Re: Stream cipher against block cipher

From: AE (hidden_at_nospam.com)
Date: 06/27/03 

Date: Fri, 27 Jun 2003 18:55:40 +0200

Douglas A. Gwyn wrote:
> AE wrote:
>
>>You can create a stream cipher from a block cipher by running it in CTR,
>>OFB or CFB mode, but it is not that simple to construct a block cipher
>>from a stream cipher.
>
> Actually it's easier.

Well - I followed the thread concerning how to create a block cipher
from a stream cipher, but nothing I've seen there looked promising:

While in theory it is possible to use a stream cipher as the round
function this reduces the number of possible stream ciphers to the ones
with fast key scheduling and that way to ones with weak key scheduling -
not an easy task to estimate the cryptographic strength of that
construction.

>>Most stream ciphers don't allow you to add a IV ...
>
> Sure they do.

Which ones? SEAL? RC4? WAKE? any LFSR-construction?

Most times the IV is concatenated with the key, allowing attacks on the
key scheduling.

>>Maybe even more important is that there is much more public knowledge
>>concerning block ciphers than concerning stream ciphers.
>
> Proof by example?

Just have a look at the papers published or on the sources you'll find
in the network - you'll find more than twice as many papers concerning
block ciphers than concerning stream ciphers.

That's surely not because there's not more to tell about stream ciphers
or because stream ciphers are easier to implement or somebody would have
managed to submit a stream cipher to Nessie that was not faulty.

AE

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