Is 3DES Broken?

[Jack Lloyd]

On Fri, Feb 04, 2005 at 07:46:39PM +0000, Ian G wrote:
> It seems that the block size of an algorithm then
> is a severe limiting factor.  Is there anyway to
> expand the effective block size of an (old 8byte)
> algorithm, in a manner akin to the TDES trick,
> and get an updated 16byte composite that neuters
> the birthday trick?
> 
> Hypothetically, by say having 2 keys and running
> 2 machines in parallel to generate a 2x blocksize.
[...]

That would make for a pretty poor cipher, considering that flipping a plaintext
bit would only change one half of the ciphertext block (assuming I'm not
misinterpreting your suggestion).

Something along the lines of DEAL should suffice for this, using the block
ciphers as round functions in a Feistel cipher. A bit slow, but it seems like a
plausible solution.

-Jack

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