TLS Working Group                                             P. Gutmann
Internet-Draft                                    University of Auckland
Intended status: Standards Track                          March 16, 2016
Expires: September 17, 2016


                   TLS 1.2 Long-term Support Profile
                      draft-gutmann-tls-lts-00.txt

Abstract

   This document specifies a profile of TLS 1.2 for long-term support,
   one that represents what's already deployed for TLS 1.2 but with the
   security holes and bugs fixed.  This represents a stable, known-good
   profile that can be deployed now to systems that can't can't roll out
   patches every month or two when the next attack on TLS is published.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on September 17, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
   2.  TLS-LTS . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The TLS-LTS Profile . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Encryption/Authentication . . . . . . . . . . . . . . . .   3
     3.2.  Message Formats . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Miscellaneous . . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Implementation Issues . . . . . . . . . . . . . . . . . .   6
     3.5.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   TLS [2] and DTLS [4], by nature of their enormous complexity and the
   inclusion of large amounts of legacy material, contain numerous
   security issues that have been known to be a problem for many years
   and that keep coming up again and again in attacks (there are simply
   too many of these to provide references for, and in any case more
   will have been published by the time you read this).  This document
   presents a minimal, known-good profile of mechanisms that defend
   against all currently-known weaknesses in TLS, that would have
   defended against them ten years ago, and that have a good chance of
   defending against them ten years from now, providing the long-term
   stability that's required by many systems in the field.

   In particular it takes inspiration from numerous published analyses
   of TLS [7] [8] [9] [10] [11] [12] [13] [14] to select a standard
   interoperable feature set that provides the best chance of long-term
   stability and resistance to attack.  This is intended for use in
   systems that need to run in a fixed configuration for a long time
   after they're deployed, with little or no ability to roll out patches
   every month or two when the next attack on TLS is published.

   Unlike the full TLS 1.2, TLS-LTS is not meant to be all things to all
   people.  It represents a fixed, safe solution that's appropriate for
   users who require a simple, secure, and long-term stable means of
   getting data from A to B.  This represents the majority of the non-
   browser use of TLS, particularly in the embedded systems that are
   most in need of a long-term stable protocol profile.



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1.1.  Conventions Used in This Document

   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 [1].

2.  TLS-LTS

   The use of TLS-LTS is negotiated via TLS/DTLS extensions as defined
   in TLS Extensions [3].  On connecting, the client includes the
   tls_lts extension in its client_hello if it wishes to use the TLS-LTS
   profile.  If the server is capable of meeting this requirement, it
   responds with an tls_lts in its server_hello.  The "extension_type"
   value for this extension SHALL be TBD (0xTBD) and the
   "extension_data" field of this extension SHALL be empty.  The client
   and server MUST NOT use the TLS-LTS profile unless both sides have
   successfully exchanged tls_lts extensions.

2.1.  Rationale

   The use of extensions precludes use with SSL 3.0, but then it's
   likely that anything still using this nearly two decades-old protocol
   will be vulnerable to any number of other attacks anyway, so there
   seems little point in bending over backwards to accomodate SSL 3.0.

3.  The TLS-LTS Profile

3.1.  Encryption/Authentication

   TLS-LTS restricts the more or less unlimited TLS 1.2 with its more
   than three hundred cipher suites, over forty ECC parameter sets, and
   zoo of supplementary algorithms, parameters, and parameter formats,
   to just two, one traditional one with DHE + AES-CBC + HMAC-SHA-256 +
   RSA-SHA-256/PSK and one ECC one with ECDHE-P256 + AES-GCM + HMAC-
   SHA-256 + ECDSA-P256-SHA-256/PSK with uncompressed points:

   o  TLS-LTS implementations MUST support
      TLS_DHE_RSA_WITH_AES_128_CBC_SHA256,
      TLS_DHE_PSK_WITH_AES_128_CBC_SHA256,
      TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
      TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256.  As the use of SHA-256 with
      RSA or ECDSA is implicit in TLS-LTS, there is no need to signal it
      via the signature_algorithms extension.  In addition the almost
      universally-ignored requirement that all certificates provided by
      the server must be signed by the algorithm(s) specified in the
      signature_algorithms extension is removed both implicitly by not
      sending the extension and explicitly by removing this requirement.
      As the use of P256 with uncompressed points is implicit in TLS-



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      LTS, there is no need to signal it via the elliptic_curves and
      ec_point_formats extensions.

       [Question: There's a gap in the suites with
        TLS_ECDHE_PSK_WITH_AES_128_GCM_SHA256 missing, although it's
        present for all manner of non-AES ciphers, should we specify
        TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256 or fill the current hole
        with TLS_ECDHE_PSK_WITH_AES_128_GCM_SHA256?].

   TLS-LTS only permits encrypt-then-MAC, not MAC-then-encrypt, fixing
   20 years of attacks on this mechanism:

   o  TLS-LTS implementations MUST implement encrypt-then-MAC [5] rather
      than the earlier MAC-then-encrypt.  As the use of encrypt-then-MAC
      is implicit in TLS-LTS, there is no need to signal it via the
      encrypt_then_mac extension.

   TLS-LTS drops the IPsec cargo-cult MAC truncation, which serves no
   obvious purpose and leads to security concerns:

   o  TLS-LTS implementations MUST use full-length MAC values (for
      example 256 bits for SHA-256).  In particular MAC values MUST NOT
      be truncated to 96 bits/12 bytes, removing the verify_data_length
      constraint in the Finished message.

   TLS-LTS recommends that implementations take measures to protect
   against side- channel attacks:

   o  Implementations SHOULD take steps to protect against timing
      attacks, for example by using constant-time implementations of
      algorithms and by using blinding for non-randomised algorithms
      like RSA.

   o  Implementations SHOULD take steps to protect against fault
      attacks, in particular for the extremely brittle ECC algorithms
      whose typical failure mode if a fault occurs is to leak the
      private key.  One simple countermeasure is to use the public key
      to verify any signatures generated before they are sent over the
      wire.

       [Question: Should the PRF be replaced with HKDF?  There's no
        pressing need for this, but it could be part of the general
        cleanup].








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3.2.  Message Formats

   TLS-LTS sends the full set of DH parameters, X9.42/FIPS 186 style,
   not p and g only, PKCS #3 style.  This allows verification of the DH
   parameters, which the current format doesn't allow:

   o  TLS-LTS implementations MUST send the DH domain parameters as { p,
      g, q } rather than { p, g }.  This makes the ServerDHParams field:

   struct {
       opaque dh_p<1..2^16-1>;
       opaque dh_g<1..2^16-1>;
       opaque dh_q<1..2^16-1>;
       opaque dh_Ys<1..2^16-1>;
       } ServerDHParams;     /* Ephemeral DH parameters */

      The domain parameters MUST be verified as specified in FIPS 186
      [6].

   TLS-LTS adds a hash of the domain parameters into the master secret
   to protect against the use of manipulated curves/domain parameters:

   o  TLS-LTS implementations MUST include a SHA-256 hash of the EDH or
      ECDH parameters in the master secret computation by concatenating
      the hash to the pre_master_secret value.  In the case of EDH, the
      value that's hashed is the ServerDHParams structure.  In the case
      of ECDH the value that's hashed is the ServerECDHParams structure.
      This means that the master_secret computation becomes:

   master_secret = PRF(pre_master_secret || param_hash, "master secret",
                       ClientHello.random + ServerHello.random)
                       [0..47];

3.3.  Miscellaneous

   TLS-LTS drops the need to send the current time in the random data,
   which serves no obvious purpose and leaks the client/server's time to
   attackers:

   o  TLS-LTS implementations SHOULD NOT include the time in the Client/
      ServerHello random data.  The data SHOULD consists entirely of
      random bytes.

   TLS-LTS drops compression and rehandshake, which have led to a number
   of attacks:

   o  TLS-LTS implementations MUST NOT implement compression or
      rehandshake.



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3.4.  Implementation Issues

   TLS-LTS requires that RSA signature verification be done as encode-
   then-compare, which fixes all known padding-manipulation issues:

   o  TLS-LTS implementations MUST verify RSA signatures by using
      encode-then-compare, meaning that they encode the expected
      signature result and perform a constant-time compare against the
      recovered signature data.

   The TLS protocol has historically and somewhat arbitrarily been
   described as a state machine, which has led to a number of
   implementation flaws when state transitions weren't very carefully
   considered and enforced.  A more logical means of representing the
   protocol is as a ladder diagram, which hardcodes the transitions into
   the diagram and removes the need to juggle a large amount of state:

   o  Implementations SHOULD consider representing/implementing the
      protocol as a ladder diagram rather than a state machine, since
      the state-diagram form has led to a number of implementation
      errors in the past which are avoided through the use of the ladder
      diagram form.

3.5.  Rationale

   A question that may be asked at this point is, why not use TLS 1.3
   instead of creating a secure profile of TLS 1.2?  The reason is that
   TLS 1.3 rolls back the 20 years of experience that we have with all
   the things that can go wrong in TLS and starts again from scratch
   with an almost entirely new protocol based on bleeding-edge/
   experimental ideas, mechanisms, and algorithms.  When SSLv3 was
   introduced, it used ideas that were 10-20 years old (DH, RSA, DES,
   and so on were all long-established algorithms, only SHA-1 was
   relatively new).  These were mature algorithms with large amounts of
   of research published on them, and yet we're still fixing issues with
   them 20 years later (the DH algorithm was published in 1976, SSLv3
   dates from 1996, and the latest DH issue, Logjam, dates from 2015.

   With TLS 1.3 we currently have zero implementation and deployment
   experience, which means that we're likely to have another 10-20 years
   of patching holes and fixing protocol and implementation problems
   ahead of us.  It's for this reason that this profile uses the decades
   of experience we have with SSL and TLS to simplify TLS 1.2 into a
   known-good subset that leverages about 15 years of analysis and 20
   years of implementation experience, rather than betting on what's
   almost an entirely new protocol based on bleeding-edge/experimental
   ideas, mechanisms, and algorithms.  The intent is to create a long-
   term stable protocol profile that can be deployed once, not deployed



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   and then patched, updated, and fixed constantly for the lifetime of
   the equipment that it's used with.

4.  Security Considerations

   This document defines a minimal, known-good subset of TLS 1.2 that
   attempts to address all known weaknesses in the protocol, mostly by
   simply removing known-insecure mechanisms but also by updating the
   ones that remain to take advantage of many years of security research
   and implementation experience.

5.  IANA Considerations

   IANA has added the extension code point TBD (0xTBD) for the tls_lts
   extension to the TLS ExtensionType values registry as specified in
   TLS [2].

6.  Acknowledgements

   The author would like to thank the members of the TLS mailing list
   for their feedback on this document.

7.  References

7.1.  Normative References

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

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

   [3]        Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions", RFC 6066, January 2011.

   [4]        Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [5]        Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, September 2014.

   [6]        "Digital Signature Standard (DSS)", FIPS 186, July 2013.








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7.2.  Informative References

   [7]        Bhargavan, K., Fournet, C., Kohlweiss, M., Pironti, A.,
              Strub, P., and S. Zanella-Beguelin, "Proving the TLS
              handshake secure (as is)", Springer-Verlag LNCS 8617,
              August 2014.

   [8]        Brzuska, C., Fischlin, M., Smart, N., Warinschi, B., and
              S. Williams, "Less is more: relaxed yet compatible
              security notions for key exchange", IACR ePrint
              archive 2012/242, April 2012.

   [9]        Dowling, B. and D. Stebila, "Modelling ciphersuite and
              version negotiation in the TLS protocol", Springer-Verlag
              LNCS 9144, June 2015.

   [10]       Firing, T., "Analysis of the Transport Layer Security
              protocol", June 2010.

   [11]       Gajek, S., Manulis, M., Pereira, O., Sadeghi, A., and J.
              Schwenk, "Universally Composable Security Analysis of
              TLS", Springer-Verlag LNCS 5324, November 2008.

   [12]       Giesen, F., Kohlar, F., and D. Stebila, "On the security
              of TLS renegotiation", ACM CCS 2013, November 2013.

   [13]       Jager, T., Kohlar, F., Schaege, S., and J. Schwenk, "On
              the security of TLS-DHE in the standard model", Springer-
              Verlag LNCS 7417, August 2012.

   [14]       Krawczyk, H., Paterson, K., and H. Wee, "On the security
              of the TLS protocol", Springer-Verlag LNCS 8042, August
              2013.

   [15]       Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", RFC XXXX, November 2013.

Author's Address

   Peter Gutmann
   University of Auckland
   Department of Computer Science
   University of Auckland
   New Zealand

   Email: pgut001@cs.auckland.ac.nz




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