[2756] in cryptography@c2.net mail archive
Re: On Mixture of Stream and Block Cipher
daemon@ATHENA.MIT.EDU (Sunder)
Wed May 27 11:15:51 1998
Date: Wed, 27 May 1998 09:39:53 -0400
From: Sunder <sunder@brainlink.com>
To: Mok-Kong Shen <mok-kong.shen@stud.uni-muenchen.de>
CC: cryptography@c2.net
Mok-Kong Shen wrote:
> According to Menezes et al., Handbook of Cryptography, block ciphers
> are memoryless while stream ciphers have memory and are also called
> state ciphers. The distinction between the two types is nevertheless
> not definitive.
If you allow yourself to bend the definitions you'll easily come up with
ways to generate stream cyphers out of block cyphers and vice versa. Just
think of a stream cypher as a very small block, and a block cypher as a
stream of very large characters. i.e. 8 bits versus 64 bits. :)
The problem is what to do with the excess space in a block cypher when you
don't have data. Sure, you could padd the data, but there comes a question
of where to pad it and how to prevent the padding from being plain text.
i.e. if your block is 64 bits, and you want to send only one byte, what
do you do? pad with all zeros? If so, a known plaintext attack could
take place at the end of the block. If you place it at the begining,
then another can take place. If you use some control byte that says
this block is X bits long, that control byte then can be used as a plain
text attack since its range will be known.
i.e. D 0 0 0 0 0 0 0 <- 0 padding at end
0 0 0 0 0 0 0 D <- 0 padding at start
1 D 0 0 0 0 0 0 <- size byte, data, padding
D 1 1 1 1 1 1 1 <- padded with size byte
D D 2 2 2 2 2 2 <- padded with size byte
2 2 2 2 D D 2 2 <- padded with size byte, but in front and back.
All of these are bad to different extents. What you'd need is a cypher in
a cypher to hide the block's padding... super encryption that won't
produce predictable plaintext. Unfortunatly, that requires you to deal with
a second key for the other cypher, and involves time penalties...
Any thoughts?
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