Deprecating TLSv1.0 and TLSv1.1
draft-ietf-tls-oldversions-deprecate-00

Internet Engineering Task Force                              K. Moriarty
Internet-Draft                                                  Dell EMC
Updates: [[List TBD]] (if approved)                           S. Farrell
Intended status: Standards Track                  Trinity College Dublin
Expires: March 18, 2019                               September 14, 2018

                    Deprecating TLSv1.0 and TLSv1.1
                draft-ietf-tls-oldversions-deprecate-00

Abstract

   This document [if approved] formally deprecates Transport Layer
   Security (TLS) versions 1.0 [RFC2246] and 1.1 [RFC4346] and moves
   these documents to the historic state.  These versions lack support
   for current and recommended cipher suites, and various government and
   industry profiles of applications using TLS now mandate avoiding
   these old TLS versions.  TLSv1.2 has been the recommended version for
   IETF protocols since 2008, providing sufficient time to transition
   away from older versions.  Products having to support older versions
   increase the attack surface unnecessarily and increase opportunities
   for misconfigurations.  Supporting these older versions also requires
   additional effort for library and product maintenance.

   This document updates the backward compatibility sections of TLS RFCs
   [[list TBD]] to prohibit fallback to TLSv1.0 and TLSv1.1.  This
   document also updates RFC 7525.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 18, 2019.

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Copyright Notice

   Copyright (c) 2018 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
   (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Support for Deprecation . . . . . . . . . . . . . . . . . . .   4
   3.  Removing Support  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Web . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Mail  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Operating Systems . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Enterprise Networks . . . . . . . . . . . . . . . . . . .   7
   5.  SHA-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Do Not Use TLSv1.0  . . . . . . . . . . . . . . . . . . . . .   8
   7.  Do Not Use TLSv1.1  . . . . . . . . . . . . . . . . . . . . .   8
   8.  Do Not Use SHA-1 in TLSv1.2 . . . . . . . . . . . . . . . . .   9
   9.  Updates to RFC7525  . . . . . . . . . . . . . . . . . . . . .   9
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     13.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   [[Text in double-square brackets is intended to be fixed as the draft
   evolves.  You've seen that we need to figure out the list of RFCs
   that this'd update in the abstract.  There is a repo for this at:
   https://github.com/tlswg/oldversions-deprecate - PRs (on the xml
   file) are welcome there.]]

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   Transport Layer Security (TLS) versions 1.0 [RFC2246] and 1.1
   [RFC4346] were superceded by TLSv1.2 [RFC5246] in 2008, which has now
   itself been superceded by TLSv1.3 [RFC8446].  It is therefore timely
   to further deprecate these old versions.  The expectation is that
   TLSv1.2 will continue to be used for many years alongside TLSv1.3.

   TLSv1.1 and TLSv1.0 are also actively being deprecated in accordance
   with guidance from government agencies (e.g.  NIST SP 80052r2
   [NIST800-52r2]) and industry consortia such as the Payment Card
   Industry Association (PCI) [PCI-TLS1].

   The primary technical reasons for deprecating these versions include:

   o  They require implementation of older cipher suites that are no
      longer desirable for cryptographic reasons, e.g.  TLSv1.0 makes
      TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA mandatory to implement
   o  Lack of support for current recommended cipher suites, especially
      using AEAD ciphers which are not supported prior to TLS 1.2.
      Note: registry entries for no-longer-desirable ciphersuites remain
      in the registries, but many TLS registries are being updated
      through [RFC8447] which denotes such entries as "not recommended."
   o  Integrity of the handshake depends on SHA-1 hash
   o  Authentication of the peers depends on SHA-1 signatures
   o  Support for four protocol versions increases the likelihood of
      misconfiguration
   o  At least one widely-used library has plans to drop TLSv1.1 and
      TLSv1.0 support in upcoming releases; products using such
      libraries would need to use older versions of the libraries to
      support TLSv1.0 and TLSv1.1, which is clearly undesirable

   Deprecation of these versions is intended to assist developers as
   additional justification to no longer support older TLS versions and
   to migrate to a minimum of TLSv1.2.  Deprecation also assists product
   teams with phasing out support for the older versions to reduce the
   attack surface and the scope of maintenance for protocols in their
   offerings.

   [[This draft is being written now so that the TLS WG chairs can just
   hit the "publication requested" button as soon as there is WG
   consensus to deprecate these ancient versions of TLS.  The authors
   however think that deprecation now is timely.]]

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP

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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Support for Deprecation

   Industry has actively followed guidance provided by NIST and the PCI
   Council to deprecate TLSv1.0 and TLSv1.1 by June 30, 2018.  TLSv1.2
   should remain a minimum baseline for TLS support at this time.

   Specific details on attacks against TLSv1.0 and TLSv1.1 as well as
   their mitigations are provided in NIST SP800-52r2 [NIST800-52r2], RFC
   7457 [RFC7457]  and other referenced RFCs.  Although the attacks have
   been mitigated, if support is dropped for future library releases for
   these versions, it is unlikely attacks found going forward will be
   mitigated in older library releases.

   NIST for example have provided the following rationale, copied with
   permission from NIST SP800-52r2 [NIST800-52r2], section 1.2 "History
   of TLS" (with references changed for RFC formatting).

      TLS 1.1, specified in [RFC4346], was developed to address
      weaknesses discovered in TLS 1.0, primarily in the areas of
      initialization vector selection and padding error processing.
      Initialization vectors were made explicit to prevent a certain
      class of attacks on the Cipher Block Chaining (CBC) mode of
      operation used by TLS.  The handling of padding errors was altered
      to treat a padding error as a bad message authentication code,
      rather than a decryption failure.  In addition, the TLS 1.1 RFC
      acknowledges attacks on CBC mode that rely on the time to compute
      the message authentication code (MAC).  The TLS 1.1 specification
      states that to defend against such attacks, an implementation must
      process records in the same manner regardless of whether padding
      errors exist.  Further implementation considerations for CBC modes
      (which were not included in RFC4346 [RFC4346]) are discussed in
      Section 3.3.2.

      TLS 1.2, specified in RFC5246 [RFC5246], made several
      cryptographic enhancements, particularly in the area of hash
      functions, with the ability to use or specify the SHA-2 family
      algorithms for hash, MAC, and Pseudorandom Function (PRF)
      computations.  TLS 1.2 also adds authenticated encryption with
      associated data (AEAD) cipher suites.

      TLS 1.3, specified in TLSv1.3 [RFC8446], represents a significant
      change to TLS that aims to address threats that have arisen over
      the years.  Among the changes are a new handshake protocol, a new
      key derivation process that uses the HMAC-based Extract-and-Expand
      Key Derivation Function (HKDF), and the removal of cipher suites

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      that use static RSA or DH key exchanges, the CBC mode of
      operation, or SHA-1.  The list of extensions that can be used with
      TLS 1.3 has been reduced considerably.

   The Canadian government treasury board have mandated that these old
   versions of TLS not be used.  [Canada]

3.  Removing Support

   [[This section can be removed upon publication - or maybe keep it?]]

   Support for TLSv1.0 has been removed by the July 2018 PCI deadline
   from the following standards, products, and services:

   o  3GPP 5G
   o  Amazon Elastic Load Balancing [Amazon]
   o  CloudFlare [CloudFlare]
   o  Digicert [Digicert]
   o  GitHub [GIT]
   o  KeyCDN [KeyCDN]
   o  PayPal [paypal]
   o  Stripe [stripe]
   o  [[Numerous web sites...]]

   Many web sites have taken the action of including the deprecation of
   TLSv1.1 into their plans for deprecating TLSv1.0 for the PCI council
   deadline.  Support for TLSv1.1 has been removed by the July 2018 PCI
   deadline from the following standards, products, and services:

   o  3GPP 5G Release 16
   o  Amazon Elastic Load Balancing [Amazon]
   o  CloudFlare [CloudFlare]
   o  GitHub [GIT]
   o  PayPal [paypal]
   o  Stripe [stripe]
   o  [[Numerous web sites...]]

4.  Usage

   [[This section can be removed upon publication - or maybe keep it?]]

4.1.  Web

   Usage statistics for TLSv1.0 and TLSv1.1 on the public web vary, but
   have been in general very low and declined further with the impending
   PCI deadline to migrate off of TLSv1.0 by June 30, 2018.  As of
   January 2018, [StackExchange] quoted 4 percent of browsers using
   TLSv1.0.

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   The number of websites supporting TLS 1.2 is still growing (+0.4%),
   and has reached 92% according to sslpulse as of June 19, 2018.
   [SSLpulse] Deprecating TLS 1.0 and TLS 1.1 will thus not have a major
   impact on browser or web server implementations.

   Figure 1 presents statistics for use of TLS versions in the web.

+----------------+----------+------+-------+-------+-------+-------+
| Name/Ref       | Date     | SSLv3|TLSv1.0|TLSv1.1|TLSv1.2|TLSv1.3|
+----------------+----------+------+-------+-------+-------+-------+
! Alexa [1]      | 20180226 |    - |   2.0 |  <0.1 |  97.9 |     - |
| Cloudflare [2] | 20180518 |  0.0 |   9.3 |   0.2 |  84.9 |   5.5 |
| Firefox [3]    | 20180709 |    - |   1.0 |     - |  94.0 |   5.0 |
| Chrome [4]     | 20180711 |    - |   0.4 |  <0.1 |    -  |     - |
+----------------+----------+------+-------+-------+-------+-------+
[1] https://scotthelme.co.uk/alexa-top-1-million-analysis-february-2018/
[2] https://www.ietf.org/mail-archive/web/tls/current/msg26578.html
[3] https://www.ietf.org/mail-archive/web/tls/current/msg26575.html
[4] https://www.ietf.org/mail-archive/web/tls/current/msg26620.html

                         Figure 1: Web Statistics

4.2.  Mail

   E-Mail uses TLS for SMTP, submission (port 587), POP/POP3 and IMAP.
   Typically email deployments lag public web deployments in terms of
   the rate of adoption of new TLS versions.  Figure 2 presents
   statistics for use of TLS versions in the email applications.

   +----------------+----------+------+-------+-------+-------+-------+
   | Name/Ref       | Date     | SSLv3|TLSv1.0|TLSv1.1|TLSv1.2|TLSv1.3|
   +----------------+----------+------+-------+-------+-------+-------+
   | Clusters [1]   | 20180316 | <0.1 |  10.6 |  <0.1 |  89.3 |     - |
   | TLSA [2]       | 20180710 |    - |   1.4 |   0.1 |  98.5 |     - |
   | UK-ESP [3]     | 20180710 |    - |  19.9 |  <0.1 |    -  |     - |
   +----------------+----------+------+-------+-------+-------+-------+
   [1] https://eprint.iacr.org/2018/299
   [2] https://www.ietf.org/mail-archive/web/tls/current/msg26603.html
   [3] https://www.ietf.org/mail-archive/web/tls/current/msg26603.html

                         Figure 2: Mail Statistics

4.3.  Operating Systems

   Figure 3 presents statistics for use of TLS versions in operating
   systems.

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   +----------------+----------+------+-------+-------+-------+-------+
   | Name/Ref       | Date     | SSLv3|TLSv1.0|TLSv1.1|TLSv1.2|TLSv1.3|
   +----------------+----------+------+-------+-------+-------+-------+
   | Windows cli [1]| 20180709 |    - | >10.0 |  ~0.3 |    -  |     - |
   | Windows svr [1]| 20180709 |    - |  ~1.5 |  ~0.0 |    -  |     - |
   | Apple [2]      | 20180709 |    - |   0.4 |     - |  99.6 |     - |
   +----------------+----------+------+-------+-------+-------+-------+
   [1] https://www.ietf.org/mail-archive/web/tls/current/msg26577.html
   [2] https://www.ietf.org/mail-archive/web/tls/current/msg26634.html

                   Figure 3: Operating System Statistics

4.4.  Enterprise Networks

   Figure 4 presents statistics for use of TLS versions in the
   enterprise networks.  The tcd.ie numbers below were the result of a
   student project and need further validation.

   +----------------+----------+------+-------+-------+-------+-------+
   | Name/Ref       | Date     | SSLv3|TLSv1.0|TLSv1.1|TLSv1.2|TLSv1.3|
   +----------------+----------+------+-------+-------+-------+-------+
   | tcd.ie [1]     | 20180713 | 18.0 |  35.0 |    0  |  45.0 |     0 |
   +----------------+----------+------+-------+-------+-------+-------+
   [1] https://www.ietf.org/mail-archive/web/tls/current/msg26633.html

                  Figure 4: Enterprise Network Statistics

5.  SHA-1

   The integrity of both TLSv1.0 and TLSv1.1 depends on a running SHA-1
   hash of the exchanged messages.  This makes it possible to perform a
   downgrade attack on the handshake by an attacker able to perform 2^77
   operations, well below the acceptable modern security margin.

   Similarly, the authentication of the handshake depends on signatures
   made using SHA-1 hash or a not stronger concatenation of MD-5 and
   SHA-1 hashes, allowing the attacker to impersonate a server when it
   is able to break the severely weakened SHA-1 hash.

   Neither TLSv1.0 nor TLSv1.1 allow the peers to select a stronger hash
   for signatures in the ServerKeyExchange or CertificateVerify
   messages, making the only upgrade path the use of a newer protocol
   version.

   See [Bhargavan2016] for additional detail.

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6.  Do Not Use TLSv1.0

   TLSv1.0 MUST NOT be used.  Negotiation of TLSv1.0 from any version of
   TLS MUST NOT be permitted.

   Any other version of TLS is more secure then TLSv1.0.  TLSv1.0 can be
   configured to prevent interception, though using the highest version
   available is preferable.

   Pragmatically, clients MUST NOT send a ClientHello with
   ClientHello.client_version set to {03,01}. Similarly, servers MUST
   NOT send a ServerHello with ServerHello.server_version set to
   {03,01}. Any party receiving a Hello message with the protocol
   version set to {03,01} MUST respond with a "protocol_version" alert
   message and close the connection.

   Historically, TLS specifications were not clear on what the record
   layer version number (TLSPlaintext.version) could contain when
   sending ClientHello.  Appendix E of [RFC5246] notes that
   TLSPlaintext.version could be selected to maximize interoperability,
   though no definitive value is identified as ideal.  That guidance is
   still applicable; therefore, TLS servers MUST accept any value
   {03,XX} (including {03,00}) as the record layer version number for
   ClientHello, but they MUST NOT negotiate TLSv1.0.

   [[Text here is derived (or stolen:-) from [RFC7568]]]

7.  Do Not Use TLSv1.1

   TLSv1.1 MUST NOT be used.  Negotiation of TLSv1.1 from any version of
   TLS MUST NOT be permitted.

   Pragmatically, clients MUST NOT send a ClientHello with
   ClientHello.client_version set to {03,02}. Similarly, servers MUST
   NOT send a ServerHello with ServerHello.server_version set to
   {03,02}. Any party receiving a Hello message with the protocol
   version set to {03,02} MUST respond with a "protocol_version" alert
   message and close the connection.

   Any newer version of TLS is more secure then TLSv1.1.  TLSv1.1 can be
   configured to prevent interception, though using the highest version
   available is preferable.  Support for TLSv1.1 is dwindling in
   libraries and will impact security going forward if mitigations for
   attacks cannot be easily addressed and supported in older libraries.

   Historically, TLS specifications were not clear on what the record
   layer version number (TLSPlaintext.version) could contain when
   sending ClientHello.  Appendix E of [RFC5246] notes that

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   TLSPlaintext.version could be selected to maximize interoperability,
   though no definitive value is identified as ideal.  That guidance is
   still applicable; therefore, TLS servers MUST accept any value
   {03,XX} (including {03,00}) as the record layer version number for
   ClientHello, but they MUST NOT negotiate TLSv1.1.

8.  Do Not Use SHA-1 in TLSv1.2

   [[This section was suggested in PR#2 for the pre-WG draft repo by
   Hubert Kario.  We're not clear if the WG would like this draft to
   include this or not, so will ask the TLS WG at the appropriate
   time.]]

   SHA-1 as a signature hash MUST NOT be used.  That means that clients
   MUST send signature_algorithms extension and that extension MUST NOT
   include pairs that include SHA-1 hash.  In particular, values {2, 1},
   {2, 2} and {2, 3} MUST NOT be present in the extension.

   Note: this does not affect cipher suites that use SHA-1 HMAC for data
   integrity as the HMAC construction is still considered secure and
   when they are used in TLSv1.2 SHA-256 is used for handshake
   integrity.

9.  Updates to RFC7525

   [[Since RFC7525 is BCP195, there'll probably be some process-fun to
   do an update of that.  Formally, it may be that this document becomes
   a new part of BCP195 I guess, but we can figure that out with chairs
   and ADs.]]

   This documents updates [RFC7525] Section 3.1.1 changing SHOULD NOT to
   MUST NOT as follows:

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

      Rationale: TLS 1.0 (published in 1999) does not support many
      modern, strong cipher suites.  In addition, TLS 1.0 lacks a per-
      record Initialization Vector (IV) for CBC-based cipher suites and
      does not warn against common padding errors.

   o  Implementations MUST NOT negotiate TLS version 1.1 [RFC4346].

      Rationale: TLS 1.1 (published in 2006) is a security improvement
      over TLS 1.0 but still does not support certain stronger cipher
      suites.

   This documents updates [RFC7525] Section 3.1.2 changing SHOULD NOT to
   MUST NOT as follows:

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   o  Implementations MUST NOT negotiate DTLS version 1.0 [RFC4347].

      Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).

10.  Security Considerations

   This document deprecates two older protocol versions for security
   reasons already described.  The attack surface is reduced when there
   are a smaller number of supported protocols and fallback options are
   removed.

11.  Acknowledgements

   Thanks to those that provided usage data, reviewed and/or improved
   this document, including: David Benjamin, David Black, Viktor
   Dukhovni, Alessandro Ghedini, Jeremy Harris, Russ Housley, Hubert
   Kario, Loganaden Velvindron, Eric Mill, Yoav Nir, Andrei Popov, Eric
   Rescorla, Yaron Sheffer, and Jakub Wilk.

12.  IANA Considerations

   [[This memo includes no request to IANA.]]

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, DOI 10.17487/RFC2246, January 1999,
              <https://www.rfc-editor.org/info/rfc2246>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <https://www.rfc-editor.org/info/rfc4346>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

13.2.  Informative References

   [Amazon]   Amazon, "Amazon Elastic Load Balancing Support Deprecated
              TLSv1.0 and TLSv1.1 https://aws.amazon.com/about-aws/
              whats-new/2017/02/elastic-load-balancing-support-for-tls-
              1-1-and-tls-1-2-pre-defined-security-policies/", 2017.

   [Bhargavan2016]
              Bhargavan, K. and G. Leuren, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH
              https://www.mitls.org/downloads/
              transcript-collisions.pdf", 2016.

   [Canada]   Treasury Board of Canada Secretariat, "Implementing HTTPS
              for Secure Web Connections: Information Technology Policy
              Implementation Notice (ITPIN)", June 2018,
              <https://www.canada.ca/en/treasury-board-
              secretariat/services/information-technology/
              policy-implementation-notices/
              implementing-https-secure-web-connections-itpin.html>.

   [CloudFlare]
              CloudFlare, "CloudFlare Deprecated TLSv1.0 and TLSv1.1
              https://blog.cloudflare.com/deprecating-old-tls-versions-
              on-cloudflare-dashboard-and-api/", 2018.

   [Digicert]
              Digicert, "Deprecating TLS 1.0 and 1.1
              https://www.digicert.com/blog/
              depreciating-tls-1-0-and-1-1/", 2018.

   [GIT]      GitHub, "GitHub Deprecates TLSv1.0 and TLSv1.1
              https://githubengineering.com/crypto-removal-notice/",
              2018.

   [KeyCDN]   KeyCDN, "Deprecating TLS 1.0 and 1.1 Enhancing Security
              for Everyone
              https://www.keycdn.com/blog/deprecating-tls-1-0-and-1-1/",
              2018.

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   [NIST800-52r2]
              National Institute of Standards and Technology, "NIST
              SP800-52r2 https://csrc.nist.gov/CSRC/media/Publications/
              sp/800-52/rev-2/draft/documents/sp800-52r2-draft.pdf",
              2018.

   [paypal]   Paypal, ""TLS1.2 and HTTP/1.1 Upgrade" https://www.paypal-
              notice.com/en/TLS-1.2-and-HTTP1.1-Upgrade/", 2018.

   [PCI-TLS1]
              PCI Security Standards Council, "Migrating from SSL and
              Early TLS https://www.pcisecuritystandards.org/documents/
              Migrating-from-SSL-Early-TLS-Info-Supp-v1_1.pdf", 2016.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
              <https://www.rfc-editor.org/info/rfc4347>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Known Attacks on Transport Layer Security (TLS) and
              Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
              February 2015, <https://www.rfc-editor.org/info/rfc7457>.

   [RFC7568]  Barnes, R., Thomson, M., Pironti, A., and A. Langley,
              "Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
              DOI 10.17487/RFC7568, June 2015,
              <https://www.rfc-editor.org/info/rfc7568>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
              <https://www.rfc-editor.org/info/rfc8447>.

   [SSLpulse]
              SSLpulse - will be deleted before publication, "SSLpulse
              https://www.ssllabs.com/ssl-pulse/", 2018.

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   [StackExchange]
              StackExchange - will be deleted before publication,
              "Stackexchange
              https://security.stackexchange.com/questions/177182/is-
              there-a-list-of-old-browsers-that-only-support-tls-1-0",
              2018.

   [stripe]   Stripe, ""Upgrading to SHA-2 and TLS 1.2"
              https://stripe.com/blog/upgrading-tls", 2018.

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Appendix A.  Change Log

   [[RFC editor: please remove this before publication.]]

   From draft-moriarty-tls-oldversions-diediedie-01 to draft-ietf-tls-
   oldversions-deprecate-00:

   o  I-Ds became RFCs 8446/8447 (old-repo PR#4, for TLS1.3)
   o  Accepted old-repo PR#5 fixing typos

   From draft-moriarty-tls-oldversions-diediedie-00 to draft-moriarty-
   tls-oldversions-diediedie-01:

   o  Added stats sent to list so far
   o  PR's #2,3
   o  a few more references
   o  added section on email

Authors' Addresses

   Kathleen Moriarty
   Dell EMC
   176 South Street
   Hopkinton
   United States

   EMail: Kathleen.Moriarty.ietf@gmail.com

   Stephen Farrell
   Trinity College Dublin
   Dublin  2
   Ireland

   Phone: +353-1-896-2354
   EMail: stephen.farrell@cs.tcd.ie

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