[dm-crypt] Some questions about cryptsetup 1.6.x

Arno Wagner arno at wagner.name
Thu Feb 13 06:57:20 CET 2014

On Wed, Feb 12, 2014 at 17:10:40 CET, Milan Broz wrote:
> On 02/12/2014 03:30 PM, Thomas Bächler wrote:
> > Am 12.02.2014 15:19, schrieb Arno Wagner:
> >> -h is the hash that the plain-text password is put through
> >> to turn it into a binary value of certain defined length.
> >> -c specifies the hash that goes into pbkdf2 for the hash
> >> iteration.
> > 
> > Are you sure?
> > 
> > I was under the impression that '-c' only affects the cipher parameter
> > passed to dm-crypt - a hash would then be relevant for cipher modes like
> > cbc-essiv, but xts-plain64 would ignore it. Thus, cryptsetup has default
> > like 'aes-cbc-essiv:sha256', since essiv needs a hash, and
> > aes-xts-plain64, since xts does not need a hash.
> > 
> > According to the manpage, -h is what is used in PBKDF2 in luksFormat
> > mode, or to hash the passphrase in plain mode.
> Yes, this is correct. The -h parameter is for LUKS header (PBKDF2 + AF splitter).
> For plain mode it means algorithm to use when hashing password.
> For -c it is cipher/mode for kernel dmcrypt (if there is a IV spec which requires
> hash like ESSIV, then it contains hashspec as parameter).
> Milan

Just added clarifications for -c and -h to the man-page. That
I was confused about their meaning shows that it was not clear
enough ;-)

Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno at wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
A good decision is based on knowledge and not on numbers. -  Plato

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