RFC 3766 (rfc3766) - Page 1 of 23

Determining Strengths For Public Keys Used For Exchanging Symmetric Keys

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Network Working Group                                           H. Orman
Request for Comments: 3766                            Purple Streak Dev.
BCP: 86                                                       P. Hoffman
Category: Best Current Practice                           VPN Consortium
                                                              April 2004


               Determining Strengths For Public Keys Used
                     For Exchanging Symmetric Keys

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   Implementors of systems that use public key cryptography to exchange
   symmetric keys need to make the public keys resistant to some
   predetermined level of attack.  That level of attack resistance is
   the strength of the system, and the symmetric keys that are exchanged
   must be at least as strong as the system strength requirements.  The
   three quantities, system strength, symmetric key strength, and public
   key strength, must be consistently matched for any network protocol
   usage.

   While it is fairly easy to express the system strength requirements
   in terms of a symmetric key length and to choose a cipher that has a
   key length equal to or exceeding that requirement, it is harder to
   choose a public key that has a cryptographic strength meeting a
   symmetric key strength requirement.  This document explains how to
   determine the length of an asymmetric key as a function of a
   symmetric key strength requirement.  Some rules of thumb for
   estimating equivalent resistance to large-scale attacks on various
   algorithms are given.  The document also addresses how changing the
   sizes of the underlying large integers (moduli, group sizes,
   exponents, and so on) changes the time to use the algorithms for key
   exchange.








Orman & Hoffman          Best Current Practice
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