BCP 86, RFC 3766 on Determining Strengths For Public Keys Used For Exchanging Symmetric Keys

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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:    [e-mail], [e-mail]
        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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Joyce K. Reynolds and Sandy Ginoza
USC/Information Sciences Institute

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