Messages in this thread |  | | Date | Sun, 21 Jul 96 05:10:45 MDT | From | (Colin Plumb) | Subject | Re: DES AND IDEA IN THE KERNEL _VERY_ BROKEN! |
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I'd like to remind folks that a cenreal tenet in cryptography is that you don't want to encrypt two messages with the same key *and IV*. It cauese information leakage. The leakage is low enough that if it happens by accident, there's no great cause for alarm, but you shouldn't do it in a regular basis.
And if you use an encrypted device on a machine which someone else can read the hard drive of, then over time, as you modify the drive, if the IV you use for encrypting each block depends only on the block number, you are creating messages (each subsequent version of the block) which are encrypted with the same key and IV. Lots of them.
Now, OFB and counter mode are *particularly* bad, as you can XOR two versions of a block together and recover the XOR of the plaintexts. Generally a bad idea. CFB mode has problems, but even CFB mode reveals the location of an alteration.
The hack used in Peter Gutmann's MS-DOS SFS device driver of using a checksum of the plaintext (and the block offset, and some per-volume unique information) is advisable. -- -Colin
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