On Tue, 8 May 2001, Oskar Sandberg wrote:
> On Tue, May 08, 2001 at 01:01:34AM -0500, Scott Gregory Miller wrote:
> > > > > * to pad data to a power of 2, you repeat the data
> > > > No. This lends an attack as well, since the attacker can try and assume
> > > > that several of the last blocks of the file are the same.
> > >
> > > Does that matter given that the mode we are using is rolling?
> > Yes, since its only rolling within a block. After two blocks you start
> > getting the same encryption (your XORing all zeros, so you're essentially
> > just encrypting as a stream cipher). I'm not saying this does give a
> > cryptanalyst any info, but it might.
>
> " * to pad data to a power of 2, you repeat the data"
>
> How does that involve XORing all zeros? Repeating the same data will not
> cause the ciphertext to repeat as far as I can tell...
>
> <>
Sorry, that was a product of me thinking about two different scenarios at
once. Namely, padding with zeros and padding with the data. As I said
though, it may give a cryptanalyist an advantage to know that the last few
blocks of a file are identical.
Scott
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