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Network Working Group                                     P. Saint-Andre
Internet-Draft                                                     Cisco
Intended status: BCP                                           J. Hodges
Expires: May 21, 2011                                             PayPal
                                                       November 17, 2010


  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-saintandre-tls-server-id-check-11

Abstract

   Many application technologies enable a secure connection 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 best current practices for representing and
   verifying the identity of application services in such interactions.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 21, 2011.

Copyright Notice

   Copyright (c) 2010 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
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   to this document.  Code Components extracted from this document must



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Applicability and Audience . . . . . . . . . . . . . . . .  5
     1.3.  Overview of Recommendations  . . . . . . . . . . . . . . .  6
     1.4.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
       1.4.1.  In Scope . . . . . . . . . . . . . . . . . . . . . . .  7
       1.4.2.  Out of Scope . . . . . . . . . . . . . . . . . . . . .  7
     1.5.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  9
     1.6.  Contributors . . . . . . . . . . . . . . . . . . . . . . . 12
     1.7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 13
   2.  Names  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.1.  Naming Application Services  . . . . . . . . . . . . . . . 13
     2.2.  DNS Domain Names . . . . . . . . . . . . . . . . . . . . . 15
     2.3.  Subject Naming in PKIX Certificates  . . . . . . . . . . . 15
   3.  Representation of Server Identity  . . . . . . . . . . . . . . 17
     3.1.  Rules  . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     3.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.  Verification of Service Identity . . . . . . . . . . . . . . . 19
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.2.  Constructing a List of Reference Identifiers . . . . . . . 19
       4.2.1.  Rules  . . . . . . . . . . . . . . . . . . . . . . . . 19
       4.2.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . 21
     4.3.  Preparing to Seeking a Match . . . . . . . . . . . . . . . 21
     4.4.  Verifying the DNS Domain Name Portion  . . . . . . . . . . 23
       4.4.1.  Checking of Traditional Domain Names . . . . . . . . . 23
       4.4.2.  Checking of Internationalized Domain Names . . . . . . 23
       4.4.3.  Checking of Wildcard Certificates  . . . . . . . . . . 23
       4.4.4.  Checking of Common Names . . . . . . . . . . . . . . . 24
     4.5.  Verifying the Application Type Portion . . . . . . . . . . 24
       4.5.1.  SRV-ID . . . . . . . . . . . . . . . . . . . . . . . . 25
       4.5.2.  URI-ID . . . . . . . . . . . . . . . . . . . . . . . . 25
     4.6.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . . 25
       4.6.1.  Case #1: Match Found . . . . . . . . . . . . . . . . . 25
       4.6.2.  Case #2: No Match Found, Pinned Certificate  . . . . . 25
       4.6.3.  Case #3: No Match Found, No Pinned Certificate . . . . 25
       4.6.4.  Fallback . . . . . . . . . . . . . . . . . . . . . . . 25
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
     5.1.  Pinned Certificates  . . . . . . . . . . . . . . . . . . . 26
     5.2.  Wildcard Certificates  . . . . . . . . . . . . . . . . . . 26
     5.3.  Internationalized Domain Names . . . . . . . . . . . . . . 27
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27



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   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Prior Art . . . . . . . . . . . . . . . . . . . . . . 33
     A.1.  IMAP, POP3, and ACAP (1999)  . . . . . . . . . . . . . . . 33
     A.2.  HTTP (2000)  . . . . . . . . . . . . . . . . . . . . . . . 34
     A.3.  LDAP (2000/2006) . . . . . . . . . . . . . . . . . . . . . 35
     A.4.  SMTP (2002/2007) . . . . . . . . . . . . . . . . . . . . . 39
     A.5.  XMPP (2004)  . . . . . . . . . . . . . . . . . . . . . . . 40
     A.6.  NNTP (2006)  . . . . . . . . . . . . . . . . . . . . . . . 41
     A.7.  NETCONF (2006/2009)  . . . . . . . . . . . . . . . . . . . 42
     A.8.  Syslog (2009)  . . . . . . . . . . . . . . . . . . . . . . 43
     A.9.  SIP (2010) . . . . . . . . . . . . . . . . . . . . . . . . 44
     A.10. SNMP (2010)  . . . . . . . . . . . . . . . . . . . . . . . 45
     A.11. GIST (2010)  . . . . . . . . . . . . . . . . . . . . . . . 46
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47



































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1.  Introduction

1.1.  Motivation

   The visible face of the Internet consists of services that employ a
   client-server architecture in which an interactive or automated
   client connects to an application service in order to retrieve or
   upload information, communicate with other entities, or access a
   broader network of services.  When a client connects to an
   application service using Transport Layer Security [TLS] or Datagram
   Transport Layer Security [DTLS], it references some conception of the
   server's identity while attempting to establish a secure connection
   (e.g., "the website at example.com").  Likewise, during TLS
   negotiation the server presents its conception 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 be sure
   that the certificate can legitimately be used to authenticate the
   connection.

   Many application technologies adhere to the pattern just outlined,
   including but not limited to the following:

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

   o  The Hypertext Transfer Protocol [HTTP], for which see also
      [HTTP-TLS]

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

   o  The Simple Mail Transfer Protocol [SMTP], for which see also
      [SMTP-AUTH] and [SMTP-TLS]

   o  The Extensible Messaging and Presence Protocol [XMPP], for which
      see also [XMPP-OLD]

   o  The Network News Transfer Protocol [NNTP], for which see also
      [NNTP-TLS]






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   o  The NETCONF Configuration Protocol [NETCONF], for which see also
      [NETCONF-SSH] and [NETCONF-TLS]

   o  The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS] and
      [SYSLOG-DTLS]

   o  The Session Initiation Protocol [SIP], for which see also
      [SIP-CERTS]

   o  The Simple Network Management Protocol [SNMP], for which see also
      [SNMP-TLS]

   o  The General Internet Signalling Transport [GIST]

   Application 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 best current practices regarding the
   implementation and deployment of secure PKIX-based authentication,
   this document specifies recommended procedures for representing and
   verifying application service identity in certificates intended for
   use in applications employing TLS.

1.2.  Applicability and Audience

   This document does not supersede the rules for certificate issuance
   or validation provided in [PKIX], which governs any certificate-
   related topic on which this document is silent (e.g., certificate
   syntax, certificate extensions such as name constraints and extended
   key usage, and handling of certification paths).  Specifically, in
   order to ensure proper authentication, application clients need to
   verify the entire certification path, because this document addresses
   only name forms in the leaf server certificate, not any name forms in
   the chain of certificates used to validate the server certificate.

   This document also does not supersede the rules for verifying service
   identity provided in specifications for existing application
   protocols, such as those mentioned under Appendix A.  However, the
   best current practices 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.  It is also expected that this document will be updated or
   obsoleted in the future as best practices for issuance and
   verification of PKIX certificates continue to evolve through more
   widespread implementation and deployment of TLS-protected application



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   services over the Internet.

   The primary audience for this document consists of application
   protocol designers, who might 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, client developers, service providers, and
   other members of technology communities that might re-use or
   "profile" the recommendations in this document when defining
   certificate issuance policies, writing software algorithms for
   identity matching, generating certificate signing requests, etc.

1.3.  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, based on
   best current practices this document provides recommended procedures
   for 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 best
   current 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.

   o  Move toward including and checking even more specific
      subjectAlternativeName extensions where appropriate for the using
      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.



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1.4.  Scope

1.4.1.  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).

1.4.2.  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 gives 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.

      Furthermore, we focus here on application service identities, not
      specific resources located at such services, e.g., a specific web
      page that can be accessed at a particular Uniform Resource
      Identifier [URI] whose authority component is the DNS domain name



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      of the application service.  We also do not address identifiers
      derived from Naming Authority Pointer (NAPTR) DNS resource records
      [NAPTR] and related technologies such as [S-NAPTR], since such
      identifiers cannot be validated in a trusted manner in the absence
      of [DNSSEC].

      Finally, we 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 syntax of OpenPGP differs essentially from that of 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 OSCP 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.

   Furthermore, this document also does not address various
   certification authority policies, such as:

   o  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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   o  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.

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

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

   o  How to certify or validate other kinds of information that might
      be included in a certificate (e.g., organization name).

   Finally, this specification is mostly silent about user interface
   issues, which in general are properly the responsibility of client
   software developers and standards development organizations dedicated
   to particular application technologies (see for example [WSC-UI]).

1.5.  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.  (Following the convention from
   [SECTERMS], each entry is preceded by a dollar sign ($) and a space.)

   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.

   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 connecting to 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



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      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; the types covered in this specification
      are:

      *  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]

      *  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, where the value includes both (i) a
         "scheme" and (ii) an "authority" component whose "host"
         subcomponent matches the "reg-name" rule (where the quoted
         terms represent the associated [ABNF] rules from [URI]); see
         [PKIX]

   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"; however that term is neither
      entirely accurate nor consistent with the terminology of common
      application protocols such as [HTTP].)








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   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 connect to the application service.
      Once a cached name association is established, the certificate is
      said to be pinned to the reference identifier and in future
      connection attempts the client simply verifies that the service's
      presented certificate matches the pinned certificate, as described
      under Section 4.6.2.  (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 the server's PKIX certificate when the client
      attempts to establish a secure connection with the server; the
      certificate can include one or more presented identifiers of
      different types.

   reference identifier:  An identifier, constructed from a source
      domain and optionally a service type, used by the client for
      matching purposes when examining presented identifiers.

   service type:  A formal identifier for the application protocol used
      to provide a particular kind of service at a domain; the service
      type typically takes the form of a Uniform Resource Identifier
      scheme [URI] or a DNS SRV Service [DNS-SRV].

   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, a
      service type enables a client to construct one or more reference
      identifiers.



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   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 in 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 composite 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 specfication
      we assume that the TLS server is an application service.

   Most security-related terms in this document are to be understood in
   the sense defined in [SECTERMS]; such terms include, but are not
   limited to, "attack", "authentication", "authorization",
   "certification authority", "certification path", "certificate",
   "credential", "identity", "self-signed certificate", "trust", "trust
   anchor", "trust chain", "validate", and "verify".

   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
   [KEYWORDS].

1.6.  Contributors

   The following individuals made important contributions to the text of
   this document: Shumon Huque, RL 'Bob' Morgan, and Kurt Zeilenga.




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1.7.  Acknowledgements

   The editors and contributors wish to thank the following individuals
   for their feedback and suggestions: Bernard Aboba, Richard Barnes,
   Uri Blumenthal, Nelson Bolyard, Kaspar Brand, Anthony Bryan, Ben
   Campbell, Scott Cantor, Wan-Teh Chang, Bil Corry, Dave Cridland, Dave
   Crocker, Cyrus Daboo, Charles Gardiner, Philip Guenther, Phillip
   Hallam-Baker, Bruno Harbulot, Wes Hardaker, David Harrington, Paul
   Hoffman, Love Hornquist Astrand, Henry Hotz, Russ Housley, Jeffrey
   Hutzelman, Simon Josefsson, Geoff Keating, John Klensin, Scott
   Lawrence, Matt McCutchen, Alexey Melnikov, Eddy Nigg, Ludwig Nussel,
   Joe Orton, Tom Petch, Yngve N. Pettersen, Tim Polk, Robert Relyea,
   Eric Rescorla, Pete Resnick, Martin Rex, Joe Salowey, Stefan
   Santesson, Jim Schaad, Rob Stradling, Michael Stroeder, Andrew
   Sullivan, Peter Sylvester, Martin Thomson, Paul Tiemann, Sean Turner,
   Nicolas Williams, Dan Wing, Dan Winship, and Stefan Winter.


2.  Names

   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 a 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 connection attempt) whereas other names are indirect because
   they are automatially resolved by the client based on user input
   (e.g., a target name resolved from a source name using DNS SRV
   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 re-used 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:



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   o  A CN-ID is direct and unrestricted.

   o  A DNS-ID is direct and unrestricted.

   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]).







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2.2.  DNS Domain Names

   For the purposes of this specification, the name of an application
   service is a DNS domain name that conforms to one of the following
   forms:

   1.  A "traditional domain name", i.e., a fully-qualified domain name
       or "FQDN" (see [DNS-CONCEPTS]) all of whose labels are "LDH
       labels" as defined 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 that can include
       appropriately encoded Unicode code points outside the traditional
       US-ASCII range (more precisely, either U-labels or A-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 of 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



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   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
   certificate contains a subjectAltName extension that includes at
   least one subjectAltName entry, because the subject alternative name
   ("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 is identified by a name or
   names carried in the subject field and/or in one of the following
   identifier types:

   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 ("www.example.com", "mail.example.net", and
   "im.example.org" 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 might contain a human-readable string for
   the service, rather than a string whose form matches that of a fully-
   qualified DNS domain name:

      CN=A Free Chat Service,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, profiles of this specification 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.






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      Implementation Note: 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 Distinguished Name (DN) is an ordered
      sequence, order is typically preserved in the DN string
      representations, although the two most prevalent 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 using terminology that
      is not directly related to the information hierarchy, 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 1.5; 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 that contains
      one and only one attribute-type-and-value pair of type Common Name
      (thus removing the possibility that an RDN might contain multiple
      AVAs of type CN, one of which would be considered "most
      specific").  Finally, although theoretically some consider the
      order of AVAs within an RDN to have meaning, in practice that rule
      is not observed, so we consider an AVA of type CN to be valid at
      any position within the subject field.


3.  Representation of Server Identity

3.1.  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.






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   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 in
       which a server is discovered by means of DNS SRV records
       [DNS-SRV] (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 in
       which a server is typically associated with a URI (e.g., this is
       true of [SIP]), then the certificate SHOULD include a URI-ID; the
       scheme SHALL be that of the protocol associated with the service
       type and the authority component SHALL be the fully-qualified DNS
       domain name of the service.

   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.  Even though many deployed clients still check for the CN-ID
       within the certificate subject field, the certificate SHOULD NOT
       represent the server's fully-qualified DNS domain name in a CN-ID
       unless a profile of this specification encourages continued
       support for the CN-ID identifier type.

   6.  The certificate MAY contain more than one DNS-ID, SRV-ID, or
       URI-ID (but SHOULD NOT contain more than one CN-ID).

   7.  Unless a profile of this specification allows continued support
       for the wildcard character '*', the fully-qualified DNS domain
       name portion of a presented identifier SHOULD NOT contain the
       wildcard character, whether as the complete left-most label
       within the identifier (following the definition of "label" from
       [DNS], e.g., "*.example.com") or as a fragment thereof (e.g.,
       *oo.example.com, f*o.example.com, or foo*.example.com).  For
       details, see Section 5.2.

3.2.  Examples

   A certificate for the website at "www.example.com" might include only
   a DNS-ID of "www.example.com" (and, strictly as a fallback for
   existing client software, a CN-ID of "www.example.com").

   A certificate for the IMAP-accessible email server at
   "mail.example.net" might include SRV-IDs of "_imap.mail.example.net"



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   and "_imaps.mail.example.net" (see [EMAIL-SRV]) and a DNS-ID of
   "mail.example.net".

   A certificate for the XMPP-compatible instant messaging server at
   "im.example.org" 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]).


4.  Verification of Service Identity

4.1.  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.

   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 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.

4.2.  Constructing a List of Reference Identifiers

4.2.1.  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



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   identifiers might be a URI that a user has typed into an interface
   (e.g., an HTTPS URL for a web site), 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 server portion of the user's account name), 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 a service type.

   The client might need to extract the source domain and 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.,
   extracting the fully-qualified DNS domain name from the authority
   component of a URI or extracting the service type from the scheme of
   a URI) or information for which the extraction is performed in a
   manner that is 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 that provides both
   mutual authentication and integrity checking).

   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
   connection.  There is only one scenario in which it is acceptable for
   an interactive client to override the recommendation in this rule and
   therefore connect to 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 4.6.4 and Section 5.1.  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
   service type, the client constructs a list of reference identifiers
   in accordance with the following rules:

   o  The list MUST 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



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      with the source domain by means of user configuration (i.e., a
      derived domain).

   o  If a server for the service type is typically discovered by means
      of DNS SRV records, then the list SHOULD include an SRV-ID.

   o  If a server for the service type is typically associated with a
      URI, then the list SHOULD include a URI-ID.

   o  The list MAY include a CN-ID, mainly for the sake of backward
      compatibility with existing infrastructure.

   The client does not need to construct the foregoing identifiers in
   the actual formats found in a certificate (e.g., as ASN.1 types), it
   only needs to construct the functional equivalent of such identifiers
   for matching purposes.

      Security Note: A client MUST NOT construct a reference identifier
      corresponding to Relative Distinguished Names (RDNs) other than
      those of type Common Name and MUST NOT check for RDNs other than
      those of type Common Name in the presented identifiers.

4.2.2.  Examples

   A web browser that is connecting via HTTPS to the website at
   "www.example.com" might have two reference identifiers: a DNS-ID of
   "www.example.com" and, as a fallback, a CN-ID of "www.example.com".

   A mail user agent that is connecting via IMAP to the email service at
   "mail.example.net" might have two reference identifiers: an SRV-ID of
   "_imaps.mail.example.net" (see [EMAIL-SRV]) and a DNS-ID of
   "mail.example.net".

   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.im.example.org", a DNS-ID of
   "im.example.org", and an XMPP-specific "XmppAddr" of "im.example.org"
   (see [XMPP]).

4.3.  Preparing to Seeking 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



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   the client SHOULD stop the search.

      Implementation Note: A client might be configured to perform
      multiple searches, i.e., to match more than one reference
      identifier; although such behavior is not forbidden by this
      document, rules for matching multiple reference identifiers are a
      matter for implementation or future specification.

      Security Note: A client MUST NOT seek a match for a reference
      identifier of CN-ID if the presented identifiers include a DNS-ID,
      SRV-ID, URI-ID, or any application-specific identifier types
      supported by the client.

   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 type portion, as
   follows:

   o  A reference identifier of type DNS-ID does not include an
      application type portion and thus can be used directly as the DNS
      domain name for comparison purposes.

   o  A reference identifier of type CN-ID also does not include an
      application type portion and thus can be used directly as the DNS
      domain name for comparison purposes; note that this document
      specifies that a CN-ID always contains a string whose form matches
      that of a DNS domain name (thus differentiating a CN-ID from a
      Common Name containing a human-friendly name).

   o  For a reference identifier of type SRV-ID, the DNS domain name
      portion is the Name and the application type portion is the
      Service.

   o  For a reference identifier of type URI-ID, the DNS domain name
      portion is the reg-name part of the authority component and the
      application type portion is the scheme name matching the [ABNF]
      "scheme" rule from [URI] (not including the ':' separator); note
      that this document specifies that a URI-ID always contains an
      authority component whose host subcomponent contains a reg-name
      (thus differentiating a URI-ID from a uniformResourceIdentifier
      entry that contains an IP address or that does not contain an
      authority component), and that extraction of the reg-name might
      necessitate normalization of the URI (as explained in [URI]).

   Detailed comparison rules for matching the DNS domain name portion
   and application type portion of the reference identifier are provided
   in the following sections.




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4.4.  Verifying 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 service type as described under Section 4.5).  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, if the client supports presented
   identifiers that contain the wildcard character '*', we define a
   supplemental rule for so-called "wildcard certificates".  We also
   specify the circumstances under which it is acceptable to check the
   "CN-ID" identifier type.

4.4.1.  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 components 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 4.4.3).

4.4.2.  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
   every label in the domain name to an A-label (as described in
   [IDNA-DEFS]) before checking the domain name.  In accordance with
   [IDNA-PROTO], a pair of A-labels MUST be compared as case-insensitive
   ASCII.  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 4.4.3).

4.4.3.  Checking of Wildcard Certificates

   A client employing this specification's rules MAY match the reference
   identifier against a presented identifier whose DNS domain name
   portion contains the wildcard character '*' as part or all of a label
   (following the definition of "label" from [DNS]).  For information
   regarding the security characteristics of wildcard certificates, see
   Section 5.2.

   If a client matches the reference identifier against a presented
   identifier whose DNS domain name portion contains the wildcard
   character '*', the following rules apply:



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   1.  The client SHOULD NOT attempt to match a presented identifier in
       which the wildcard character comprises a label other than the
       left-most label (e.g., do not match bar.*.example.net).

   2.  If the wildcard character is the only character of the left-most
       label in the presented identifier, the client SHOULD NOT compare
       against anything but the left-most label of the reference
       identifier (e.g., *.example.com would match foo.example.com but
       not bar.foo.example.com or example.com).

   3.  The client MAY match a presented identifier in which the wildcard
       character is not the only character of the label (e.g.,
       baz*.example.net and *baz.example.net and b*z.example.net would
       be taken to match baz1.example.net and foobaz.example.net and
       buzz.example.net, respectively).

4.4.4.  Checking of Common Names

   As noted, a client MUST NOT seek a match for a reference identifier
   of CN-ID if the presented identifiers include a DNS-ID, SRV-ID,
   URI-ID, or any application-specific identifier types supported by the
   client.

   Therefore, if and only if the presented identifiers do not include a
   DNS-ID, SRV-ID, URI-ID, or any application-specific identifier types
   supported by the client, then the client MAY as a last resort check
   for a string whose form matches that of a fully-qualified DNS domain
   name in the CN-ID.  If the client chooses to compare a reference
   identifier of type CN-ID against that string, it MUST follow the
   comparison rules for the DNS domain name portion of an identifier of
   type DNS-ID, SRV-ID, or URI-ID, as described under Section 4.4.1,
   Section 4.4.2, and Section 4.4.3.

4.5.  Verifying the Application Type Portion

   When checking identifiers of type SRV-ID and URI-ID, a client SHOULD
   check not only the domain name but also the service type of the
   service to which it connects.  This is a best practice because
   typically a client is not designed to connect to all kinds of
   services using all possible application protocols, but instead is
   designed to connect to one kind of service, such as websites, email
   services, or instant messaging services.

   The service type is verified by means of either an SRV-ID or URI-ID.







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4.5.1.  SRV-ID

   The service name portion of an SRV-ID (e.g., "xmpp") 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.

4.5.2.  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.

4.6.  Outcome

   The outcome of the checking procedure is one of the following cases.

4.6.1.  Case #1: Match Found

   If the client has found a presented identifier that matches a
   reference identifier, matching has succeeded.  In this case, the
   client MUST use the matched reference identifier as the validated
   identity of the application service.

4.6.2.  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 connection attempt (as "pinning"
   is explained under Section 1.5), and the presented certificate
   matches the pinned certificate (including the context as described
   under Section 5.1), then the service identity check succeeds.

4.6.3.  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 connection attempt, then the client MUST proceed
   as described under Section 4.6.4.

4.6.4.  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



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   and automatically terminate the connection with a bad certificate
   error; this behavior is preferable because it prevents users from
   inadvertently bypassing security protections in hostile situations.

      Security Note: Some interactive clients give advanced users the
      option of proceeding with acceptance despite the identity
      mismatch, thereby "pinning" the certificate to one of the
      reference identifiers in the list constructed by the client for
      this connection attempt.  Although this behavior can be
      appropriate in certain specialized circumstances, in general it
      ought to be exposed only to advanced users.  Even then it needs to
      be handled with extreme caution, for example by first encouraging
      even an advanced user to terminate the connection and, if the
      advanced user chooses to proceed anyway, by forcing the user to
      view the entire certification path and only then allowing the user
      to pin the certificate (on a temporary or permanent basis, at the
      user's option).

   Otherwise, if the client is an automated application not directly
   controlled by a human user, then it SHOULD terminate the connection
   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.


5.  Security Considerations

5.1.  Pinned Certificates

   As defined under Section 1.5, 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 the that 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 service type, the service's derived
   domain and port number, and any other relevant information provided
   by the user or associated by the client).

5.2.  Wildcard Certificates

   This document states that the wildcard character '*' SHOULD NOT be
   included in presented identifiers but MAY be checked by application
   clients (mainly for the sake of backward compatibility with existing
   infrastructure); as a result, the rules provided in this document are



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   more restrictive than the rules for many existing application
   technologies (see Appendix A).  Several security considerations
   justify tightening the rules:

   o  The inclusion of the wildcard character in certificates has led to
      homograph attacks involving non-ASCII characters that look similar
      to characters commonly included in HTTPS URLs, such as "/" and
      "?"; for discussion, see for example [Defeating-SSL] (beginning at
      slide 91).

   o  The ability to obtain a certificate containing the wildcard
      character might broaden the range of applications services that an
      attacker could forge, thus increasing (a) the chances that
      attackers would attempt to steal credentials needed for obtaining
      certificates and (b) the potential damage that would result from a
      successful attack.

   o  Specifications for existing application technologies are not clear
      about whether the wildcard character is allowed only as the
      complete left-most label (e.g., *.example.com), some fragment of
      the left-most label (e.g., foo*.example.com, f*o.example.com, or
      *oo.example.com), or even all or part of a label other than the
      left-most label (e.g., www.*.example.com or www.foo*.example.com);
      nor are they clear about whether a presented identifier can
      include more than one instance of the wildcard character (e.g.,
      f*b*r.example.com or *.*.example.com).  These ambiguities might
      introduce exploitable differences in identity checking behavior
      among client implementations and necessitate overly complex and
      inefficient identity checking algorithms.

   Notwithstanding the foregoing security considerations, profiles of
   this specification can legitimately encourage continued support for
   the wildcard character if they have good reasons to do so, such as
   backward compatibility with deployed infrastructure.

5.3.  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].


6.  IANA Considerations

   This document specifies no actions for the IANA.


7.  References



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7.1.  Normative References

   [DNS]      Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [DNS-CONCEPTS]
              Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [DNS-SRV]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [IDNA-DEFS]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [IDNA-PROTO]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, August 2010.

   [KEYWORDS]
              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [LDAP-DN]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names",
              RFC 4514, June 2006.

   [PKIX]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [SRVNAME]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, August 2007.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

7.2.  Informative References

   [ABNF]     Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.




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

   [DNS-CASE]
              Eastlake, D., "Domain Name System (DNS) Case Insensitivity
              Clarification", RFC 4343, January 2006.

   [DNSSEC]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [DTLS]     Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [EMAIL-SRV]
              Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access services", draft-daboo-srv-email-05
              (work in progress), May 2010.

   [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>.

   [GIST]     Schulzrinne, H. and R. Hancock, "GIST: General Internet
              Signalling Transport", RFC 5971, October 2010.

   [HTTP]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [HTTP-TLS]
              Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [IMAP]     Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [IDNA2003]
              Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [IP]       Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.



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   [IPv6]     Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [IPSEC]    Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [LDAP]     Sermersheim, J., "Lightweight Directory Access Protocol
              (LDAP): The Protocol", RFC 4511, June 2006.

   [LDAP-AUTH]
              Harrison, R., "Lightweight Directory Access Protocol
              (LDAP): Authentication Methods and Security Mechanisms",
              RFC 4513, June 2006.

   [LDAP-SCHEMA]
              Sciberras, A., "Lightweight Directory Access Protocol
              (LDAP): Schema for User Applications", RFC 4519,
              June 2006.

   [LDAP-TLS]
              Hodges, J., Morgan, R., and M. Wahl, "Lightweight
              Directory Access Protocol (v3): Extension for Transport
              Layer Security", RFC 2830, May 2000.

   [NAPTR]    Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

   [NETCONF]  Enns, R., "NETCONF Configuration Protocol", RFC 4741,
              December 2006.

   [NETCONF-SSH]
              Wasserman, M. and T. Goddard, "Using the NETCONF
              Configuration Protocol over Secure SHell (SSH)", RFC 4742,
              December 2006.

   [NETCONF-TLS]
              Badra, M., "NETCONF over Transport Layer Security (TLS)",
              RFC 5539, May 2009.

   [NNTP]     Feather, C., "Network News Transfer Protocol (NNTP)",
              RFC 3977, October 2006.

   [NNTP-TLS]
              Murchison, K., Vinocur, J., and C. Newman, "Using
              Transport Layer Security (TLS) with Network News Transfer
              Protocol (NNTP)", RFC 4642, October 2006.




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   [OCSP]     Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
              Adams, "X.509 Internet Public Key Infrastructure Online
              Certificate Status Protocol - OCSP", RFC 2560, June 1999.

   [OPENPGP]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

   [POP3]     Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [PRIVATE]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [PKIX-OLD]
              Housley, R., Ford, W., Polk, T., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and CRL
              Profile", RFC 2459, January 1999.

   [S-NAPTR]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, January 2005.

   [SECTERMS]
              Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

   [SIP]      Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [SIP-CERTS]
              Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
              Certificates in the Session Initiation Protocol (SIP)",
              RFC 5922, June 2010.

   [SMTP]     Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [SMTP-AUTH]
              Siemborski, R. and A. Melnikov, "SMTP Service Extension
              for Authentication", RFC 4954, July 2007.

   [SMTP-TLS]
              Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.




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   [SNMP]     Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [SNMP-TLS]
              Hardaker, W., "Transport Layer Security (TLS) Transport
              Model for the Simple Network Management Protocol (SNMP)",
              RFC 5953, August 2010.

   [SYSLOG]   Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

   [SYSLOG-DTLS]
              Salowey, J., Petch, T., Gerhards, R., and H. Feng,
              "Datagram Transport Layer Security (DTLS) Transport
              Mapping for Syslog", RFC 6012, October 2010.

   [SYSLOG-TLS]
              Miao, F., Ma, Y., and J. Salowey, "Transport Layer
              Security (TLS) Transport Mapping for Syslog", RFC 5425,
              March 2009.

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

   [USINGTLS]
              Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [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>.

   [X.500]    International Telecommunications Union, "Information
              Technology - Open Systems Interconnection - The Directory:
              Overview of concepts, models and services", ITU-
              T Recommendation X.500, ISO Standard 9594-1, August 2005.

   [X.501]    International Telecommunications Union, "Information
              Technology - Open Systems Interconnection - The Directory:
              Models", ITU-T Recommendation X.501, ISO Standard 9594-2,
              August 2005.



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   [X.509]    International Telecommunications Union, "Information
              Technology - Open Systems Interconnection - The Directory:
              Public-key and attribute certificate frameworks", ITU-
              T Recommendation X.509, ISO Standard 9594-8, August 2005.

   [X.520]    International Telecommunications Union, "Information
              Technology - Open Systems Interconnection - The Directory:
              Selected attribute types", ITU-T Recommendation X.509,
              ISO Standard 9594-6, August 2005.

   [X.690]    International Telecommunications Union, "Information
              Technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T Recommendation
              X.690, ISO Standard 8825-1, August 2008.

   [XMPP]     Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-19 (work
              in progress), November 2010.

   [XMPP-OLD]
              Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.


Appendix A.  Prior Art

   (This section is non-normative.)

   The recommendations in this document are an abstraction from
   recommendations in specifications for a wide range of application
   protocols.  For the purpose of comparison and to delineate the
   history of thinking about application service identity verification
   within the IETF, this informative section gathers together prior art
   by including the exact text from various RFCs (the only modifications
   are changes to the names of several references to maintain coherence
   with the main body of this document, and the elision of irrelevant
   text as marked by the characters "[...]").

A.1.  IMAP, POP3, and ACAP (1999)

   In 1999, [USINGTLS] specified the following text regarding
   application service identity verification in IMAP, POP3, and ACAP:

   ######

   2.4.  Server Identity Check




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   During the TLS negotiation, the client MUST check its understanding
   of the server hostname against the server's identity as presented in
   the server Certificate message, in order to prevent man-in-the-middle
   attacks.  Matching is performed according to these rules:

   o  The client MUST use the server hostname it used to open the
      connection as the value to compare against the server name as
      expressed in the server certificate.  The client MUST NOT use any
      form of the server hostname derived from an insecure remote source
      (e.g., insecure DNS lookup).  CNAME canonicalization is not done.
   o  If a subjectAltName extension of type dNSName is present in the
      certificate, it SHOULD be used as the source of the server's
      identity.
   o  Matching is case-insensitive.
   o  A "*" wildcard character MAY be used as the left-most name
      component in the certificate.  For example, *.example.com would
      match a.example.com, foo.example.com, etc. but would not match
      example.com.
   o  If the certificate contains multiple names (e.g. more than one
      dNSName field), then a match with any one of the fields is
      considered acceptable.

   If the match fails, the client SHOULD either ask for explicit user
   confirmation, or terminate the connection and indicate the server's
   identity is suspect.

   ######

A.2.  HTTP (2000)

   In 2000, [HTTP-TLS] specified the following text regarding
   application service identity verification in HTTP:

   ######

   3.1.  Server Identity

   In general, HTTP/TLS requests are generated by dereferencing a URI.
   As a consequence, the hostname for the server is known to the client.
   If the hostname is available, the client MUST check it against the
   server's identity as presented in the server's Certificate message,
   in order to prevent man-in-the-middle attacks.

   If the client has external information as to the expected identity of
   the server, the hostname check MAY be omitted.  (For instance, a
   client may be connecting to a machine whose address and hostname are
   dynamic but the client knows the certificate that the server will
   present.)  In such cases, it is important to narrow the scope of



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   acceptable certificates as much as possible in order to prevent man
   in the middle attacks.  In special cases, it may be appropriate for
   the client to simply ignore the server's identity, but it must be
   understood that this leaves the connection open to active attack.

   If a subjectAltName extension of type dNSName is present, that MUST
   be used as the identity.  Otherwise, the (most specific) Common Name
   field in the Subject field of the certificate MUST be used.  Although
   the use of the Common Name is existing practice, it is deprecated and
   Certification Authorities are encouraged to use the dNSName instead.

   Matching is performed using the matching rules specified by
   [PKIX-OLD].  If more than one identity of a given type is present in
   the certificate (e.g., more than one dNSName name, a match in any one
   of the set is considered acceptable.)  Names may contain the wildcard
   character * which is considered to match any single domain name
   component or component fragment.  E.g., *.a.com matches foo.a.com but
   not bar.foo.a.com. f*.com matches foo.com but not bar.com.

   In some cases, the URI is specified as an IP address rather than a
   hostname.  In this case, the iPAddress subjectAltName must be present
   in the certificate and must exactly match the IP in the URI.

   If the hostname does not match the identity in the certificate, user
   oriented clients MUST either notify the user (clients MAY give the
   user the opportunity to continue with the connection in any case) or
   terminate the connection with a bad certificate error.  Automated
   clients MUST log the error to an appropriate audit log (if available)
   and SHOULD terminate the connection (with a bad certificate error).
   Automated clients MAY provide a configuration setting that disables
   this check, but MUST provide a setting which enables it.

   Note that in many cases the URI itself comes from an untrusted
   source.  The above-described check provides no protection against
   attacks where this source is compromised.  For example, if the URI
   was obtained by clicking on an HTML page which was itself obtained
   without using HTTP/TLS, a man in the middle could have replaced the
   URI.  In order to prevent this form of attack, users should carefully
   examine the certificate presented by the server to determine if it
   meets their expectations.

   ######

A.3.  LDAP (2000/2006)

   In 2000, [LDAP-TLS] specified the following text regarding
   application service identity verification in LDAP:




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   ######

   3.6.  Server Identity Check

   The client MUST check its understanding of the server's hostname
   against the server's identity as presented in the server's
   Certificate message, in order to prevent man-in-the-middle attacks.

   Matching is performed according to these rules:

   o  The client MUST use the server hostname it used to open the LDAP
      connection as the value to compare against the server name as
      expressed in the server's certificate.  The client MUST NOT use
      the server's canonical DNS name or any other derived form of name.
   o  If a subjectAltName extension of type dNSName is present in the
      certificate, it SHOULD be used as the source of the server's
      identity.
   o  Matching is case-insensitive.
   o  The "*" wildcard character is allowed.  If present, it applies
      only to the left-most name component.

   E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not
   bar.com.  If more than one identity of a given type is present in the
   certificate (e.g. more than one dNSName name), a match in any one of
   the set is considered acceptable.

   If the hostname does not match the dNSName-based identity in the
   certificate per the above check, user-oriented clients SHOULD either
   notify the user (clients MAY give the user the opportunity to
   continue with the connection in any case) or terminate the connection
   and indicate that the server's identity is suspect.  Automated
   clients SHOULD close the connection, returning and/or logging an
   error indicating that the server's identity is suspect.

   Beyond the server identity checks described in this section, clients
   SHOULD be prepared to do further checking to ensure that the server
   is authorized to provide the service it is observed to provide.  The
   client MAY need to make use of local policy information.

   ######

   In 2006, [LDAP-AUTH] specified the following text regarding
   application service identity verification in LDAP:

   ######

   3.1.3.  Server Identity Check




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   In order to prevent man-in-the-middle attacks, the client MUST verify
   the server's identity (as presented in the server's Certificate
   message).  In this section, the client's understanding of the
   server's identity (typically the identity used to establish the
   transport connection) is called the "reference identity".

   The client determines the type (e.g., DNS name or IP address) of the
   reference identity and performs a comparison between the reference
   identity and each subjectAltName value of the corresponding type
   until a match is produced.  Once a match is produced, the server's
   identity has been verified, and the server identity check is
   complete.  Different subjectAltName types are matched in different
   ways.  Sections 3.1.3.1 - 3.1.3.3 explain how to compare values of
   various subjectAltName types.

   The client may map the reference identity to a different type prior
   to performing a comparison.  Mappings may be performed for all
   available subjectAltName types to which the reference identity can be
   mapped; however, the reference identity should only be mapped to
   types for which the mapping is either inherently secure (e.g.,
   extracting the DNS name from a URI to compare with a subjectAltName
   of type dNSName) or for which the mapping is performed in a secure
   manner (e.g., using [DNSSEC], or using user- or admin-configured
   host-to-address/address-to-host lookup tables).

   The server's identity may also be verified by comparing the reference
   identity to the Common Name (CN) [LDAP-SCHEMA] value in the last
   Relative Distinguished Name (RDN) of the subject field of the
   server's certificate (where "last" refers to the DER-encoded order,
   not the order of presentation in a string representation of DER-
   encoded data).  This comparison is performed using the rules for
   comparison of DNS names in Section 3.1.3.1, below, with the exception
   that no wildcard matching is allowed.  Although the use of the Common
   Name value is existing practice, it is deprecated, and Certification
   Authorities are encouraged to provide subjectAltName values instead.
   Note that the TLS implementation may represent DNs in certificates
   according to X.500 or other conventions.  For example, some X.500
   implementations order the RDNs in a DN using a left-to-right (most
   significant to least significant) convention instead of LDAP's right-
   to-left convention.

   If the server identity check fails, user-oriented clients SHOULD
   either notify the user (clients may give the user the opportunity to
   continue with the LDAP session in this case) or close the transport
   connection and indicate that the server's identity is suspect.
   Automated clients SHOULD close the transport connection and then
   return or log an error indicating that the server's identity is
   suspect or both.



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   Beyond the server identity check described in this section, clients
   should be prepared to do further checking to ensure that the server
   is authorized to provide the service it is requested to provide.  The
   client may need to make use of local policy information in making
   this determination.

   3.1.3.1.  Comparison of DNS Names

   If the reference identity is an internationalized domain name,
   conforming implementations MUST convert it to the ASCII Compatible
   Encoding (ACE) format as specified in Section 4 of RFC 3490
   [IDNA2003] before comparison with subjectAltName values of type
   dNSName.  Specifically, conforming implementations MUST perform the
   conversion operation specified in Section 4 of RFC 3490 as follows:

   o  in step 1, the domain name SHALL be considered a "stored string";
   o  in step 3, set the flag called "UseSTD3ASCIIRules";
   o  in step 4, process each label with the "ToASCII" operation; and
   o  in step 5, change all label separators to U+002E (full stop).

   After performing the "to-ASCII" conversion, the DNS labels and names
   MUST be compared for equality according to the rules specified in
   Section 3 of RFC3490.

   The '*' (ASCII 42) wildcard character is allowed in subjectAltName
   values of type dNSName, and then only as the left-most (least
   significant) DNS label in that value.  This wildcard matches any
   left-most DNS label in the server name.  That is, the subject
   *.example.com matches the server names a.example.com and
   b.example.com, but does not match example.com or a.b.example.com.

   3.1.3.2.  Comparison of IP Addresses

   When the reference identity is an IP address, the identity MUST be
   converted to the "network byte order" octet string representation
   [IP] [IPv6].  For IP Version 4, as specified in RFC 791, the octet
   string will contain exactly four octets.  For IP Version 6, as
   specified in RFC 2460, the octet string will contain exactly sixteen
   octets.  This octet string is then compared against subjectAltName
   values of type iPAddress.  A match occurs if the reference identity
   octet string and value octet strings are identical.

   3.1.3.3.  Comparison of Other subjectName Types

   Client implementations MAY support matching against subjectAltName
   values of other types as described in other documents.

   ######



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A.4.  SMTP (2002/2007)

   In 2002, [SMTP-TLS] specified the following text regarding
   application service identity verification in SMTP:

   ######

   4.1 Processing After the STARTTLS Command

   [...]

   The decision of whether or not to believe the authenticity of the
   other party in a TLS negotiation is a local matter.  However, some
   general rules for the decisions are:

   o  A SMTP client would probably only want to authenticate an SMTP
      server whose server certificate has a domain name that is the
      domain name that the client thought it was connecting to.

   [...]

   ######

   In 2006, [SMTP-AUTH] specified the following text regarding
   application service identity verification in SMTP:

   ######

   14.  Additional Requirements When Using SASL PLAIN over TLS

   [...]

   After a successful [TLS] negotiation, the client MUST check its
   understanding of the server hostname against the server's identity as
   presented in the server Certificate message, in order to prevent man-
   in-the-middle attacks.  If the match fails, the client MUST NOT
   attempt to authenticate using the SASL PLAIN mechanism.  Matching is
   performed according to the following rules:

      The client MUST use the server hostname it used to open the
      connection as the value to compare against the server name as
      expressed in the server certificate.  The client MUST NOT use any
      form of the server hostname derived from an insecure remote source
      (e.g., insecure DNS lookup).  CNAME canonicalization is not done.
      If a subjectAltName extension of type dNSName is present in the
      certificate, it SHOULD be used as the source of the server's
      identity.




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      Matching is case-insensitive.
      A "*" wildcard character MAY be used as the leftmost name
      component in the certificate.  For example, *.example.com would
      match a.example.com, foo.example.com, etc., but would not match
      example.com.
      If the certificate contains multiple names (e.g., more than one
      dNSName field), then a match with any one of the fields is
      considered acceptable.

   ######

A.5.  XMPP (2004)

   In 2004, [XMPP-OLD] specified the following text regarding
   application service identity verification in XMPP:

   ######

   14.2.  Certificate Validation

   When an XMPP peer communicates with another peer securely, it MUST
   validate the peer's certificate.  There are three possible cases:

   Case #1:  The peer contains an End Entity certificate which appears
      to be certified by a certification path terminating in a trust
      anchor (as described in Section 6.1 of [PKIX]).
   Case #2:  The peer certificate is certified by a Certificate
      Authority not known to the validating peer.
   Case #3:  The peer certificate is self-signed.

   In Case #1, the validating peer MUST do one of two things:
   1.  Verify the peer certificate according to the rules of [PKIX].
       The certificate SHOULD then be checked against the expected
       identity of the peer following the rules described in [HTTP-TLS],
       except that a subjectAltName extension of type "xmpp" MUST be
       used as the identity if present.  If one of these checks fails,
       user-oriented clients MUST either notify the user (clients MAY
       give the user the opportunity to continue with the connection in
       any case) or terminate the connection with a bad certificate
       error.  Automated clients SHOULD terminate the connection (with a
       bad certificate error) and log the error to an appropriate audit
       log.  Automated clients MAY provide a configuration setting that
       disables this check, but MUST provide a setting that enables it.
   2.  The peer SHOULD show the certificate to a user for approval,
       including the entire certification path.  The peer MUST cache the
       certificate (or some non-forgeable representation such as a
       hash).  In future connections, the peer MUST verify that the same
       certificate was presented and MUST notify the user if it has



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       changed.

   In Case #2 and Case #3, implementations SHOULD act as in (2) above.

   ######

   At the time of this writing, [XMPP] refers to this document for rules
   regarding application service identity verification in XMPP.

A.6.  NNTP (2006)

   In 2006, [NNTP-TLS] specified the following text regarding
   application service identity verification in NNTP:

   ######

   5.  Security Considerations

   [...]

   During the TLS negotiation, the client MUST check its understanding
   of the server hostname against the server's identity as presented in
   the server Certificate message, in order to prevent man-in-the-middle
   attacks.  Matching is performed according to these rules:

   o  The client MUST use the server hostname it used to open the
      connection (or the hostname specified in TLS "server_name"
      extension [TLS]) as the value to compare against the server name
      as expressed in the server certificate.  The client MUST NOT use
      any form of the server hostname derived from an insecure remote
      source (e.g., insecure DNS lookup).  CNAME canonicalization is not
      done.
   o  If a subjectAltName extension of type dNSName is present in the
      certificate, it SHOULD be used as the source of the server's
      identity.
   o  Matching is case-insensitive.
   o  A "*" wildcard character MAY be used as the left-most name
      component in the certificate.  For example, *.example.com would
      match a.example.com, foo.example.com, etc., but would not match
      example.com.
   o  If the certificate contains multiple names (e.g., more than one
      dNSName field), then a match with any one of the fields is
      considered acceptable.

   If the match fails, the client SHOULD either ask for explicit user
   confirmation or terminate the connection with a QUIT command and
   indicate the server's identity is suspect.




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   Additionally, clients MUST verify the binding between the identity of
   the servers to which they connect and the public keys presented by
   those servers.  Clients SHOULD implement the algorithm in Section 6
   of [PKIX] for general certificate validation, but MAY supplement that
   algorithm with other validation methods that achieve equivalent
   levels of verification (such as comparing the server certificate
   against a local store of already-verified certificates and identity
   bindings).

   ######

A.7.  NETCONF (2006/2009)

   In 2006, [NETCONF-SSH] specified the following text regarding
   application service identity verification in NETCONF:

   ######

   6.  Security Considerations

   The identity of the server MUST be verified and authenticated by the
   client according to local policy before password-based authentication
   data or any configuration or state data is sent to or received from
   the server.  The identity of the client MUST also be verified and
   authenticated by the server according to local policy to ensure that
   the incoming client request is legitimate before any configuration or
   state data is sent to or received from the client.  Neither side
   should establish a NETCONF over SSH connection with an unknown,
   unexpected, or incorrect identity on the opposite side.

   ######

   In 2009, [NETCONF-TLS] specified the following text regarding
   application service identity verification in NETCONF:

   ######

   3.1.  Server Identity

   During the TLS negotiation, the client MUST carefully examine the
   certificate presented by the server to determine if it meets the
   client's expectations.  Particularly, the client MUST check its
   understanding of the server hostname against the server's identity as
   presented in the server Certificate message, in order to prevent man-
   in-the-middle attacks.

   Matching is performed according to the rules below (following the
   example of [NNTP-TLS]):



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   o  The client MUST use the server hostname it used to open the
      connection (or the hostname specified in the TLS "server_name"
      extension [TLS]) as the value to compare against the server name
      as expressed in the server certificate.  The client MUST NOT use
      any form of the server hostname derived from an insecure remote
      source (e.g., insecure DNS lookup).  CNAME canonicalization is not
      done.
   o  If a subjectAltName extension of type dNSName is present in the
      certificate, it MUST be used as the source of the server's
      identity.
   o  Matching is case-insensitive.
   o  A "*" wildcard character MAY be used as the leftmost name
      component in the certificate.  For example, *.example.com would
      match a.example.com, foo.example.com, etc., but would not match
      example.com.
   o  If the certificate contains multiple names (e.g., more than one
      dNSName field), then a match with any one of the fields is
      considered acceptable.

   If the match fails, the client MUST either ask for explicit user
   confirmation or terminate the connection and indicate the server's
   identity is suspect.

   Additionally, clients MUST verify the binding between the identity of
   the servers to which they connect and the public keys presented by
   those servers.  Clients SHOULD implement the algorithm in Section 6
   of [PKIX] for general certificate validation, but MAY supplement that
   algorithm with other validation methods that achieve equivalent
   levels of verification (such as comparing the server certificate
   against a local store of already-verified certificates and identity
   bindings).

   If the client has external information as to the expected identity of
   the server, the hostname check MAY be omitted.

   ######

A.8.  Syslog (2009)

   In 2009, [SYSLOG-TLS] specified the following text regarding
   application service identity verification in Syslog:

   ######

   5.2.  Subject Name Authorization

   Implementations MUST support certification path validation [PKIX].
   In addition, they MUST support specifying the authorized peers using



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   locally configured host names and matching the name against the
   certificate as follows.

   o  Implementations MUST support matching the locally configured host
      name against a dNSName in the subjectAltName extension field and
      SHOULD support checking the name against the common name portion
      of the subject distinguished name.
   o  The '*' (ASCII 42) wildcard character is allowed in the dNSName of
      the subjectAltName extension (and in common name, if used to store
      the host name), but only as the left-most (least significant) DNS
      label in that value.  This wildcard matches any left-most DNS
      label in the server name.  That is, the subject *.example.com
      matches the server names a.example.com and b.example.com, but does
      not match example.com or a.b.example.com.  Implementations MUST
      support wildcards in certificates as specified above, but MAY
      provide a configuration option to disable them.
   o  Locally configured names MAY contain the wildcard character to
      match a range of values.  The types of wildcards supported MAY be
      more flexible than those allowed in subject names, making it
      possible to support various policies for different environments.
      For example, a policy could allow for a trust-root-based
      authorization where all credentials issued by a particular CA
      trust root are authorized.
   o  If the locally configured name is an internationalized domain
      name, conforming implementations MUST convert it to the ASCII
      Compatible Encoding (ACE) format for performing comparisons, as
      specified in Section 7 of [PKIX].
   o  Implementations MAY support matching a locally configured IP
      address against an iPAddress stored in the subjectAltName
      extension.  In this case, the locally configured IP address is
      converted to an octet string as specified in [PKIX], Section
      4.2.1.6.  A match occurs if this octet string is equal to the
      value of iPAddress in the subjectAltName extension.

   ######

A.9.  SIP (2010)

   In 2010, [SIP-CERTS] specified the following text regarding
   application service identity verification in SIP:

   ######

   7.2.  Comparing SIP Identities

   When an implementation (either client or server) compares two values
   as SIP domain identities:




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      Implementations MUST compare only the DNS name component of each
      SIP domain identifier; an implementation MUST NOT use any scheme
      or parameters in the comparison.
      Implementations MUST compare the values as DNS names, which means
      that the comparison is case insensitive as specified by
      [DNS-CASE].  Implementations MUST handle Internationalized Domain
      Names (IDNs) in accordance with Section 7.2 of [PKIX].
      Implementations MUST match the values in their entirety:
         Implementations MUST NOT match suffixes.  For example,
         "foo.example.com" does not match "example.com".
         Implemenations MUST NOT match any form of wildcard, such as a
         leading "." or "*." with any other DNS label or sequence of
         labels.  For example, "*.example.com" matches only
         "*.example.com" but not "foo.example.com".  Similarly,
         ".example.com" matches only ".example.com", and does not match
         "foo.example.com."
            [HTTP-TLS] allows the dNSName component to contain a
            wildcard; e.g., "DNS:*.example.com".  [PKIX], while not
            disallowing this explicitly, leaves the interpretation of
            wildcards to the individual specification.  [SIP] does not
            provide any guidelines on the presence of wildcards in
            certificates.  Through the rule above, this document
            prohibits such wildcards in certificates for SIP domains.

   ######

A.10.  SNMP (2010)

   In 2010, [SNMP-TLS] specified the following text regarding
   application service identity verification in SNMP:

   ######

   If the server's presented certificate has passed certification path
   validation [PKIX] to a configured trust anchor, and an active row
   exists with a zero-length snmpTlstmAddrServerFingerprint value, then
   the snmpTlstmAddrServerIdentity column contains the expected host
   name.  This expected host name is then compared against the server's
   certificate as follows:

   o  Implementations MUST support matching the expected host name
      against a dNSName in the subjectAltName extension field and MAY
      support checking the name against the CommonName portion of the
      subject distinguished name.

   o  The '*' (ASCII 0x2a) wildcard character is allowed in the dNSName
      of the subjectAltName extension (and in common name, if used to
      store the host name), but only as the left-most (least



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      significant) DNS label in that value.  This wildcard matches any
      left-most DNS label in the server name.  That is, the subject
      *.example.com matches the server names a.example.com and
      b.example.com, but does not match example.com or a.b.example.com.
      Implementations MUST support wildcards in certificates as
      specified above, but MAY provide a configuration option to disable
      them.

   o  If the locally configured name is an internationalized domain
      name, conforming implementations MUST convert it to the ASCII
      Compatible Encoding (ACE) format for performing comparisons, as
      specified in Section 7 of [PKIX].

   If the expected host name fails these conditions then the connection
   MUST be closed.

   ######

A.11.  GIST (2010)

   In 2010, [GIST] specified the following text regarding application
   service identity verification in the General Internet Signalling
   Transport:

   ######

   5.7.3.1.  Identity Checking in TLS

   After TLS authentication, a node MUST check the identity presented by
   the peer in order to avoid man-in-the-middle attacks, and verify that
   the peer is authorised to take part in signalling at the GIST layer.
   The authorisation check is carried out by comparing the presented
   identity with each Authorised Peer Database (APD) entry in turn, as
   discussed in Section 4.4.2.  This section defines the identity
   comparison algorithm for a single APD entry.

   For TLS authentication with X.509 certificates, an identity from the
   DNS namespace MUST be checked against each subjectAltName extension
   of type dNSName present in the certificate.  If no such extension is
   present, then the identity MUST be compared to the (most specific)
   Common Name in the Subject field of the certificate.  When matching
   DNS names against dNSName or Common Name fields, matching is case-
   insensitive.  Also, a "*" wildcard character MAY be used as the left-
   most name component in the certificate or identity in the APD.  For
   example, *.example.com in the APD would match certificates for
   a.example.com, foo.example.com, *.example.com, etc., but would not
   match example.com.  Similarly, a certificate for *.example.com would
   be valid for APD identities of a.example.com, foo.example.com,



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   *.example.com, etc., but not example.com.

   Additionally, a node MUST verify the binding between the identity of
   the peer to which it connects and the public key presented by that
   peer.  Nodes SHOULD implement the algorithm in Section 6 of [PKIX]
   for general certificate validation, but MAY supplement that algorithm
   with other validation methods that achieve equivalent levels of
   verification (such as comparing the server certificate against a
   local store of already-verified certificates and identity bindings).

   For TLS authentication with pre-shared keys, the identity in the
   psk_identity_hint (for the server identity, i.e. the Responding node)
   or psk_identity (for the client identity, i.e. the Querying node)
   MUST be compared to the identities in the APD.

   ######


Authors' Addresses

   Peter Saint-Andre
   Cisco
   1899 Wyknoop Street, Suite 600
   Denver, CO  80202
   USA

   Phone: +1-303-308-3282
   Email: psaintan@cisco.com


   Jeff Hodges
   PayPal
   2211 North First Street
   San Jose, California  95131
   US

   Email: Jeff.Hodges@PayPal.com














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