* Re: [dm-crypt] Cryptsetup FAQ is confusing re: key-file length and newlines
2020-07-15 20:49 ` [dm-crypt] Cryptsetup FAQ is confusing re: key-file length and newlines John Wiersba
@ 2020-07-16 15:08 ` Arno Wagner
0 siblings, 0 replies; 2+ messages in thread
From: Arno Wagner @ 2020-07-16 15:08 UTC (permalink / raw)
To: dm-crypt
Hi John,
On Wed, Jul 15, 2020 at 22:49:09 CEST, John Wiersba wrote:
> The FAQ as of today (2020/7/15) states
>
> Make sure no trailing newline (0x0a) is contained in the input key
> file, or the passphrase will not work because the whole file is used as
> input.
>
> But then a few lines later it suggests
>
> head -c 256 /dev/random > keyfile
>
> Obviously if /dev/random is used, it's possible that the keyfile will
> end with a trailing newline.
> I think you're trying to distinguish between
> 1. A file which contains a human-readable passphrase which could also
> be entered interactively, and
> 2. A file which contains random bytes.
You are overthinking this, I think.
If you create the keyfile via a text-editor, make sure you have no
trailing newling (unless you _want_ that traling newline to be part
of the passphrase). Many UNIX editors add that trailing newline
when saving a file automatically and then you have a character in
there you do not see but which is part of the passphrase.
This comment just simplifies debugging the problem.
If you create a new random keyfile, whatever bytes are in
there are fine. A random keyfile will contain (almost certainly ;-)
a lot of characters you cannot enter interactively anyways,
hence this does not have "interactive entry" as use-case.
[...]
> Additionally, I see lots of guidance on the length of a keyfile which
> uses magic numbers, both on the internet and also in the FAQ. Examples
> are the value 256 above, and the parameters bs=512 count=8 for dd. If
> I understand the FAQ correctly, the actual advice is
>
> Plain dm-crypt: Use > 80 bit. ... If paranoid, add at least 20 bit.
>
> This implies (taking the worst case) that
>
> head -c 13 /dev/random
>
> should be sufficient (13 * 8 bytes = 104 bits > 81+20 bits), and 256
> bytes is "overkill". I do understand that some reasonable amount of
> overkill is essentially "free" and therefore can be used "just in
> case".
Yes. When it costs you nothing, use more. When it costs you something,
what you quoted gives you a generally reasonable trade-off.
Regards,
Arno
> Did I understand these two concepts correctly, and if so, could you
> clarify the FAQ?
> Thanks!
> -- John Wiersba
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--
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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