[dm-crypt] couple of questions related to using crypto map

[dm-crypt] couple of questions related to using crypto map

[Fourhundred Thecat]

Hello,

after some experiments, I came up with following 4 questions. I think
that the answer to all of them is yes, but I would like to confirm this
with the experts here:

1)
A crypto map aes-xts-plain64:sha512 with a random 512bit key is created
over a block device. When zeroes are written to the device through the
crypto map (encrypting), does it generate quality random data on the
block device (comparable to /dev/urandom)?

2)
The same crypto map over a block device, but the block devices contains
zeroes. When I read data, will it generate quality random data?
(comparable to /dev/urandom or a random number generator?)

3)
If yes should it be the fastest way to generate random data on a typical
Linux computer, compared to reading /dev/urandom? (especially when
having AES instruction support in the CPU)

4)
If the data obtained by reading from the zeroed device through that
crypto map (aes-xts-plain64:sha512) is written back to the block device
(using the same crypto map and key), will you get the original data? (in
this example zeros). Does the crypto map work in both directions?
Encrypting and decrypting when write and read back, and also when read
and write back?

could somebody please give me some feedback?

thanks,

Re: [dm-crypt] couple of questions related to using crypto map

[Arno Wagner]

On Mon, Jun 29, 2020 at 02:13:21 CEST, Fourhundred Thecat wrote:
> Hello,
> 
> after some experiments, I came up with following 4 questions. I think
> that the answer to all of them is yes, but I would like to confirm this
> with the experts here:
> 
> 1)
> A crypto map aes-xts-plain64:sha512 with a random 512bit key is created
> over a block device. When zeroes are written to the device through the
> crypto map (encrypting), does it generate quality random data on the
> block device (comparable to /dev/urandom)?

Well, yes and no. Yes, if you just ise it once. No, as it gets
written to disk and that is it.
 
> 2)
> The same crypto map over a block device, but the block devices contains
> zeroes. When I read data, will it generate quality random data?
> (comparable to /dev/urandom or a random number generator?)

See above.

> 3)
> If yes should it be the fastest way to generate random data on a typical
> Linux computer, compared to reading /dev/urandom? (especially when
> having AES instruction support in the CPU)

No. /dev/urandom has gotten a lot faster. 

> 4)
> If the data obtained by reading from the zeroed device through that
> crypto map (aes-xts-plain64:sha512) is written back to the block device
> (using the same crypto map and key), will you get the original data? (in
> this example zeros). 

That is how disk encryption works, at least when you do not have
per-sector metadata and LUKS does not.

Regards,
Arno
-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno@wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
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