Recommendations for Secure Use of TLS and DTLS
draft-sheffer-tls-bcp-00

Network Working Group                                         Y. Sheffer
Internet-Draft                                                  Porticor
Intended status: BCP                                   September 8, 2013
Expires: March 12, 2014

             Recommendations for Secure Use of TLS and DTLS
                        draft-sheffer-tls-bcp-00

Abstract

   Over the last few years there have been several serious attacks on
   TLS, including attacks on its most commonly used ciphers and modes of
   operation.  This document offers recommendations on securely using
   the TLS and DTLS protocols, given existing standards and
   implementations.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 12, 2014.

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Table of Contents

   1.          Introduction  . . . . . . . . . . . . . . . . . . . . . 3
   1.1.        Conventions used in this document . . . . . . . . . . . 3
   2.          Attacks on TLS  . . . . . . . . . . . . . . . . . . . . 3
   2.1.        BEAST . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.2.        Lucky Thirteen  . . . . . . . . . . . . . . . . . . . . 4
   2.3.        Attacks on RC4  . . . . . . . . . . . . . . . . . . . . 4
   2.4.        Compression Attacks: CRIME and BREACH . . . . . . . . . 4
   3.          Selection Criteria  . . . . . . . . . . . . . . . . . . 4
   4.          Recommendations . . . . . . . . . . . . . . . . . . . . 5
   4.1.        Details . . . . . . . . . . . . . . . . . . . . . . . . 5
   5.          Implementation Status . . . . . . . . . . . . . . . . . 5
   6.          Security Considerations . . . . . . . . . . . . . . . . 6
   6.1.        AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . 6
   6.2.        Downgrade Attacks . . . . . . . . . . . . . . . . . . . 6
   7.          IANA Considerations . . . . . . . . . . . . . . . . . . 6
   8.          References  . . . . . . . . . . . . . . . . . . . . . . 6
   8.1.        Normative References  . . . . . . . . . . . . . . . . . 6
   8.2.        Informative References  . . . . . . . . . . . . . . . . 7
   Appendix A. Appendix: Change Log  . . . . . . . . . . . . . . . . . 8
   A.1.        -00 . . . . . . . . . . . . . . . . . . . . . . . . . . 8
               Author's Address  . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   Over the last few years there have been several major attacks on TLS
   [RFC5246], including attacks on its most commonly used ciphers and
   modes of operation.  Details are given in Section 2, but suffice it
   to say that both AES-CBC and RC4, which together make up for most
   current usage, have been seriously attacked in the context of TLS.

   Given these issues, there is need for IETF guidance on how TLS can be
   used securely.  Unlike most IETF documents, this is guidance for
   deployers rather than for implementers.  In fact the recommendations
   below call for the use of widely implemented algorithms, which are
   not seeing widespread use today.

   This recommendation applies to both TLS and DTLS.  TLS 1.3, when it
   is standardized and deployed in the field, should resolve the current
   vulnerabilities while providing significantly better functionality,
   and will very likely obsolete the current document.

1.1.  Conventions used in this document

   [[Are we normative?  This section might go away.]]

   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 [RFC2119].

2.  Attacks on TLS

   This section lists the attacks that motivated the current
   recommendation.  This is not intended to be an extensive survey of
   TLS's security.

   While there are widely deployed mitigations for some of the attacks
   listed below, we believe that their root causes necessitate a more
   systemic solution.

2.1.  BEAST

   The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation
   of CBC (that is, predictable IV) to decrypt parts of a packet, and
   specifically shows how this can be used to decrypt HTTP cookies when
   run over TLS.

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2.2.  Lucky Thirteen

   A consequence of the MAC-then-encrypt design is the existence of
   padding oracle attacks [Padding-Oracle].  A recent incarnation of
   these attacks is the Lucky Thirteen attack [CBC-Attack], a timing
   side-channel attack that allows the attacker to decrypt arbitrary
   ciphertext.

2.3.  Attacks on RC4

   The RC4 algorithm [RC4] has been used with TLS (and previously, SSL)
   for many years.  Attacks have also been known for a long time, e.g.
   [RC4-Attack-FMS].  But recent attacks [RC4-Attack] have weakened this
   algorithm even more.  See [I-D.popov-tls-prohibiting-rc4] for more
   details.

2.4.  Compression Attacks: CRIME and BREACH

   The CRIME attack [CRIME] allows an active attacker to decrypt
   cyphertext (specifically, cookies) when TLS is used with protocol-
   level compression.  The attack is a consequence of the TLS MAC-then-
   encrypt approach.

   The BREACH attack [BREACH] makes similar use of HTTP-level
   compression which is much more prevalent than compression at the TLS
   level, to decrypt secret data passed in the HTTP response.

   While the former attack can be mitigated by disabling TLS
   compression, we are not aware of mitigations at the protocol level to
   the latter attack, and so application-level mitigations are needed.
   For example, implementations of HTTP that use CSRF tokens will need
   to randomize them even when the recommendations of the current
   document are adopted.

   [[Is it possible to affect some length hiding using TLS 1.2 as
   specified today, i.e. without draft-pironti-tls-length-hiding-01, and
   using available APIs?]]

3.  Selection Criteria

   Given the above attacks, we are proposing that deployers opt for a
   specific ciphersuite when negotiating TLS.  We have used the
   following criteria when framing our recommendations:

   o  The ciphersuite must be secure in default use, and should not
      require any additional security measures beyond those defined in
      the standard.

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   o  The ciphersuite must be widely implemented, i.e. available in a
      large percentage of popular cryptographic libraries.
   o  The ciphersuite must have undergone a significant amount of
      analysis, and the algorithm and mode of operation must both be
      standardized by relevant organizations.
   o  We prefer ciphersuites that provide client-side privacy and
      perfect forward secrecy, i.e. those that use ephemeral Diffie-
      Hellman.
   o  When there are multiple key sizes available, we have chosen the
      current industry standard, 128 bits of strength.  Of course
      deployers are free to opt for a stronger ciphersuite.

4.  Recommendations

   Based on the criteria above, we recommend using as a preferred
   ciphersuite the following:

   o  TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288]

   It is noted that the above ciphersuite is an authenticated encryption
   (AEAD) algorithm [RFC5116], and therefore requires the use of TLS
   1.2.

4.1.  Details

   We recommend that clients include this cipher suite as the first
   proposal to any server, unless they have prior knowledge that the
   server cannot respond to a TLS 1.2 client_hello message.

   We recommend that servers prefer this ciphersuite (or a similar but
   stronger one) whenever it is proposed, even if it is not the first
   proposal.

   Note that other profiles of TLS 1.2 exist that use different
   ciphersuites.  For example, [RFC6460] defines a profile that uses the
   TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
   TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 ciphersuites.

5.  Implementation Status

   Since this document does not propose a new protocol or a new
   ciphersuite, we do not provide a full implementation status, as per
   [RFC6982].  However it is useful to list some known existing
   implementations of the recommended ciphersuite(s).

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   +----------+----------------+--------------+------------------------+
   | Category |    Software    |     As Of    |         Comment        |
   |          |                |    Version   |                        |
   +----------+----------------+--------------+------------------------+
   |  Library |     OpenSSL    |     1.0.1    |                        |
   |          |     GnuTLS     |              |                        |
   |          |       NSS      |    3.11.1    |                        |
   |  Browser |    Internet    |    IE8 on    |                        |
   |          |    Explorer    |   Windows 7  |                        |
   |          |     Firefox    |              |           TBD          |
   |          |     Chrome     |              |           TBD          |
   |          |     Safari     |              |           TBD          |
   |    Web   |     Apache     |      ??      |                        |
   |  server  |  (mod_gnutls)  |              |                        |
   |          |     Apache     |      ??      |                        |
   |          |    (mod_ssl)   |              |                        |
   |          |      Nginx     | 1.0.9, 1.1.6 |  With a recent version |
   |          |                |              |       of OpenSSL       |
   +----------+----------------+--------------+------------------------+

6.  Security Considerations

6.1.  AES-GCM

   Please refer to [RFC5246], Sec. 11 for general security
   considerations when using TLS 1.2, and to [RFC5288], Sec. 6 for
   security considerations that apply specifically to AES-GCM when used
   with TLS.

6.2.  Downgrade Attacks

   [[Do we need to disallow some protocol variants, e.g.  SSL 3.0, so
   that there are no downgrade attacks possible?]]

7.  IANA Considerations

   This document requires no IANA actions.

8.  References

8.1.  Normative References

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

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

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              August 2008.

8.2.  Informative References

   [I-D.popov-tls-prohibiting-rc4]
              Popov, A., "Prohibiting RC4 Cipher Suites",
              draft-popov-tls-prohibiting-rc4-00 (work in progress),
              August 2013.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

   [RFC6460]  Salter, M. and R. Housley, "Suite B Profile for Transport
              Layer Security (TLS)", RFC 6460, January 2012.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              July 2013.

   [CBC-Attack]
              AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
              the TLS and DTLS Record Protocols", IEEE Symposium on
              Security and Privacy , 2013.

   [BEAST]    Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
              2011, <http://packetstormsecurity.com/files/105499/
              Browser-Exploit-Against-SSL-TLS.html>.

   [CRIME]    Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty
              Security Conference 2012, 2012.

   [BREACH]   Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
              2013, <http://breachattack.com/>.

   [RC4]      Schneier, B., "Applied Cryptography: Protocols,
              Algorithms, and Source Code in C, 2nd Ed.", 1996.

   [RC4-Attack-FMS]
              Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
              Key Scheduling Algorithm of RC4", Selected Areas in
              Cryptography , 2001.

   [RC4-Attack]

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              ISOBE, T., OHIGASHI, T., WATANABE, Y., and M. MORII, "Full
              Plaintext Recovery Attack on Broadcast RC4", International
              Workshop on Fast Software Encryption , 2013.

   [Padding-Oracle]
              Vaudenay, S., ""Security Flaws Induced by CBC Padding
              Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
              2002, <http://www.iacr.org/cryptodb/archive/2002/
              EUROCRYPT/2850/2850.pdf>.

Appendix A.  Appendix: Change Log

   Note to RFC Editor: please remove this section before publication.

A.1.  -00

   o  Initial version.

Author's Address

   Yaron Sheffer
   Porticor
   29 HaHarash St.
   Hod HaSharon  4501303
   Israel

   Email: yaronf.ietf@gmail.com

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