Re: [dm-crypt] How to compress LUKS2 header?

[Arno Wagner <arno@wagner.name>]

On Fri, Dec 27, 2019 at 17:20:11 CET, Michael Kjörling wrote:
> On 27 Dec 2019 10:56 -0500, from gebser@mousecar.com (ken):
> > Compressing a file is one step in the encryption of that file.  So if
> > your LUKS2 header file is encrypted, it's also already compressed. 
> > Using ZIP on it would yield no further compression.
> 
> No, encryption does not imply compression. Rather, trying to compress
> ciphertext is a largely pointless exercise if the encryption is any
> good in the first place; therefore, _if_ you're going to compress the
> data you're encrypting (keeping in mind that doing so is not always a
> good idea; see compression oracle attacks), then you need to compress
> first, then encrypt, not the other way around.

Well, actually compressing encrypted data is very easy. It just 
has one mandatory condition that has to be met: You have to be able
to decrypt. Moden ciphers make sure you can only do that when
you have the key. 


> I'm pretty sure the LUKS header backup isn't compressed.

It is not. Also, the anti-forensic stripes are not encyption.
They are basically an all-or-nothing transformation and that 
is different form encryption.

Regards,
Arno

> -- 
> Michael Kjörling • https://michael.kjorling.se • michael@kjorling.se
>  “Remember when, on the Internet, nobody cared that you were a dog?”
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-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno@wagner.name
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