[dm-crypt] Using plain64/plain IV (initialisation vector) in dm-crypt

Milan Broz mbroz at redhat.com
Tue Jul 27 10:46:44 CEST 2010


This thread is going crazy... :)

1) Facts about using plain IV generator:

- "plain" IV is 32bit only, supported by all kernels
- you should avoid using it for >2TB devices
- it will remain this way because of backward compatibility (howgh:-)

- "plain64" is fully 64bit, available since kernel 2.6.33
- for device < 2TB it produces the same output as "plain"

=> use plain64 for new devices if you want to use tweakable encryption
mode like XTS (or LRW), e.g. cryptsetup -c aes-xts-plain64

p.s.
Never use plain* IV for CBC mode, use ESSIV there.
<joke>If you are using ECB mode, you are lost anyway.</joke>

2) crypsetup should have always safe defaults.
It is aes-cbc-essiv:sha256 with 256bit key currently.


3) For the resize - we cannot catch all situations, someone can
dd LUKS disk to another bigger volume without "resize" command.

Tools will suggest using plain64 but it cannot force it.


> So you guess the the 1TB limit could be actually ...

Forgot about 1TB limit, it is different XTS only problem.
We mixed up two unrelated problems here.

Milan


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