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RFC 6125

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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)

Part 1 of 3, p. 1 to 17
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Internet Engineering Task Force (IETF)                    P. Saint-Andre
Request for Comments: 6125                                         Cisco
Category: Standards Track                                      J. Hodges
ISSN: 2070-1721                                                   PayPal
                                                              March 2011


  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)

Abstract

   Many application technologies enable secure communication between two
   entities by means of Internet Public Key Infrastructure Using X.509
   (PKIX) certificates in the context of Transport Layer Security (TLS).
   This document specifies procedures for representing and verifying the
   identity of application services in such interactions.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6125.

Copyright Notice

   Copyright (c) 2011 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
   (http://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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Audience . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  How to Read This Document  . . . . . . . . . . . . . . . .  4
     1.4.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
     1.5.  Overview of Recommendations  . . . . . . . . . . . . . . .  5
     1.6.  Generalization from Current Technologies . . . . . . . . .  6
     1.7.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       1.7.1.  In Scope . . . . . . . . . . . . . . . . . . . . . . .  7
       1.7.2.  Out of Scope . . . . . . . . . . . . . . . . . . . . .  7
     1.8.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  9
   2.  Naming of Application Services . . . . . . . . . . . . . . . . 13
     2.1.  Naming Application Services  . . . . . . . . . . . . . . . 13
     2.2.  DNS Domain Names . . . . . . . . . . . . . . . . . . . . . 14
     2.3.  Subject Naming in PKIX Certificates  . . . . . . . . . . . 15
       2.3.1.  Implementation Notes . . . . . . . . . . . . . . . . . 17
   3.  Designing Application Protocols  . . . . . . . . . . . . . . . 18
   4.  Representing Server Identity . . . . . . . . . . . . . . . . . 18
     4.1.  Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     4.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Requesting Server Certificates . . . . . . . . . . . . . . . . 21
   6.  Verifying Service Identity . . . . . . . . . . . . . . . . . . 21
     6.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 21
     6.2.  Constructing a List of Reference Identifiers . . . . . . . 22
       6.2.1.  Rules  . . . . . . . . . . . . . . . . . . . . . . . . 22
       6.2.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . 24
     6.3.  Preparing to Seek a Match  . . . . . . . . . . . . . . . . 25
     6.4.  Matching the DNS Domain Name Portion . . . . . . . . . . . 26
       6.4.1.  Checking of Traditional Domain Names . . . . . . . . . 27
       6.4.2.  Checking of Internationalized Domain Names . . . . . . 27
       6.4.3.  Checking of Wildcard Certificates  . . . . . . . . . . 27
       6.4.4.  Checking of Common Names . . . . . . . . . . . . . . . 28
     6.5.  Matching the Application Service Type Portion  . . . . . . 28
       6.5.1.  SRV-ID . . . . . . . . . . . . . . . . . . . . . . . . 29
       6.5.2.  URI-ID . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.6.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . . 29
       6.6.1.  Case #1: Match Found . . . . . . . . . . . . . . . . . 29
       6.6.2.  Case #2: No Match Found, Pinned Certificate  . . . . . 29
       6.6.3.  Case #3: No Match Found, No Pinned Certificate . . . . 30
       6.6.4.  Fallback . . . . . . . . . . . . . . . . . . . . . . . 30
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 30
     7.1.  Pinned Certificates  . . . . . . . . . . . . . . . . . . . 30
     7.2.  Wildcard Certificates  . . . . . . . . . . . . . . . . . . 31
     7.3.  Internationalized Domain Names . . . . . . . . . . . . . . 32
     7.4.  Multiple Identifiers . . . . . . . . . . . . . . . . . . . 32
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 33

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   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 34
     10.2. Informative References . . . . . . . . . . . . . . . . . . 34
   Appendix A.  Sample Text . . . . . . . . . . . . . . . . . . . . . 40
   Appendix B.  Prior Art . . . . . . . . . . . . . . . . . . . . . . 42
     B.1.  IMAP, POP3, and ACAP (1999)  . . . . . . . . . . . . . . . 42
     B.2.  HTTP (2000)  . . . . . . . . . . . . . . . . . . . . . . . 43
     B.3.  LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 44
     B.4.  SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 47
     B.5.  XMPP (2004)  . . . . . . . . . . . . . . . . . . . . . . . 49
     B.6.  NNTP (2006)  . . . . . . . . . . . . . . . . . . . . . . . 50
     B.7.  NETCONF (2006/2009)  . . . . . . . . . . . . . . . . . . . 51
     B.8.  Syslog (2009)  . . . . . . . . . . . . . . . . . . . . . . 52
     B.9.  SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 54
     B.10. SNMP (2010)  . . . . . . . . . . . . . . . . . . . . . . . 55
     B.11. GIST (2010)  . . . . . . . . . . . . . . . . . . . . . . . 55

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.

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   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.  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:

   o  Protocol designers might want to first read the checklist in
      Section 3.

   o  Certification authorities might want to first read the
      recommendations for representation of server identity in
      Section 4.

   o  Service providers might want to first read the recommendations for
      requesting of server certificates in Section 5.

   o  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.

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1.4.  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].

   This document also does not supersede the rules for verifying service
   identity provided in specifications for existing application
   protocols published prior to this document, such as those excerpted
   under Appendix B.  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.

1.5.  Overview of Recommendations

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

   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:

   o  Move away from including and checking strings that look like
      domain names in the subject's Common Name.

   o  Move toward including and checking DNS domain names via the
      subjectAlternativeName extension designed for that purpose:
      dNSName.

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

   o  Move away from the issuance of so-called wildcard certificates
      (e.g., a certificate containing an identifier for
      "*.example.com").

   These suggestions are not entirely consistent with all practices that
   are currently followed by certification authorities, client
   developers, and service providers.  However, they reflect the best
   aspects of current practices and are expected to become more widely
   adopted in the coming years.

1.6.  Generalization from Current Technologies

   This document attempts to generalize best practices from the many
   application technologies that currently use PKIX certificates with
   TLS.  Such technologies include, but are not limited to:

   o  The Internet Message Access Protocol [IMAP] and the Post Office
      Protocol [POP3]; see also [USINGTLS]

   o  The Hypertext Transfer Protocol [HTTP]; see also [HTTP-TLS]

   o  The Lightweight Directory Access Protocol [LDAP]; see also
      [LDAP-AUTH] and its predecessor [LDAP-TLS]

   o  The Simple Mail Transfer Protocol [SMTP]; see also [SMTP-AUTH] and
      [SMTP-TLS]

   o  The Extensible Messaging and Presence Protocol [XMPP]; see also
      [XMPP-OLD]

   o  The Network News Transfer Protocol [NNTP]; see also [NNTP-TLS]

   o  The NETCONF Configuration Protocol [NETCONF]; see also
      [NETCONF-SSH] and [NETCONF-TLS]

   o  The Syslog Protocol [SYSLOG]; see also [SYSLOG-TLS] and
      [SYSLOG-DTLS]

   o  The Session Initiation Protocol [SIP]; see also [SIP-CERTS]

   o  The Simple Network Management Protocol [SNMP]; see also [SNMP-TLS]

   o  The General Internet Signalling Transport [GIST]

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   However, as noted, this document does not supersede the rules for
   verifying service identity provided in specifications for those
   application protocols.

1.7.  Scope

1.7.1.  In Scope

   This document applies only to service identities associated with
   fully qualified DNS domain names, only to TLS and DTLS (or the older
   Secure Sockets Layer (SSL) technology), 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:

   o  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.

   o  Identifiers other than fully qualified DNS domain names.

      Some certification authorities issue server certificates based on
      IP addresses, but preliminary evidence indicates that such
      certificates are a very small percentage (less than 1%) of issued
      certificates.  Furthermore, IP addresses are not necessarily
      reliable identifiers for application services because of the
      existence of private internets [PRIVATE], host mobility, multiple
      interfaces on a given host, Network Address Translators (NATs)
      resulting in different addresses for a host from different
      locations on the network, the practice of grouping many hosts
      together behind a single IP address, etc.  Most fundamentally,
      most users find DNS domain names much easier to work with than IP
      addresses, which is why the domain name system was designed in the
      first place.  We prefer to define best practices for the much more
      common use case and not to complicate the rules in this
      specification.

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      Furthermore, we focus here 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).

      We also do not discuss attributes unrelated to DNS domain names,
      such as those defined in [X.520] and other such specifications
      (e.g., organizational attributes, geographical attributes, company
      logos, and the like).

   o  Security protocols other than [TLS], [DTLS], or the older Secure
      Sockets Layer (SSL) technology.

      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.

   o  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.

   o  Certification authority policies, such as:

      *  What types or "classes" of certificates to issue and whether to
         apply different policies for them (e.g., allow the wildcard
         character in certificates issued to individuals who have
         provided proof of identity but do not allow the wildcard
         character in "Extended Validation" certificates [EV-CERTS]).

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      *  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).

   o  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.

   o  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

   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.

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   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].

   attribute-type-and-value pair:  A colloquial name for the ASN.1-based
      construction comprising a Relative Distinguished Name (RDN), which
      itself is a building-block component of Distinguished Names.  See
      Section 2 of [LDAP-DN].

   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.

   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.

      *  CN-ID = a Relative Distinguished Name (RDN) in the certificate
         subject field that contains one and only one attribute-type-
         and-value pair of type Common Name (CN), where the value
         matches the overall form of a domain name (informally, dot-
         separated letter-digit-hyphen labels); see [PKIX] and also
         [LDAP-SCHEMA]

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      *  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
      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,

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      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.

   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 -- in addition to, or in place of, identifiers
      that may be embedded within or provided as a certificate's subject
      field.

   subject field:  The subject field of a PKIX certificate identifies
      the entity associated with the public key stored in the subject
      public key field (see Section 4.1.2.6 of [PKIX]).

   subject name:  In an overall sense, a subject's name(s) can be
      represented by or in the subject field, the subjectAltName
      extension, or both (see [PKIX] for details).  More specifically,
      the term often refers to the name of a PKIX certificate's subject,
      encoded as the X.501 type Name and conveyed in a certificate's
      subject field (see Section 4.1.2.6 of [PKIX]).

   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 RFC
   2119 [KEYWORDS].

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:

   o  A CN-ID is direct and unrestricted.

   o  A DNS-ID is direct and unrestricted.

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   o  An SRV-ID can be either direct or (more typically) indirect, and
      is restricted.

   o  A URI-ID is direct and restricted.

   We summarize this taxonomy in the following table.

   +-----------+-----------+---------------+
   |           |  Direct   |  Restricted   |
   +-----------+-----------+---------------+
   |  CN-ID    |  Yes      |  No           |
   +-----------+-----------+---------------+
   |  DNS-ID   |  Yes      |  No           |
   +-----------+-----------+---------------+
   |  SRV-ID   |  Either   |  Yes          |
   +-----------+-----------+---------------+
   |  URI-ID   |  Yes      |  Yes          |
   +-----------+-----------+---------------+

   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:

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   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

   In theory, the Internet Public Key Infrastructure using X.509 [PKIX]
   employs the global directory service model defined in [X.500] and
   [X.501].  Under that model, information is held in a directory
   information base (DIB) and entries in the DIB are organized in a
   hierarchy called the directory information tree (DIT).  An object or
   alias entry in that hierarchy consists of a set of attributes (each
   of which has a defined type and one or more values) and is uniquely
   identified by a Distinguished Name (DN).  The DN of an entry is
   constructed by combining the Relative Distinguished Names of its
   superior entries in the tree (all the way down to the root of the
   DIT) with one or more specially nominated attributes of the entry
   itself (which together comprise the Relative Distinguished Name (RDN)
   of the entry, so-called because it is relative to the Distinguished
   Names of the superior entries in the tree).  The entry closest to the
   root is sometimes referred to as the "most significant" entry, and
   the entry farthest from the root is sometimes referred to as the
   "least significant" entry.  An RDN is a set (i.e., an unordered
   group) of attribute-type-and-value pairs (see also [LDAP-DN]), each
   of which asserts some attribute about the entry.

   In practice, the certificates used in [X.509] and [PKIX] borrow key
   concepts from X.500 and X.501 (e.g., DNs and RDNs) to identify
   entities, but such certificates are not necessarily part of a global
   directory information base.  Specifically, the subject field of a
   PKIX certificate is an X.501 type Name that "identifies the entity
   associated with the public key stored in the subject public key
   field" (see Section 4.1.2.6 of [PKIX]).  However, it is perfectly
   acceptable for the subject field to be empty, as long as the

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   certificate contains a subject alternative name ("subjectAltName")
   extension that includes at least one subjectAltName entry, because
   the subjectAltName extension allows various identities to be bound to
   the subject (see Section 4.2.1.6 of [PKIX]).  The subjectAltName
   extension itself is a sequence of typed entries, where each type is a
   distinct kind of identifier.

   For our purposes, an application service can be identified by a name
   or names carried in the subject field (i.e., a CN-ID) and/or in one
   of the following identifier types within subjectAltName entries:

   o  DNS-ID

   o  SRV-ID

   o  URI-ID

   Existing certificates often use a CN-ID in the subject field to
   represent a fully qualified DNS domain name; for example, consider
   the following three subject names, where the attribute of type Common
   Name contains a string whose form matches that of a fully qualified
   DNS domain name ("im.example.org", "mail.example.net", and
   "www.example.com", respectively):

      CN=im.example.org,O=Example Org,C=GB

      C=CA,O=Example Internetworking,CN=mail.example.net

      O=Examples-R-Us,CN=www.example.com,C=US

   However, the Common Name is not strongly typed because a Common Name
   might contain a human-friendly string for the service, rather than a
   string whose form matches that of a fully qualified DNS domain name
   (a certificate with such a single Common Name will typically have at
   least one subjectAltName entry containing the fully qualified DNS
   domain name):

      CN=A Free Chat Service,O=Example Org,C=GB

   Or, a certificate's subject might contain both a CN-ID as well as
   another common name attribute containing a human-friendly string:

      CN=A Free Chat Service,CN=im.example.org,O=Example Org,C=GB

   In general, this specification recommends and prefers use of
   subjectAltName entries (DNS-ID, SRV-ID, URI-ID, etc.) over use of the
   subject field (CN-ID) where possible, as more completely described in
   the following sections.  However, specifications that reuse this one

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   can legitimately encourage continued support for the CN-ID identifier
   type if they have good reasons to do so, such as backward
   compatibility with deployed infrastructure (see, for example,
   [EV-CERTS]).

2.3.1.  Implementation Notes

   Confusion sometimes arises from different renderings or encodings of
   the hierarchical information contained in a certificate.

   Certificates are binary objects and are encoded using the
   Distinguished Encoding Rules (DER) specified in [X.690].  However,
   some implementations generate displayable (a.k.a. printable)
   renderings of the certificate issuer, subject field, and
   subjectAltName extension, and these renderings convert the DER-
   encoded sequences into a "string representation" before being
   displayed.  Because a certificate subject field (of type Name
   [X.509], the same as for a Distinguished Name (DN) [X.501]) is an
   ordered sequence, order is typically preserved in subject string
   representations, although the two most prevalent subject (and DN)
   string representations differ in employing left-to-right vs. right-
   to-left ordering.  However, because a Relative Distinguished Name
   (RDN) is an unordered group of attribute-type-and-value pairs, the
   string representation of an RDN can differ from the canonical DER
   encoding (and the order of attribute-type-and-value pairs can differ
   in the RDN string representations or display orders provided by
   various implementations).  Furthermore, various specifications refer
   to the order of RDNs in DNs or certificate subject fields using
   terminology that is implicitly related to an information hierarchy
   (which may or may not actually exist), such as "most specific" vs.
   "least specific", "left-most" vs. "right-most", "first" vs. "last",
   or "most significant" vs. "least significant" (see, for example,
   [LDAP-DN]).

   To reduce confusion, in this specification we avoid such terms and
   instead use the terms provided under Section 1.8; in particular, we
   do not use the term "(most specific) Common Name field in the subject
   field" from [HTTP-TLS] and instead state that a CN-ID is a Relative
   Distinguished Name (RDN) in the certificate subject containing one
   and only one attribute-type-and-value pair of type Common Name (thus
   removing the possibility that an RDN might contain multiple AVAs
   (Attribute Value Assertions) of type CN, one of which could be
   considered "most specific").

   Finally, although theoretically some consider the order of RDNs
   within a subject field to have meaning, in practice that rule is
   often not observed.  An AVA of type CN is considered to be valid at
   any position within the subject field.


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