Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)
RFC 7919

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Internet Engineering Task Force (IETF)                        D. Gillmor
Request for Comments: 7919                                          ACLU
Updates: 2246, 4346, 4492, 5246                              August 2016
Category: Standards Track
ISSN: 2070-1721

      Negotiated Finite Field Diffie-Hellman Ephemeral Parameters
                   for Transport Layer Security (TLS)

Abstract

   Traditional finite-field-based Diffie-Hellman (DH) key exchange
   during the Transport Layer Security (TLS) handshake suffers from a
   number of security, interoperability, and efficiency shortcomings.
   These shortcomings arise from lack of clarity about which DH group
   parameters TLS servers should offer and clients should accept.  This
   document offers a solution to these shortcomings for compatible peers
   by using a section of the TLS "Supported Groups Registry" (renamed
   from "EC Named Curve Registry" by this document) to establish common
   finite field DH parameters with known structure and a mechanism for
   peers to negotiate support for these groups.

   This document updates TLS versions 1.0 (RFC 2246), 1.1 (RFC 4346),
   and 1.2 (RFC 5246), as well as the TLS Elliptic Curve Cryptography
   (ECC) extensions (RFC 4492).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7919.

Gillmor                      Standards Track                    [Page 1]
RFC 7919                Negotiated FFDHE for TLS             August 2016

Copyright Notice

   Copyright (c) 2016 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
   described in the Simplified BSD License.

Gillmor                      Standards Track                    [Page 2]
RFC 7919                Negotiated FFDHE for TLS             August 2016

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.2.  Vocabulary  . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Named Group Overview  . . . . . . . . . . . . . . . . . . . .   5
   3.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Client Local Policy on Custom Groups  . . . . . . . . . .   7
   4.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Optimizations . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Checking the Peer's Public Key  . . . . . . . . . . . . .   9
     5.2.  Short Exponents . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  Table Acceleration  . . . . . . . . . . . . . . . . . . .  10
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .  10
     6.1.  Preference Ordering . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     8.1.  Negotiation Resistance to Active Attacks  . . . . . . . .  12
     8.2.  Group Strength Considerations . . . . . . . . . . . . . .  13
     8.3.  Finite Field DHE Only . . . . . . . . . . . . . . . . . .  13
     8.4.  Deprecating Weak Groups . . . . . . . . . . . . . . . . .  14
     8.5.  Choice of Groups  . . . . . . . . . . . . . . . . . . . .  14
     8.6.  Timing Attacks  . . . . . . . . . . . . . . . . . . . . .  14
     8.7.  Replay Attacks from Non-negotiated FFDHE  . . . . . . . .  15
     8.8.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .  15
     8.9.  False Start . . . . . . . . . . . . . . . . . . . . . . .  15
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
     9.1.  Client Fingerprinting . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     10.2.  Informative References . . . . . . . . . . . . . . . . .  17