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BCP 86
RFC 3766
Title: Determining Strengths For Public Keys Used
For Exchanging Symmetric Keys
Author(s): H. Orman, P. Hoffman
Status: Best Current Practice
Date: April 2004
Mailbox: hilarie@purplestreak.com, paul.hoffman@vpnc.org
Pages: 23
Characters: 55939
SeeAlso: BCP 86
I-D Tag: draft-orman-public-key-lengths-08.txt
URL: ftp://ftp.rfc-editor.org/in-notes/rfc3766.txt
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.
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.
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Authors, for further information.
Joyce K. Reynolds and Sandy Ginoza
USC/Information Sciences Institute
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