# Symmetric alternative to public-key crypto for key-exchange?

*arachnidster_at_gmail.com*

**Date:** 08/30/05

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Date: 29 Aug 2005 22:10:37 -0700

I remember seeing a note somewhere to the effect that a symmetric

cipher could be used for key exchange instead of a public key one, as

follows:

1) Bob encrypts the key to be exchanged with his key and sends it to

Alice

2) Alice encrypts this message with her key and sends it back to Bob

3) Bob decrypts this message with his key, and sends it back to Alice

4) Alice decrypts the message, revealing the key

This requires a couple of properties from the symmetric cipher used: It

must be true that D(b, D(a, E(b, E(a, message)))) = message, and it

must not be possible to recover the key given the intermediate messages

transmitted between Alice and Bob (for example, with a standard stream

cipher, XORing subsequent messages would reveal both Alice and Bob's

keystreams, and hence the original message).

My question is this: is anyone familiar with a cipher that meets these

criteria?

**Next message:**Regis: "Re: Re-secured Algorithm?"**Previous message:**Regis: "Re: My my, how time flies ...... it's been about "1 hour" -- anyone cracked CryptoSMS yet?"**Next in thread:**Paul Rubin: "Re: Symmetric alternative to public-key crypto for key-exchange?"**Reply:**Paul Rubin: "Re: Symmetric alternative to public-key crypto for key-exchange?"**Reply:**mobius30: "Re: Symmetric alternative to public-key crypto for key-exchange?"**Reply:**Valery Pryamikov: "Re: Symmetric alternative to public-key crypto for key-exchange?"**Reply:**Twittering One: "Re: Symmetric alternative to public-key crypto for key-exchange?"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ] [ attachment ]