draft-ietf-acme-caa-07.txt		draft-ietf-acme-caa-08.txt
	ACME Working Group H. Landau		ACME Working Group H. Landau
	Internet-Draft May 20, 2019		Internet-Draft May 31, 2019
	Intended status: Standards Track		Intended status: Standards Track
	Expires: November 21, 2019		Expires: December 2, 2019
	CAA Record Extensions for Account URI and ACME Method Binding		CAA Record Extensions for Account URI and ACME Method Binding
	draft-ietf-acme-caa-07		draft-ietf-acme-caa-08
	Abstract		Abstract
	The Certification Authority Authorization (CAA) DNS record allows a		The Certification Authority Authorization (CAA) DNS record allows a
	domain to communicate issuance policy to Certification Authorities		domain to communicate issuance policy to Certification Authorities
	(CAs), but only allows a domain to define policy with CA-level		(CAs), but only allows a domain to define policy with CA-level
	granularity. However, the CAA specification also provides facilities		granularity. However, the CAA specification also provides facilities
	for extension to admit more granular, CA-specific policy. This		for extension to admit more granular, CA-specific policy. This
	specification defines two such parameters, one allowing specific		specification defines two such parameters, one allowing specific
	accounts of a CA to be identified by URI and one allowing specific		accounts of a CA to be identified by URI and one allowing specific
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	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 https://datatracker.ietf.org/drafts/current/.		Drafts is at https://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 November 21, 2019.		This Internet-Draft will expire on December 2, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2019 IETF Trust and the persons identified as the		Copyright (c) 2019 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
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	3.1. Use with ACME . . . . . . . . . . . . . . . . . . . . . . 3		3.1. Use with ACME . . . . . . . . . . . . . . . . . . . . . . 3
	3.2. Use without ACME . . . . . . . . . . . . . . . . . . . . 3		3.2. Use without ACME . . . . . . . . . . . . . . . . . . . . 3
	4. Extensions to the CAA Record: validationmethods Parameter . . 4		4. Extensions to the CAA Record: validationmethods Parameter . . 4
	5. Security Considerations . . . . . . . . . . . . . . . . . . . 4		5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
	5.1. Limited to CAs Processing CAA Records . . . . . . . . . . 4		5.1. Limited to CAs Processing CAA Records . . . . . . . . . . 4
	5.2. Restrictions Ineffective without CA Recognition . . . . . 5		5.2. Restrictions Ineffective without CA Recognition . . . . . 5
	5.3. Mandatory Consistency in CA Recognition . . . . . . . . . 5		5.3. Mandatory Consistency in CA Recognition . . . . . . . . . 5
	5.4. URI Ambiguity . . . . . . . . . . . . . . . . . . . . . . 6		5.4. URI Ambiguity . . . . . . . . . . . . . . . . . . . . . . 6
	5.5. Authorization Freshness . . . . . . . . . . . . . . . . . 6		5.5. Authorization Freshness . . . . . . . . . . . . . . . . . 6
	5.6. Use with and without DNSSEC . . . . . . . . . . . . . . . 6		5.6. Use with and without DNSSEC . . . . . . . . . . . . . . . 6
	5.7. Restrictions Supercedable by DNS Delegation . . . . . . . 7		5.7. Restrictions Supercedable by DNS Delegation . . . . . . . 8
	5.8. Misconfiguration Hazards . . . . . . . . . . . . . . . . 8		5.8. Misconfiguration Hazards . . . . . . . . . . . . . . . . 8
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	7. Normative References . . . . . . . . . . . . . . . . . . . . 8		7. Normative References . . . . . . . . . . . . . . . . . . . . 8
	Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 8		Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 9
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9		Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
	1. Introduction		1. Introduction
	This specification defines two parameters for the "issue" and		This specification defines two parameters for the "issue" and
	"issuewild" properties of the Certification Authority Authorization		"issuewild" properties of the Certification Authority Authorization
	(CAA) DNS resource record [RFC6844]. The first, "accounturi", allows		(CAA) DNS resource record [RFC6844]. The first, "accounturi", allows
	authorization conferred by a CAA policy to be restricted to specific		authorization conferred by a CAA policy to be restricted to specific
	accounts of a CA, which are identified by URIs. The second,		accounts of a CA, which are identified by URIs. The second,
	"validationmethods", allows the set of validation methods supported		"validationmethods", allows the set of validation methods supported
	by a CA to validate domain control to be limited to a subset of the		by a CA to validate domain control to be limited to a subset of the
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	described in BCP 14 [RFC2119] [RFC8174] when, and only when, they		described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
	appear in all capitals, as shown here.		appear in all capitals, as shown here.
	3. Extensions to the CAA Record: accounturi Parameter		3. Extensions to the CAA Record: accounturi Parameter
	A CAA parameter "accounturi" is defined for the "issue" and		A CAA parameter "accounturi" is defined for the "issue" and
	"issuewild" properties defined by [RFC6844]. The value of this		"issuewild" properties defined by [RFC6844]. The value of this
	parameter, if specified, MUST be a URI [RFC3986] identifying a		parameter, if specified, MUST be a URI [RFC3986] identifying a
	specific CA account.		specific CA account.
	"CA account" means an object maintained by a specific CA representing		"CA account" means an object, maintained by a specific CA and which
	a specific entity, or group of related entities, which may request		may request the issuance of certificates, which represents a specific
	the issuance of certificates.		entity or group of related entities.
	The presence of this parameter constrains the property to which it is		The presence of this parameter constrains the property to which it is
	attached. Where a CAA property has an "accounturi" parameter, a CA		attached. Where a CAA property has an "accounturi" parameter, a CA
	MUST only consider that property to authorize issuance in the context		MUST only consider that property to authorize issuance in the context
	of a given certificate issuance request if the CA recognises the URI		of a given certificate issuance request if the CA recognises the URI
	specified as identifying the account making that request.		specified as identifying the account making that request.
	A property without an "accounturi" parameter matches any account. A		A property without an "accounturi" parameter matches any account. A
	property with an invalid or unrecognised "accounturi" parameter is		property with an invalid or unrecognised "accounturi" parameter is
	unsatisfiable. A property with multiple "accounturi" parameters is		unsatisfiable. A property with multiple "accounturi" parameters is
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	splitting the CA into two domain names for the purposes of CAA		splitting the CA into two domain names for the purposes of CAA
	processing. For example, a CA "example.com" with an ACME-based		processing. For example, a CA "example.com" with an ACME-based
	issuance system and a non-ACME-based issuance system could recognise		issuance system and a non-ACME-based issuance system could recognise
	only "acme.example.com" for the former and "example.com" for the		only "acme.example.com" for the former and "example.com" for the
	latter, and then implement support for the "accounturi" and		latter, and then implement support for the "accounturi" and
	"validationmethods" parameters for "acme.example.com" only.		"validationmethods" parameters for "acme.example.com" only.
	A CA which is unable to ensure consistent processing of the		A CA which is unable to ensure consistent processing of the
	"accounturi" or "validationmethods" parameters for a given CA domain		"accounturi" or "validationmethods" parameters for a given CA domain
	name as specifiable in CAA "issue" or "issuewild" properties MUST NOT		name as specifiable in CAA "issue" or "issuewild" properties MUST NOT
	implement support for these parameters. Failure to do so will result		implement support for these parameters. Failure to do so would
	in an implementation of these parameters which does not provide		result in an implementation of these parameters which does not
	effective security.		provide effective security.
	5.4. URI Ambiguity		5.4. URI Ambiguity
	Suppose that CA A recognises "a.example.com" as identifying itself,		Suppose that CA A recognises "a.example.com" as identifying itself,
	CA B is a subsidiary of CA A which recognises both "a.example.com"		CA B is a subsidiary of CA A which recognises both "a.example.com"
	and "b.example.com" as identifying itself.		and "b.example.com" as identifying itself.
	Suppose that both CA A and CA B issue account URIs of the form		Suppose that both CA A and CA B issue account URIs of the form
	"account-id:1234"		"urn:example:account-id:1234"
	If the CA domain name in a CAA record is specified as "a.example.com"		If the CA domain name in a CAA record is specified as "a.example.com"
	then this could be construed as identifying account number 1234 at CA		then this could be construed as identifying account number 1234 at CA
	A or at CA B. These may be different accounts, creating ambiguity.		A or at CA B. These may be different accounts, creating ambiguity.
	Thus, CAs MUST ensure that the URIs they recognise as pertaining to a		Thus, CAs MUST ensure that the URIs they recognise as pertaining to a
	specific account of that CA are unique within the scope of all domain		specific account of that CA are unique within the scope of all domain
	names which they recognise as identifying that CA for the purpose of		names which they recognise as identifying that CA for the purpose of
	CAA record validation.		CAA record validation.
	CAs MUST satisfy this requirement by using URIs which include an		CAs SHOULD satisfy this requirement by using URIs which include an
	authority:		authority (see Section 3.2 of [RFC3986]):
	"https://a.example.com/account/1234"		"https://a.example.com/account/1234"
	5.5. Authorization Freshness		5.5. Authorization Freshness
	The CAA specification governs the act of issuance by a CA. In some		The CAA specification governs the act of issuance by a CA. In some
	cases, a CA may establish authorization for an account to request		cases, a CA may establish authorization for an account to request
	certificate issuance for a specific domain separately to the act of		certificate issuance for a specific domain separately to the act of
	issuance itself. Such authorization may occur substantially prior to		issuance itself. Such authorization may occur substantially prior to
	a certificate issuance request. The CAA policy expressed by a domain		a certificate issuance request. The CAA policy expressed by a domain
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	issue certificates in a manner inconsistent with the presently		issue certificates in a manner inconsistent with the presently
	published CAA policy.		published CAA policy.
	CAs SHOULD adopt practices to reduce the risk of such circumstances.		CAs SHOULD adopt practices to reduce the risk of such circumstances.
	Possible countermeasures include issuing authorizations with very		Possible countermeasures include issuing authorizations with very
	limited validity periods, such as an hour, or revalidating the CAA		limited validity periods, such as an hour, or revalidating the CAA
	policy for a domain at certificate issuance time.		policy for a domain at certificate issuance time.
	5.6. Use with and without DNSSEC		5.6. Use with and without DNSSEC
	Where a domain chooses to secure its nameservers using DNSSEC, the		The "domain validation" model of validation commonly used for
	authenticity of its DNS data can be assured, providing that a given		certificate issuance cannot ordinarily protect against adversaries
	CA makes all DNS resolutions via an appropriate, trusted DNSSEC-		who can conduct global man-in-the-middle attacks against a particular
	validating resolver. A domain can use this property to protect		domain. A global man-in-the-middle attack is an attack which can
	itself from the threat posed by a global adversary capable of		intercept traffic to or from a given domain, regardless of the origin
	performing man-in-the-middle attacks, which is not ordinarily		or destination of that traffic. Such an adversary can intercept all
	mitigated by the "domain validation" model.		validation traffic initiated by a CA and thus appear to have control
			of the given domain.
			Where a domain is signed using DNSSEC, the authenticity of its DNS
			data can be assured, providing that a given CA makes all DNS
			resolutions via a trusted DNSSEC-validating resolver. A domain can
			use this property to protect itself from the threat posed by an
			adversary capable of performing a global man-in-the-middle attack
			against that domain.
	In order to facilitate this, a CA validation process must either rely		In order to facilitate this, a CA validation process must either rely
	solely on information obtained via DNSSEC, or meaningfully bind the		solely on information obtained via DNSSEC, or meaningfully bind the
	other parts of the validation transaction using material obtained via		other parts of the validation transaction using material obtained via
	DNSSEC.		DNSSEC.
	The CAA parameters described in this specification can be used to		The CAA parameters described in this specification can be used to
	ensure that only validation methods meeting these criteria are used.		ensure that only validation methods meeting these criteria are used.
	In particular, a domain secured via DNSSEC SHOULD either:		In particular, a domain secured via DNSSEC SHOULD either:
	1. Use the "accounturi" parameter to ensure that only accounts which		1. Use the "accounturi" parameter to ensure that only accounts which
	it controls are authorized to obtain certificates, or		it controls are authorized to obtain certificates, or
	2. Exclusively use validation methods which rely solely on		2. Exclusively use validation methods which rely solely on
	information obtained via DNSSEC, and use the "validationmethods"		information obtained via DNSSEC, and use the "validationmethods"
	parameter to ensure that only such methods are used.		parameter to ensure that only such methods are used.
			A CA supporting the "accounturi" or "validationmethods" parameters
			MUST perform CAA validation using a trusted, DNSSEC-validating
			resolver.
			"Trusted" in this context means that the CA both trusts the resolver
			itself and ensures that the communications path between the resolver
			and the system performing CAA validation are secure. It is
			RECOMMENDED that a CA ensure this by using a DNSSEC-validating
			resolver running on the same machine as the system performing CAA
			validation.
	Use of the "accounturi" or "validationmethods" parameters does not		Use of the "accounturi" or "validationmethods" parameters does not
	confer additional security against an attacker capable of performing		confer additional security against an attacker capable of performing
	a man-in-the-middle attack against all validation attempts made by a		a man-in-the-middle attack against all validation attempts made by a
	given CA which is authorized by CAA where:		given CA which is authorized by CAA where:
	1. A domain does not secure its nameservers using DNSSEC, or		1. A domain does not secure its nameservers using DNSSEC, or
	2. That CA does not perform CAA validation using a trusted DNSSEC-		2. That CA does not perform CAA validation using a trusted DNSSEC-
	validating resolver.		validating resolver.
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	do not validate CAA records, or which do not do so using a trusted		do not validate CAA records, or which do not do so using a trusted
	DNSSEC-validating resolver, regardless of whether those CAs are		DNSSEC-validating resolver, regardless of whether those CAs are
	authorized by CAA or not; see Section 5.1.		authorized by CAA or not; see Section 5.1.
	In these cases, the "accounturi" and "validationmethods" parameters		In these cases, the "accounturi" and "validationmethods" parameters
	still provide an effective means of administrative control over		still provide an effective means of administrative control over
	issuance, except where control over DNS is subdelegated (see below).		issuance, except where control over DNS is subdelegated (see below).
	5.7. Restrictions Supercedable by DNS Delegation		5.7. Restrictions Supercedable by DNS Delegation
	Because CAA records are located during validation by walking up the		CAA records are located during validation by walking up the DNS
	DNS hierarchy until one or more records are found, the use of the		hierarchy until one or more records are found. CAA records are
	"accounturi" and "validationmethods" parameters, or any CAA policy,		therefore not an effective way of restricting or controlling issuance
	is not an effective way to restrict or control issuance for		for subdomains of a domain, where control over those subdomains is
	subdomains of a domain, where control over those subdomains is
	delegated to another party (such as via DNS delegation or by		delegated to another party (such as via DNS delegation or by
	providing limited access to manage subdomain DNS records).		providing limited access to manage subdomain DNS records).
	5.8. Misconfiguration Hazards		5.8. Misconfiguration Hazards
	Because they express a restrictive security policy, misconfiguration		Because the "accounturi" and "validationmethods" parameters express
	of the "accounturi" or "validationmethods" parameters may result in		restrictive security policies, misconfiguration of said parameters
	legitimate issuance requests being refused.		may result in legitimate issuance requests being refused.
	6. IANA Considerations		6. IANA Considerations
	None. As per the CAA specification, the parameter namespace for the		None. As per the CAA specification, the parameter namespace for the
	CAA "issue" and "issuewild" properties has CA-defined semantics.		CAA "issue" and "issuewild" properties has CA-defined semantics and
	This document merely specifies a RECOMMENDED semantic for parameters		the identifiers within that namespace may be freely and arbitrarily
	of the names "accounturi" and "validationmethods".		assigned by a CA. This document merely specifies recommended
			semantics for parameters of the names "accounturi" and
			"validationmethods", which CAs may choose to adopt.
	7. Normative References		7. Normative References
	[I-D.ietf-acme-acme]		[I-D.ietf-acme-acme]
	Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.		Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
	Kasten, "Automatic Certificate Management Environment		Kasten, "Automatic Certificate Management Environment
	(ACME)", draft-ietf-acme-acme-18 (work in progress),		(ACME)", draft-ietf-acme-acme-18 (work in progress),
	December 2018.		December 2018.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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