[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 draft-ietf-uta-tls-bcp

tls                                                           Y. Sheffer
Internet-Draft                                                  Porticor
Intended status: BCP                                             R. Holz
Expires: March 24, 2014                                              TUM
                                                      September 20, 2013


             Recommendations for Secure Use of TLS and DTLS
                        draft-sheffer-tls-bcp-01

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on March 24, 2014.

Copyright Notice

   Copyright (c) 2013 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



Sheffer & Holz           Expires March 24, 2014                 [Page 1]


Internet-Draft             TLS Recommendations            September 2013


   described in the Simplified BSD License.


Table of Contents

   1.          Introduction . . . . . . . . . . . . . . . . . . . . .  3
   1.1.        Conventions used in this document  . . . . . . . . . .  3
   2.          Attacks on TLS . . . . . . . . . . . . . . . . . . . .  3
   2.1.        BEAST  . . . . . . . . . . . . . . . . . . . . . . . .  4
   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.        Summary  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.2.        Cipher Suite Negotiation Details . . . . . . . . . . .  6
   4.3.        Downgrade Attacks  . . . . . . . . . . . . . . . . . .  6
   4.4.        Alternatives . . . . . . . . . . . . . . . . . . . . .  6
   5.          Implementation Status  . . . . . . . . . . . . . . . .  7
   6.          Security Considerations  . . . . . . . . . . . . . . .  8
   6.1.        AES-GCM  . . . . . . . . . . . . . . . . . . . . . . .  8
   6.2.        Perfect Forward Secrecy (PFS)  . . . . . . . . . . . .  8
   6.3.        Session Resumption . . . . . . . . . . . . . . . . . .  9
   7.          IANA Considerations  . . . . . . . . . . . . . . . . .  9
   8.          Acknowledgements . . . . . . . . . . . . . . . . . . .  9
   9.          References . . . . . . . . . . . . . . . . . . . . . .  9
   9.1.        Normative References . . . . . . . . . . . . . . . . .  9
   9.2.        Informative References . . . . . . . . . . . . . . . . 10
   Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . . 12
   A.1.        -01  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   A.2.        -00  . . . . . . . . . . . . . . . . . . . . . . . . . 12
               Authors' Addresses . . . . . . . . . . . . . . . . . . 12



















Sheffer & Holz           Expires March 24, 2014                 [Page 2]


Internet-Draft             TLS Recommendations            September 2013


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, as well as for implementers.  In fact the recommendations
   below call for the use of widely implemented algorithms, which are
   not seeing widespread use today.

   Rather than standardizing new mechanisms in TLS, our goal is to
   recommend a few already-specified mechanisms and cipher suites, and
   to encourage the industry to use them in order to improve the overall
   security of TLS-protected network traffic.  When picking these
   mechanisms, we consider their security, their technical maturity and
   interoperability, as well as their prevalence at the time of writing.

   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.

   Our knowledge about the strength of various algorithms and feasible
   attacks can change quickly, and experience shows that a crypto BCP is
   a point-in-time statement more than other BCPs.  Readers are advised
   to seek out any errata or udpates that apply to this document.

1.1.  Conventions used in this document

   [[Are we normative?  Currently we're not and 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
   recommendations.  This is not intended to be an extensive survey of
   TLS's security.

   While there are widely deployed mitigations for some of the attacks



Sheffer & Holz           Expires March 24, 2014                 [Page 3]


Internet-Draft             TLS Recommendations            September 2013


   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.

2.2.  Lucky Thirteen

   A consequence of the MAC-then-encrypt design in all current versions
   of TLS 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],
   [RC4-Attack-AlF]) 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 BREACH attack [BREACH] makes similar use of HAdded TTP-level
   compression, which is much more prevalent than compression at the TLS
   level, to decrypt secret data passed in the HTTP response.

   The former attack can be mitigated by disabling TLS compression, as
   recommended below.  We are not aware of mitigations at the protocol
   level to the latter attack, and so application-level mitigations are
   needed (see [BREACH]).  For example, implementations of HTTP that use
   CSRF tokens will need to randomize them even when the recommendations
   of the current document are adopted.


3.  Selection Criteria

   Given the above attacks, we are proposing that deployers opt for a
   specific cipher suite when negotiating TLS.  We have used the



Sheffer & Holz           Expires March 24, 2014                 [Page 4]


Internet-Draft             TLS Recommendations            September 2013


   following criteria when framing our recommendations:

   o  The cipher suite must be secure in default use, and should not
      require any additional security measures beyond those defined in
      the standard.
   o  The cipher suite must be widely implemented, i.e. available in a
      large percentage of popular cryptographic libraries.
   o  The cipher suite 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 cipher suites that provide client-side privacy and
      perfect forward secrecy, i.e. those that use ephemeral Diffie-
      Hellman.  See Section 6.2 for more details.
   o  As currently specified and implemented, elliptic curve groups are
      preferable over modular DH groups: they are easier and safer to
      use within TLS.
   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 cipher suite.


4.  Recommendations

   Following are recommendations for people implementing and deploying
   client and server-side TLS.

4.1.  Summary

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

   o  TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5829]

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

   We recommend using 2048-bit server certificates, with a SHA-256
   fingerprint.  See [CAB-Baseline] for more details.

   [RFC4492] allows clients and servers to negotiate ECDH parameters
   (curves).  We recommend that clients and servers prefer verifiably
   random curves (specifically Brainpool P-256, brainpoolp256r1
   [I-D.merkle-tls-brainpool]), and fall back to the commonly used NIST
   P-256 (secp256r1) [RFC4492].  In addition, clients should send an
   ec_point_formats extension with a single element, "uncompressed".

   We recommend to always disable TLS-level compression ([RFC5246], Sec.



Sheffer & Holz           Expires March 24, 2014                 [Page 5]


Internet-Draft             TLS Recommendations            September 2013


   6.2.2).

   Finally, we recommend that clients disable fallback to SSLv3 (see
   Section 4.3).

4.2.  Cipher Suite Negotiation Details

   We recommend that clients include the above 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 cipher suite (or a similar but
   stronger one) whenever it is proposed, even if it is not the first
   proposal.

   Both clients and servers should include the "Supported Elliptic
   Curves" extension [RFC4492].

   Clients are of course free to offer stronger cipher suites, e.g.
   using AES-256; when they do, the server should prefer the stronger
   cipher suite unless there are reasons (e.g. performance) to choose
   otherwise.

   Note that other profiles of TLS 1.2 exist that use different cipher
   suites.  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 cipher suites.

   This document is not an application profile standard, in the sense of
   Sec. 9 of [RFC5246].  As a result, clients and servers are still
   required to support the TLS mandatory cipher suite,
   TLS_RSA_WITH_AES_128_CBC_SHA.

4.3.  Downgrade Attacks

   Some client implementations revert to SSLv3 if the server rejected
   higher versions of SSL/TLS.  This fallback can be forced by a MITM
   attacker.  Moreover, IP scans [[reference?]] show that SSLv3-only
   servers amount to about 3% of the current server population.  As a
   result, we recommend that by default, clients should avoid falling
   back to SSLv3.

4.4.  Alternatives

   Elliptic Curves Cryptography is not universally deployed for several
   reasons, including its complexity compared to modular arithmetic and
   longstanding IPR concerns.  On the other hand, there are two related
   issues hindering effective use of modular Diffie-Hellman cipher



Sheffer & Holz           Expires March 24, 2014                 [Page 6]


Internet-Draft             TLS Recommendations            September 2013


   suites in TLS:

   o  There are no protocol mechanisms to negotiate the DH groups or
      parameter lengths supported by client and server.
   o  There are widely deployed client implementations that reject
      received DH parameters, if they are longer than 1024 bits.

   We note that with DHE and ECDHE cipher suites, the TLS master key
   only depends on the Diffie Hellman parameters and not on the strength
   the the RSA certificate; moreover, 1024 bits DH parameters are
   generally considered insufficient at this time.

   Because of the above, we recommend using (in priority order):

   1.  Elliptic Curve DHE with negotiated parameters, as described in
       Section 4.1.
   2.  TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
       Diffie-Hellman parameters.
   3.  The same cipher suite, with 1024-bit parameters.

   With modular ephemeral DH, deployers should carefully evaluate
   interoperability vs. security considerations when configuring their
   TLS endpoints.


5.  Implementation Status

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

   +----------+--------------+---------------------+-------------------+
   | Category |   Software   |    As Of Version    |      Comment      |
   +----------+--------------+---------------------+-------------------+
   |  Library |    OpenSSL   |        1.0.1        |                   |
   |          |    GnuTLS    |                     |                   |
   |          |      NSS     |        3.11.1       |                   |
   |  Browser |   Internet   |   IE8 on Windows 7  |                   |
   |          |   Explorer   |                     |                   |
   |          |    Firefox   |         TBD         |                   |
   |          |    Chrome    | TLS 1.2 and AES-GCM |                   |
   |          |              |  expected in Chrome |                   |
   |          |              |          30         |                   |
   |          |    Safari    |         TBD         |                   |
   |    Web   |    Apache    |          ??         |                   |
   |  server  | (mod_gnutls) |                     |                   |




Sheffer & Holz           Expires March 24, 2014                 [Page 7]


Internet-Draft             TLS Recommendations            September 2013


   |          |    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.  Perfect Forward Secrecy (PFS)

   PFS is a defense against an attacker who records encrypted
   conversations where the session keys are only encrypted with the
   communicating parties' long-term keys.  Should the attacker be able
   to obtain these long-term keys at some point later in the future, he
   will be able to decrypt the session keys and thus the entire
   conversation.  In the context of TLS and DTLS, such compromise of
   long-term keys is not entirely implausible.  It can happen, for
   example, due to:

   o  A client or server being attacked by some other attack vector, and
      the private key retrieved.
   o  A long-term key retrieved from a device that has been sold or
      otherwise decommissioned without prior wiping.
   o  A long-term key used on a device as a default key [Heninger2012].
   o  A key generated by a Trusted Third Party like a CA, and later
      retrieved from it either by extortion or compromise
      [Soghoian2011].
   o  A cryptographic break-through, or the use of asymmetric keys with
      insufficient length [Kleinjung2010].

   PFS ensures in such cases that the session keys cannot be determined
   even by an attacker who obtains the long-term keys some time after
   the conversation.  It also protects against an attacker who is in
   possession of the long-term keys, but remains passive during the
   conversation.

   PFS is generally achieved by using the Diffie-Hellman scheme to
   derive session keys.  The Diffie-Hellman scheme has both parties
   maintain private secrets and send parameters over the network as



Sheffer & Holz           Expires March 24, 2014                 [Page 8]


Internet-Draft             TLS Recommendations            September 2013


   modular powers over certain cyclic groups.  The properties of the so-
   called Discrete Logarithm Problem (DLP) allow to derive the session
   keys without an eavesdropper being able to do so.  There is currently
   no known attack against DLP if sufficiently large parameters are
   chosen.

   Unfortunately, many TLS/DTLS cipher suites were defined that do not
   enable PFS, e.g.  TLS_RSA_WITH_AES_256_CBC_SHA256.  We thus advocate
   strict use of PFS-only ciphers.  These are listed in Section
   Section 4.1.

6.3.  Session Resumption

   TBD, https://www.imperialviolet.org/2013/06/27/botchingpfs.html.


7.  IANA Considerations

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

   This document requires no IANA actions.


8.  Acknowledgements

   We would like to thank Stephen Farrell, Simon Josefsson, Yoav Nir,
   Kenny Paterson, Patrick Pelletier, and Rich Salz for their review.
   Thanks to Brian Smith whose "browser cipher suites" page is a great
   resource.  Finally, Thanks to all others who commented on the TLS and
   other lists and are not mentioned here by name.

   The document was prepared using the lyx2rfc tool, created by Nico
   Williams.


9.  References

9.1.  Normative References

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

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 2006.

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



Sheffer & Holz           Expires March 24, 2014                 [Page 9]


Internet-Draft             TLS Recommendations            September 2013


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

   [RFC5829]  Brown, A., Clemm, G., and J. Reschke, "Link Relation Types
              for Simple Version Navigation between Web Resources",
              RFC 5829, April 2010.

   [I-D.merkle-tls-brainpool]
              Merkle, J. and M. Lochter, "ECC Brainpool Curves for
              Transport Layer Security (TLS)",
              draft-merkle-tls-brainpool-04 (work in progress),
              July 2013.

9.2.  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.



Sheffer & Holz           Expires March 24, 2014                [Page 10]


Internet-Draft             TLS Recommendations            September 2013


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

   [RC4-Attack-AlF]
              AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,
              and J. Schuldt, "On the Security of RC4 in TLS", Usenix
              Security Symposium 2013, 2013, <https://www.usenix.org/
              conference/usenixsecurity13/security-rc4-tls>.

   [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>.

   [CAB-Baseline]
              "Baseline Requirements for the Issuance and Management of
              Publicly-Trusted Certificates Version 1.1.6", 2013,
              <https://www.cabforum.org/documents.html>.

   [TLS-IANA]
              "Transport Layer Security (TLS) Parameters - TLS Cipher
              Suite Registry", <https://www.iana.org/assignments/
              tls-parameters/tls-parameters.xhtml#tls-parameters-4>.

   [Heninger2012]
              Heninger, N., Durumeric, Z., Wustrow, E., and J.
              Halderman, "Mining Your Ps and Qs: Detection of Widespread
              Weak Keys in Network Devices", Usenix Security
              Symposium 2012, 2012.

   [Kleinjung2010]
              Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",
              CRYPTO 10, 2010.

   [Soghoian2011]
              Soghoian, C. and S. Stamm, "Certified lies: Detecting and
              defeating government interception attacks against SSL.",
              Proc. 15th Int. Conf. Financial Cryptography and Data
              Security , 2011.




Sheffer & Holz           Expires March 24, 2014                [Page 11]


Internet-Draft             TLS Recommendations            September 2013


Appendix A.  Appendix: Change Log

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

A.1.  -01

   o  Clarified our motivation in the introduction.
   o  Added a section justifying the need for PFS.
   o  Added recommendations for RSA and DH parameter lengths.  Moved
      from DHE to ECDHE, with a discussion on whether/when DHE is
      appropriate.
   o  Recommendation to avoid fallback to SSLv3.
   o  Initial information about browser support - more still needed!
   o  More clarity on compression.
   o  Client can offer stronger cipher suites.
   o  Discussion of the regular TLS mandatory cipher suite.

A.2.  -00

   o  Initial version.


Authors' Addresses

   Yaron Sheffer
   Porticor
   29 HaHarash St.
   Hod HaSharon  4501303
   Israel

   Email: yaronf.ietf@gmail.com


   Ralph Holz
   Technische Universitaet Muenchen
   Boltzmannstr. 3
   Garching  85748
   Germany

   Email: holz@net.in.tum.de











Sheffer & Holz           Expires March 24, 2014                [Page 12]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/