TLS Client Puzzles Extension
draft-nygren-tls-client-puzzles-01

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Network Working Group                                          E. Nygren
Internet-Draft                                                    S. Erb
Intended status: Standards Track                     Akamai Technologies
Expires: December 30, 2016                                   A. Biryukov
                                                         D. Khovratovich
                                                University of Luxembourg
                                                           June 28, 2016

                      TLS Client Puzzles Extension
                   draft-nygren-tls-client-puzzles-01

Abstract

   Client puzzles allow a TLS server to defend itself against asymmetric
   DDoS attacks.  In particular, it allows a server to request clients
   perform a selected amount of computation prior to the server
   performing expensive cryptographic operations.  This allows servers
   to employ a layered defense that represents an improvement over pure
   rate-limiting strategies.

   Client puzzles are implemented as an extension to TLS 1.3
   [I-D.ietf-tls-tls13] wherein a server can issue a HelloRetryRequest
   containing the puzzle as an extension.  The client must then resend
   its ClientHello with the puzzle results in the extension.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on December 30, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Internet-Draft               Client Puzzles                    June 2016

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Table of Contents

   1.  Overview and rationale  . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Handshake Changes . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  The ClientPuzzleExtension Message . . . . . . . . . . . .   5
   4.  Usage by Servers  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Proposed Client Puzzles . . . . . . . . . . . . . . . . . . .   6
     5.1.  Cookie Client Puzzle Type . . . . . . . . . . . . . . . .   7
     5.2.  SHA-256 CPU Puzzle Type . . . . . . . . . . . . . . . . .   7
     5.3.  SHA-512 CPU Puzzle Type . . . . . . . . . . . . . . . . .   8
     5.4.  Equihash: Memory-hard Generalized Birthday Problem Puzzle
           Type  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Overview and rationale

   Adversaries can exploit the design of the TLS protocol to craft
   powerful asymmetric DDOS attacks.  Once an attacker has opened a TCP
   connection, the attacker can transmit effectively static content that
   causes the server to perform expensive cryptographic operations.
   Rate limiting offers one possible defense against this type of
   attack; however, pure rate limiting systems represent an incomplete
   solution:

   1.  Rate limiting systems work best when a small number of bots are
       attacking a single server.  Rate limiting is much more difficult
       when a large number of bots are directing small amounts of
       traffic to each member of a large distributed pool of servers.

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