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

**From:** Valery Pryamikov (*valery_at_nospam.harper.no*)

**Date:** 08/31/05

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Date: Wed, 31 Aug 2005 07:32:07 +0200

<arachnidster@gmail.com> wrote:

*>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:
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*>
*

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

*> Alice
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*> 2) Alice encrypts this message with her key and sends it back to Bob
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*> 3) Bob decrypts this message with his key, and sends it back to Alice
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*> 4) Alice decrypts the message, revealing the key
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*>
*

*> This requires a couple of properties from the symmetric cipher used: It
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*> must be true that D(b, D(a, E(b, E(a, message)))) = message, and it
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*> must not be possible to recover the key given the intermediate messages
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*> transmitted between Alice and Bob (for example, with a standard stream
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*> cipher, XORing subsequent messages would reveal both Alice and Bob's
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*> keystreams, and hence the original message).
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*>
*

Stream ciphers would do it, as well as block ciphers in CTR/OFB (or other

streaming operation mode).

-Valery.

http://www.harper.no/valery

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