Network Working Group S. Turner Internet Draft IECA Updates: 5246 (once approved) Tim Polk Intended Status: BCP NIST Expires: December 25, 2010 June 25, 2010 Prohibiting SSL Version 3.0 and Earlier draft-turner-ssl-must-not-01.txt Abstract This document requires that when TLS clients and servers establish connections that they never negotiate the use of Secure Sockets Layer (SSL) version 3.0 or earlier. In addition, this document recommends that TLS clients and servers not use TLS 1.0/SSL 3.1. This document updates the backward compatibility sections found in the Transport Security Layer (TLS) Protocol, RFC 5246. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 25, 2010. Turner & Polk Expires December 25, 2010 [Page 1]
Internet-Draft Prohibiting SSL May 2010 Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction Many protocols specified in the IETF rely on Transport Layer Security (TLS) [TLS1.0][TLS1.1][TLS1.2] 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] and/or 3.0 [SSL3]; however, these versions will not provide the expected level of security. Both SSL version 2.0 and 3.0 have known deficiencies. This document describes those deficiencies, and it requires TLS clients and servers never negotiate the use of SSL version 2.0 or 3.0. TLS has evolved from TLS 1.0 through 1.2. One change, which is addressed in Section 4, adopted by TLS 1.1 mitigates an attack against CBC modes used by TLS 1.0. As a result of this concern with TLS 1.0 this document recommends that TLS clients and servers never negotiate the use of TLS 1.0. Note that TLS 1.0 and SSL 3.1 are equivalent. This document updates the backward compatibility sections found in the Transport Security Layer (TLS) Protocol, RFC 5246. 1.1. Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [WORDS]. 2. SSL2 Deficiencies SSL version 2.0 [SSL2] deficiencies include: o Message authentication uses MD5 [MD5]. MD5 is universally accepted as a broken hash algorithm. Turner and Polk Expires December 25, 2010 [Page 2]
Internet-Draft Prohibiting SSL May 2010 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 algorithm. 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. SSL3 Deficiencies SSL version 3.0 [SSL3] has a primary key derivation function (KDF) deficiency. The SSL 3.0 KDF, which is used by all SSL 3.0 cipher suites, depends entirely upon the MD5 hash function [MD5] for half of the master key set up during the SSL key exchange. MD5 is not collision resistant and the pre-image resistance has been weakened by recent attacks. 4. TLS 1.0 Deficiencies TLS 1.0 [TLS1.0] is an improvement over SSL 2.0 and 3.0; however, it too has deficiencies: o TLS 1.0 did not have explicit initialization vectors (IVs) in CBC mode. Explicit IVs were added to TLS 1.1 to prevent the attacks described in [CBC-Issues]. o Some TLS 1.0 implementations incorrectly fail the handshake when there is data (e.g., extensions) following the ClientHello that they do not understand. This was a problem addressed in [RFC5746]. 5. Use TLS 1.0 and Later Versions Because of the deficiencies noted in the previous sections, TLS implementations MUST NOT support SSL 3.0 or earlier. Support for TLS 1.2 is RECOMMENDED. To ensure interoperability, TLS implementations MUST support TLS 1.0 and SHOULD support TLS 1.1 and 1.2. That is, TLS clients and servers MUST NOT request, offer, or use SSL 2.0 or SSL 3.0 and SHOULD offer TLS 1.2. The specific changes to [TLS1.2] 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. Turner and Polk Expires December 25, 2010 [Page 3]
Internet-Draft Prohibiting SSL May 2010 o TLS clients MUST NOT use {3,0} in ClientHello.client_version. o TLS servers MUST NOT accept {3,0} in ClientHello.client_version; it MUST send a "protocol_version" alert message and close the connection. o TLS clients SHOULD offer {3,3} in ClientHello.client_version. 6. IANA Considerations None. 7. Security Considerations The security considerations of [TLS1.0] [TLS1.1] [TLS1.2] apply; no new security considerations are introduced by this document. Since all recent TLS implementations support at least TLS 1.0/SSL 3.1, the risk of denial of service from the lack of interoperability with earlier versions is considered minimal. 8. Acknowledgements The idea for this document was inspired by discussions between Peter Saint Andre, Simon Josefsson, and others on the XMPP mailing list. 9. References 9.1. Normative References [TLS1.0] Dierks, T. and C. Allen, "The Transport Layer Security (TLS) Protocol Version 1.0", RFC 2246, January 1999. [TLS1.1] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4246, April 2006. [TLS1.2] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [WORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Turner and Polk Expires December 25, 2010 [Page 4]
Internet-Draft Prohibiting SSL May 2010 9.2. Informative References [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [RFC5746] Rescorla, E., Ray, M., Dispensa, S., Oskov, N., "Transport Layer Security (TLS) Renegotiation Indication Extension", RFC 5746, February 2010. [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications Corp., Feb 9, 1995. [SSL3] A. Freier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol", Netscape Communications Corp., Nov 18, 1996. [CBC-Issues] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures", http://www.openssl.org/~bodo/tls-cbc.txt. Authors' Addresses Sean Turner IECA, Inc. 3057 Nutley Street, Suite 106 Fairfax, VA 22031 USA EMail: turners@ieca.com Tim Polk National Institute of Standards and Technology 100 Bureau Drive, Mail Stop 8930 Gaithersburg, MD 20899-8930 USA EMail: tim.polk@nist.gov Turner and Polk Expires December 25, 2010 [Page 5]
