Internet Engineering Task Force                               D. Gillmor
Internet-Draft                                                      ACLU
Updates: 4492, 5246, 4346, 2246 (if                       March 28, 2015
         approved)
Intended status: Informational
Expires: September 29, 2015


  Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for TLS
                  draft-ietf-tls-negotiated-ff-dhe-08

Abstract

   Traditional finite-field-based Diffie-Hellman (DH) key exchange
   during the 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 "EC Named Curve Registry" to establish common
   finite-field DH parameters with known structure and a mechanism for
   peers to negotiate support for these groups.

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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   This Internet-Draft will expire on September 29, 2015.

Copyright Notice

   Copyright (c) 2015 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



Gillmor                Expires September 29, 2015               [Page 1]

Internet-Draft          Negotiated-FF-DHE-for-TLS             March 2015


   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Vocabulary  . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Named Group Overview  . . . . . . . . . . . . . . . . . . . .   4
   3.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Optimizations . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Checking the Peer's Public Key  . . . . . . . . . . . . .   7
     5.2.  Short Exponents . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Table Acceleration  . . . . . . . . . . . . . . . . . . .   8
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   8
     6.1.  Preference Ordering . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     9.1.  Negotiation resistance to active attacks  . . . . . . . .  10
     9.2.  Group strength considerations . . . . . . . . . . . . . .  11
     9.3.  Finite-Field DHE only . . . . . . . . . . . . . . . . . .  11
     9.4.  Deprecating weak groups . . . . . . . . . . . . . . . . .  11
     9.5.  Choice of groups  . . . . . . . . . . . . . . . . . . . .  12
     9.6.  Timing attacks  . . . . . . . . . . . . . . . . . . . . .  12
     9.7.  Replay attacks from non-negotiated FFDHE  . . . . . . . .  12
     9.8.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .  13
     9.9.  False Start . . . . . . . . . . . . . . . . . . . . . . .  13
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14
     10.1.  Client fingerprinting  . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .