At 09:55 2005-02-03 -0500, John Kelsey wrote:
>From: "Steven M. Bellovin" <[EMAIL PROTECTED]>
>Sent: Feb 2, 2005 1:39 PM
>To: bear <[EMAIL PROTECTED]>
>Cc: Aram Perez <[EMAIL PROTECTED]>, Cryptography <cryptography@metzdowd.com>
>Subject: Re: Is 3DES Broken?
...
>>I think you meant ECB mode?
>No, I meant CBC -- there's a birthday paradox attack to watch out for.
Yep. In fact, there's a birthday paradox problem for all the standard
chaining modes at around 2^{n/2}.
For CBC and CFB, this ends up leaking information about the XOR of a
couple plaintext blocks at a time; for OFB and counter mode, it ends up
making the keystream distinguishable from random. Also, most of the
security proofs for block cipher constructions (like the secure CBC-MAC
schemes) limit the number of blocks to some constant factor times 2^{n/2}.
I'm surprised that no-one has said that ECB mode is "unsafe at any speed".
Greg.
Greg Rose INTERNET: [EMAIL PROTECTED]
Qualcomm Incorporated VOICE: +1-858-651-5733 FAX: +1-858-651-5766
5775 Morehouse Drive http://people.qualcomm.com/ggr/
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