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None                                                      P. Saint-Andre
Internet-Draft                                                     Cisco
Intended status: Standards Track                             K. Zeilenga
Expires: March 4, 2010                                     Isode Limited
                                                               J. Hodges
                                                                  PayPal
                                                               R. Morgan
                                                               Internet2
                                                         August 31, 2009


         Server Identity Verification in Application Protocols
                draft-saintandre-tls-server-id-check-01

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on March 4, 2010.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Abstract

   Technologies such as Transport Layer Security (TLS) and IPsec enable
   a secure connection between two entities (a "client" and a "server")
   using X.509 certificates.  This document specifies recommended
   procedures for checking the identity of the server in such an
   interaction.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Verification Process . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Comparison Rules . . . . . . . . . . . . . . . . . . . . .  6
       3.2.1.  Domain Names . . . . . . . . . . . . . . . . . . . . .  6
       3.2.2.  IP Addresses . . . . . . . . . . . . . . . . . . . . .  7
       3.2.3.  Email Addresses  . . . . . . . . . . . . . . . . . . .  7
       3.2.4.  SIP Addresses  . . . . . . . . . . . . . . . . . . . .  8
       3.2.5.  JabberIDs  . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Outcome  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12























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1.  Introduction

   Technologies such as Transport Layer Security [TLS] and [IPSEC]
   enable a secure connection between two entities using the Internet
   X.509 Public Key Infrastructure (PKI) as described in [X509].  In
   such interactions, the entity that initiates the connection is called
   a "client" and the entity that receives the connection is called a
   "server".

      Note: The terms "client" and "server" as used here refer to
      security roles, not application roles; a server in the context of
      TLS or IPSec might be a "client" (i.e., a user agent) in the
      context of an application protocol as deployed on the Internet.

   If a client wishes to connect to a server securely, it needs to check
   the identity of the server so that it can determine if the server is
   what it claims to be, verify that there is no attacker in the middle,
   etc.  Typically this is done by correlating the information presented
   in the server's certificate with information available about the
   server contained in the Domain Name System (DNS).

   Different application protocols that make use of the client-server
   pattern for security purposes have traditionally specified their own
   procedures for checking server identities.  Examples include but are
   not limited to:

   o  The Hypertext Transfer Protocol [HTTP], for which see also
      [HTTP-TLS]
   o  The Internet Message Access Protocol [IMAP] and the Post Office
      Protocol [POP3], for which see also [USINGTLS]
   o  The Lightweight Directory Access Protocol [LDAP], for which see
      also [LDAP-AUTH] and its predecesor [LDAP-TLS]
   o  The NETCONF Configuration Protocol [NETCONF], for which see also
      [NETCONF-SSH] and [NETCONF-TLS]
   o  The Network News Transfer Protocol [NNTP], for which see also
      [NNTP-TLS]
   o  The Session Initiation Protocol [SIP], for which see also
      [SIP-CERTS]
   o  The Simple Mail Transfer Protocol [SMTP], for which see also
      [SMTP-AUTH] and [SMTP-TLS]
   o  The Syslog Protocol [SYSLOG], for which see also [SYSLOG-TLS]
   o  The Extensible Messaging and Presence Protocol [XMPP], for which
      see also [XMPPBIS]

   Unfortunately, this divergence of approaches has caused some
   confusion among developers and protocol designers.  Therefore this
   document specifies recommended identity checking procedures for
   application protocols produced within the Internet Standards Process,



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   for the purpose of codifying secure authentication practices.

   Note: This document is currently limited in scope to the presentation
   of identities in X.509 certificates as issued in the context of the
   Public Key Infrastructure (PKI) and as applied to Transport Layer
   Security [TLS]; a future version of this document might address X.509
   certificates as issued outside the context of the PKI, non-X.509
   public keys such as OpenPGP keys, presentation of identities in ways
   other than in the certificate itself (e.g., certificate fingerprints
   for Secure Shell as described in [SSH] or for Datagram Transport
   Layer Security DTLS and Secure Real-time Transport Protocol as
   described in [DTLS-SRTP]), and applications other than TLS.


2.  Conventions

   The following capitalized keywords are to be interpreted as described
   in [TERMS]: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT";
   "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY",
   "OPTIONAL".

   Most security-related terms are to be understood in the sense defined
   in [SECTERMS]; such terms include, but are not limited to,
   "assurance", "attack", "authentication", "authorization",
   "certificate", "certification authority", "confidentiality",
   "credential", "downgrade", "encryption", "fingerprint", "hash value",
   "identity", "integrity", "signature", "security perimeter", "self-
   signed certificate", "sign", "spoof", "tamper", "trust", "trust
   anchor", "trust chain", "validate", "verify".

   In addition, we define the following terms to assist in understanding
   the process of verifying identity:

   identity set:  The set of identities that are presented by the server
      to the client (in the form of the server's X.509 certificate) when
      the client is attempts to establish a secure connection to the
      server.
   identity type:  The "natural kind" of identity to which a presented
      identity or reference identity belongs.  For example, the
      reference identity might be a domain name, an IPv4 or IPv6
      address, an email address, a SIP address, a JabberID, or some
      other type (this specification does not yet provide a complete
      taxonomy of identity types).  In the case of domain names, the
      reference identity MUST NOT contain the wildcard character '*'
      (ASCII 42) in the left-most (least significant) domain name
      component or component fragment.





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   presented identity:  A single member of the identity set.
   reference identity:  The client's conception of the server's identity
      before it attempts to establish a secure connection to the server;
      this is the identity that the client expects the server to present
      and to which the client makes reference when attempting to verify
      the server's identity.


3.  Verification Process

   When a client connects to a server, it MUST verify the server's
   identity (in order to prevent passive and active attacks against the
   connection).  By "verify identity" we mean that the client needs to
   establish that at least one of the identities in the identity set
   matches the reference identity.

3.1.  Overview

   At a high level, the client verifies the server identity in
   accordance with the following rules:

   1.  Before connecting to the server, the client determines the
       identity type of the reference identity.
   2.  During the process of attempting to establish a secure
       connection, the server MUST present its identity set to the
       client in the form of an X.509 certificate [X509].
   3.  Upon being presented with the server's identity set, the client
       MUST check the reference identity against the presented
       identities for the purpose of finding a match.  To do so, the
       client iterates through all of the subjectAltName extensions it
       recognizes in the server's certificate (potentially in an
       application-specific preference order) and compares the value of
       each extension against the reference identity until it has either
       produced a match or exhausted the identities in the identity set
       (comparison rules for matching particular identity types are
       provided under Section 3.2, including fallbacks to several
       subjectName fields).
   4.  Before attempting to find a match in relation to a particular
       presented identity, the client MAY map the reference identity to
       a different identity type.  Such a mapping MAY be performed for
       any available subjectAltName type to which the reference identity
       can be mapped; however, the reference identity SHOULD be mapped
       only 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-
       configured or admin-configured host-to-address/address-to-host
       lookup tables).



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   5.  If the identity set has more than one member, a match with any of
       the presented identities is acceptable.

      Note: Beyond the server identity check described in this section,
      clients might complete further checking to ensure that the server
      is authorized to provide the service it is requested to provide.
      The client might need to make use of local policy information in
      making this determination.

3.2.  Comparison Rules

3.2.1.  Domain Names

   If the reference identity is a domain name as defined by [RFC1034]
   and [RFC1035] for "traditional" domain names or by [IDNA] for
   internationalized domain names, then the client can match the
   reference identity against subjectAltName extensions of type dNSName
   and SRVName [SRVNAME] according to the following rules.

   If the reference identity is a "traditional" domain name, then
   matching of reference identity against the presented identity is
   performed by comparing the set of domain components using a case-
   insensitive ASCII comparison.

   If the reference identity is an internationalized domain name, then
   an implementation MUST convert the reference identity to the ASCII
   Compatible Encoding (ACE) format as specified in Section 4 of [IDNA]
   before comparison with subjectAltName values of type dNSName;
   specifically, the conversion operation specified in Section 4 of
   [IDNA] MUST be performed 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
   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 [IDNA].

   A dNSName MAY contain the wildcard character '*' (ASCII 42).  The
   wildcard character applies only to the left-most (least significant)
   domain name component or component fragment and matches any single
   component or component fragment.  For instance, a dNSName of
   *.example.com matches foo.example.com but not bar.foo.example.com or
   example.com itself; similarly, a dNSName of baz*.example.net matches
   baz1.example.net and baz2.example.net but not qux.example.net or
   example.net itself.



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   In addition to checking the subjectAltName extensions of type dNSName
   and SRVNAME, the client MAY as a fallback check the value of the
   Common Name (CN) (see [LDAP-SCHEMA]) as presented in the subjectName
   component of the server's X.509 certificate.  In existing
   certificates, the CN is often used for encapsulating a domain name;
   for example, consider the following subjectName:

   cn=www.example.com, ou=Web Services, c=GB

   Here the Common Name is "www.example.com" and the client could choose
   to compare the reference identity against that CN.

   When comparing the referenced identity against the Common Name, the
   client MUST follow the comparison rules described above for
   subjectAltName extensions of type dNSName and SRVName, with the
   exception that no wildcard matching is allowed.

   In order to match domain names, a client MUST NOT check Relative
   Distinguished Names (RDNs) other than the Common Name; in particular,
   this means that a series of Domain Component (DC) attributes MUST NOT
   be checked (because the order of Domain Components is not guaranteed,
   certain attacks are possible if DC attributes are checked).

3.2.2.  IP Addresses

   If the reference identity is an IP address as defined by [IP] or
   [IPv6], then the client can match the reference identity against
   subjectAltName extensions of type iPaddress according to the
   following rules.

   The reference identity MUST be converted to the "network byte order"
   octet string representation; for IP Version 4 the octet string will
   contain exactly four octets, and for IP Version 6 the octet string
   will contain exactly sixteen octets.  The client then compares this
   octet string, where a match occurs if the reference identity and
   presented identity octet strings are identical.

3.2.3.  Email Addresses

   If the reference identity is an email address as defined by [EMAIL],
   then the client SHOULD compare the reference identity against the
   value of the "rfc822Name" subjectAltName extension described in
   [X509].

   The client MAY also compare the reference identity against the value
   of the "E" attribute of the subjectName as described in [CRMF].





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3.2.4.  SIP Addresses

   If the reference identity is a SIP address as defined by [SIP], then
   the client SHOULD compare map the reference identity to a domain name
   or email address and proceed as described for those identity types,
   or proceed as described in [SIP-CERTS].

3.2.5.  JabberIDs

   If the reference identity is a JabberID as defined by [XMPP], then
   the client SHOULD compare the reference identity against the value of
   the "id-on-xmppAddr" subjectAltName extension of type otherName
   described in [XMPP], or proceed as described in [XMPPBIS].

3.3.  Outcome

   The outcome of the checking procedure is one of the following:

   Case #1:  The client finds at least one presented identity that
      matches the reference identity; the entity MUST use this as the
      validated identity of the server.
   Case #2:  The client finds no subjectAltName that matches the
      reference identity but a human user has permanently accepted the
      certificate during a previous connection attempt; the client MUST
      verify that the cached certificate was presented and MUST notify
      the user if the certificate has changed since the last time that a
      secure connection was successfully negotiated.
   Case #3:  The client finds no subjectAltName that matches the
      reference identity and a human user has not permanently accepted
      the certificate during a previous connection attempt; the client
      MUST NOT use the presented identity (if any) as the validated
      identity of the server and instead MUST proceed as described in
      the next section.  Instead, if the client is a user-oriented
      application, then it MUST either (1) automatically terminate the
      connection with a bad certificate error or (2) show the
      certificate (including the entire certificate chain) to the user
      and give the user the choice of terminating the connecting or
      accepting the certificate temporarily (i.e., for this connection
      attempt only) or permanently (i.e., for all future connection
      attempts) and then continuing with the connection; if a user
      permanently accepts a certificate in this way, the client MUST
      cache the certificate (or some non-forgeable representation such
      as a hash value) and in future connection attempts behave as in
      Case #2.  (It is the resposibility of the human user to verify the
      hash value or fingerprint of the certificate with the peer over a
      trusted communication layer.)  If the client is an automated
      application, then it SHOULD terminate the connection with a bad
      certificate error and log the error to an appropriate audit log;



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      an automated application MAY provide a configuration setting that
      disables this check, but MUST provide a setting that enables the
      check.


4.  Security Considerations

   To follow.


5.  IANA Considerations

   This document has no actions for the IANA.


6.  References

6.1.  Normative References

   [IDNA]     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.

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

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

   [X509]     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.

6.2.  Informative References

   [CRMF]     Schaad, J., "Internet X.509 Public Key Infrastructure
              Certificate Request Message Format (CRMF)", RFC 4211,
              September 2005.

   [DTLS-SRTP]
              McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for  Secure
              Real-time Transport Protocol (SRTP)",
              draft-ietf-avt-dtls-srtp-07 (work in progress),



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              February 2009.

   [EMAIL]    Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [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.

   [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.

   [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.




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   [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.

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

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [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 B. Laboratories, "Domain
              Certificates in the Session Initiation Protocol (SIP)",
              draft-ietf-sip-domain-certs-04 (work in progress),
              May 2009.

   [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.

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

   [SSH]      Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)



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              Protocol Architecture", RFC 4251, January 2006.

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

   [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.

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

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

   [XMPPBIS]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", draft-ietf-xmpp-3920bis-01 (work
              in progress), August 2009.


Authors' Addresses

   Peter Saint-Andre
   Cisco

   Email: psaintan@cisco.com


   Kurt D. Zeilenga
   Isode Limited

   Email: Kurt.Zeilenga@Isode.COM


   Jeff Hodges
   PayPal

   Email: Jeff.Hodges@KingsMountain.com









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   RL 'Bob' Morgan
   UWashington/Internet2

   Email: rlmorgan@washington.edu















































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