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

Document Type Active Internet-Draft (uta WG)
Authors Peter Saint-Andre  , Jeff Hodges  , Rich Salz 
Last updated 2021-09-08
Replaces draft-ietf-uta-use-san
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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
                                                     Akamai Technologies
                                                        8 September 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

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.

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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   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."

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   This Internet-Draft will expire on 12 March 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
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   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

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     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.  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.

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   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

   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.

1.3.  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
      extension is used.

   *  References to the X.500 directory, the survey of prior art, and
      the sample text in Appendix A have been removed.

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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.

   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.

1.5.  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.

   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].

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   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

   To orient the reader, this section provides an informational overview
   of the recommendations contained in this document.

   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.

   For the secondary audiences, in essence this document encourages
   certification authorities, application service providers, and
   application client developers to coalesce on the following practices:

   *  Stop including and checking strings that look like domain names in
      the subject's Common Name.

   *  Check DNS domain names via the subjectAlternativeName extension
      designed for that purpose: dNSName.

   *  Move toward including and checking even more specific
      subjectAlternativeName extensions where appropriate for using the
      protocol (e.g., uniformResourceIdentifier and the otherName form
      SRVName).

   *  Constrain and simplify the validation of wildcard certificates
      (e.g., a certificate containing an identifier for
      "*.example.com").

1.7.  Scope

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1.7.1.  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.

      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.

   *  Identifiers other than fully qualified DNS domain names.

      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).

   *  Security protocols other than [TLS] or [DTLS].

      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.

   *  Keys or certificates employed outside the context of PKIX-based
      systems.

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      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.

   *  Certification authority policies, such as:

      -  What types or "classes" of certificates to issue and whether to
         apply different policies for them.

      -  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.

      -  Which identifiers to include (e.g., whether to include SRV-IDs
         or URI-IDs as defined in the body of this specification).

      -  How to certify or validate fully qualified DNS domain names and
         application service types.

      -  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.

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      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

   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].

   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
      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.

   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.

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   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.

      *  DNS-ID = a subjectAltName entry of type dNSName; see [PKIX]

      *  SRV-ID = a subjectAltName entry of type otherName whose name
         form is SRVName; see [SRVNAME]

      *  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]

   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].)

   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
      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

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      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
      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.

   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
      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.

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   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".

   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.

2.1.  Naming Application Services

   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").

   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.

   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.

   Therefore, we can categorize the identifier types of interest as
   follows:

   *  A DNS-ID is direct and unrestricted.

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   *  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

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       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

   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.

   *  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 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
      technologies use URIs to identify application services, but do not
      rely on representation of those URIs in PKIX certificates by means
      of URI-IDs.

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   *  If your technology disallows the wildcard character in DNS domain
      names, then state the wildcard certificates as defined in this
      document are not supported.

4.  Representing Server Identity

   This section provides rules and guidelines 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.

   1.  The certificate SHOULD include a "DNS-ID" if possible 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).

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   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.

   5.  The certificate MAY contain more than one DNS-ID, SRV-ID, or URI-
       ID as further explained under Section 7.4.

4.2.  Examples

   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".

   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
   <sip:voice.example.edu>.  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".

   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]).

5.  Requesting Server Certificates

   This section provides rules and guidelines for service providers
   regarding the information to include in certificate signing requests
   (CSRs).

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   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 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.

   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]).

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):

   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.

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   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.

   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.

6.2.  Constructing a List of Reference Identifiers

6.2.1.  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
   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.

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   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
   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 <sips:alice@example.net>, a client
   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.

   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:

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   *  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).

   *  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
   its list of reference identifiers.

6.2.2.  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".

   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.)

   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]).

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   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]).

6.3.  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".

   *  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 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

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      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.

6.4.  Matching the DNS Domain Name Portion

   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".

6.4.1.  Checking of Traditional Domain Names

   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

   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).

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6.4.3.  Checking of Wildcard Certificates

   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:

   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].

   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

   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.

   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.

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6.5.1.  SRV-ID

   The application service name portion of an SRV-ID (e.g., "imaps")
   MUST be matched in a case-insensitive manner, in accordance with
   [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.

6.6.1.  Case #1: Match Found

   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.

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6.6.4.  Fallback

   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.

   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
   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.

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).

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7.2.  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.

7.3.  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].

7.4.  Multiple 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.

   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

8.1.  Normative References

   [DNS-CONCEPTS]
              Mockapetris, P.V., "Domain names - concepts and
              facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034,
              November 1987, <https://doi.org/10.17487/RFC1034>.

   [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,
              <https://doi.org/10.17487/RFC2782>.

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   [DNS-WILDCARDS]
              Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
              <https://doi.org/10.17487/RFC4592>.

   [IDNA-DEFS]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://doi.org/10.17487/RFC5890>.

   [IDNA-PROTO]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <https://doi.org/10.17487/RFC5891>.

   [LDAP-DN]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names",
              RFC 4514, DOI 10.17487/RFC4514, June 2006,
              <https://doi.org/10.17487/RFC4514>.

   [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,
              <https://doi.org/10.17487/RFC5280>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://doi.org/10.17487/RFC2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://doi.org/10.17487/RFC8174>.

   [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,
              <https://doi.org/10.17487/RFC4985>.

   [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,
              <https://doi.org/10.17487/RFC3986>.

8.2.  Informative References

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   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://doi.org/10.17487/RFC5234>.

   [Defeating-SSL]
              Marlinspike, M., "New Tricks for Defeating SSL in
              Practice", BlackHat DC, February 2009,
              <http://www.blackhat.com/presentations/bh-dc-
              09/Marlinspike/BlackHat-DC-09-Marlinspike-Defeating-
              SSL.pdf>.

   [DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case
              Insensitivity Clarification", RFC 4343,
              DOI 10.17487/RFC4343, January 2006,
              <https://doi.org/10.17487/RFC4343>.

   [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,
              <https://doi.org/10.17487/RFC4033>.

   [DTLS]     Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://doi.org/10.17487/RFC6347>.

   [EMAIL-SRV]
              Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <https://doi.org/10.17487/RFC6186>.

   [EV-CERTS] CA/Browser Forum, "Guidelines For The Issuance And
              Management Of Extended Validation Certificates", October
              2009, <http://www.cabforum.org/Guidelines_v1_2.pdf>.

   [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,
              <https://doi.org/10.17487/RFC7230>.

   [HTTP-TLS] Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://doi.org/10.17487/RFC2818>.

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   [HTTPSbytes]
              Sokol, J. and R. Hansen, "HTTPS Can Byte Me", BlackHat Abu
              Dhabi, November 2010, <https://media.blackhat.com/bh-ad-
              10/Hansen/Blackhat-AD-2010-Hansen-Sokol-HTTPS-Can-Byte-Me-
              slides.pdf>.

   [IPSEC]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://doi.org/10.17487/RFC4301>.

   [NAPTR]    Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, DOI 10.17487/RFC3403, October 2002,
              <https://doi.org/10.17487/RFC3403>.

   [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,
              <https://doi.org/10.17487/RFC6960>.

   [OPENPGP]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <https://doi.org/10.17487/RFC4880>.

   [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, <https://doi.org/10.17487/RFC3958>.

   [SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://doi.org/10.17487/RFC4949>.

   [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,
              <https://doi.org/10.17487/RFC3261>.

   [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,
              <https://doi.org/10.17487/RFC5922>.

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   [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,
              <https://doi.org/10.17487/RFC5630>.

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://doi.org/10.17487/RFC8446>.

   [US-ASCII] American National Standards Institute, "Coded Character
              Set - 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

   [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, <https://doi.org/10.17487/RFC6125>.

   [WSC-UI]   Saldhana, A. and T. Roessler, "Web Security Context: User
              Interface Guidelines", World Wide Web Consortium LastCall 
              WD-wsc-ui-20100309, March 2010,
              <http://www.w3.org/TR/2010/WD-wsc-ui-20100309>.

   [XMPP]     Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://doi.org/10.17487/RFC6120>.

Acknowledgements

   We gratefully acknowledge everyone who contributed to the previous
   version of this document, [VERIFY].

Authors' Addresses

   Peter Saint-Andre
   Mozilla
   United States of America

   Email: stpeter@mozilla.com

   Jeff Hodges
   Google
   United States of America

   Email: jdhodges@google.com

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   Rich Salz
   Akamai Technologies
   United States of America

   Email: rsalz@akamai.com

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