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Title : Determining Strengths For Public Keys Used For
Exchanging Symmetric Keys
Author(s) : H. Orman, P. Hoffman
Filename : draft-orman-public-key-lengths-02.txt
Pages : 14
Date : 22-Mar-01
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 the length of a symmetric
key. 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.
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