Re: Stream cipher against block cipher

From: AE (nospam_at_hidden.com)
Date: 07/07/03 

Date: Mon, 07 Jul 2003 16:10:41 +0200

The problem is that just placing the IV at the beginning of the stream
and not doing anything else - like when using a block cipher in CBC or
CFB mode - is completely useless and doesn't change the remaining
keystream at all.

Since there is no feedback with plain- or ciphertext there is no other
way to get different keystreams than to change the key itself :-(

Skipping a variable number of bytes at the beginning is useless, so
there's no way to re-use a key in a way that would give me a good feeling:

Even if the problem of the weak key-scheduling of RC4 can be solved by
dropping a number of bytes this doesn't change the fact that
key-material is exposed and RC4 was never designed to defeat attacks on
the key-scheduling.

>From my point of view one should either find a way to exchange the whole
key in a secure way every time a new message is encrypted or use a block
cipher in CTR or OFB mode.

Mrsjunecarey wrote:
> The context of this thread is the method suggested by Doug Gwyn of transmitting
> the IV in the clear before the ciphertext.
>
> That is the method I'm referring to, and you can use it with RC4. But as you
> rightly mention, there are security problems associated with that technique.
>
>
>>There's indeed no way just to add an IV to RC4. Using a nounce means
>>changing the key and - since the nounce has to be transmitted in clear -
>>exposing part of the key-material to an attacker, which allows her to
>>mount key-scheduling attacks.
>>
>>AE
>
>

Relevant Pages

  • Re: Another Dumb Idea for Debunking...
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  • Re: Another Dumb Idea for Debunking...
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  • Re: About VMPC
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  • Re: Securing ARC4
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