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Versions: (draft-sheffer-tls-bcp) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7525

UTA                                                           Y. Sheffer
Internet-Draft                                                  Porticor
Intended status: Best Current Practice                           R. Holz
Expires: December 26, 2014                                           TUM
                                                          P. Saint-Andre
                                                           June 24, 2014

             Recommendations for Secure Use of TLS and DTLS


   Transport Layer Security (TLS) and Datagram Transport Security Layer
   (DTLS) are widely used to protect data exchanged over application
   protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP.  Over the
   last few years, several serious attacks on TLS have emerged,
   including attacks on its most commonly used cipher suites and modes
   of operation.  This document provides recommendations for improving
   the security of both software implementations and deployed services
   that use TLS and DTLS.

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 December 26, 2014.

Copyright Notice

   Copyright (c) 2014 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

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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Protocol Versions . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Fallback to SSL . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Always Use TLS  . . . . . . . . . . . . . . . . . . . . .   4
     3.4.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . .   5
     3.5.  Public Key Length . . . . . . . . . . . . . . . . . . . .   6
     3.6.  Compression . . . . . . . . . . . . . . . . . . . . . . .   7
     3.7.  Session Resumption  . . . . . . . . . . . . . . . . . . .   7
     3.8.  Renegotiation . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Detailed Guidelines . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Cipher Suite Negotiation Details  . . . . . . . . . . . .   7
     4.2.  Alternative Cipher Suites . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     6.1.  AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .   9
     6.3.  Certificate Revocation  . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Appendix: Change Log . . . . . . . . . . . . . . . .  13
     A.1.  draft-ietf-tls-bcp-01 . . . . . . . . . . . . . . . . . .  13
     A.2.  draft-ietf-tls-bcp-00 . . . . . . . . . . . . . . . . . .  13
     A.3.  draft-sheffer-tls-bcp-02  . . . . . . . . . . . . . . . .  13
     A.4.  draft-sheffer-tls-bcp-01  . . . . . . . . . . . . . . . .  13
     A.5.  draft-sheffer-tls-bcp-00  . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Transport Layer Security (TLS) and Datagram Transport Security Layer
   (DTLS) are widely used to protect data exchanged over application
   protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP.  Over the
   last few years, several serious attacks on TLS have emerged,
   including attacks on its most commonly used cipher suites and modes
   of operation.  For instance, both AES-CBC and RC4, which together

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   comprise most current usage, have been attacked in the context of
   TLS.  A companion document [I-D.sheffer-uta-tls-attacks] provides
   detailed information about these attacks.

   Because of these attacks, those who implement and deploy TLS and DTLS
   need updated guidance on how TLS can be used securely.  Note that
   this document provides guidance for deployed services, as well as
   software implementations.  In fact, this document calls for the
   deployment of algorithms that are widely implemented but not yet
   widely deployed.

   The recommendations herein take into consideration the security of
   various mechanisms, their technical maturity and interoperability,
   and their prevalence in implementatios at the time of writing.  These
   recommendations apply 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 this document.

   These are minimum recommendations for the general use of TLS.
   Individual specifications may have stricter requirements related to
   one or more aspects of the protocol, and based on their particular
   circumstances.  When that is the case, implementers MUST adhere to
   those stricter requirements.

   Community knowledge about the strength of various algorithms and
   feasible attacks can change quickly, and experience shows that a
   security BCP is a point-in-time statement.  Readers are advised to
   seek out any errata or updates that apply to this document.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Recommendations

3.1.  Protocol Versions

   It is important both to stop using old, less secure versions of SSL/
   TLS and to start using modern, more secure versions.  Therefore:

   o  Implementations MUST NOT negotiate SSL version 2.

      Rationale: SSLv2 has serious security vulnerabilities [RFC6176].

   o  Implementations SHOULD NOT negotiate SSL version 3.

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      Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
      plugged some significant security holes, but did not support
      strong cipher suites.

   o  Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246].

      Rationale: TLS 1.0 (published in 1999) includes a way to downgrade
      the connection to SSLv3 and does not support more modern, strong
      cipher suites.

   o  Implementations MAY negotiate TLS version 1.1 [RFC4346].

      Rationale: TLS 1.1 (published in 2006) prevents downgrade attacks
      to SSL, but does not support certain stronger cipher suites.

   o  Implementations MUST support, and prefer to negotiate, TLS version
      1.2 [RFC5246].

      Rationale: Several stronger cipher suites are available only with
      TLS 1.2 (published in 2008).

   As of the date of this writing, the latest version of TLS is 1.2.
   When TLS is updated to a newer version, this document will be updated
   to recommend support for the latest version.  If this document is not
   updated in a timely manner, it can be assumed that support for the
   latest version of TLS is recommended.

3.2.  Fallback to SSL

   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 only about 3% of the current web server population.
   Therefore, by default clients SHOULD NOT fall back from TLS to SSLv3.

3.3.  Always Use TLS

   Combining unprotected and TLS-protected communication opens the way
   to SSL Stripping and similar attacks.  In cases where an application
   protocol allows implementations or deployments a choice between
   strict TLS configuration and dynamic upgrade from unencrypted to TLS-
   protected traffic (such as STARTTLS), clients and servers SHOULD
   prefer strict TLS configuration.

   When applicable, Web servers SHOULD advertise that they are willing
   to accept TLS-only clients, using the HTTP Strict Transport Security
   (HSTS) header [RFC6797].

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3.4.  Cipher Suites

   It is important both to stop using old, insecure cipher suites and to
   start using modern, more secure cipher suites.  Therefore:

   o  Implementations MUST NOT negotiate the NULL cipher suites.

      Rationale: The NULL cipher suites offer no encryption whatsoever
      and thus are completely insecure.

   o  Implementations MUST NOT negotiate RC4 cipher suites

      Rationale: The RC4 stream cipher has a variety of cryptographic
      weaknesses, as documented in [I-D.popov-tls-prohibiting-rc4].

   o  Implementations MUST NOT negotiate cipher suites offering only so-
      called "export-level" encryption (including algorithms with 40
      bits or 56 bits of security).

      Rationale: These cipher suites are deliberately "dumbed down" and
      are very easy to break.

   o  Implementations SHOULD NOT negotiate cipher suites that use
      algorithms offering less than 128 bits of security.  Note that
      some legacy cipher suites (e.g. 168-bit 3DES) have an effective
      key length which is smaller than their nominal key length.  Such
      cipher suites should be evaluated accoring to their effective key

      Rationale: Although these cipher suites are not actively subject
      to breakage, their useful life is short enough that stronger
      cipher suites are desirable.

   o  Implementations SHOULD prefer cipher suites that use algorithms
      with at least 128 (and, if possible, 256) bits of security.

      Rationale: Although the useful life of such cipher suites is
      unknown, it is probably at least several years for the 128-bit
      ciphers and "until the next fundamental technology breakthrough"
      for 256-bit ciphers.

   o  Implementations MUST support, and SHOULD prefer to negotiate,
      cipher suites offering forward secrecy, such as those in the
      "EDH", "DHE", and "ECDHE" families.

      Rationale: Forward secrecy (sometimes called "perfect forward
      secrecy") prevents the recovery of information that was encrypted

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      with older session keys, thus limiting the amount of time during
      which attacks can be successful.

   Given the foregoing considerations, implementation of the following
   cipher suites is RECOMMENDED (see [RFC5289] for details):





   We suggest that TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 be preferred in

   Unfortunately, those cipher suites are supported only in TLS 1.2
   since they are authenticated encryption (AEAD) algorithms [RFC5116].
   A future version of this document might recommend cipher suites for
   earlier versions of TLS.

   [RFC4492] allows clients and servers to negotiate ECDH parameters
   (curves).  Clients and servers SHOULD prefer verifiably random curves
   (specifically Brainpool P-256, brainpoolp256r1 [RFC7027]), and fall
   back to the commonly used NIST P-256 (secp256r1) curve [RFC4492].  In
   addition, clients SHOULD send an ec_point_formats extension with a
   single element, "uncompressed".

3.5.  Public Key Length

   Because Diffie-Hellman keys of 1024 bits are estimated to be roughly
   equivalent to 80-bit symmetric keys, it is better to use longer keys
   for the "DH" family of cipher suites.  Unfortunately, some existing
   software cannot handle (or cannot easily handle) key lengths greater
   than 1024 bits.  The most common workaround for these systems is to
   prefer the "ECDHE" family of cipher suites instead of the "DH"
   family, then use longer keys.  Key lengths of at least 2048 bits are
   RECOMMENDED, since they are estimated to be roughly equivalent to
   112-bit symmetric keys and might be sufficient for at least the next
   10 years.

   In addition to 2048-bit server certificates, the use of SHA-256
   fingerprints is RECOMMENDED (see [CAB-Baseline] for more details).
   Clients SHOULD indicate to servers that they request SHA-256, by
   using the "Signature Algorithms" extension defined in TLS 1.2.

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   Note: The foregoing recommendations are preliminary and will likely
   be corrected and enhanced in a future version of this document.

3.6.  Compression

   Implementations and deployments SHOULD disable TLS-level compression
   ([RFC5246], Sec. 6.2.2), because it has been subject to security

3.7.  Session Resumption

   If TLS session resumption is used, care ought to be taken to do so
   safely.  In particular, the resumption information (either session
   IDs [RFC5246] or session tickets [RFC5077]) needs to be authenticated
   and encrypted to prevent modification or eavesdropping by an
   attacker.  For session tickets, a strong cipher suite MUST be used
   when encrypting the ticket (as least as strong as the main TLS cipher
   suite); ticket keys MUST be changed regularly, e.g. once every week,
   so as not to negate the effect of forward secrecy.  Session ticket
   validity SHOULD be limited to a reasonable duration (e.g. 1 day), so
   as not to negate the benefits of forward secrecy.

3.8.  Renegotiation

   Where handshake renegotiation is implemented, both clients and
   servers MUST implement the renegotiation_info extension, as defined
   in [RFC5746].

4.  Detailed Guidelines

   The following sections provide more detailed information about the
   recommendations listed above.

4.1.  Cipher Suite Negotiation Details

   Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 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.

   Servers SHOULD 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

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   cipher suite unless there are compelling reasons (e.g., seriously
   degraded 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_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,

4.2.  Alternative Cipher Suites

   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
   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 [RFC5289]

   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.

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5.  IANA Considerations

   This document requests no actions of IANA.

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.  Forward Secrecy

   Forward secrecy (also often called Perfect Forward Secrecy or "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

   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

   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

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   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

   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.

6.3.  Certificate Revocation

   Unfortunately there is currently no effective, Internet-scale
   mechanism to affect certificate revocation:

   o  Certificate Revocation Lists (CRLs) are non-scalable and therefore
      often unused.

   o  The On-Line Certification Status Prorocol (OCSP) presents both
      scaling and privacy issues when used for heavy traffic Web
      servers.  In addition, clients typically "soft-fail", meaning they
      do not abort the TLS connection if the OCSP server does not

   o  OCSP stapling (Sec. 8 of [RFC6066]) resolves the operational
      issues with OCSP, but is still ineffective in the presence of a
      MITM atacker, because they can simply ignore the client's request.

   o  Proprietary mechanisms that embed revocation lists in the Web
      browser's configuration database cannot scale beyond a small
      number of the most heavily used Web servers.

   The current consensus appears to be that OCSP stapling, combined with
   a "must staple" mechanism similar to HSTS, would finally resolve this
   problem.  But such a mechanism has not been standardized yet.

7.  Acknowledgements

   We would like to thank Stephen Farrell, Simon Josefsson, Johannes
   Merkle, Yoav Nir, Kenny Paterson, Patrick Pelletier, Tom Ritter 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.

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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.

   [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.

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

   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with
              SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              August 2008.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, February 2010.

   [RFC6176]  Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
              (SSL) Version 2.0", RFC 6176, March 2011.

   [RFC7027]  Merkle, J. and M. Lochter, "Elliptic Curve Cryptography
              (ECC) Brainpool Curves for Transport Layer Security
              (TLS)", RFC 7027, October 2013.

8.2.  Informative References

              "Baseline Requirements for the Issuance and Management of
              Publicly-Trusted Certificates Version 1.1.6", 2013,

              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.

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              Popov, A., "Prohibiting RC4 Cipher Suites", draft-popov-
              tls-prohibiting-rc4-02 (work in progress), April 2014.

              Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Current Attacks on TLS and DTLS", draft-sheffer-uta-tls-
              attacks-00 (work in progress), February 2014.

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

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

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

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6101]  Freier, A., Karlton, P., and P. Kocher, "The Secure
              Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
              August 2011.

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

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797, November 2012.

              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.

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Appendix A.  Appendix: Change Log

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

A.1.  draft-ietf-tls-bcp-01

   o  Clarified that specific TLS-using protocols may have stricter

   o  Changed TLS 1.0 from MAY to SHUOLD NOT.

   o  Added discussion of "optional TLS" and HSTS.

   o  Recommended use of the Signature Algorithm and Renegotiation Info

   o  Use of a strong cipher for a resumption ticket: changed SHOULD to

   o  Added an informational discussion of certificate revocation, but
      no recommendations.

A.2.  draft-ietf-tls-bcp-00

   o  Initial WG version, with only updated references.

A.3.  draft-sheffer-tls-bcp-02

   o  Reorganized the content to focus on recommendations.

   o  Moved description of attacks to a separate document (draft-

   o  Strengthened recommendations regarding session resumption.

A.4.  draft-sheffer-tls-bcp-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

   o  Recommendation to avoid fallback to SSLv3.

   o  Initial information about browser support - more still needed!

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   o  More clarity on compression.

   o  Client can offer stronger cipher suites.

   o  Discussion of the regular TLS mandatory cipher suite.

A.5.  draft-sheffer-tls-bcp-00

   o  Initial version.

Authors' Addresses

   Yaron Sheffer
   29 HaHarash St.
   Hod HaSharon  4501303

   Email: yaronf.ietf@gmail.com

   Ralph Holz
   Technische Universitaet Muenchen
   Boltzmannstr. 3
   Garching  85748

   Email: holz@net.in.tum.de

   Peter Saint-Andre

   Email: ietf@stpeter.im

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