--- 1/draft-ietf-uta-rfc6125bis-02.txt 2021-10-13 13:13:10.958078633 -0700 +++ 2/draft-ietf-uta-rfc6125bis-03.txt 2021-10-13 13:13:11.010079930 -0700 @@ -1,32 +1,34 @@ Network Working Group P. Saint-Andre Internet-Draft Mozilla Obsoletes: 6125 (if approved) J. Hodges Intended status: Standards Track Google -Expires: 12 March 2022 R. Salz +Expires: 16 April 2022 R. Salz Akamai Technologies - 8 September 2021 + 13 October 2021 Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS) - draft-ietf-uta-rfc6125bis-02 + draft-ietf-uta-rfc6125bis-03 Abstract Many application technologies enable secure communication between two entities by means of Transport Layer Security (TLS) with Internet Public Key Infrastructure Using X.509 (PKIX) certificates. This document specifies procedures for representing and verifying the identity of application services in such interactions. + This document obsoletes RFC 6125. + Discussion Venues This note is to be removed before publishing as an RFC. Discussion of this document takes place on the Using TLS in Applications Working Group mailing list (uta@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/uta/. Source for this draft and an issue tracker can be found at https://github.com/richsalz/draft-ietf-uta-rfc6125bis. @@ -38,1337 +40,1063 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on 12 March 2022. + + This Internet-Draft will expire on 16 April 2022. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3 - 1.2. Audience . . . . . . . . . . . . . . . . . . . . . . . . 4 - 1.3. Changes since RFC 6125 . . . . . . . . . . . . . . . . . 4 - 1.4. How to Read This Document . . . . . . . . . . . . . . . . 5 - 1.5. Applicability . . . . . . . . . . . . . . . . . . . . . . 5 - 1.6. Overview of Recommendations . . . . . . . . . . . . . . . 6 - 1.7. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 1.7.1. In Scope . . . . . . . . . . . . . . . . . . . . . . 7 - 1.7.2. Out of Scope . . . . . . . . . . . . . . . . . . . . 7 - 1.8. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9 - 2. Naming of Application Services . . . . . . . . . . . . . . . 12 - 2.1. Naming Application Services . . . . . . . . . . . . . . . 12 - 2.2. DNS Domain Names . . . . . . . . . . . . . . . . . . . . 13 - 2.3. Subject Naming in PKIX Certificates . . . . . . . . . . . 14 - 3. Designing Application Protocols . . . . . . . . . . . . . . . 14 - 4. Representing Server Identity . . . . . . . . . . . . . . . . 15 - 4.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 15 - 4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 16 - 5. Requesting Server Certificates . . . . . . . . . . . . . . . 16 - 6. Verifying Service Identity . . . . . . . . . . . . . . . . . 17 - 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 17 - 6.2. Constructing a List of Reference Identifiers . . . . . . 18 - 6.2.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 18 - 6.2.2. Examples . . . . . . . . . . . . . . . . . . . . . . 20 - 6.3. Preparing to Seek a Match . . . . . . . . . . . . . . . . 21 - 6.4. Matching the DNS Domain Name Portion . . . . . . . . . . 22 - 6.4.1. Checking of Traditional Domain Names . . . . . . . . 22 - 6.4.2. Checking of Internationalized Domain Names . . . . . 22 - 6.4.3. Checking of Wildcard Certificates . . . . . . . . . . 23 - - 6.5. Matching the Application Service Type Portion . . . . . . 23 - 6.5.1. SRV-ID . . . . . . . . . . . . . . . . . . . . . . . 24 - 6.5.2. URI-ID . . . . . . . . . . . . . . . . . . . . . . . 24 - 6.6. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 24 - 6.6.1. Case #1: Match Found . . . . . . . . . . . . . . . . 24 - 6.6.2. Case #2: No Match Found, Pinned Certificate . . . . . 24 - 6.6.3. Case #3: No Match Found, No Pinned Certificate . . . 24 - 6.6.4. Fallback . . . . . . . . . . . . . . . . . . . . . . 25 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 25 - 7.1. Pinned Certificates . . . . . . . . . . . . . . . . . . . 25 - 7.2. Wildcard Certificates . . . . . . . . . . . . . . . . . . 26 - 7.3. Internationalized Domain Names . . . . . . . . . . . . . 26 - 7.4. Multiple Identifiers . . . . . . . . . . . . . . . . . . 26 - 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 - 8.1. Normative References . . . . . . . . . . . . . . . . . . 26 - 8.2. Informative References . . . . . . . . . . . . . . . . . 27 - Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 30 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30 + 1.2. Changes since RFC 6125 . . . . . . . . . . . . . . . . . 3 + 1.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 4 + 1.4. Overview of Recommendations . . . . . . . . . . . . . . . 4 + 1.5. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 1.5.1. In Scope . . . . . . . . . . . . . . . . . . . . . . 5 + 1.5.2. Out of Scope . . . . . . . . . . . . . . . . . . . . 5 + 1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6 + 2. Naming of Application Services . . . . . . . . . . . . . . . 9 + 3. Designing Application Protocols . . . . . . . . . . . . . . . 10 + 4. Representing Server Identity . . . . . . . . . . . . . . . . 11 + 4.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . 11 + 4.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . 11 + 5. Requesting Server Certificates . . . . . . . . . . . . . . . 12 + 6. Verifying Service Identity . . . . . . . . . . . . . . . . . 13 + 6.1. Constructing a List of Reference Identifiers . . . . . . 13 + 6.1.1. Rules . . . . . . . . . . . . . . . . . . . . . . . . 13 + 6.1.2. Examples . . . . . . . . . . . . . . . . . . . . . . 15 + 6.2. Preparing to Seek a Match . . . . . . . . . . . . . . . . 15 + 6.3. Matching the DNS Domain Name Portion . . . . . . . . . . 16 + 6.4. Matching the Application Service Type Portion . . . . . . 17 + 6.5. Outcome . . . . . . . . . . . . . . . . . . . . . . . . . 18 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 + 7.1. Wildcard Certificates . . . . . . . . . . . . . . . . . . 19 + 7.2. Internationalized Domain Names . . . . . . . . . . . . . 19 + 7.3. Multiple Presented Identifiers . . . . . . . . . . . . . 20 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 + 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 + 9.1. Normative References . . . . . . . . . . . . . . . . . . 20 + 9.2. Informative References . . . . . . . . . . . . . . . . . 21 + Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 1. Introduction 1.1. Motivation The visible face of the Internet largely consists of services that employ a client-server architecture in which an interactive or - automated client communicates with an application service in order to - retrieve or upload information, communicate with other entities, or - access a broader network of services. When a client communicates - with an application service using Transport Layer Security [TLS] or - Datagram Transport Layer Security [DTLS], it references some notion - of the server's identity (e.g., "the website at example.com") while - attempting to establish secure communication. Likewise, during TLS - negotiation, the server presents its notion of the service's identity - in the form of a public-key certificate that was issued by a - certification authority (CA) in the context of the Internet Public - Key Infrastructure using X.509 [PKIX]. Informally, we can think of - these identities as the client's "reference identity" and the - server's "presented identity" (these rough ideas are defined more - precisely later in this document through the concept of particular - identifiers). In general, a client needs to verify that the server's - presented identity matches its reference identity so it can - authenticate the communication. - - Many application technologies adhere to the pattern just outlined. - Such protocols have traditionally specified their own rules for - representing and verifying application service identity. - Unfortunately, this divergence of approaches has caused some - confusion among certification authorities, application developers, - and protocol designers. - - Therefore, to codify secure procedures for the implementation and - deployment of PKIX-based authentication, this document specifies - recommended procedures for representing and verifying application - service identity in certificates intended for use in application - protocols employing TLS. - -1.2. Audience + automated client communicates with an application service. When a + client communicates with an application service using Transport Layer + Security [TLS] or Datagram Transport Layer Security [DTLS], it has + some notion of the server's identity (e.g., "the website at + example.com") while attempting to establish secure communication. + Likewise, during TLS negotiation, the server presents its notion of + the service's identity in the form of a public-key certificate that + was issued by a certification authority (CA) in the context of the + Internet Public Key Infrastructure using X.509 [PKIX]. Informally, + we can think of these identities as the client's "reference identity" + and the server's "presented identity" (more formal definitions are + given later). A client needs to verify that the server's presented + identity matches its reference identity so it can authenticate the + communication. - The primary audience for this document consists of application - protocol designers, who can reference this document instead of - defining their own rules for the representation and verification of - application service identity. Secondarily, the audience consists of - certification authorities, service providers, and client developers - from technology communities that might reuse the recommendations in - this document when defining certificate issuance policies, generating - certificate signing requests, or writing software algorithms for - identity matching. + This document defines procedures for how clients do this + verification. It therefore implicitly defines requirements on other + parties, such as the CA's that issue certificates, the service + administrators requesting them, and the protocol designers defining + how things are named. -1.3. Changes since RFC 6125 +1.2. Changes since RFC 6125 This document revises and obsoletes [VERIFY] based on the decade of experience and changes since it was first published. The major changes, in no particular order, include: * All references have been updated to the current latest version. * The TLS SNI extension is no longer new, it is commonplace. * The only legal place for a certificate wildcard name is as the left-most component in a domain name. - * It is no longer allowed to use the commonName RDN, known as "CN- - ID", to represent the server identity; only the subjectAltNames + * It is no longer allowed to use the commonName RDN, known as CN-ID, + to represent the server identity; only the subjectAltNames extension is used. * References to the X.500 directory, the survey of prior art, and the sample text in Appendix A have been removed. -1.4. How to Read This Document - - This document is longer than the authors would have liked because it - was necessary to carefully define terminology, explain the underlying - concepts, define the scope, and specify recommended behavior for both - certification authorities and application software implementations. - The following sections are of special interest to various audiences: - - * Protocol designers might want to first read the checklist in - Section 3. - - * Certification authorities might want to first read the - recommendations for representation of server identity in - Section 4. - - * Service providers might want to first read the recommendations for - requesting of server certificates in Section 5. - - * Software implementers might want to first read the recommendations - for verification of server identity in Section 6. + * Detailed discussion of pinning (configuring use of a certificate + that doesn't match the criteria in this document) has been + removed. - The sections on terminology (Section 1.8), naming of application - services (Section 2), document scope (Section 1.7), and the like - provide useful background information regarding the recommendations - and guidelines that are contained in the above-referenced sections, - but are not absolutely necessary for a first reading of this - document. + * The sections detailing different target audiences and which + sections to read (first) have been removed. -1.5. Applicability +1.3. Applicability This document does not supersede the rules for certificate issuance - or validation provided in [PKIX]. Therefore, [PKIX] is authoritative - on any point that might also be discussed in this document. - Furthermore, [PKIX] also governs any certificate-related topic on - which this document is silent, including but not limited to - certificate syntax, certificate extensions such as name constraints - and extended key usage, and handling of certification paths. + or validation specified by [PKIX]. That document also governs any + certificate-related topic on which this document is silent. This + includes certificate syntax, certificate extensions such as name + constraints or extended key usage, and handling of certification + paths. This document addresses only name forms in the leaf "end entity" - server certificate, not any name forms in the chain of certificates - used to validate the server certificate. Therefore, in order to - ensure proper authentication, application clients need to verify the - entire certification path per [PKIX]. - - This document also does not supersede the rules for verifying service - identity provided in specifications for existing application - protocols published prior to this document. However, the procedures - described here can be referenced by future specifications, including - updates to specifications for existing application protocols if the - relevant technology communities agree to do so. - -1.6. Overview of Recommendations + server certificate. It does not address the name forms in the chain + of certificates used to validate a cetrificate, let alone creating or + checking the validity of such a chain. In order to ensure proper + authentication, applications need to verify the entire certification + path as per [PKIX]. - To orient the reader, this section provides an informational overview - of the recommendations contained in this document. +1.4. Overview of Recommendations The previous version of this specification, [VERIFY], surveyed the current practice from many IETF standards and tried to generalize best practices. This document takes the lessons learned in the past - decade and codifies them as best practices. - - For the primary audience of application protocol designers, this - document provides recommended procedures for the representation and - verification of application service identity within PKIX certificates - used in the context of TLS. + decade and codifies them. he rules are brief: - For the secondary audiences, in essence this document encourages - certification authorities, application service providers, and - application client developers to coalesce on the following practices: + * Only check DNS domain names via the subjectAlternativeName + extension designed for that purpose: dNSName. - * Stop including and checking strings that look like domain names in - the subject's Common Name. + * Allow use of even more specific subjectAlternativeName extensions + where appropriate such as uniformResourceIdentifier and the + otherName form SRVName. - * Check DNS domain names via the subjectAlternativeName extension - designed for that purpose: dNSName. + * Constrain wildcard certificates so that the wildcard can only be + the left-most component of a domain name. - * Move toward including and checking even more specific - subjectAlternativeName extensions where appropriate for using the - protocol (e.g., uniformResourceIdentifier and the otherName form - SRVName). + * Do not include or check strings that look like domain names in the + subject's Common Name. - * Constrain and simplify the validation of wildcard certificates - (e.g., a certificate containing an identifier for - "*.example.com"). +1.5. Scope -1.7. Scope -1.7.1. In Scope +1.5.1. In Scope This document applies only to service identities associated with - fully qualified DNS domain names, only to TLS and DTLS, and only to - PKIX-based systems. As a result, the scenarios described in the - following section are out of scope for this specification (although - they might be addressed by future specifications). - -1.7.2. Out of Scope - - The following topics are out of scope for this specification: - - * Client or end-user identities. + FQDNs only to TLS and DTLS, and only to PKIX-based systems. - Certificates representing client or end-user identities (e.g., the - rfc822Name identifier) can be used for mutual authentication - between a client and server or between two clients, thus enabling - stronger client-server security or end-to-end security. However, - certification authorities, application developers, and service - operators have less experience with client certificates than with - server certificates, thus giving us fewer models from which to - generalize and a less solid basis for defining best practices. + TLS uses the words client and server, where the client is the entity + that initiates the connection. In many cases, this models common + practice, such as a browser connecting to a Web origin. For the sake + of clarity, and to follow the usage in [TLS] and related + specifications, we will continue to use to use the terms client and + server in this document. Note that these are TLS-layer roles, and + that the application protocol could support the TLS server making + requests to the TLS client after the TLS handshake; these is no + requirement that the roles at the application layer match the TLS- + layer. - * Identifiers other than fully qualified DNS domain names. + At the time of this writing, other protocols such as [QUIC] and + Network Time Security ([NTS]) use TLS as a service to do the initial + establishment of cryptographic key material. Such services MUST also + follow the rules specified here. - For example, this specification does not discuss IP addresses or - other attributes within a certificate beyond the subjectAltName - extension. The focus of this document is on application service - identities, not specific resources located at such services. - Therefore this document discusses Uniform Resource Identifiers - [URI] only as a way to communicate a DNS domain name (via the URI - "host" component or its equivalent), not as a way to communicate - other aspects of a service such as a specific resource (via the - URI "path" component) or parameters (via the URI "query" - component). +1.5.2. Out of Scope - * Security protocols other than [TLS] or [DTLS]. + The following topics are out of scope for this specification: - Although other secure, lower-layer protocols exist and even employ - PKIX certificates at times (e.g., IPsec [IPSEC]), their use cases - can differ from those of TLS-based and DTLS-based application - technologies. Furthermore, application technologies have less - experience with IPsec than with TLS, thus making it more difficult - to gather feedback on proposed best practices. + * Security protocols other than [TLS] or [DTLS] except as described + above. * Keys or certificates employed outside the context of PKIX-based systems. - Some deployed application technologies use a web of trust model - based on or similar to OpenPGP [OPENPGP], or use self-signed - certificates, or are deployed on networks that are not directly - connected to the public Internet and therefore cannot depend on - Certificate Revocation Lists (CRLs) or the Online Certificate - Status Protocol [OCSP] to check CA-issued certificates. However, - the method for binding a public key to an identifier in OpenPGP - differs essentially from the method in X.509, the data in self- - signed certificates has not been certified by a third party in any - way, and checking of CA-issued certificates via CRLs or OCSP is - critically important to maintaining the security of PKIX-based - systems. Attempting to define best practices for such - technologies would unduly complicate the rules defined in this - specification. + * Client or end-user identities. Certificates representing client + identities other than that described above, such as rfc822Name, + are beyond the scope of this document. - * Certification authority policies, such as: + * Identifiers other than FQDNs. Identifiers such as IP address are + not discussed. In addition, the focus of this document is on + application service identities, not specific resources located at + such services. Therefore this document discusses Uniform Resource + Identifiers [URI] only as a way to communicate a DNS domain name + (via the URI "host" component or its equivalent), not other + aspects of a service such as a specific resource (via the URI + "path" component) or parameters (via the URI "query" component). - - What types or "classes" of certificates to issue and whether to - apply different policies for them. + * Certification authority policies. This includes items such as the + following: - - Whether to issue certificates based on IP addresses (or some - other form, such as relative domain names) in addition to fully - qualified DNS domain names. + - How to certify or validate FQDNs and application service types + (see [ACME] for some definition of this). - - Which identifiers to include (e.g., whether to include SRV-IDs - or URI-IDs as defined in the body of this specification). + - Issuing certificates with additional identifiers such as IP + address or relative domain name, in addition to FQDNs. - - How to certify or validate fully qualified DNS domain names and - application service types. + - Types or "classes" of certificates to issue and whether to + apply different policies for them. - How to certify or validate other kinds of information that might be included in a certificate (e.g., organization name). - * Resolution of DNS domain names. - - Although the process whereby a client resolves the DNS domain name - of an application service can involve several steps (e.g., this is - true of resolutions that depend on DNS SRV resource records, - Naming Authority Pointer (NAPTR) DNS resource records [NAPTR], and - related technologies such as [S-NAPTR]), for our purposes we care - only about the fact that the client needs to verify the identity - of the entity with which it communicates as a result of the - resolution process. Thus the resolution process itself is out of - scope for this specification. - - * User interface issues. + * Resolution of DNS domain names. Although the process whereby a + client resolves the DNS domain name of an application service can + involve several steps, for our purposes we care only about the + fact that the client needs to verify the identity of the entity + with which it communicates as a result of the resolution process. + Thus the resolution process itself is out of scope for this + specification. - In general, such issues are properly the responsibility of client - software developers and standards development organizations - dedicated to particular application technologies (see, for - example, [WSC-UI]). + * User interface issues. In general, such issues are properly the + responsibility of client software developers and standards + development organizations dedicated to particular application + technologies (see, for example, [WSC-UI]). -1.8. Terminology +1.6. Terminology Because many concepts related to "identity" are often too vague to be actionable in application protocols, we define a set of more concrete terms for use in this specification. application service: A service on the Internet that enables interactive and automated clients to connect for the purpose of retrieving or uploading information, communicating with other entities, or connecting to a broader network of services. application service provider: An organization or individual that hosts or deploys an application service. application service type: A formal identifier for the application protocol used to provide a particular kind of application service - at a domain; the application service type typically takes the form - of a Uniform Resource Identifier scheme [URI] or a DNS SRV Service - [DNS-SRV]. + at a domain. This often apepars as a URI scheme [URI] or a DNS + SRV Service [DNS-SRV]. automated client: A software agent or device that is not directly controlled by a human user. delegated domain: A domain name or host name that is explicitly - configured for communicating with the source domain, by either (a) - the human user controlling an interactive client or (b) a trusted - administrator. In case (a), one example of delegation is an - account setup that specifies the domain name of a particular host - to be used for retrieving information or connecting to a network, - which might be different from the server portion of the user's - account name (e.g., a server at mailhost.example.com for + configured for communicating with the source domain, by either the + human user controlling an interactive client or a trusted + administrator. For example, a server at mailhost.example.com for connecting to an IMAP server hosting an email address of - juliet@example.com). In case (b), one example of delegation is an - admin-configured host-to-address/address-to-host lookup table. + user@example.com. derived domain: A domain name or host name that a client has derived from the source domain in an automated fashion (e.g., by means of a [DNS-SRV] lookup). identifier: A particular instance of an identifier type that is either presented by a server in a certificate or referenced by a client for matching purposes. identifier type: A formally defined category of identifier that can be included in a certificate and therefore that can also be used for matching purposes. For conciseness and convenience, we define the following identifier types of interest, which are based on those found in the PKIX specification [PKIX] and various PKIX - extensions. + extensions: - * DNS-ID = a subjectAltName entry of type dNSName; see [PKIX] + * DNS-ID: a subjectAltName entry of type dNSName - * SRV-ID = a subjectAltName entry of type otherName whose name + * SRV-ID: a subjectAltName entry of type otherName whose name form is SRVName; see [SRVNAME] - * URI-ID = a subjectAltName entry of type + * URI-ID: a subjectAltName entry of type uniformResourceIdentifier whose value includes both (i) a "scheme" and (ii) a "host" component (or its equivalent) that matches the "reg-name" rule (where the quoted terms represent - the associated [ABNF] productions from [URI]); see [PKIX] and - [URI] + the associated [ABNF] productions from [URI]) An entry which + does not have both the scheme and host is not a valid URI-ID + and MUST be ignored. interactive client: A software agent or device that is directly - controlled by a human user. (Other specifications related to - security and application protocols, such as [WSC-UI], often refer - to this entity as a "user agent".) - - pinning: The act of establishing a cached name association between - the application service's certificate and one of the client's - reference identifiers, despite the fact that none of the presented - identifiers matches the given reference identifier. Pinning is - accomplished by allowing a human user to positively accept the - mismatch during an attempt to communicate with the application - service. Once a cached name association is established, the - certificate is said to be pinned to the reference identifier and - in future communication attempts the client simply verifies that - the service's presented certificate matches the pinned - certificate, as described under Section 6.6.2. (A similar - definition of "pinning" is provided in [WSC-UI].) + controlled by a human user, commonly known as a "user agent." PKIX: PKIX is a short name for the Internet Public Key - Infrastructure using X.509 defined in RFC 5280 [PKIX], which - comprises a profile of the X.509v3 certificate specifications and + Infrastructure using X.509 defined in [PKIX]. That document + provides a profile of the X.509v3 certificate specifications and X.509v2 certificate revocation list (CRL) specifications for use in the Internet. - PKIX-based system: A software implementation or deployed service - that makes use of X.509v3 certificates and X.509v2 certificate - revocation lists (CRLs). - - PKIX certificate: An X.509v3 certificate generated and employed in - the context of PKIX. - - presented identifier: An identifier that is presented by a server to - a client within a PKIX certificate when the client attempts to - establish secure communication with the server; the certificate + presented identifier: An identifier presented by a server to a + client within a PKIX certificate when the client attempts to + establish secure communication with the server. The certificate can include one or more presented identifiers of different types, and if the server hosts more than one domain then the certificate might present distinct identifiers for each domain. - reference identifier: An identifier, constructed from a source - domain and optionally an application service type, used by the - client for matching purposes when examining presented identifiers. + reference identifier: An identifier used by the client when + examining presented identifiers. It is constructed from the + source domain, and optionally an application service type. Relative Distinguished Name (RDN): The ASN.1-based construction comprising a Relative Distinguished Name (RDN), which itself is a building-block component of Distinguished Names. See [LDAP-DN], Section 2. - source domain: The fully qualified DNS domain name that a client - expects an application service to present in the certificate - (e.g., "www.example.com"), typically input by a human user, - configured into a client, or provided by reference such as in a - hyperlink. The combination of a source domain and, optionally, an + source domain: The FQDN that a client expects an application service + to present in the certificate. This is typically input by a human + user, configured into a client, or provided by reference such as + URL. The combination of a source domain and, optionally, an application service type enables a client to construct one or more reference identifiers. subjectAltName entry: An identifier placed in a subjectAltName extension. subjectAltName extension: A standard PKIX certificate extension [PKIX] enabling identifiers of various types to be bound to the certificate subject. subject name: In this specification, the term refers to the name of a PKIX certificate's subject, encoded in a certificate's subject field (see [PKIX], Section 4.1.2.6). - TLS client: An entity that assumes the role of a client in a - Transport Layer Security [TLS] negotiation. In this specification - we generally assume that the TLS client is an (interactive or - automated) application client; however, in application protocols - that enable server-to-server communication, the TLS client could - be a peer application service. - - TLS server: An entity that assumes the role of a server in a - Transport Layer Security [TLS] negotiation; in this specification - we assume that the TLS server is an application service. - - Most security-related terms in this document are to be understood in - the sense defined in [SECTERMS]; such terms include, but are not - limited to, "attack", "authentication", "authorization", - "certification authority", "certification path", "certificate", - "credential", "identity", "self-signed certificate", "trust", "trust - anchor", "trust chain", "validate", and "verify". + Security-related terms used in this document, but not defined here or + in [PKIX] should be understood in the the sense defined in + [SECTERMS]. Such terms include "attack", "authentication", + "identity", "trust", "validate", and "verify". The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. Naming of Application Services - This section discusses naming of application services on the - Internet, followed by a brief tutorial about subject naming in PKIX. + This document assumes that the name of an application service is + based on a DNS domain name (e.g., example.com) -- supplemented in + some circumstances by an application service type (e.g., "the IMAP + server at example.com"). The DNS name conforms to one of the + following forms: -2.1. Naming Application Services + 1. A "traditional domain name", i.e., a FQDN (see [DNS-CONCEPTS]) + all of whose labels are "LDH labels" as described in [IDNA-DEFS]. + Informally, such labels are constrained to [US-ASCII] letters, + digits, and the hyphen, with the hyphen prohibited in the first + character position. Additional qualifications apply (refer to + the above-referenced specifications for details), but they are + not relevant here. - This specification assumes that the name of an application service is - based on a DNS domain name (e.g., "example.com") -- supplemented in - some circumstances by an application service type (e.g., "the IMAP - server at example.com"). + 2. An "internationalized domain name", a DNS domain name that + includes at least one label containing appropriately encoded + Unicode code points outside the traditional US-ASCII range. That + is, it contains at least one U-label or A-label, but otherwise + may contain any mixture of NR-LDH labels, A-labels, or U-labels, + as described in [IDNA-DEFS] and the associated documents. From the perspective of the application client or user, some names - are direct because they are provided directly by a human user (e.g., - via runtime input, prior configuration, or explicit acceptance of a - client communication attempt), whereas other names are indirect - because they are automatically resolved by the client based on user - input (e.g., a target name resolved from a source name using DNS SRV - or NAPTR records). This dimension matters most for certificate - consumption, specifically verification as discussed in this document. + are _direct_ because they are provided directly by a human user. + This includes runtime input, prior configuration, or explicit + acceptance of a client communication attempt. Other names are + _indirect_ because they are automatically resolved by the application + based on user input, such as a target name resolved from a source + name using DNS SRV or [NAPTR] records). The distinction matters most + for certificate consumption, specifically verification as discussed + in this document. From the perspective of the application service, some names are - unrestricted because they can be used in any type of service (e.g., a - certificate might be reused for both the HTTP service and the IMAP - service at example.com), whereas other names are restricted because - they can be used in only one type of service (e.g., a special-purpose - certificate that can be used only for an IMAP service). This - dimension matters most for certificate issuance. + _unrestricted_ because they can be used in any type of service, such + as a single certificate being used for both the HTTP and IMAP + services at the host example.com. Other names are _restricted_ + because they can only be used for one type of service, such as a + special-purpose certificate that can only be used for an IMAP + service. This distinction matters most for certificate issuance. - Therefore, we can categorize the identifier types of interest as - follows: + We can categorize the supported identifier types as follows: * A DNS-ID is direct and unrestricted. * An SRV-ID is typically indirect but can be direct, and is restricted. * A URI-ID is direct and restricted. - When implementing software, deploying services, and issuing - certificates for secure PKIX-based authentication, it is important to - keep these distinctions in mind. In particular, best practices - differ somewhat for application server implementations, application - client implementations, application service providers, and - certification authorities. Ideally, protocol specifications that - reference this document will specify which identifiers are mandatory- - to-implement by servers and clients, which identifiers ought to be - supported by certificate issuers, and which identifiers ought to be - requested by application service providers. Because these - requirements differ across applications, it is impossible to - categorically stipulate universal rules (e.g., that all software - implementations, service providers, and certification authorities for - all application protocols need to use or support DNS-IDs as a - baseline for the purpose of interoperability). - - However, it is preferable that each application protocol will at - least define a baseline that applies to the community of software - developers, application service providers, and CAs actively using or - supporting that technology (one such community, the CA/Browser Forum, - has codified such a baseline for "Extended Validation Certificates" - in [EV-CERTS]). - -2.2. DNS Domain Names - - For the purposes of this specification, the name of an application - service is (or is based on) a DNS domain name that conforms to one of - the following forms: - - 1. A "traditional domain name", i.e., a fully qualified DNS domain - name or "FQDN" (see [DNS-CONCEPTS]) all of whose labels are "LDH - labels" as described in [IDNA-DEFS]. Informally, such labels are - constrained to [US-ASCII] letters, digits, and the hyphen, with - the hyphen prohibited in the first character position. - Additional qualifications apply (please refer to the above- - referenced specifications for details), but they are not relevant - to this specification. - - 2. An "internationalized domain name", i.e., a DNS domain name that - conforms to the overall form of a domain name (informally, dot- - separated letter-digit-hyphen labels) but includes at least one - label containing appropriately encoded Unicode code points - outside the traditional US-ASCII range. That is, it contains at - least one U-label or A-label, but otherwise may contain any - mixture of NR-LDH labels, A-labels, or U-labels, as described in - [IDNA-DEFS] and the associated documents. - -2.3. Subject Naming in PKIX Certificates - - For our purposes, an application service can be identified by a name - or names carried in one or more of the following identifier types - within subjectAltName entries: - - * DNS-ID - - * SRV-ID - - * URI-ID + It is important to keep these distinctions in mind, as best practices + for the deployment and use of the identifiers differ. As a further + example, cross-protocol attacks such as [ALPACA] are possibile when + two different protocol services use the same certificate. This can + be addressed by using restricted identifiers, or telling services to + not share certificates. Protocol specifications MUST specify which + identifiers are mandatory-to-implement and SHOULD provide operational + guidance when necessary. The Common Name RDN MUST NOT be used to identify a service. Reasons for this include: * It is not strongly typed and therefore suffers from ambiguities in interpretation. * It can appear more than once in the Subject Name. For similar reasons, other RDN's within the Subject Name MUST NOT be used to identify a service. 3. Designing Application Protocols - This section provides guidelines for designers of application - protocols, in the form of a checklist to follow when reusing the - recommendations provided in this document. + This section defines how protocol designers should reference this + document, which MUST be a normative reference in their specification. + The technology MUST only use the identifiers defined in this + document. Its specification MAY choose to allow only one of them. - * If your technology does not use DNS SRV records to resolve the DNS - domain names of application services then consider stating that - SRV-ID as defined in this document is not supported. Note that - many existing application technologies use DNS SRV records to - resolve the DNS domain names of application services, but do not - rely on representations of those records in PKIX certificates by - means of SRV-IDs as defined in [SRVNAME]. + If the technology does not use DNS SRV records to resolve the DNS + domain names of application services then its specification MUST + state that SRV-ID as defined in this document is not supported. Note + that many existing application technologies use DNS SRV records to + resolve the DNS domain names of application services, but do not rely + on representations of those records in PKIX certificates by means of + SRV-IDs as defined in [SRVNAME]. - * If your technology does not use use URIs to identify application - services, then consider stating that URI-ID as defined in this - document is not supported. Note that many existing application + If the technology does not use URI's to identify application + services, then its specification MUST state that URI-ID as defined in + this document is not supported. Note that many existing application technologies use URIs to identify application services, but do not - rely on representation of those URIs in PKIX certificates by means - of URI-IDs. + rely on representation of those URIs in PKIX certificates by means of + URI-IDs. - * If your technology disallows the wildcard character in DNS domain - names, then state the wildcard certificates as defined in this - document are not supported. + A technology MAY disallow the use of the wildcard character in DNS + names. If it does so, then the specification MUST state that + wildcard certificates as defined in this document are not supported. 4. Representing Server Identity - This section provides rules and guidelines for issuers of - certificates. + This section provides instructions for issuers of certificates. 4.1. Rules - When a certification authority issues a certificate based on the - fully qualified DNS domain name at which the application service - provider will provide the relevant application, the following rules - apply to the representation of application service identities. The - reader needs to be aware that some of these rules are cumulative and - can interact in important ways that are illustrated later in this - document. + When a certification authority issues a certificate based on the FQDN + at which the application service provider will provide the relevant + application, the following rules apply to the representation of + application service identities. Note that some of these rules are + cumulative and can interact in important ways that are illustrated + later in this document. - 1. The certificate SHOULD include a "DNS-ID" if possible as a - baseline for interoperability. + 1. The certificate MUST include a "DNS-ID" as a baseline for + interoperability. 2. If the service using the certificate deploys a technology for which the relevant specification stipulates that certificates ought to include identifiers of type SRV-ID (e.g., this is true of [XMPP]), then the certificate SHOULD include an SRV-ID. 3. If the service using the certificate deploys a technology for which the relevant specification stipulates that certificates ought to include identifiers of type URI-ID (e.g., this is true - of [SIP] as specified by [SIP-CERTS], but not true of [HTTP] - since [HTTP-TLS] does not describe usage of a URI-ID for HTTP - services), then the certificate SHOULD include a URI-ID. The - scheme SHALL be that of the protocol associated with the - application service type and the "host" component (or its - equivalent) SHALL be the fully qualified DNS domain name of the - service. A specification that reuses this one MUST specify which - URI schemes are to be considered acceptable in URI-IDs contained - in PKIX certificates used for the application protocol (e.g., - "sip" but not "sips" or "tel" for SIP as described in [SIP-SIPS], - or perhaps http and https for HTTP as might be described in a - future specification). + of [SIP] as specified by [SIP-CERTS]), then the certificate + SHOULD include a URI-ID. The scheme MUST be that of the protocol + associated with the application service type and the "host" + component (or its equivalent) MUST be the FQDN of the service. + The application protocol specification MUST specify which URI + schemes are acceptable in URI-IDs contained in PKIX certificates + used for the application protocol (e.g., sip but not sips or tel + for SIP as described in [SIP-SIPS]). - 4. The certificate MAY include other application-specific - identifiers for types that were defined before publication of - [SRVNAME] (e.g., XmppAddr for [XMPP]) or for which service names - or URI schemes do not exist; however, such application-specific - identifiers are not applicable to all application technologies - and therefore are out of scope for this specification. + 4. The certificate MAY contain more than one DNS-ID, SRV-ID, or URI- + ID as further explained under Section 7.3. - 5. The certificate MAY contain more than one DNS-ID, SRV-ID, or URI- - ID as further explained under Section 7.4. + 5. The certificate MAY include other application-specific + identifiers for compatibility with a deployed base. Such + identifiers are out of scope for this specification. 4.2. Examples - Consider a simple website at "www.example.com", which is not + Consider a simple website at www.example.com, which is not discoverable via DNS SRV lookups. Because HTTP does not specify the use of URIs in server certificates, a certificate for this service - might include only a DNS-ID of "www.example.com". + might include only a DNS-ID of www.example.com. - Consider an IMAP-accessible email server at the host - "mail.example.net" servicing email addresses of the form - "user@example.net" and discoverable via DNS SRV lookups on the - application service name of "example.net". A certificate for this - service might include SRV-IDs of "_imap.example.net" and - "_imaps.example.net" (see [EMAIL-SRV]) along with DNS-IDs of - "example.net" and "mail.example.net". + Consider an IMAP-accessible email server at the host mail.example.net + servicing email addresses of the form user@example.net and + discoverable via DNS SRV lookups on the application service name of + example.net. A certificate for this service might include SRV-IDs of + _imap.example.net and _imaps.example.net (see [EMAIL-SRV]) along with + DNS-IDs of example.net and mail.example.net. Consider a SIP-accessible voice-over-IP (VoIP) server at the host - "voice.example.edu" servicing SIP addresses of the form - "user@voice.example.edu" and identified by a URI of + voice.example.edu servicing SIP addresses of the form + user@voice.example.edu and identified by a URI of . A certificate for this service would - include a URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]) along - with a DNS-ID of "voice.example.edu". + include a URI-ID of sip:voice.example.edu (see [SIP-CERTS]) along + with a DNS-ID of voice.example.edu. Consider an XMPP-compatible instant messaging (IM) server at the host - "im.example.org" servicing IM addresses of the form - "user@im.example.org" and discoverable via DNS SRV lookups on the - "im.example.org" domain. A certificate for this service might - include SRV-IDs of "_xmpp-client.im.example.org" and "_xmpp- - server.im.example.org" (see [XMPP]), a DNS-ID of "im.example.org", - and an XMPP-specific "XmppAddr" of "im.example.org" (see [XMPP]). + im.example.org servicing IM addresses of the form user@im.example.org + and discoverable via DNS SRV lookups on the im.example.org domain. A + certificate for this service might include SRV-IDs of _xmpp- + client.im.example.org and _xmpp-server.im.example.org (see [XMPP]), a + DNS-ID of im.example.org. For backward compatibility, it may also + have an XMPP-specific XmppAddr of im.example.org (see [XMPP]). 5. Requesting Server Certificates - This section provides rules and guidelines for service providers - regarding the information to include in certificate signing requests - (CSRs). + This section provides instructions for service providers regarding + the information to include in certificate signing requests (CSRs). + In general, service providers SHOULD request certificates that + include all of the identifier types that are required or recommended + for the application service type that will be secured using the + certificate to be issued. - In general, service providers are encouraged to request certificates - that include all of the identifier types that are required or - recommended for the application service type that will be secured - using the certificate to be issued. + If the certificate will be used for only a single type of application + service, the service provider SHOULD request a certificate that + includes a DNS-ID and, if appropriate for the application service + type, an SRV-ID or URI-ID that limits the deployment scope of the + certificate to only the defined application service type. If the certificate might be used for any type of application service, - then the service provider is encouraged to request a certificate that - includes only a DNS-ID. - - If the certificate will be used for only a single type of application - service, then the service provider is encouraged to request a - certificate that includes a DNS-ID and, if appropriate for the - application service type, an SRV-ID or URI-ID that limits the - deployment scope of the certificate to only the defined application - service type. + then the service provider SHOULD to request a certificate that + includes only a DNS-ID. Again, because of multi-protocol attacks + this practice is discouraged; this can be mitigated by providing only + one service on a host. - If a service provider offering multiple application service types - (e.g., a World Wide Web service, an email service, and an instant - messaging service) wishes to limit the applicability of certificates - using SRV-IDs or URI-IDs, then the service provider is encouraged to - request multiple certificates, i.e., one certificate per application - service type. Conversely, the service provider is discouraged from - requesting a single certificate containing multiple SRV-IDs or URI- - IDs identifying each different application service type. This - guideline does not apply to application service type "bundles" that - are used to identify manifold distinct access methods to the same - underlying application (e.g., an email application with access - methods denoted by the application service types of "imap", "imaps", - "pop3", "pop3s", and "submission" as described in [EMAIL-SRV]). + If a service provider offersmultiple application service types and + wishes to limit the applicability of certificates using SRV-IDs or + URI-IDs, they SHOULD request multiple certificates, rather than a + single certificate containing multiple SRV-IDs or URI-IDs each + identifying ia different application service type. This rule does + not apply to application service type "bundles" that identify + distinct access methods to the same underlying application such as an + email application with access methods denoted by the application + service types of imap, imaps, pop3, pop3s, and submission as + described in [EMAIL-SRV]. 6. Verifying Service Identity - This section provides rules and guidelines for implementers of - application client software regarding algorithms for verification of - application service identity. - -6.1. Overview - At a high level, the client verifies the application service's - identity by performing the actions listed below (which are defined in - the following subsections of this document): + identity by performing the following actions: 1. The client constructs a list of acceptable reference identifiers based on the source domain and, optionally, the type of service to which the client is connecting. 2. The server provides its identifiers in the form of a PKIX certificate. 3. The client checks each of its reference identifiers against the - presented identifiers for the purpose of finding a match. - - 4. When checking a reference identifier against a presented - identifier, the client matches the source domain of the - identifiers and, optionally, their application service type. + presented identifiers for the purpose of finding a match. When + checking a reference identifier against a presented identifier, + the client matches the source domain of the identifiers and, + optionally, their application service type. - Naturally, in addition to checking identifiers, a client might - complete further checks to ensure that the server is authorized to - provide the requested service. However, such checking is not a - matter of verifying the application service identity presented in a - certificate, and therefore methods for doing so (e.g., consulting - local policy information) are out of scope for this document. + Naturally, in addition to checking identifiers, a client should + perform further checks, such as expiration and revocation, to ensure + that the server is authorized to provide the requested service. Such + checking is not a matter of verifying the application service + identity presented in a certificate, however, and methods for doing + so are therefore out of scope for this document. -6.2. Constructing a List of Reference Identifiers +6.1. Constructing a List of Reference Identifiers -6.2.1. Rules +6.1.1. Rules The client MUST construct a list of acceptable reference identifiers, and MUST do so independently of the identifiers presented by the service. The inputs used by the client to construct its list of reference identifiers might be a URI that a user has typed into an interface (e.g., an HTTPS URL for a website), configured account information - (e.g., the domain name of a particular host or URI used for - retrieving information or connecting to a network, which might be + (e.g., the domain name of a host for retrieving email, which might be different from the DNS domain name portion of a username), a hyperlink in a web page that triggers a browser to retrieve a media object or script, or some other combination of information that can yield a source domain and an application service type. The client might need to extract the source domain and application service type from the input(s) it has received. The extracted data MUST include only information that can be securely parsed out of the - inputs (e.g., parsing the fully qualified DNS domain name out of the - "host" component (or its equivalent) of a URI or deriving the - application service type from the scheme of a URI) or information - that is derived in a manner not subject to subversion by network - attackers (e.g., pulling the data from a delegated domain that is - explicitly established via client or system configuration, resolving - the data via [DNSSEC], or obtaining the data from a third-party - domain mapping service in which a human user has explicitly placed - trust and with which the client communicates over a connection or - association that provides both mutual authentication and integrity - checking). These considerations apply only to extraction of the - source domain from the inputs; naturally, if the inputs themselves + inputs, such as parsing the FQDN out of the "host" component or + deriving the application service type from the scheme of a URI. + Other possibilities include pulling the data from a delegated domain + that is explicitly established via client or system configuration, + resolving the data via [DNSSEC], or obtaining the data from a third- + party domain mapping service in which a human user has explicitly + placed trust and with which the client communicates over a connection + or association that provides both mutual authentication and integrity + checking. These considerations apply only to extraction of the + source domain from the inputs. Naturally, if the inputs themselves are invalid or corrupt (e.g., a user has clicked a link provided by a malicious entity in a phishing attack), then the client might end up communicating with an unexpected application service. For example, given an input URI of , a client - would derive the application service type "sip" from the scheme and - parse the domain name "example.net" from the host component. + would derive the application service type sip from the scheme and + parse the domain name example.net from the host component. - Each reference identifier in the list SHOULD be based on the source - domain and SHOULD NOT be based on a derived domain (e.g., a host name - or domain name discovered through DNS resolution of the source - domain). This rule is important because only a match between the - user inputs and a presented identifier enables the client to be sure - that the certificate can legitimately be used to secure the client's - communication with the server. There is only one scenario in which - it is acceptable for an interactive client to override the - recommendation in this rule and therefore communicate with a domain - name other than the source domain: because a human user has "pinned" - the application service's certificate to the alternative domain name - as further discussed under Section 6.6.4 and Section 7.1. In this - case, the inputs used by the client to construct its list of - reference identifiers might include more than one fully qualified DNS - domain name, i.e., both (a) the source domain and (b) the alternative - domain contained in the pinned certificate. + Each reference identifier in the list MUST be based on the source + domain and MUST NOT be based on a derived domain such as a domain + name discovered through DNS resolution of the source domain. This + rule is important because only a match between the user inputs and a + presented identifier enables the client to be sure that the + certificate can legitimately be used to secure the client's + communication with the server. - Using the combination of fully qualified DNS domain name(s) and - application service type, the client constructs a list of reference - identifiers in accordance with the following rules: + Using the combination of FQDN(s) and application service type, the + client MUST construct its list of reference identifiers in accordance + with the following rules: * The list SHOULD include a DNS-ID. A reference identifier of type - DNS-ID can be directly constructed from a fully qualified DNS - domain name that is (a) contained in or securely derived from the - inputs (i.e., the source domain), or (b) explicitly associated - with the source domain by means of user configuration (i.e., a - derived domain). + DNS-ID can be directly constructed from a FQDN that is (a) + contained in or securely derived from the inputs, or (b) + explicitly associated with the source domain by means of user + configuration. * If a server for the application service type is typically discovered by means of DNS SRV records, then the list SHOULD include an SRV-ID. * If a server for the application service type is typically associated with a URI for security purposes (i.e., a formal protocol document specifies the use of URIs in server certificates), then the list SHOULD include a URI-ID. Which identifier types a client includes in its list of reference - identifiers is a matter of local policy. For example, in certain - deployment environments, a client that is built to connect only to a - particular kind of service (e.g., only IM services) might be - configured to accept as valid only certificates that include an SRV- - ID for that application service type; in this case, the client would - include only SRV-IDs matching the application service type in its - list of reference identifiers (not, for example, DNS-IDs). By - contrast, a more lenient client (even one built to connect only to a - particular kind of service) might include both SRV-IDs and DNS-IDs in + identifiers, and their priority, is a matter of local policy. For + example, a client that is built to connect only to a particular kind + of service might be configured to accept as valid only certificates + that include an SRV-ID for that application service type. By + contrast, a more lenient client, even if built to connect only to a + particular kind of service, might include both SRV-IDs and DNS-IDs in its list of reference identifiers. -6.2.2. Examples +6.1.2. Examples A web browser that is connecting via HTTPS to the website at - "www.example.com" would have a single reference identifier: a DNS-ID - of "www.example.com". + www.example.com would have a single reference identifier: a DNS-ID of + www.example.com. A mail user agent that is connecting via IMAPS to the email service - at "example.net" (resolved as "mail.example.net") might have three - reference identifiers: an SRV-ID of "_imaps.example.net" (see - [EMAIL-SRV]), and DNS-IDs of "example.net" and "mail.example.net". - (A legacy email user agent would not support [EMAIL-SRV] and - therefore would probably be explicitly configured to connect to - "mail.example.net", whereas an SRV-aware user agent would derive - "example.net" from an email address of the form "user@example.net" - but might also accept "mail.example.net" as the DNS domain name - portion of reference identifiers for the service.) + at example.net (resolved as mail.example.net) might have three + reference identifiers: an SRV-ID of _imaps.example.net (see + [EMAIL-SRV]), and DNS-IDs of example.net and mail.example.net. An + email user agentthat does not support [EMAIL-SRV] would probably be + explicitly configured to connect to mail.example.net, whereas an SRV- + aware user agent would derive example.net from an email address of + the form user@example.net but might also accept mail.example.net as + the DNS domain name portion of reference identifiers for the service. A voice-over-IP (VoIP) user agent that is connecting via SIP to the - voice service at "voice.example.edu" might have only one reference - identifier: a URI-ID of "sip:voice.example.edu" (see [SIP-CERTS]). + voice service at voice.example.edu might have only one reference + identifier: a URI-ID of sip:voice.example.edu (see [SIP-CERTS]). An instant messaging (IM) client that is connecting via XMPP to the - IM service at "im.example.org" might have three reference - identifiers: an SRV-ID of "_xmpp-client.im.example.org" (see [XMPP]), - a DNS-ID of "im.example.org", and an XMPP-specific "XmppAddr" of - "im.example.org" (see [XMPP]). + IM service at im.example.org might have three reference identifiers: + an SRV-ID of _xmpp-client.im.example.org (see [XMPP]), a DNS-ID of + im.example.org, and an XMPP-specific XmppAddr of im.example.org (see + [XMPP]). -6.3. Preparing to Seek a Match +6.2. Preparing to Seek a Match Once the client has constructed its list of reference identifiers and has received the server's presented identifiers in the form of a PKIX certificate, the client checks its reference identifiers against the presented identifiers for the purpose of finding a match. The search fails if the client exhausts its list of reference identifiers without finding a match. The search succeeds if any presented identifier matches one of the reference identifiers, at which point the client SHOULD stop the search. Before applying the comparison rules provided in the following sections, the client might need to split the reference identifier into its DNS domain name portion and its application service type portion, as follows: - * A reference identifier of type DNS-ID does not include an - application service type portion and thus can be used directly as - the DNS domain name for comparison purposes. As an example, a - DNS-ID of "www.example.com" would result in a DNS domain name - portion of "www.example.com". + * A DNS-ID reference identifier MUST be used directly as the DNS + domain name and there is no application service type. - * For a reference identifier of type SRV-ID, the DNS domain name - portion is the Name and the application service type portion is - the Service. As an example, an SRV-ID of "_imaps.example.net" - would be split into a DNS domain name portion of "example.net" and - an application service type portion of "imaps" (mapping to an - application protocol of IMAP as explained in [EMAIL-SRV]). + * For an SRV-ID reference identifier, the DNS domain name portion is + the Name and the application service type portion is the Service. + For example, an SRV-ID of _imaps.example.net has a DNS domain name + portion of example.net and an application service type portion of + imaps, which maps to the IMAP application protocol as explained in + [EMAIL-SRV]. * For a reference identifier of type URI-ID, the DNS domain name - portion is the "reg-name" part of the "host" component (or its - equivalent) and the application service type portion is the - application service type associated with the scheme name matching - the [ABNF] "scheme" rule from [URI] (not including the ':' - separator). As previously mentioned, this document specifies that - a URI-ID always contains a "host" component (or its equivalent) - containing a "reg-name". (Matching only the "reg-name" rule from - [URI] limits verification to DNS domain names, thereby - differentiating a URI-ID from a uniformResourceIdentifier entry - that contains an IP address or a mere host name, or that does not - contain a "host" component at all.) Furthermore, note that - extraction of the "reg-name" might necessitate normalization of - the URI (as explained in [URI]). As an example, a URI-ID of - "sip:voice.example.edu" would be split into a DNS domain name - portion of "voice.example.edu" and an application service type of - "sip" (associated with an application protocol of SIP as explained - in [SIP-CERTS]). - - Detailed comparison rules for matching the DNS domain name portion - and application service type portion of the reference identifier are - provided in the following sections. + portion is the "reg-name" part of the "host" component and the + application service type portion is the scheme, as defind above. + Matching only the "reg-name" rule from [URI] limits verification + to DNS domain names, thereby differentiating a URI-ID from a + uniformResourceIdentifier entry that contains an IP address or a + mere host name, or that does not contain a "host" component at + all. Furthermore, note that extraction of the "reg-name" might + necessitate normalization of the URI (as explained in [URI]). For + example, a URI-ID of sip:voice.example.edu would be split into a + DNS domain name portion of voice.example.edu and an application + service type of sip (associated with an application protocol of + SIP as explained in [SIP-CERTS]). -6.4. Matching the DNS Domain Name Portion + A client MUST match the DNS name, and if an application service type + is present it MUST also match the service type as well. These are + described below. - The client MUST match the DNS domain name portion of a reference - identifier according to the following rules (and SHOULD also check - the application service type as described under Section 6.5). The - rules differ depending on whether the domain to be checked is a - "traditional domain name" or an "internationalized domain name" (as - defined under Section 2.2). Furthermore, to meet the needs of - clients that support presented identifiers containing the wildcard - character "*", we define a supplemental rule for such "wildcard - certificates". +6.3. Matching the DNS Domain Name Portion -6.4.1. Checking of Traditional Domain Names + This section describes how the client must determine if the the + presented DNS name matches the reference DNS name. The rules differ + depending on whether the domain to be checked is a "traditional + domain name" or an "internationalized domain name" (as defined under + Section 2). For clients that support names containing the wildcard + character "*", this section also specifies a supplemental rule for + such "wildcard certificates". This section uses the description of + labels and domain names in [DNS-CONCEPTS]. If the DNS domain name portion of a reference identifier is a "traditional domain name", then matching of the reference identifier - against the presented identifier is performed by comparing the set of - domain name labels using a case-insensitive ASCII comparison, as - clarified by [DNS-CASE] (e.g., "WWW.Example.Com" would be lower-cased - to "www.example.com" for comparison purposes). Each label MUST match - in order for the names to be considered to match, except as - supplemented by the rule about checking of wildcard labels - (Section 6.4.3). - -6.4.2. Checking of Internationalized Domain Names + against the presented identifier MUST be performed by comparing the + set of domain name labels using a case-insensitive ASCII comparison, + as clarified by [DNS-CASE]. For example, WWW.Example.Com would be + lower-cased to www.example.com for comparison purposes. Each label + MUST match in order for the names to be considered to match, except + as supplemented by the rule about checking of wildcard labels given + below. If the DNS domain name portion of a reference identifier is an - internationalized domain name, then an implementation MUST convert - any U-labels [IDNA-DEFS] in the domain name to A-labels before - checking the domain name. In accordance with [IDNA-PROTO], A-labels - MUST be compared as case-insensitive ASCII. Each label MUST match in - order for the domain names to be considered to match, except as - supplemented by the rule about checking of wildcard labels - (Section 6.4.3; but see also Section 7.2 regarding wildcards in - internationalized domain names). - -6.4.3. Checking of Wildcard Certificates + internationalized domain name, then the client MUST convert any + U-labels [IDNA-DEFS] in the domain name to A-labels before checking + the domain name. In accordance with [IDNA-PROTO], A-labels MUST be + compared as case-insensitive ASCII. Each label MUST match in order + for the domain names to be considered to match, except as + supplemented by the rule about checking of wildcard labels given + below. - A client MAY match the reference identifier against a presented - identifier whose DNS domain name portion contains the wildcard - character "*" in a label (following the description of labels and - domain names in [DNS-CONCEPTS]), provided these requirements are met: + If the technology specification supports wildcards, then the client + MUST match the reference identifier against a presented identifier + whose DNS domain name portion contains the wildcard character "*" in + a label provided these requirements are met: 1. There is only one wildcard character. 2. The wildcard character appears only as the content of the left- most label. - 3. The wildcard character is not embedded in an A-label or U-label - [IDNA-DEFS] of an internationalized domain name [IDNA-PROTO]. + If the requirements are not met, the presented identifier is invalid + and MUST be ignored. A wildcard in a presented identifier can only match exactly one label in a reference identifier. Note that this is not the same as DNS wildcard matching, where the "*" label always matches at least one whole label and sometimes more. See [DNS-CONCEPTS], Section 4.3.3 and [DNS-WILDCARDS]. For information regarding the security characteristics of wildcard - certificates, see Section 7.2. - -6.5. Matching the Application Service Type Portion + certificates, see Section 7.1. - When a client checks identifiers of type SRV-ID and URI-ID, it MUST - check not only the DNS domain name portion of the identifier but also - the application service type portion. The client does this by - splitting the identifier into the DNS domain name portion and the - application service type portion (as described under Section 6.3), - then checking both the DNS domain name portion (as described under - Section 6.4) and the application service type portion as described in - the following subsections. +6.4. Matching the Application Service Type Portion - Implementation Note: An identifier of type SRV-ID or URI-ID provides - an application service type portion to be checked, but that portion - is combined only with the DNS domain name portion of the SRV-ID or - URI-ID itself. For example, if a client's list of reference - identifiers includes an SRV-ID of "_xmpp-client.im.example.org" and a - DNS-ID of "apps.example.net", the client would check (a) the - combination of an application service type of "xmpp-client" and a DNS - domain name of "im.example.org" and (b) a DNS domain name of - "apps.example.net". However, the client would not check (c) the - combination of an application service type of "xmpp-client" and a DNS - domain name of "apps.example.net" because it does not have an SRV-ID - of "_xmpp-client.apps.example.net" in its list of reference - identifiers. + The rules for matching the application service type deopend on + whether the identifier is an SRV-ID or a URI-ID. -6.5.1. SRV-ID + These identifiers provide an application service type portion to be + checked, but that portion is combined only with the DNS domain name + portion of the SRV-ID or URI-ID itself. For example, if a client's + list of reference identifiers includes an SRV-ID of _xmpp- + client.im.example.org and a DNS-ID of apps.example.net, the client + would check both the combination of an application service type of + xmpp-client and a DNS domain name of im.example.org and a DNS domain + name of apps.example.net. However, the client would not check the + combination of an application service type of xmpp-client and a DNS + domain name of apps.example.net because it does not have an SRV-ID of + _xmpp-client.apps.example.net in its list of reference identifiers. - The application service name portion of an SRV-ID (e.g., "imaps") + If the identifier is an SRV-ID, then the application service name MUST be matched in a case-insensitive manner, in accordance with - [DNS-SRV]. Note that the "_" character is prepended to the service + [DNS-SRV]. Note that the _ character is prepended to the service identifier in DNS SRV records and in SRV-IDs (per [SRVNAME]), and thus does not need to be included in any comparison. -6.5.2. URI-ID - - The scheme name portion of a URI-ID (e.g., "sip") MUST be matched in - a case-insensitive manner, in accordance with [URI]. Note that the - ":" character is a separator between the scheme name and the rest of - the URI, and thus does not need to be included in any comparison. - -6.6. Outcome - - The outcome of the matching procedure is one of the following cases. + If the identifier is a URI-ID, then the scheme name portion MUST be + matched in a case-insensitive manner, in accordance with [URI]. Note + that the : character is a separator between the scheme name and the + rest of the URI, and thus does not need to be included in any + comparison. -6.6.1. Case #1: Match Found +6.5. Outcome If the client has found a presented identifier that matches a reference identifier, then the service identity check has succeeded. In this case, the client MUST use the matched reference identifier as the validated identity of the application service. -6.6.2. Case #2: No Match Found, Pinned Certificate - If the client does not find a presented identifier matching any of - the reference identifiers but the client has previously pinned the - application service's certificate to one of the reference identifiers - in the list it constructed for this communication attempt (as - "pinning" is explained under Section 1.8), and the presented - certificate matches the pinned certificate (including the context as - described under Section 7.1), then the service identity check has - succeeded. - -6.6.3. Case #3: No Match Found, No Pinned Certificate - - If the client does not find a presented identifier matching any of - the reference identifiers and the client has not previously pinned - the certificate to one of the reference identifiers in the list it - constructed for this communication attempt, then the client MUST - proceed as described under Section 6.6.4. + the reference identifiers then the client MUST proceed as described + as follows. -6.6.4. Fallback + If the client is an automated application not directly controlled by + a human user, then it SHOULD terminate the communication attempt with + a bad certificate error and log the error appropriately. The + application MAY provide a configuration setting to disable this + behavior, but it MUST enable it by default. If the client is an interactive client that is directly controlled by a human user, then it SHOULD inform the user of the identity mismatch and automatically terminate the communication attempt with a bad - certificate error; this behavior is preferable because it prevents - users from inadvertently bypassing security protections in hostile - situations. + certificate error in order to prevent users from inadvertently + bypassing security protections in hostile situations. - Some interactive clients give advanced users the option of proceeding - with acceptance despite the identity mismatch. Although this - behavior can be appropriate in certain specialized circumstances, it - needs to be handled with extreme caution, for example by first - encouraging even an advanced user to terminate the communication - attempt and, if the advanced user chooses to proceed anyway, by + Some interactive clients MAY give advanced users the option of + proceeding with acceptance despite the identity mismatch. Although + this behavior can be appropriate in certain specialized + circumstances, it needs to be handled with extreme caution, for + example by first encouraging even an advanced user to terminate the + communication attempt and, if they choose to proceed anyway, by forcing the user to view the entire certification path before proceeding. - Otherwise, if the client is an automated application not directly - controlled by a human user, then it SHOULD terminate the - communication attempt with a bad certificate error and log the error - appropriately. An automated application MAY provide a configuration - setting that disables this behavior, but MUST enable the behavior by - default. + The application MAY also present the user with the ability to accept + the presented certificate as valid for subsequent connections. Such + ad-hoc "pinning" SHOULD NOT restrict future connections to just the + pinned certificate. Local policy that statically enforces a given + certificate for a given peer is best made available only as prior + configuration, rather than a just-in-time override for a failed + connection. 7. Security Considerations -7.1. Pinned Certificates - - As defined under Section 1.8, a certificate is said to be "pinned" to - a DNS domain name when a user has explicitly chosen to associate a - service's certificate with that DNS domain name despite the fact that - the certificate contains some other DNS domain name (e.g., the user - has explicitly approved "apps.example.net" as a domain associated - with a source domain of "example.com"). The cached name association - MUST take account of both the certificate presented and the context - in which it was accepted or configured (where the "context" includes - the chain of certificates from the presented certificate to the trust - anchor, the source domain, the application service type, the - service's derived domain and port number, and any other relevant - information provided by the user or associated by the client). - -7.2. Wildcard Certificates +7.1. Wildcard Certificates Wildcard certificates, those that have an identifier with "*" as the left-most DNS label, automatically vouch for any single-label host names within their domain, but not multiple levels of domains. This can be convenient for administrators but also poses the risk of vouching for rogue or buggy hosts. See for example [Defeating-SSL] (beginning at slide 91) and [HTTPSbytes] (slides 38-40). - Protection against a wildcard that identifies a so-called "public - suffix" (e.g., "*.co.uk" or "*.com") is beyond the scope of this - document. + Protection against a wildcard that identifies a public suffix + [Public-Suffix], such as *.co.uk or *.com, is beyond the scope of + this document. -7.3. Internationalized Domain Names +7.2. Internationalized Domain Names - Allowing internationalized domain names can lead to the inclusion of - visually similar (so-called "confusable") characters in certificates; - for discussion, see for example [IDNA-DEFS]. + Allowing internationalized domain names can lead to visually similar + characters, also referred to as "confusables", being included within + certificates. For discussion, see for example [IDNA-DEFS], + Section 4.4 and [UTS-39]. -7.4. Multiple Identifiers +7.3. Multiple Presented Identifiers A given application service might be addressed by multiple DNS domain names for a variety of reasons, and a given deployment might service - multiple domains or protocols. The client SHOULD use the TLS Server - Name Identification (SNI) extension as discussed in [TLS], - Section 4.4.2.2. + multiple domains or protocols. TLS Extensions such as TLS Server + Name Identification (SNI), discussed in [TLS], Section 4.4.2.2, and + Application Layer Protocol Negotiation (ALPN), discussed in [ALPN], + provide a way for the application to indicate the desired identifier + and protocol to the server, which can be used to select the most + appropriate certificate. To accommodate the workaround that was needed before the development of the SNI extension, this specification allows multiple DNS-IDs, SRV-IDs, or URI-IDs in a certificate. -8. References +8. IANA Considerations -8.1. Normative References + This document has no actions for IANA. + +9. References + +9.1. Normative References [DNS-CONCEPTS] - Mockapetris, P.V., "Domain names - concepts and - facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, - November 1987, . + Mockapetris, P., "Domain names - concepts and facilities", + STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, + . [DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, DOI 10.17487/RFC2782, February 2000, - . + . [DNS-WILDCARDS] Lewis, E., "The Role of Wildcards in the Domain Name System", RFC 4592, DOI 10.17487/RFC4592, July 2006, - . + . [IDNA-DEFS] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010, - . + . [IDNA-PROTO] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/RFC5891, August 2010, - . + . [LDAP-DN] Zeilenga, K., Ed., "Lightweight Directory Access Protocol (LDAP): String Representation of Distinguished Names", RFC 4514, DOI 10.17487/RFC4514, June 2006, - . + . [PKIX] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, - . + . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, - . + . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, - May 2017, . + May 2017, . [SRVNAME] Santesson, S., "Internet X.509 Public Key Infrastructure Subject Alternative Name for Expression of Service Name", RFC 4985, DOI 10.17487/RFC4985, August 2007, - . + . [URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, - . + . -8.2. Informative References +9.2. Informative References [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, - . + . + + [ACME] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. + Kasten, "Automatic Certificate Management Environment + (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, + . + + [ALPACA] Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D., + Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel, + "ALPACA: Application Layer Protocol Confusion - Analyzing + and Mitigating Cracks in TLS Authentication", September + 2021, . + + [ALPN] Friedl, S., Popov, A., Langley, A., and E. Stephan, + "Transport Layer Security (TLS) Application-Layer Protocol + Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, + July 2014, . [Defeating-SSL] Marlinspike, M., "New Tricks for Defeating SSL in Practice", BlackHat DC, February 2009, . [DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case Insensitivity Clarification", RFC 4343, DOI 10.17487/RFC4343, January 2006, - . + . [DNSSEC] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005, - . + . [DTLS] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, - January 2012, . + January 2012, . [EMAIL-SRV] Daboo, C., "Use of SRV Records for Locating Email Submission/Access Services", RFC 6186, DOI 10.17487/RFC6186, March 2011, - . - - [EV-CERTS] CA/Browser Forum, "Guidelines For The Issuance And - Management Of Extended Validation Certificates", October - 2009, . - - [HTTP] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer - Protocol (HTTP/1.1): Message Syntax and Routing", - RFC 7230, DOI 10.17487/RFC7230, June 2014, - . - - [HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818, - DOI 10.17487/RFC2818, May 2000, - . + . [HTTPSbytes] Sokol, J. and R. Hansen, "HTTPS Can Byte Me", BlackHat Abu Dhabi, November 2010, . - [IPSEC] Kent, S. and K. Seo, "Security Architecture for the - Internet Protocol", RFC 4301, DOI 10.17487/RFC4301, - December 2005, . - [NAPTR] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain Name System (DNS) Database", RFC 3403, DOI 10.17487/RFC3403, October 2002, - . + . - [OCSP] Santesson, S., Myers, M., Ankney, R., Malpani, A., - Galperin, S., and C. Adams, "X.509 Internet Public Key - Infrastructure Online Certificate Status Protocol - OCSP", - RFC 6960, DOI 10.17487/RFC6960, June 2013, - . + [NTS] Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R. + Sundblad, "Network Time Security for the Network Time + Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020, + . - [OPENPGP] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. - Thayer, "OpenPGP Message Format", RFC 4880, - DOI 10.17487/RFC4880, November 2007, - . + [Public-Suffix] + "Public Suffix List", 2020, . - [S-NAPTR] Daigle, L. and A. Newton, "Domain-Based Application - Service Location Using SRV RRs and the Dynamic Delegation - Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958, - January 2005, . + [QUIC] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure + QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, + . [SECTERMS] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, - . + . [SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, - . + . [SIP-CERTS] Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain Certificates in the Session Initiation Protocol (SIP)", RFC 5922, DOI 10.17487/RFC5922, June 2010, - . + . [SIP-SIPS] Audet, F., "The Use of the SIPS URI Scheme in the Session Initiation Protocol (SIP)", RFC 5630, DOI 10.17487/RFC5630, October 2009, - . + . [TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, - . + . [US-ASCII] American National Standards Institute, "Coded Character Set - 7-bit American Standard Code for Information Interchange", ANSI X3.4, 1986. + [UTS-39] Davis, M. and M. Suignard, "Unicode Security Mechanisms", + n.d., . + [VERIFY] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March - 2011, . + 2011, . [WSC-UI] Saldhana, A. and T. Roessler, "Web Security Context: User - Interface Guidelines", World Wide Web Consortium LastCall - WD-wsc-ui-20100309, March 2010, - . + Interface Guidelines", August 2010, + . [XMPP] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, - March 2011, . + March 2011, . Acknowledgements We gratefully acknowledge everyone who contributed to the previous - version of this document, [VERIFY]. + version of this document, [VERIFY]. Thanks also to Carsten Bormann + for converting the previous document to Markdown so that we could + more easily use Martin Thomson's i-d-template software. Authors' Addresses Peter Saint-Andre Mozilla United States of America Email: stpeter@mozilla.com Jeff Hodges