LUKS --hash vs. --cipher whatever:hash

Hello,

I'm interested in the difference between, --hash sha256 and --cipher whatever:sha256.

Couldn't find anything in The Fine Manual, so any pointers in the right direction would be much appreciated.

Thanks,
Eric

Hi,

For LUKS mode (luksFormat / luksOpen):

--hash is for LUKS header hash (by default is it sha1), IOW the hash used for anti-forensic splitter and volume key obfuscation (passphrase is used to unlock LUKS keyslots whe is volume key stored. Volume key is always generated from random generator during format.)

sha256 in cipher specification is useful only for ESSIV initialization vector. e.g. aes-cbc-essiv:sha256 - means cipher AES in CBC mode and with ESSIV initialization vector which uses sha256 (IV is derived from key using sha256 hash).

(For othes modes, like XTS, you do not need to use ESSIV because the problem of predictable IV is solved by mode itself - so you can have e.g. aes-xts-plain64. For LUKS header you must use some hash always. It is part of the specification - for more info see project pages
http://code.google.com/p/cryptsetup/ - specification bookmark.)

For plain mode (cryptsetup create command):
--hash means which hash alg. will be used to generate key. (passphrase is hashed and its output is directly used as key).

cipher specification is the same as in LUKS mode.

Milan

Thanks for the replies.

Is there any benefit to hash size / algorithm strength? It seems like afsplitter will ensure the split key is the required length regardless of the hash output.

Does using sha256 over some other hash outputting only 128 bits offer any practical benefit, other than decreasing the likelyhood of two IVs being the same?

Thanks for the link, interesting reading.

Thanks,
Eric