< draft-ietf-stir-certificates-00.txt		draft-ietf-stir-certificates-01.txt >
	Network Working Group J. Peterson		STIR Working Group J. Peterson
	Internet-Draft NeuStar		Internet-Draft NeuStar
	Intended status: Standards Track S. Turner		Intended status: Standards Track S. Turner
	Expires: April 26, 2015 IECA		Expires: September 25, 2015 IECA
	October 23, 2014		March 24, 2015
	Secure Telephone Identity Credentials: Certificates		Secure Telephone Identity Credentials: Certificates
	draft-ietf-stir-certificates-00.txt		draft-ietf-stir-certificates-01.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 April 26, 2015.		This Internet-Draft will expire on September 22, 2015.
	Copyright Notice		Copyright Notice
	Copyright (c) 2014 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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	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		Table of Contents
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
	3. Enrollment and Authorization . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
	3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
	3.2. Provisioning Private Keying Material . . . . . . . . . . 5		3. Enrollment and Authorization . . . . . . . . . . . . . . . . . 3
	4. Acquiring Credentials to Verify Signatures . . . . . . . . . 5		3.1. Certificate Scope and Structure . . . . . . . . . . . . . 4
	4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6		3.2. Provisioning Private Keying Material . . . . . . . . . . . 5
	4.2. Certificate Freshness and Revocation . . . . . . . . . . 8		4. Acquiring Credentials to Verify Signatures . . . . . . . . . . 5
	5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8		4.1. Verifying Certificate Scope . . . . . . . . . . . . . . . 6
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		4.2. Certificate Freshness and Revocation . . . . . . . . . . . 7
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 8		5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
	8. Informative References . . . . . . . . . . . . . . . . . . . 9		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10		7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
			8. Informative References . . . . . . . . . . . . . . . . . . . . 10
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
	1. Introduction		1. Introduction
	As is discussed in the STIR problem statement [13], the primary		As is discussed in the STIR problem statement [13], the primary
	enabler of robocalling, vishing, swatting and related attacks is the		enabler of robocalling, vishing, swatting and related attacks is the
	capability to impersonate a calling party number. The starkest		capability to impersonate a calling party number. The starkest
	examples of these attacks are cases where automated callees on the		examples of these attacks are cases where automated callees on the
	PSTN rely on the calling number as a security measure, for example to		Public Switched Telephone Network (PSTN) rely on the calling number
	access a voicemail system. Robocallers use impersonation as a means		as a security measure, for example to access a voicemail system.
	of obscuring identity; while robocallers can, in the ordinary PSTN,		Robocallers use impersonation as a means of obscuring identity; while
	block (that is, withhold) their caller identity, callees are less		robocallers can, in the ordinary PSTN, block (that is, withhold)
	likely to pick up calls from blocked identities, and therefore		their caller identity, callees are less likely to pick up calls from
	appearing to calling from some number, any number, is preferable.		blocked identities, and therefore appearing to calling from some
	Robocallers however prefer not to call from a number that can trace		number, any number, is preferable. Robocallers however prefer not to
	back to the robocaller, and therefore they impersonate numbers that		call from a number that can trace back to the robocaller, and
	are not assigned to them.		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 [7]. While telephone numbers have long been a part of the X.509
	standard, the certificates described in this document may contain		standard, the certificates described in this document may contain
	telephone number blocks or ranges, and accordingly it uses an		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 [15]. An authentication
	service must be operated by an entity enrolled with the certificate		service must be operated by an entity enrolled with the certification
	authority (see Section 3), whereas a verifier need only trust the		authority (see Section 3), whereas a verifier need only trust the
	root certificate of the authority, and have a means to acquire and		root certificate of the authority, and have a means to acquire and
	validate certificates.		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 [1] and RFC 6919 [2].
	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 certificate authority		The first enrollment model is one where the certification authority
	acts in concert with national numbering authorities to issue		(CA) 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 certificate 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 deferenced by the recipient)
	as a means of verifying that a user has control of terminal		as a means of verifying that a user has control of terminal
	corresponding to that number. Checks of this form are frequently		corresponding to that number. Checks of this form are frequently
	used in commercial systems today to validate telephone numbers		used in commercial systems today to validate telephone numbers
	provided by users. This is comparable to existing enrollment systems		provided by users. This is comparable to existing enrollment systems
	used by some certificate authorities for issuing S/MIME credentials		used by some certificate authorities for issuing S/MIME credentials
	for email by verifying that the party applying for a credential		for email by verifying that the party applying for a credential
	receives mail at the email address in question.		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
	PKIs who use the name constraints extension [3]; the root CA		Public Key Infrastructures (PKIs) who use the name constraints
	delegates names in sales to the sales department CA, names in		extension [3]; the root CA delegates names in sales to the sales
	development to the development CA, etc. As lengthy certificate		department CA, names in development to the development CA, etc. As
	delegation chains are brittle, however, and can cause delays in the		lengthy certificate delegation chains are brittle, however, and can
	verification process, this document considers optimizations to reduce		cause delays in the verification process, this document considers
	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.
	skipping to change at page 5, line 4 ¶		skipping to change at page 5, line 8 ¶
	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
	RFC 5280 but the telephone number authorization extension is defined		[7] but the telephone number authorization extension is defined in
	in this document.		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 [15], they must possess a private key corresponding to a
	certificate with authority over the calling number. This		certificate with authority over the calling number. This
	specification does not require that any particular entity sign		specification does not require that any particular entity sign
	requests, only that it be an entity with an appropriate private key;		requests, only that it be an entity with an appropriate private key;
	the authentication service role may be instantiated by any entity in		the authentication service role may be instantiated by any entity in
	a SIP network. For a certificate granting authority only over a		a SIP network. For a certificate granting authority only over a
	skipping to change at page 6, line 23 ¶		skipping to change at page 6, line 25 ¶
	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 TN Authorization List certificate extension is identified by the		The telephone number (TN) Authorization List certificate extension is
	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
	skipping to change at page 7, line 10 ¶		skipping to change at page 6, line 38 ¶
	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 8, line 20 ¶		skipping to change at page 7, line 37 ¶
	4.2. Certificate Freshness and Revocation		4.2. Certificate Freshness and Revocation
	The problem of certificate freshness gains a new wrinkle in the		The problem of certificate freshness gains a new wrinkle in the
	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 the signer		number), the verifier must determine that the entity that signed is
	is still a proper authority for that number.		still a proper authority for that number.
	This document therefore recommends the use of OCSP in high-volume		To verify the status of the certificate, the verifier needs the
	environments for validating the freshness of certificates, per [12].		certificate, which is included with the call, and they need to:
	[TBD - depending on our algorithm choices this profile may need to be
	further profiled.]		o Rely on short-lived certificates and not check the certificate's
			status, or
			o 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].
			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.
			When relying on status information, the verifier needs to obtain the
			status information but before that can happen the verifier needs to
			know where to locate it. Placing the location of the status
			information in the certificate makes the certificate 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]. 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) because CAs issue one with all of their revoked 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 the
			CRL though introduces other issues (i.e., does the RP have all of the
			CRL partitions). CAs in this system will likely all create CRLs for
			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
			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 is authorized for the
			telephone number that the verifier is validating. There's two ways
			to ask the authorization question:
			o For this certificate, is the following number currently in its
			scope of validity?
			o What are the numbers associated with this certificate?
			The former seems to lend itself to piggybacking on the status
			mechanism; since the verifier is already asking an authority about
			the certificate's status why not use that mechanism instead of
			creating a new service that requires additional round trips. Like
			most PKIX-developed protocols, OCSP is extensible; OCSP supports
			request extensions (OCSP supports sending multiple requests at once)
			and per-request extensions. It seems unlikely that the verifier will
			be requesting authorization checks on multiple callers in one request
			so a per-request extension is what is needed. But, support for any
			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
			based on the X.509 v3 certificate extensions: 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 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
			interaction and could be initiated through a URI included 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 then
			follow the URI to ascertain whether the list of TNs authorized for
			use by the caller. There are obviously some privacy considerations
			with this approach.
			The need to check the authorizations in another round-trip is also
			something to consider because it will add to the call setup time.
			OCSP implementations commonly pre-generate responses and to speed up
			HTTPS connections the server provides OCSP responses for each
			certificate in their hierarchy. If possible, both of these OCSP
			concepts should be adopted.
	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. While not all possible
	categories of verifiers could implement such behavior, some sort of		categories of verifiers could implement such behavior, some sort of
	event-driven notification of certificate status is another potential		event-driven notification of certificate status is another potential
	subject of future work.		subject of future work.
	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.		This memo includes no request to IANA at this time. If we define an
			OCSP extension or AIA access method then we'll need an OID from the
			PKIX.
	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 [5], in particular
	its Security Considerations.		its Security Considerations.
	8. Informative References		8. Informative References
	[1] Bradner, S., "Key words for use in RFCs to Indicate		[1] Bradner, S., "Key words for use in RFCs to Indicate
End of changes. 23 change blocks.
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