DANE                                                           R. Barnes
Internet-Draft                                          BBN Technologies
Intended status: Informational                            April 19, 2011
Expires: October 21, 2011

    Use Cases and Requirements for DNS-based Authentication of Named


   Many current applications use the certificate-based authentication
   features in TLS to allow clients to verify that a connected server
   properly represents a desired domain name.  Traditionally, this
   authentication has been based on PKIX trust hierarchies, rooted in
   well-known CAs, but additional information can be provided via the
   DNS itself.  This document describes a set of use cases in which the
   DNS and DNSSEC could be used to make assertions that support the TLS
   authentication process.

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   This Internet-Draft will expire on October 21, 2011.

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   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.1.  CA Constraints  . . . . . . . . . . . . . . . . . . . . . . 4
     3.2.  Certificate Constraints . . . . . . . . . . . . . . . . . . 5
     3.3.  Domain-Issued Certificates  . . . . . . . . . . . . . . . . 5
   4.  Other Requirements  . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   Transport-Layer Security or TLS is used as the basis for security
   features in many modern Internet applications [RFC5246].  It is used
   as the basis for secure HTTP and secure email
   [RFC2818][RFC2595][RFC3207], and provides hop-by-hop security in
   real-time multimedia and instant-messaging protocols

   One feature that is common to most uses of TLS is the use of
   certificates to authenticate domain names for services.  The TLS
   client begins the TLS connection process with the goal of connecting
   to a server with a specific domain name.  After locating the server
   via an A or AAAA record, the client conducts a TLS handshake with the
   server, during which the server presents a PKIX certificate for
   itself [RFC5280].  Based on this certificate, the client decides
   whether the server properly represents the desired domain name, and
   thus whether to proceed with the TLS connection or not.

   In most current applications, this decision process is based on PKIX
   validation and name matching.  The client validates that the
   certificate chains to a trust anchor [RFC5280], and that the desired
   domain name is contained in the certificate [RFC6125].  Within this
   framework, bindings between public keys and domain names are asserted
   by PKIX CAs.  Authentication decisions based on these bindings rely
   on the authority of these CAs.

   The DNS is built to provide information about domain names, and with
   the advent of DNSSEC [RFC1034][RFC4033], it is possible for this
   information to be provided securely (in the sense that clients can
   verify that DNS information was provided by the domain owner).  One
   of the goals of the current DANE working group is to develop
   technologies for using the DNS to provide additional information to
   inform the TLS domain authentication process.  This document
   describes a set of use cases that capture specific goals for using
   the DNS in this way, and a set of requirements that the ultimate DANE
   mechanism should satisfy.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document also makes use of standard PKIX, DNSSEC, and TLS
   terminology.  See RFC 5280 [RFC5280], RFC 4033 [RFC4033], and RFC
   5246 [RFC5246], respectively, for these terms.

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3.  Use Cases

   In this section, we describe the two major use cases that the DANE
   mechanism should support.  This list is not intended to represent all
   possible ways that the DNS can be used to support TLS authentication.
   Rather it represents the specific cases that comprise the initial
   goal for DANE.

   In the below use cases, we will refer to the following dramatis

   Alice  The operator of a TLS-based service on the host
      alice.example.com, and administrator of the corresponding DNS

   Bob  A client connecting to alice.example.com

   Charlie  A well-known CA that issues certificates with domain names
      as identifiers

3.1.  CA Constraints

   Alice runs a website on alice.example.com and has obtained a
   certificate from the well-known CA Charlie.  She is concerned about
   mis-issued certificates and would like to provide a mechanism for
   visitors to her site to know that they should expect
   alice.example.com to use a certificate issued by the CA that she uses
   (Charlie) and not another CA.

   When Bob connects to alice.example.com, he uses this mechanism to
   verify that that the certificate presented by the server was issued
   by the proper CA, Charlie.  Bob also performs the normal PKIX
   validation procedure for this certificate, in particular verifying
   that the certificate chains to a trust anchor.

   Because these constraints do not increase the scope of PKIX-based
   assertions about domains, there is not a strict requirement for
   DNSSEC.  Deletion of records removes the protection provided by this
   constraint, but the client is still protected by CA practices (as
   now).  Injected or modified false records are not useful unless the
   attacker can also obtain an unauthorized certificate.  In the worst
   case, tampering with these constraints degrades security to the level
   that is now standard.

   Continuing to require PKIX validation also limits the degree to which
   DNS operators can interfere with TLS authentication through this
   mechanism.  As above, even if a DNS operator falsifies DANE records,
   it cannot masquerade as the target server unless it can also obtain

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   an unauthorized certificate.

3.2.  Certificate Constraints

   Alice runs a website on alice.example.com and has obtained a
   certificate from the well-known CA Charlie.  She is concerned about
   certificates being issued by Charlie as well as other CAs.  She would
   like to provide a way for visitors to her site to know that they
   should expect alice.example.com to present the specific certificate
   issued by Charlie.

   When Bob connects to alice.example.com, he uses this mechanism to
   verify that that the certificate presented by the server is the
   correct certificate.  Bob also performs the normal PKIX validation
   procedure for this certificate, in particular verifying that the
   certificate chains to a trust anchor.

   The security considerations for this case are the same as for the "CA
   Constraints" case above.

3.3.  Domain-Issued Certificates

   Alice would like to be able to use generate and use certificates for
   her website on alice.example.com without involving an external CA at
   all.  Alice can generate her own certificates today, making self-
   signed certificates and possibly certificates subordinate to those
   certificates.  When Bob receives such a certificate, however, he
   doesn't have a way to verify that the issuer of the certificate is
   actually Alice.  This concerns him as an attacker could present a
   different certificate and perform a man in the middle attack.  Bob
   would like to protect against this.

   Alice would thus like to have a mechanism for visitors to her site to
   know that the certificates she issues are actually hers.  When Bob
   connects to alice.example.com, he uses this mechanism to verify that
   the certificate presented by the server was issued by Alice.  Since
   Bob can bind certificates to Alice in this way, he can use Alice's CA
   as a trust anchor for purposes of validating certificates for
   alice.example.com.  Alice can additionally recommend that clients
   accept only her certificates using the CA constraints described

   Providing trust anchor material in this way clearly requires DNSSEC,
   since corrupted or injected records could be used by an attacker to
   cause clients to trust an attacker's certificate.  Deleted records
   will only result in connection failure and denial of service,
   although this could result on a bid-down to an unsecured protocol,
   depending on the application.

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   By the same token, this use case puts the most power in the hands of
   DNS operators.  Since the operator of the appropriate DNS zone has de
   facto control over the content and signing of the zone, he can create
   false DANE records that bind a malicious party's certificate to a
   domain.  This is not a significant incremental risk relative to the
   current PKIX-based system, however, since it is possible for domain
   operators to obtain certificates for domains under some well-known
   certificate authorities today.

4.  Other Requirements

   In addition to supporting the above use cases, the DANE mechanism
   must satisfy several lower-level operational and protocol
   requirements and goals.

   Multiple Ports:  DANE should be able to support multiple services
      with different credentials on the same named host, distinguished
      by port number.

   Simple Key Management:  DANE must have a mode in which the domain
      owner only needs to maintain a single long-lived public/private
      key pair.

   Hard Failure:  Clients must be required to refuse to proceed with a
      connection to a site if DANE indicates a security error.

   Encapsulation:  If there is a DANE record for the name
      alice.example.com, it must only affect services hosted at

   Predictability:  Client behavior in response to DANE records must be
      spelled out in the DANE specification as precisely as possible.

   Minimal Dependencies:  It should be possible for a site to deploy
      DANE without also deploying anything else, except DNSSEC.

   Minimal Options:  Ideally, DANE should have only one operating mode.
      Practically, DANE should have as few operating modes as possible.

   Wild Cards and CNAME:  The mechanism for DANE record distribution
      should be compatible with the use of DNS wild cards and CNAME
      records for setting default properties for domains and redirecting

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5.  Acknowledgements

   Thanks to Eric Rescorla for the initial formulation of the use cases,
   Zack Weinberg and Phillip Hallam-Baker for contributing other
   requirements, and the whole DANE working group for helpful comments
   on the mailing list.

6.  IANA Considerations

   This document makes no request of IANA.

7.  Security Considerations

   The primary focus of this document is the enhancement of TLS
   authentication procedures using the DNS.  The general effect of such
   mechanisms is to increase the role of DNS operators in authentication
   processes, either in place of or in addition to traditional third-
   party actors such as commercial certificate authorities.  The
   specific security implications of the respective use cases are
   discussed in their respective sections above.

8.  References

8.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  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.

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8.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC3261]  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.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6125]  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, March 2011.

Author's Address

   Richard Barnes
   BBN Technologies
   9861 Broken Land Parkway
   Columbia, MD  21046

   Phone: +1 410 290 6169
   Email: rbarnes@bbn.com

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