String, e.g.: UTF8String (SIZE (1..15)) (FROM ("0123456789"))		UTF8String, e.g.: UTF8String (SIZE (1..15)) (FROM ("0123456789")) ]
	The TN Authorization List certificate extension indicates the		The TN Authorization List certificate extension indicates the
	authorized phone numbers for the call setup signer. It indicates one		authorized phone numbers for the call setup signer. It indicates one
	or more blocks of telephone number entries that have been authorized		or more blocks of telephone number entries that have been authorized
	for use by the call setup signer. There are three ways to identify		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		the block: 1) a Service Provider Identifier (SPID) can be used to
	indirectly name all of the telephone numbers associated with that		indirectly name all of the telephone numbers associated with that
	service provider, 2) telephone numbers can be listed in a range, and		service provider, 2) telephone numbers can be listed in a range, and
	3) a single telephone number can be listed.		3) a single telephone number can be listed.
	skipping to change at page 7, line 41 ¶		skipping to change at page 8, line 24 ¶
	telephone number context, because verifiers must establish not only		telephone number context, because verifiers must establish not only
	that a certificate remains valid, but also that the certificate's		that a certificate remains valid, but also that the certificate's
	scope contains the telephone number that the verifier is validating.		scope contains the telephone number that the verifier is validating.
	Dynamic changes to number assignments can occur due to number		Dynamic changes to number assignments can occur due to number
	portability, for example. So even if a verifier has a valid cached		portability, for example. So even if a verifier has a valid cached
	certificate for a telephone number (or a range containing the		certificate for a telephone number (or a range containing the
	number), the verifier must determine that the entity that signed is		number), the verifier must determine that the entity that signed is
	still a proper authority for that number.		still a proper authority for that number.
	To verify the status of the certificate, the verifier needs the		To verify the status of the certificate, the verifier needs the
	certificate, which is included with the call, and they need to:		certificate, which is included with the call, and then would need to
			either:
	o Rely on short-lived certificates and not check the certificate's		Rely on short-lived certificates and not check the certificate's
	status, or		status, or
	o Rely on status information from the authority; there are three		Rely on status information from the authority
	common mechanisms employed by CAs:
	* Certificate Revocation Lists (CRLs) [7],
	* Online Certificate Status Protocol (OCSP) [RFC6560], and
	* Server-based Certificate Validation Protocol (SCVP) [RFC5055].
	The tradeoff between short lived certificates and using status		The tradeoff between short lived certificates and using status
	information is the former's burden is on the front end (i.e.,		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.,		enrollment) and the latter's burden is on the back end (i.e.,
	verification). Both impact call setup time, but it is assumed that		verification). Both impact call setup time, but it is assumed that
	performing enrollment for each call is more of an impact that using		performing enrollment for each call is more of an impact that using
	status information. This document therefore recommends relying on		status information. This document therefore recommends relying on
	status information.		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		When relying on status information, the verifier needs to obtain the
	status information but before that can happen the verifier needs to		status information - but before that can happen, the verifier needs
	know where to locate it. Placing the location of the status		to know where to locate it. Placing the location of the status
	information in the certificate makes the certificate larger but it		information in the certificate makes the certificate larger, but it
	eases the client workload. The CRL Distribution Point certificate		eases the client workload. The CRL Distribution Point certificate
	extension includes the location of the CRL and the Authority		extension includes the location of the CRL and the Authority
	Information Access certificate extension includes the location of		Information Access certificate extension includes the location of
	OCSP and/or SCVP servers; both of these extensions are defined in		OCSP and/or SCVP servers; both of these extensions are defined in
	[7]. In all cases, the status information location is provided in		[RFC5280]. In all cases, the status information location is provided
	the form of an URI.		in the form of an URI.
	CRLs are an obviously attractive solution because they are supported		CRLs are an obviously attractive solution because they are supported
	by every CA. CRLs have a reputation of being quite large (10s of		by every CA. CRLs have a reputation of being quite large (10s of
	MBytes) because CAs issue one with all of their revoked certificates		MBytes), because CAs maintain and issue one monolithic CRL with all
	but CRLs do support a variety of mechanisms to scope the size of the		of their revoked certificates, but CRLs do support a variety of
	CRLs based on revocation reasons (e.g., key compromise vs CA		mechanisms to scope the size of the CRLs based on revocation reasons
	compromise), user certificates only, and CA certificates only as well		(e.g., key compromise vs CA compromise), user certificates only, and
	as just operationally deciding to keep the CRLs small. Scoping the		CA certificates only as well as just operationally deciding to keep
	CRL though introduces other issues (i.e., does the RP have all of the		the CRLs small. However, scoping the CRL introduces other issues
	CRL partitions). CAs in this system will likely all create CRLs for		(i.e., does the RP have all of the CRL partitions).
	audit purposes but it not recommended that they be relying upon for
	status information. Instead, one of the two "online" options is
	recommended. 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 OCSP responses have three possible values: good,
	revoked, or unknown.
	[TBD] HVE OCSP requires SHA-1 be used as the hash algorithm, we're		CAs in the STIR architecture will likely all create CRLs for audit
	obviously going to change this to be SHA-256.		purposes, but it NOT RECOMMENDED that they be relying upon for status
			information. Instead, one of the two "online" options is 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 [RFC6960],
			incorporating some (but not all) of the optimizations of [RFC5019].
	[TBD] What would happen in the unknown case?		4.2.2. Using OCSP with STIR Certificates
	The wrinkle here is that OCSP only provides status information it		Certificates compliant with this specification therefore SHOULD
	does not indicate whether the certificate's is authorized for the		include a URL pointing to an OCSP service in the Authority
	telephone number that the verifier is validating. There's two ways		Information Access (AIA) certificate extension, via the "id-ad-ocsp"
	to ask the authorization question:		accessMethod specified in [RFC5280]. Baseline OCSP however supports
			only three possible response values: good, revoked, or unknown. With
			some extension, OCSP would not indicate whether the certificate is
			authorized for a particular telephone number that the verifier is
			validating.
	o For this certificate, is the following number currently in its		[TBD] What would happen in the unknown case? Can we profile OCSP
	scope of validity?		usage so that unknown is never returned for our extension?
	o What are the numbers associated with this certificate?		At a high level, there are two ways that a client might pose this
			authorization question:
	The former seems to lend itself to piggybacking on the status		For this certificate, is the following number currently in its
			scope of validity?
			What are all the telephone numbers associated with this
			certificate, or this certificate subject?
			Only the former lends itself to piggybacking on the OCSP status
	mechanism; since the verifier is already asking an authority about		mechanism; since the verifier is already asking an authority about
	the certificate's status why not use that mechanism instead of		the certificate's status, why not reuse that mechanism, instead of
	creating a new service that requires additional round trips. Like		creating a new service that requires additional round trips? Like
	most PKIX-developed protocols, OCSP is extensible; OCSP supports		most PKIX-developed protocols, OCSP is extensible; OCSP supports
	request extensions (OCSP supports sending multiple requests at once)		request extensions (including sending multiple requests at once) and
	and per-request extensions. It seems unlikely that the verifier will		per-request extensions. It seems unlikely that the verifier will be
	be requesting authorization checks on multiple callers in one request		requesting authorization checks on multiple telephone numbers in one
	so a per-request extension is what is needed. But, support for any		request, so a per-request extension is what is needed.
	particular extension is optional and the HVE OCSP profile [12]
	prohibits the use of per-request extensions so there is some
	additional work required to modify existing OCSP responders.
	The extension mechanism itself is fairly straightforward and it's		[TBD] HVE OCSP requires SHA-1 be used as the hash algorithm,
	based on the X.509 v3 certificate extensions: an OID, a criticality		we're6960 obviously going to change this to be SHA-256.
	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 the OCSP responder doesn't understand the
	extension stop processing), and the syntax can be anything we desire.
	Applying the KISS principle, the syntax could simply be the TN being
	asserted by caller. The responder could then determine whether the
	TN asserted in the OCSP per-request extension is still authorized for
	the certificate referred to in the certificate request field; the
	reference is a tuple of hash algorithm, issuer name hash, issuer key
	hash, and serial number.
	The second option seems more like a query response type of		The requirement to consult OCSP in real time results in a network
	interaction and could be initiated through a URI included in the		round-trip time of day, which is something to consider because it
	certificate. Luckily, the AIA extension supports such a mechanism;		will add to the call setup time. OCSP server implementations
	it's an OID to identify the "access method" and an "access location",		commonly pre-generate responses, and to speed up HTTPS connections,
	which would most most likely be a URI. The verifier would then		servers often provide OCSP responses for each certificate in their
	follow the URI to ascertain whether the list of TNs authorized for		hierarchy. If possible, both of these OCSP concepts should be
	use by the caller. There are obviously some privacy considerations		adopted for use with STIR.
	with this approach.
	The need to check the authorizations in another round-trip is also		4.2.2.1. OCSP Extension Specification
	something to consider because it will add to the call setup time.
	OCSP implementations commonly pre-generate responses and to speed up		The extension mechanism for OCSP follows X.509 v3 certificate
	HTTPS connections the server provides OCSP responses for each		extensions, and thus requires an OID, a criticality flag, and ASN.1
	certificate in their hierarchy. If possible, both of these OCSP		syntax as defined by the OID. The criticality specified here is
	concepts should be adopted.		optional: per [RFC6960] Section 4.4, support for all OCSP extensions
			is optional. If the OCSP server does not understand the requested
			extension, it will still provide the baseline validation of the
			certificate itself. Moreover, in practical STIR deployments, the
			issuer of the 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 OCSP TNQuery extension is included as one of the
			requestExtensions in requests. It may also appear in the
			responseExtensions. When an OCSP server includes a number in the
			responseExtensions, this informs the client that the certificate is
			still valid for the number that appears in the TNQuery extension
			field. If the TNQuery is absent from a response to a query
			containing a TNQuery in its requestExtensions, then the server is not
			able to validate that the number is still in the scope of authority
			of the certificate.
			id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD }
			TNQuery ::= E164Number
			Note that HVE OCSP profile [RFC5019] prohibits the use of per-request
			extensions. As it is anticipated that STIR will use OCSP in 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 environment with some HVE
			optimizations. [More TBD]
	Ideally, once a certificate has been acquired by a verifier, some		Ideally, once a certificate has been acquired by a verifier, some
	sort of asynchronous mechanism could notify and update the verifier		sort of asynchronous mechanism could notify and update the verifier
	if the scope of the certificate changes. While not all possible		if the scope of the certificate changes so that verifiers could
	categories of verifiers could implement such behavior, some sort of		implement a cache. While not all possible categories of verifiers
	event-driven notification of certificate status is another potential		could implement such behavior, some sort of event-driven notification
	subject of future work.		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		5. Acknowledgments
	Russ Housley, Brian Rosen, Cullen Jennings and Eric Rescorla provided		Russ Housley, Brian Rosen, Cullen Jennings and Eric Rescorla provided
	key input to the discussions leading to this document.		key input to the discussions leading to this document.
	6. IANA Considerations		6. IANA Considerations
	This memo includes no request to IANA at this time. If we define an		This document makes use of object identifiers for the TN Certificate
	OCSP extension or AIA access method then we'll need an OID from the		Extension defined in Section 4.1, TN-HVE OCSP extension in
	PKIX.		Section 4.2.2.1, and the TN by reference AIA access 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 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		7. Security Considerations
	This document is entirely about security. For further information on		This document is entirely about security. For further information on
	certificate security and practices, see RFC 3280 [5], in particular		certificate security and practices, see RFC 3280 [RFC3280], in
	its Security Considerations.		particular its Security Considerations.
	8. Informative References		8. Informative References
	[1] Bradner, S., "Key words for use in RFCs to Indicate		[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.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[2] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words		[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
	for Use in RFCs to Indicate Requirement Levels", RFC 6919,		Locators", RFC 2392, August 1998.
	April 1 2013.
	[3] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,		[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
			[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
	A., Peterson, J., Sparks, R., Handley, M., and E.		A., Peterson, J., Sparks, R., Handley, M., and E.
	Schooler, "SIP: Session Initiation Protocol", RFC 3261,		Schooler, "SIP: Session Initiation Protocol", RFC 3261,
	June 2002.		June 2002.
	[4] Rosenberg, J. and H. Schulzrinne, "Session Initiation		[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
	Protocol (SIP): Locating SIP Servers", RFC 3263, June		Protocol (SIP): Locating SIP Servers", RFC 3263, June
	2002.		2002.
	[5] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet		[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
	X.509 Public Key Infrastructure Certificate and		X.509 Public Key Infrastructure Certificate and
	Certificate Revocation List (CRL) Profile", RFC 3280,		Certificate Revocation List (CRL) Profile", RFC 3280,
	April 2002.		April 2002.
	[6] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.		[RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online
			Certificate Status Protocol (OCSP) Profile for High-Volume
			Environments", RFC 5019, September 2007.
	[7] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,		[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		Housley, R., and W. Polk, "Internet X.509 Public Key
	Infrastructure Certificate and Certificate Revocation List		Infrastructure Certificate and Certificate Revocation List
	(CRL) Profile", RFC 5280, May 2008.		(CRL) Profile", RFC 5280, May 2008.
	[8] Turner, S., "Asymmetric Key Packages", RFC 5958, August		[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, August
	2010.		2010.
	[9] Freed, N. and N. Borenstein, "Multipurpose Internet Mail		[RFC6919] Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
	Extensions (MIME) Part Two: Media Types", RFC 2046,		for Use in RFCs to Indicate Requirement Levels", RFC 6919,
	November 1996.		April 2013.
	[10] Levinson, E., "Content-ID and Message-ID Uniform Resource		[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
	Locators", RFC 2392, August 1998.		Galperin, S., and C. Adams, "X.509 Internet Public Key
			Infrastructure Online Certificate Status Protocol - OCSP",
			RFC 6960, June 2013.
	[11] Pritikin, M., Yee, P., and D. Harkins, "Enrollment over		[RFC7030] Pritikin, M., Yee, P., and D. Harkins, "Enrollment over
	Secure Transport", RFC 7030, October 2013.		Secure Transport", RFC 7030, October 2013.
	[12] Deacon, A. and R. Hurst, "The Lightweight Online		[RFC7299] Housley, R., "Object Identifier Registry for the PKIX
	Certificate Status Protocol (OCSP) Profile for High-Volume		Working Group", RFC 7299, July 2014.
	Environments", RFC 5019, September 2007.
	[13] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure		[X.680] USC/Information Sciences Institute, "Information
	Telephone Identity Problem Statement and Requirements",		Technology - Abstract Syntax Notation One.", ITU-T X.680,
	draft-ietf-stir-problem-statement-05 (work in progress),		ISO/IEC 8824-1:2002, 2002.
	May 2014.
	[14] Peterson, J., "Retargeting and Security in SIP: A		[X.681] USC/Information Sciences Institute, "Information
	Framework and Requirements", draft-peterson-sipping-		Technology - Abstract Syntax Notation One: Information
	retarget-00 (work in progress), February 2005.		Object Specification", ITU-T X.681, ISO/IEC 8824-2:2002,
			2002.
	[15] Peterson, J., Jennings, C., and E. Rescorla,		[X.682] USC/Information Sciences Institute, "Information
	"Authenticated Identity Management in the Session		Technology - Abstract Syntax Notation One: Constraint
	Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-02		Specification", ITU-T X.682, ISO/IEC 8824-3:2002, 2002.
	(work in progress), October 2014.
			[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 normative ASN.1 [X.680] definitions for
			the structures described in this specification using ASN.1, as
			defined in [X.680] through [X.683].
			TBD
	Authors' Addresses		Authors' Addresses
	Jon Peterson		Jon Peterson
	Neustar, Inc.		Neustar, Inc.
	1800 Sutter St Suite 570		1800 Sutter St Suite 570
	Concord, CA 94520		Concord, CA 94520
	US		US
	Email: jon.peterson@neustar.biz		Email: jon.peterson@neustar.biz
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