Re: .NET DES CFB - useless interface?

From: Peter Gutmann (pgut001_at_cs.auckland.ac.nz)
Date: 11/22/04 

Date: 22 Nov 2004 09:04:58 GMT

dominicsmith501@hotmail.com (Dominic) writes:

>I always thought that the main point of CFB mode was that it allowed
>you to use a block cipher like a stream cipher.

>eg if you set the feedback size to 8 bits, you can put a byte in and
>get a byte out. etc

>The .NET framework implementation offers CFB mode for DES, and it even
>lets you set the feedback size.

>But unfortunately the TransformBlock method seems to insist that you
>pass in data which is an exact number of block lengths in size. And it
>also insists that the block length is 64 bits (ie the block size for
>DES).

>This basically makes it impossible to use CFB mode to stream data in
>any smaller chunks than you could do with CBC anyway.

That problem dates back to CryptoAPI 1.0, circa 1996, and yes, it does make
the stream modes completely useless. I don't know about .NET, but in
CryptoAPI the feedback also defaults to CFB-8 rather than CFB-64.

Peter.

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