Re: Stream cipher against block cipher

From: aditya (truman_at_rediffmail.com)
Date: 06/30/03 

Date: 30 Jun 2003 05:19:02 -0700

AE <hidden@nospam.com> wrote in message news:<3EFDD393.9010601@nospam.com>...
> Douglas A. Gwyn wrote:
> > AE wrote:
> >
> >> This doesn't make a block cipher.
> >
> > Sure it does. What do *you* think the definition of
> > "block cipher" is?
>
> A block cipher is a codebook.
>
> Using SEAL, RC4 or any of the LFSR-constructions I know to meet this
> goal would mean to use the same part of the keystream for every block,
> this way reducing a potentially strong stream cipher to vigenere encryption.
>
> In addition I'd expect a block cipher to make sure that every plaintext
> bit and every key bit effects every output bit with some probability.
>
> That's surely a requirement you can't meet with a single pass of any
> stream cipher except you are defining your blocksize as the wordsize of
> the stream cipher.
>
> That's per definition possible, but I don't think it's that a good idea
> to use RC4 as a blockcipher with 1 bit blocksize :o)
>

You guys seem to be having an interesting conversation out here. Whats
a block cipher? At times it helps to get back to the basics.....so can
I add? A block cipher encrypts plaintext in blocks. While a stream
cipher encrypts plaintext 1-bit or 1-byte at a time.
In ECB mode(at least), with the same key and the text block- the block
cipher is expected to generate the same cipher EVERY TIME.
While the stream cipher may or may not generate the same cipher under
the same circumstances.
as a beginner in the subject this is what I know. Correct me if I am
wrong.
:)
regards.
Aditya K

Relevant Pages