Is 3DES Broken?

[james hughes]

On Jan 31, 2005, at 10:38 PM, Steven M. Bellovin wrote:
> When using CBC mode, one should not encrypt more than 2^32 64-bit
> blocks under a given key.  That comes to ~275G bits, which means that
> on a GigE link running flat out you need to rekey at least every 5
> minutes, which is often impractical.  Since I've seen Gigabit Ethernet
> cards for <US$25, this bears thinking about -- and while 10GigE is
> still too expensive for most people, its prices are dropping rapidly.
> With 10GigE, you'd have to rekey every 27.5 seconds...
>
> For reference purposes, with AES you'd be safe for 2^64*128 bits.
> That's a Big Number of seconds.
>
> 		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb

I would also like to reinforce Prof. Bellovin's comment that the 3DES 
block size is too small.

In bulk storage system encryption, 3DES will require rekey every 
~~65GBytes. Most PC's have more than this.

With AES the number is ~250 Exabytes (which is 250 billion gigabytes).

Thanks!

jim


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