Internet DRAFT - draft-turner-ssl-must-not


Network Working Group                                         S. Turner 
Internet Draft                                                     IECA 
Updates: 5246 (once approved)                                  Tim Polk 
Intended Status: Standards Track                                   NIST 
Expires: January 26, 2011                                 July 26, 2010 
                        Prohibiting SSL Version 2.0 


   This document requires that when TLS clients and servers establish 
   connections that they never negotiate the use of Secure Sockets Layer 
   (SSL) version 2.0.  This document updates the backward compatibility 
   sections found in the Transport Security Layer (TLS) Protocol, RFC 

Status of this Memo 

   This Internet-Draft is submitted in full conformance with the 
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   from IETF Documents or IETF Contributions published or made publicly 
   available before November 10, 2008. 

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   This Internet-Draft will expire on January 26, 2009. 

Copyright Notice 

   Copyright (c) 2010 IETF Trust and the persons identified as the 
   document authors. All rights reserved. 
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   This document is subject to BCP 78 and the IETF Trust's Legal 
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   ( in effect on the date of 
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1. Introduction 

   Many protocols specified in the IETF rely on Transport Layer Security 
   (TLS) [TLS] for security services.  This is a good thing, but some 
   TLS clients and servers also support negotiating the use of SSL 
   version 2.0 [SSL2]; however, this version does not provide the 
   expected level of security. SSL version 2.0 has known deficiencies. 
   This document describes those deficiencies, and it requires TLS 
   clients and servers never negotiate the use of SSL version 2.0. 

   This document updates the backward compatibility sections found in 
   the Transport Security Layer (TLS) Protocol [TLS] and earlier 

1.1. Requirements Terminology 

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 
   "OPTIONAL" in this document are to be interpreted as described in 

2. SSL 2.0 

   SSL version 2.0 [SSL2] deficiencies include: 

    o Message authentication uses MD5 [MD5].  Most security-aware users 
     have already moved away from any of MD5 

    o Handshake messages are not protected.  This permits a man-in-the-
     middle to trick the client into picking a weaker cipher suite than 
     they would normally choose. 

    o Message integrity and message encryption use the same key, which 
     is a problem if the client and server negotiate a weak encryption 

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    o Sessions can be easily terminated.  A man-in-the-middle can easily 
     insert a TCP FIN to close the session and the peer is unable to 
     determine whether or not it was a legitimate end of the session. 

3. Changes to TLS 

   Because of the deficiencies noted in the previous sections, TLS 
   implementations MUST NOT support SSL 2.0.  The specific changes to 
   [TLS], including earlier versions, are as follows: 

    o TLS clients MUST NOT use SSL 2.0 ClientHello messages. 

    o TLS servers MUST NOT accept SSL 2.0 ClientHello messages. 

4. IANA Considerations 


5. Security Considerations 

   This entire document is about security considerations. 

6. Acknowledgements 

   The idea for this document was inspired by discussions between Peter 
   Saint Andre, Simon Josefsson, and others on the XMPP mailing list.  
   We would also like to thank Paul Hoffman, Yaron Sheffer, and Nikos 
   Mavrogiannopoulos, Yngve Pettersen, Marsh Ray, and Martin Rex for 
   reviews and comments. 

7. References 

7.1. Normative References 

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

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

7.2. Informative References 

   [MD5]            Rivest, R., "The MD5 Message-Digest Algorithm", RFC 
                    1321, April 1992.  

   [SSL2]           Hickman, Kipp, "The SSL Protocol", Netscape 
                    Communications Corp., Feb 9, 1995. 
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   [I-D.turner-md5-seccon-update] Turner, S., and L. Chen, "Updated 
                    Security Considerations for the MD5 Message-Digest 
                    Algorithm", draft-turner-md5-seccon-update, work-in-

 Authors' Addresses 

   Sean Turner 
   IECA, Inc. 
   3057 Nutley Street, Suite 106 
   Fairfax, VA 22031 


   Tim Polk 
   National Institute of Standards and Technology 
   100 Bureau Drive, Mail Stop 8930 
   Gaithersburg, MD 20899-8930 



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