Service Names in TLS
draft-ietf-uta-rfc6125bis-06

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                          
Obsoletes: 6125 (if approved)                                  J. Hodges
Intended status: Standards Track                                        
Expires: 27 November 2022                                        R. Salz
                                                     Akamai Technologies
                                                             26 May 2022

                          Service Names in TLS
                      draft-ietf-uta-rfc6125bis-06

Abstract

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

   This document obsoletes RFC 6125.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Using TLS in
   Applications Working Group mailing list (uta@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/uta/.

   Source for this draft and an issue tracker can be found at
   https://github.com/richsalz/draft-ietf-uta-rfc6125bis.

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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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 27 November 2022.

Copyright Notice

   Copyright (c) 2022 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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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Applicability . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Overview of Recommendations . . . . . . . . . . . . . . .   4
     1.4.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       1.4.1.  In Scope  . . . . . . . . . . . . . . . . . . . . . .   4
       1.4.2.  Out of Scope  . . . . . . . . . . . . . . . . . . . .   5
     1.5.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Naming of Application Services  . . . . . . . . . . . . . . .   8
   3.  Designing Application Protocols . . . . . . . . . . . . . . .  10
   4.  Representing Server Identity  . . . . . . . . . . . . . . . .  10
     4.1.  Rules . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Requesting Server Certificates  . . . . . . . . . . . . . . .  12
   6.  Verifying Service Identity  . . . . . . . . . . . . . . . . .  12
     6.1.  Constructing a List of Reference Identifiers  . . . . . .  13
       6.1.1.  Rules . . . . . . . . . . . . . . . . . . . . . . . .  13
       6.1.2.  Examples  . . . . . . . . . . . . . . . . . . . . . .  14
     6.2.  Preparing to Seek a Match . . . . . . . . . . . . . . . .  15
     6.3.  Matching the DNS Domain Name Portion  . . . . . . . . . .  16
     6.4.  Matching the Application Service Type Portion . . . . . .  17
     6.5.  Outcome . . . . . . . . . . . . . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     7.1.  Wildcard Certificates . . . . . . . . . . . . . . . . . .  19
     7.2.  Internationalized Domain Names  . . . . . . . . . . . . .  19
     7.3.  Multiple Presented Identifiers  . . . . . . . . . . . . .  19
     7.4.  Multiple Reference Identifiers  . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20

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     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Appendix A.  Changes from RFC 6125  . . . . . . . . . . . . . . .  24
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

1.1.  Motivation

   The visible face of the Internet largely consists of services that
   employ a client-server architecture in which a client communicates
   with an application service.  When a client communicates with an
   application service using [TLS], [DTLS], or a protocol built on
   those, it has some notion of the server's identity (e.g., "the
   website at example.com") while attempting to establish secure
   communication.  Likewise, during TLS negotiation, the server presents
   its notion of the service's identity in the form of a public-key
   certificate that was issued by a certificate authority (CA) in the
   context of the Internet Public Key Infrastructure using X.509 [PKIX].
   Informally, we can think of these identities as the client's
   "reference identity" and the server's "presented identity"; more
   formal definitions are given later.  A client needs to verify that
   the server's presented identity matches its reference identity so it
   can deterministically and automatically authenticate the
   communication.

   This document defines procedures for how clients do this
   verification.  It therefore also defines requirements on other
   parties, such as the certificate authorities that issue certificates,
   the service administrators requesting them, and the protocol
   designers defining how things are named.

   This document obsoletes RFC 6125.  Changes from RFC 6125 are
   described under Appendix A.

1.2.  Applicability

   This document does not supersede the rules for certificate issuance
   or validation specified by [PKIX].  That document also governs any
   certificate-related topic on which this document is silent.  This
   includes certificate syntax, extensions such as name constraints or
   extended key usage, and handling of certification paths.

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   This document addresses only name forms in the leaf "end entity"
   server certificate.  It does not address the name forms in the chain
   of certificates used to validate a cetrificate, let alone creating or
   checking the validity of such a chain.  In order to ensure proper
   authentication, applications need to verify the entire certification
   path as per [PKIX].

1.3.  Overview of Recommendations

   The previous version of this specification, [VERIFY], surveyed the
   then-current practice from many IETF standards and tried to
   generalize best practices (see Appendix A [VERIFY] for details).
   This document takes the lessons learned since then and codifies them.
   The rules are brief:

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

   *  Allow use of even more specific subjectAlternativeName extensions
      where appropriate such as uniformResourceIdentifier and the
      otherName form SRVName.

   *  Wildcard support is now the default.  Constrain wildcard
      certificates so that the wildcard can only be the complete left-
      most component of a domain name.

   *  Do not include or check strings that look like domain names in the
      subject's Common Name.

1.4.  Scope

1.4.1.  In Scope

   This document applies only to service identities that meet these
   three characteristics: associated with fully-qualified domain names
   (FQDNs), used with TLS and DTLS, and are PKIX-based.

   TLS uses the words client and server, where the client is the entity
   that initiates the connection.  In many cases, this is consistent
   with common practice, such as a browser connecting to a Web origin.
   For the sake of clarity, and to follow the usage in [TLS] and related
   specifications, we will continue to use the terms client and server
   in this document.  However, these are TLS-layer roles, and the
   application protocol could support the TLS server making requests to
   the TLS client after the TLS handshake; these is no requirement that
   the roles at the application layer match the TLS layer.

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   At the time of this writing, other protocols such as [QUIC] and
   Network Time Security ([NTS]) use DTLS or TLS to do the initial
   establishment of cryptographic key material.  The rules specified
   here apply to such services, as well.

1.4.2.  Out of Scope

   The following topics are out of scope for this specification:

   *  Security protocols other than [TLS] or [DTLS] except as described
      above.

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

   *  Client or end-user identities.  Certificates representing client
      identities other than as described above, such as rfc822Name, are
      beyond the scope of this document.

   *  Identifiers other than FQDNs.  Identifiers such as IP address are
      not discussed.  In addition, the focus of this document is on
      application service identities, not specific resources located at
      such services.  Therefore this document discusses Uniform Resource
      Identifiers [URI] only as a way to communicate a DNS domain name
      (via the URI "host" component or its equivalent), not other
      aspects of a service such as a specific resource (via the URI
      "path" component) or parameters (via the URI "query" component).

   *  Certification authority policies.  This includes items such as the
      following:

      -  How to certify or validate FQDNs and application service types
         (see [ACME] for some definition of this).

      -  Issuance of certificates with identifiers such as IP addresses
         instead of or in addition to FQDNs.

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

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

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   *  Resolution of DNS domain names.  Although the process whereby a
      client resolves the DNS domain name of an application service can
      involve several steps, for our purposes we care only about the
      fact that the client needs to verify the identity of the entity
      with which it communicates as a result of the resolution process.
      Thus the resolution process itself is out of scope for this
      specification.

   *  User interface issues.  In general, such issues are properly the
      responsibility of client software developers and standards
      development organizations dedicated to particular application
      technologies (see, for example, [WSC-UI]).

1.5.  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.

   application service:  A service on the Internet that enables 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 entity that hosts or deploys an
      application service.

   application service type:  A formal identifier for the application
      protocol used to provide a particular kind of application service
      at a domain.  This often appears as a URI scheme [URI], DNS SRV
      Service [DNS-SRV], or an ALPN [ALPN] identifier.

   delegated domain:  A domain name or host name that is explicitly
      configured for communicating with the source domain, either by the
      human user controlling the client or by a trusted administrator.
      For example, a server at mail.example.net could be a delegated
      domain for connecting to an IMAP server hosting an email address
      of user@example.net.

   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

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      be included in a certificate and therefore that can also be used
      for matching purposes.  For conciseness and convenience, we define
      the following identifier types of interest:

      *  DNS-ID: a subjectAltName entry of type dNSName as defined in
         [PKIX].

      *  SRV-ID: a subjectAltName entry of type otherName whose name
         form is SRVName, as defined in [SRVNAME].

      *  URI-ID: a subjectAltName entry of type
         uniformResourceIdentifier as defined in [PKIX].  This entry
         MUST include both a "scheme" and a "host" component (or its
         equivalent) that matches the "reg-name" rule (where the quoted
         terms represent the associated [ABNF] productions from [URI]).
         If the entry does not have both, it is not a valid URI-ID and
         MUST be ignored.

   PKIX:  The short name for the Internet Public Key Infrastructure
      using X.509 defined in [PKIX].  That document provides a profile
      of the X.509v3 certificate specifications and X.509v2 certificate
      revocation list (CRL) specifications for use in the Internet.

   presented identifier:  An identifier presented by a server to a
      client within a PKIX certificate when the client attempts to
      establish secure communication with the server.  The certificate
      can include one or more presented identifiers of different types,
      and if the server hosts more than one domain then the certificate
      might present distinct identifiers for each domain.

   reference identifier:  An identifier used by the client when
      examining presented identifiers.  It is constructed from the
      source domain, and optionally an application service type.

   Relative Distinguished Name (RDN):  An ASN.1-based construction which
      itself is a building-block component of Distinguished Names.  See
      [LDAP-DN], Section 2.

   source domain:  The FQDN that a client expects an application service
      to present in the certificate.  This is typically input by a human
      user, configured into a client, or provided by reference such as a
      URL.  The combination of a source domain and, optionally, an
      application service type enables a client to construct one or more
      reference identifiers.

   subjectAltName entry:  An identifier placed in a subjectAltName
      extension.

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   subjectAltName extension:  A standard PKIX extension enabling
      identifiers of various types to be bound to the certificate
      subject.

   subjectName:  The name of a PKIX certificate's subject, encoded in a
      certificate's subject field (see [PKIX], Section 4.1.2.6).

   Security-related terms used in this document, but not defined here or
   in [PKIX] should be understood in the the sense defined in
   [SECTERMS].  Such terms include "attack", "authentication",
   "identity", "trust", "validate", and "verify".

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Naming of Application Services

   This document assumes that the name of an application service is
   based on a DNS domain name (e.g., example.com) -- supplemented in
   some circumstances by an application service type (e.g., "the IMAP
   server at example.net").  The DNS name conforms to one of the
   following forms:

   1.  A "traditional domain name", i.e., a FQDN (see [DNS-CONCEPTS])
       all of whose labels are "LDH labels" as described in [IDNA-DEFS].
       Informally, such labels are constrained to [US-ASCII] letters,
       digits, and the hyphen, with the hyphen prohibited in the first
       character position.  Additional qualifications apply (refer to
       the above-referenced specifications for details), but they are
       not relevant here.

   2.  An "internationalized domain name", i.e., a DNS domain name that
       includes at least one label containing appropriately encoded
       Unicode code points outside the traditional US-ASCII range.  That
       is, it contains at least one U-label or A-label, but otherwise
       may contain any mixture of NR-LDH labels, A-labels, or U-labels,
       as described in [IDNA-DEFS] and the associated documents.

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   From the perspective of the application client or user, some names
   are _direct_ because they are provided directly by a human user.
   This includes runtime input, prior configuration, or explicit
   acceptance of a client communication attempt.  Other names are
   _indirect_ because they are automatically resolved by the application
   based on user input, such as a target name resolved from a source
   name using DNS SRV or [NAPTR] records.  The distinction matters most
   for certificate consumption, specifically verification as discussed
   in this document.

   From the perspective of the application service, some names are
   _unrestricted_ because they can be used in any type of service, such
   as a single certificate being used for both the HTTP and IMAP
   services at the host example.com.  Other names are _restricted_
   because they can only be used for one type of service, such as a
   special-purpose certificate that can only be used for an IMAP
   service.  This distinction matters most for certificate issuance.

   We can categorize the three identifier types as follows:

   *  A DNS-ID is direct and unrestricted.

   *  An SRV-ID is typically indirect but can be direct, and is
      restricted.

   *  A URI-ID is direct and restricted.

   It is important to keep these distinctions in mind, because best
   practices for the deployment and use of the identifiers differ.  Note
   that cross-protocol attacks such as [ALPACA] are possibile when two
   different protocol services use the same certificate.  This can be
   addressed by using restricted identifiers, or deploying services so
   that they do not share certificates.  Protocol specifications MUST
   specify which identifiers are mandatory-to-implement and SHOULD
   provide operational guidance when necessary.

   The Common Name RDN MUST NOT be used to identify a service.  Reasons
   for this include:

   *  It is not strongly typed and therefore suffers from ambiguities in
      interpretation.

   *  It can appear more than once in the subjectName.

   For similar reasons, other RDN's within the subjectName MUST NOT be
   used to identify a service.

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3.  Designing Application Protocols

   This section defines how protocol designers should reference this
   document, which would typically be a normative reference in their
   specification.  Its specification MAY choose to allow only one of the
   identifier types defined here.

   If the technology does not use DNS SRV records to resolve the DNS
   domain names of application services then its specification MUST
   state that SRV-ID as defined in this document is not supported.  Note
   that many existing application technologies use DNS SRV records to
   resolve the DNS domain names of application services, but do not rely
   on representations of those records in PKIX certificates by means of
   SRV-IDs as defined in [SRVNAME].

   If the technology does not use URIs to identify application services,
   then its specification MUST state that URI-ID as defined in this
   document is not supported.  Note that many existing application
   technologies use URIs to identify application services, but do not
   rely on representation of those URIs in PKIX certificates by means of
   URI-IDs.

   A technology MAY disallow the use of the wildcard character in DNS
   names.  If it does so, then the specification MUST state that
   wildcard certificates as defined in this document are not supported.

4.  Representing Server Identity

   This section provides instructions for issuers of certificates.

4.1.  Rules

   When a certificate authority issues a certificate based on the FQDN
   at which the application service provider will provide the relevant
   application, the following rules apply to the representation of
   application service identities.  Note that some of these rules are
   cumulative and can interact in important ways that are illustrated
   later in this document.

   1.  The certificate SHOULD include a "DNS-ID" as a baseline for
       interoperability.

   2.  If the service using the certificate deploys a technology for
       which the relevant specification stipulates that certificates
       ought to include identifiers of type SRV-ID (e.g., [XMPP]), then
       the certificate SHOULD include an SRV-ID.

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   3.  If the service using the certificate deploys a technology for
       which the relevant specification stipulates that certificates
       ought to include identifiers of type URI-ID (e.g., [SIP] as
       specified by [SIP-CERTS]), then the certificate SHOULD include a
       URI-ID.  The scheme MUST be that of the protocol associated with
       the application service type and the "host" component (or its
       equivalent) MUST be the FQDN of the service.  The application
       protocol specification MUST specify which URI schemes are
       acceptable in URI-IDs contained in PKIX certificates used for the
       application protocol (e.g., sip but not sips or tel for SIP as
       described in [SIP-SIPS]).

   4.  The certificate MAY contain more than one DNS-ID, SRV-ID, or URI-
       ID as further explained under Section 7.3.

   5.  The certificate MAY include other application-specific
       identifiers for compatibility with a deployed base.  Such
       identifiers are out of scope for this specification.

4.2.  Examples

   Consider a simple website at www.example.com, which is not
   discoverable via DNS SRV lookups.  Because HTTP does not specify the
   use of URIs in server certificates, a certificate for this service
   might include only a DNS-ID of www.example.com.

   Consider an IMAP-accessible email server at the host mail.example.net
   servicing email addresses of the form user@example.net and
   discoverable via DNS SRV lookups on the application service name of
   example.net.  A certificate for this service might include SRV-IDs of
   _imap.example.net and _imaps.example.net (see [EMAIL-SRV]) along with
   DNS-IDs of example.net and mail.example.net.

   Consider a SIP-accessible voice-over-IP (VoIP) server at the host
   voice.example.edu servicing SIP addresses of the form
   user@voice.example.edu and identified by a URI of
   <sip:voice.example.edu>.  A certificate for this service would
   include a URI-ID of sip:voice.example.edu (see [SIP-CERTS]) along
   with a DNS-ID of voice.example.edu.

   Consider an XMPP-compatible instant messaging (IM) server at the host
   im.example.org servicing IM addresses of the form user@im.example.org
   and discoverable via DNS SRV lookups on the im.example.org domain.  A
   certificate for this service might include SRV-IDs of _xmpp-
   client.im.example.org and _xmpp-server.im.example.org (see [XMPP]), a
   DNS-ID of im.example.org.  For backward compatibility, it may also
   have an XMPP-specific XmppAddr of im.example.org (see [XMPP]).

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5.  Requesting Server Certificates

   This section provides instructions for service providers regarding
   the information to include in certificate signing requests (CSRs).
   In general, service providers SHOULD request certificates that
   include all of the identifier types that are required or recommended
   for the application service type that will be secured using the
   certificate to be issued.

   If the certificate will be used for only a single type of application
   service, the service provider SHOULD request a certificate that
   includes a DNS-ID and, if appropriate for the application service
   type, an SRV-ID or URI-ID that limits the deployment scope of the
   certificate to only the defined application service type.

   If the certificate might be used for any type of application service,
   then the service provider SHOULD request a certificate that includes
   only a DNS-ID.  Again, because of multi-protocol attacks this
   practice is discouraged; this can be mitigated by deploying only one
   service on a host.

   If a service provider offers multiple application service types and
   wishes to limit the applicability of certificates using SRV-IDs or
   URI-IDs, they SHOULD request multiple certificates, rather than a
   single certificate containing multiple SRV-IDs or URI-IDs each
   identifying a different application service type.  This rule does not
   apply to application service type "bundles" that identify distinct
   access methods to the same underlying application such as an email
   application with access methods denoted by the application service
   types of imap, imaps, pop3, pop3s, and submission as described in
   [EMAIL-SRV].

6.  Verifying Service Identity

   At a high level, the client verifies the application service's
   identity by performing the following actions:

   1.  The client constructs a list of acceptable reference identifiers
       based on the source domain and, optionally, the type of service
       to which the client is connecting.

   2.  The server provides its identifiers in the form of a PKIX
       certificate.

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   3.  The client checks each of its reference identifiers against the
       presented identifiers for the purpose of finding a match.  When
       checking a reference identifier against a presented identifier,
       the client matches the source domain of the identifiers and,
       optionally, their application service type.

   Naturally, in addition to checking identifiers, a client should
   perform further checks, such as expiration and revocation, to ensure
   that the server is authorized to provide the requested service.
   Because such checking is not a matter of verifying the application
   service identity presented in a certificate, methods for doing so are
   out of scope for this document.

6.1.  Constructing a List of Reference Identifiers

6.1.1.  Rules

   The client MUST construct a list of acceptable reference identifiers,
   and MUST do so independently of the identifiers presented by the
   service.

   The inputs used by the client to construct its list of reference
   identifiers might be a URI that a user has typed into an interface
   (e.g., an HTTPS URL for a website), configured account information
   (e.g., the domain name of a host for retrieving email, which might be
   different from the DNS domain name portion of a username), a
   hyperlink in a web page that triggers a browser to retrieve a media
   object or script, or some other combination of information that can
   yield a source domain and an application service type.

   The client might need to extract the source domain and application
   service type from the input(s) it has received.  The extracted data
   MUST include only information that can be securely parsed out of the
   inputs, such as parsing the FQDN out of the "host" component or
   deriving the application service type from the scheme of a URI.
   Other possibilities include pulling the data from a delegated domain
   that is explicitly established via client or system configuration or
   resolving the data via [DNSSEC].  These considerations apply only to
   extraction of the source domain from the inputs.  Naturally, if the
   inputs themselves are invalid or corrupt (e.g., a user has clicked a
   link provided by a malicious entity in a phishing attack), then the
   client might end up communicating with an unexpected application
   service.

   For example, given an input URI of <sip:alice@example.net>, a client
   would derive the application service type sip from the scheme and
   parse the domain name example.net from the host component.

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   Each reference identifier in the list MUST be based on the source
   domain and MUST NOT be based on a derived domain such as a domain
   name discovered through DNS resolution of the source domain.  This
   rule is important because only a match between the user inputs and a
   presented identifier enables the client to be sure that the
   certificate can legitimately be used to secure the client's
   communication with the server.  This removes DNS and DNS resolution
   from the attack surface.

   Using the combination of FQDN(s) and application service type, the
   client MUST construct its list of reference identifiers in accordance
   with the following rules:

   *  The list SHOULD include a DNS-ID.  A reference identifier of type
      DNS-ID can be directly constructed from a FQDN that is (a)
      contained in or securely derived from the inputs, or (b)
      explicitly associated with the source domain by means of user
      configuration.

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

   *  If a server for the application service type is typically
      associated with a URI for security purposes (i.e., a formal
      protocol document specifies the use of URIs in server
      certificates), then the list SHOULD include a URI-ID.

   Which identifier types a client includes in its list of reference
   identifiers, and their priority, is a matter of local policy.  For
   example, a client that is built to connect only to a particular kind
   of service might be configured to accept as valid only certificates
   that include an SRV-ID for that application service type.  By
   contrast, a more lenient client, even if built to connect only to a
   particular kind of service, might include both SRV-IDs and DNS-IDs in
   its list of reference identifiers.

6.1.2.  Examples

   The following examples are for illustrative purposes only and are not
   intended to be comprehensive.

   1.  A web browser that is connecting via HTTPS to the website at
       www.example.com would have a single reference identifier: a DNS-
       ID of www.example.com.

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   2.  A mail user agent that is connecting via IMAPS to the email
       service at example.net (resolved as mail.example.net) might have
       three reference identifiers: an SRV-ID of _imaps.example.net (see
       [EMAIL-SRV]), and DNS-IDs of example.net and mail.example.net.
       An email user agent that does not support [EMAIL-SRV] would
       probably be explicitly configured to connect to mail.example.net,
       whereas an SRV-aware user agent would derive example.net from an
       email address of the form user@example.net but might also accept
       mail.example.net as the DNS domain name portion of reference
       identifiers for the service.

   3.  A voice-over-IP (VoIP) user agent that is connecting via SIP to
       the voice service at voice.example.edu might have only one
       reference identifier: a URI-ID of sip:voice.example.edu (see
       [SIP-CERTS]).

   4.  An instant messaging (IM) client that is connecting via XMPP to
       the IM service at im.example.org might have three reference
       identifiers: an SRV-ID of _xmpp-client.im.example.org (see
       [XMPP]), a DNS-ID of im.example.org, and an XMPP-specific
       XmppAddr of im.example.org (see [XMPP]).

   In all of these cases, presented identifiers that do not match the
   reference identifier(s) would be rejected; for instance:

   *  With regard to the first example a DNS-ID of "web.example.com"
      would be rejected because the DNS domain name portion does not
      match "www.example.com".

   *  With regard to the third example, a URI-ID of
      "sip:www.example.edu" would be rejected because the DNS domain
      name portion does not match "voice.example.edu" and a DNS-ID of
      "voice.example.edu" would be rejected because it lacks the
      appropriate application service type portion (i.e., it does not
      specify a "sip:" URI).

6.2.  Preparing to Seek a Match

   Once the client has constructed its list of reference identifiers and
   has received the server's presented identifiers, the client checks
   its reference identifiers against the presented identifiers for the
   purpose of finding a match.  The search fails if the client exhausts
   its list of reference identifiers without finding a match.  The
   search succeeds if any presented identifier matches one of the
   reference identifiers, at which point the client SHOULD stop the
   search.

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   Before applying the comparison rules provided in the following
   sections, the client might need to split the reference identifier
   into its DNS domain name portion and its application service type
   portion, as follows:

   *  A DNS-ID reference identifier MUST be used directly as the DNS
      domain name and there is no application service type.

   *  For an SRV-ID reference identifier, the DNS domain name portion is
      the Name and the application service type portion is the Service.
      For example, an SRV-ID of _imaps.example.net has a DNS domain name
      portion of example.net and an application service type portion of
      imaps, which maps to the IMAP application protocol as explained in
      [EMAIL-SRV].

   *  For a reference identifier of type URI-ID, the DNS domain name
      portion is the "reg-name" part of the "host" component and the
      application service type portion is the scheme, as defined above.
      Matching only the "reg-name" rule from [URI] limits verification
      to DNS domain names, thereby differentiating a URI-ID from a
      uniformResourceIdentifier entry that contains an IP address or a
      mere host name, or that does not contain a "host" component at
      all.  Furthermore, note that extraction of the "reg-name" might
      necessitate normalization of the URI (as explained in [URI]).  For
      example, a URI-ID of sip:voice.example.edu would be split into a
      DNS domain name portion of voice.example.edu and an application
      service type of sip (associated with an application protocol of
      SIP as explained in [SIP-CERTS]).

   A client MUST match the DNS name, and if an application service type
   is present it MUST also match the service type as well.  These are
   described below.

6.3.  Matching the DNS Domain Name Portion

   This section describes how the client must determine if the presented
   DNS name matches the reference DNS name.  The rules differ depending
   on whether the domain to be checked is a traditional domain name or
   an internationalized domain name, as defined in Section 2.  For
   clients that support names containing the wildcard character "*",
   this section also specifies a supplemental rule for such "wildcard
   certificates".  This section uses the description of labels and
   domain names in [DNS-CONCEPTS].

   If the DNS domain name portion of a reference identifier is a
   traditional domain name, then matching of the reference identifier
   against the presented identifier MUST be performed by comparing the
   set of domain name labels using a case-insensitive ASCII comparison,

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   as clarified by [DNS-CASE].  For example, WWW.Example.Com would be
   lower-cased to www.example.com for comparison purposes.  Each label
   MUST match in order for the names to be considered to match, except
   as supplemented by the rule about checking of wildcard labels given
   below.

   If the DNS domain name portion of a reference identifier is an
   internationalized domain name, then the client MUST convert any
   U-labels [IDNA-DEFS] in the domain name to A-labels before checking
   the domain name.  In accordance with [IDNA-PROTO], A-labels MUST be
   compared as case-insensitive ASCII.  Each label MUST match in order
   for the domain names to be considered to match, except as
   supplemented by the rule about checking of wildcard labels given
   below.

   If the technology specification supports wildcards, then the client
   MUST match the reference identifier against a presented identifier
   whose DNS domain name portion contains the wildcard character "*" in
   a label provided these requirements are met:

   1.  There is only one wildcard character.

   2.  The wildcard character appears only as the complete content of
       the left-most label.

   If the requirements are not met, the presented identifier is invalid
   and MUST be ignored.

   A wildcard in a presented identifier can only match exactly one label
   in a reference identifier.  Note that this is not the same as DNS
   wildcard matching, where the "*" label always matches at least one
   whole label and sometimes more.  See [DNS-CONCEPTS], Section 4.3.3
   and [DNS-WILDCARDS].

   For information regarding the security characteristics of wildcard
   certificates, see Section 7.1.

6.4.  Matching the Application Service Type Portion

   The rules for matching the application service type depend on whether
   the identifier is an SRV-ID or a URI-ID.

   These identifiers provide an application service type portion to be
   checked, but that portion is combined only with the DNS domain name
   portion of the SRV-ID or URI-ID itself.  For example, if a client's
   list of reference identifiers includes an SRV-ID of _xmpp-
   client.im.example.org and a DNS-ID of apps.example.net, the client
   MUST check both the combination of an application service type of

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   xmpp-client and a DNS domain name of im.example.org and a DNS domain
   name of apps.example.net.  However, the client MUST NOT check the
   combination of an application service type of xmpp-client and a DNS
   domain name of apps.example.net because it does not have an SRV-ID of
   _xmpp-client.apps.example.net in its list of reference identifiers.

   If the identifier is an SRV-ID, then the application service name
   MUST be matched in a case-insensitive manner, in accordance with
   [DNS-SRV].  Note that the _ character is prepended to the service
   identifier in DNS SRV records and in SRV-IDs (per [SRVNAME]), and
   thus does not need to be included in any comparison.

   If the identifier is a URI-ID, then the scheme name portion MUST be
   matched in a case-insensitive manner, in accordance with [URI].  Note
   that the : character is a separator between the scheme name and the
   rest of the URI, and thus does not need to be included in any
   comparison.

6.5.  Outcome

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

   If the client does not find a presented identifier matching any of
   the reference identifiers, then the client MUST proceed as described
   as follows.

   If the client is an automated application, then it SHOULD terminate
   the communication attempt with a bad certificate error and log the
   error appropriately.  The application MAY provide a configuration
   setting to disable this behavior, but it MUST enable it by default.

   If the client is one that is directly controlled by a human user,
   then it SHOULD inform the user of the identity mismatch and
   automatically terminate the communication attempt with a bad
   certificate error in order to prevent users from inadvertently
   bypassing security protections in hostile situations.  Such clients
   MAY give advanced users the option of proceeding with acceptance
   despite the identity mismatch.  Although this behavior can be
   appropriate in certain specialized circumstances, it needs to be
   handled with extreme caution, for example by first encouraging even
   an advanced user to terminate the communication attempt and, if they
   choose to proceed anyway, by forcing the user to view the entire
   certification path before proceeding.

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   The application MAY also present the user with the ability to accept
   the presented certificate as valid for subsequent connections.  Such
   ad-hoc "pinning" SHOULD NOT restrict future connections to just the
   pinned certificate.  Local policy that statically enforces a given
   certificate for a given peer SHOULD made available only as prior
   configuration, rather than a just-in-time override for a failed
   connection.

7.  Security Considerations

7.1.  Wildcard Certificates

   Wildcard certificates automatically vouch for any single-label host
   names within their domain, but not multiple levels of domains.  This
   can be convenient for administrators but also poses the risk of
   vouching for rogue or buggy hosts.  See for example [Defeating-SSL]
   (beginning at slide 91) and [HTTPSbytes] (slides 38-40).

   Protection against a wildcard that identifies a public suffix
   [Public-Suffix], such as *.co.uk or *.com, is beyond the scope of
   this document.

7.2.  Internationalized Domain Names

   Allowing internationalized domain names can lead to visually similar
   characters, also referred to as "confusables", being included within
   certificates.  For discussion, see for example [IDNA-DEFS],
   Section 4.4 and [UTS-39].

7.3.  Multiple Presented Identifiers

   A given application service might be addressed by multiple DNS domain
   names for a variety of reasons, and a given deployment might service
   multiple domains or protocols.  TLS Extensions such as TLS Server
   Name Indication (SNI), discussed in [TLS], Section 4.4.2.2, and
   Application Layer Protocol Negotiation (ALPN), discussed in [ALPN],
   provide a way for the application to indicate the desired identifier
   and protocol to the server, which it can then use to select the most
   appropriate certificate.

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   This specification allows multiple DNS-IDs, SRV-IDs, or URI-IDs in a
   certificate.  As a result, an application service can use the same
   certificate for multiple hostnames, such as when a client does not
   support the TLS SNI extension, or for multiple protocols, such as
   SMTP and HTTP, on a single hostname.  Note that the set of names in a
   certificate is the set of names that could be affected by a
   compromise of any other server named in the set: the strength of any
   server in the set of names is determined by the weakest of those
   servers that offer the names.

   The way to mitigate this risk is to limit the number of names that
   any server can speak for, and to ensure that all servers in the set
   have a strong minimum configuration as described in [RFC7525bis].

7.4.  Multiple Reference Identifiers

   This specification describes how a client may construct multiple
   acceptable reference identifiers, and may match any of those
   reference identifiers with the set of presented identifiers.  [PKIX],
   Section 4.2.1.10 describes a mechanism to allow CA certificates to be
   constrained in the set of presented identifiers that they may include
   within server certificates.  However, these constraints only apply to
   the explicitly enumerated name forms.  For example, a CA that is only
   name constrained for DNS-IDs is not constrained for SRV-IDs and URI-
   IDs, unless those name forms are also explicitly included within the
   name constraints extension.

   A client that constructs multiple reference identifiers of different
   types, such as both DNS-ID and SRV-IDs, as described in
   Section 6.1.1, SHOULD take care to ensure that CAs issuing such
   certificates are appropriately constrained.  This MAY take the form
   of local policy through agreement with the issuing CA, or MAY be
   enforced by the client requiring that if one form of presented
   identifier is constrained, such as a dNSName name constraint for DNS-
   IDs, then all other forms of acceptable reference identities are also
   constrained, such as requiring a uniformResourceIndicator name
   constraint for URI-IDs.

8.  IANA Considerations

   This document has no actions for IANA.

9.  References

9.1.  Normative References

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   [DNS-CONCEPTS]
              Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/rfc/rfc1034>.

   [DNS-SRV]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/rfc/rfc2782>.

   [DNS-WILDCARDS]
              Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
              <https://www.rfc-editor.org/rfc/rfc4592>.

   [IDNA-DEFS]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5890>.

   [IDNA-PROTO]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <https://www.rfc-editor.org/rfc/rfc5891>.

   [LDAP-DN]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names",
              RFC 4514, DOI 10.17487/RFC4514, June 2006,
              <https://www.rfc-editor.org/rfc/rfc4514>.

   [PKIX]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC7525bis]
              Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", Work in Progress, Internet-Draft, draft-ietf-uta-

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              rfc7525bis-06, 24 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              rfc7525bis-06>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [SRVNAME]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, DOI 10.17487/RFC4985, August 2007,
              <https://www.rfc-editor.org/rfc/rfc4985>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

9.2.  Informative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

   [ACME]     Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8555>.

   [ALPACA]   Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D.,
              Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel,
              "ALPACA: Application Layer Protocol Confusion - Analyzing
              and Mitigating Cracks in TLS Authentication", September
              2021, <https://alpaca-attack.com/ALPACA.pdf>.

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.

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

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   [DNS-CASE] Eastlake 3rd, D., "Domain Name System (DNS) Case
              Insensitivity Clarification", RFC 4343,
              DOI 10.17487/RFC4343, January 2006,
              <https://www.rfc-editor.org/rfc/rfc4343>.

   [DNSSEC]   Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/rfc/rfc4033>.

   [DTLS]     Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.

   [EMAIL-SRV]
              Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <https://www.rfc-editor.org/rfc/rfc6186>.

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

   [NAPTR]    Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, DOI 10.17487/RFC3403, October 2002,
              <https://www.rfc-editor.org/rfc/rfc3403>.

   [NTS]      Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,
              <https://www.rfc-editor.org/rfc/rfc8915>.

   [Public-Suffix]
              "Public Suffix List", 2020, <https://publicsuffix.org>.

   [QUIC]     Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9001>.

   [SECTERMS] Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/rfc/rfc4949>.

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   [SIP]      Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/rfc/rfc3261>.

   [SIP-CERTS]
              Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
              Certificates in the Session Initiation Protocol (SIP)",
              RFC 5922, DOI 10.17487/RFC5922, June 2010,
              <https://www.rfc-editor.org/rfc/rfc5922>.

   [SIP-SIPS] Audet, F., "The Use of the SIPS URI Scheme in the Session
              Initiation Protocol (SIP)", RFC 5630,
              DOI 10.17487/RFC5630, October 2009,
              <https://www.rfc-editor.org/rfc/rfc5630>.

   [TLS]      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

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

   [UTS-39]   Davis, M. and M. Suignard, "Unicode Security Mechanisms",
              n.d., <https://unicode.org/reports/tr39>.

   [VERIFY]   Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/rfc/rfc6125>.

   [WSC-UI]   Saldhana, A. and T. Roessler, "Web Security Context: User
              Interface Guidelines", August 2010,
              <https://www.w3.org/TR/2010/REC-wsc-ui-20100812/>.

   [XMPP]     Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/rfc/rfc6120>.

Appendix A.  Changes from RFC 6125

   This document revises and obsoletes [VERIFY] based on the decade of
   experience and changes since it was published.  The major changes, in
   no particular order, include:

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   *  The only legal place for a certificate wildcard is as the complete
      left-most component in a domain name.

   *  The server identity can only be expressed in the subjectAltNames
      extension; it is no longer valid to use the commonName RDN, known
      as CN-ID in [VERIFY].

   *  Detailed discussion of pinning (configuring use of a certificate
      that doesn't match the criteria in this document) has been removed
      and replaced with two paragraphs in Section 6.5.

   *  The sections detailing different target audiences and which
      sections to read (first) have been removed.

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

   *  All references have been updated to the current latest version.

   *  The TLS SNI extension is no longer new, it is commonplace.

   *  Additional text on multiple identifiers, and their security
      considerations, has been added.

Acknowledgements

   We gratefully acknowledge everyone who contributed to the previous
   version of this document, [VERIFY].  Thanks also to Carsten Bormann
   for converting the previous document to Markdown so that we could
   more easily use Martin Thomson's i-d-template software.

   In addition to discussion on the mailing list, the following people
   contributed significant changes: Viktor Dukhovni, Jim Fenton, Olle
   Johansson, and Ryan Sleevi.

Authors' Addresses

   Peter Saint-Andre
   United States of America
   Email: stpeter@stpeter.im

   Jeff Hodges
   United States of America
   Email: netwerkeddude@gmail.com

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   Rich Salz
   Akamai Technologies
   United States of America
   Email: rsalz@akamai.com

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