Full disk encryption without a boot partition

Alex Hornung alex at alexhornung.com
Thu Dec 27 23:48:45 PST 2012

On 27/12/12 22:40, mhca12 wrote:
> On Thu, Dec 27, 2012 at 11:00 PM, Alex Hornung <alex at alexhornung.com> wrote:
>> On 27/12/12 22:13, mhca12 wrote:
>>> On Thu, Dec 27, 2012 at 10:08 PM, Alex Hornung <alex at alexhornung.com> wrote:
>>>> On 26/12/12 22:19, mhca12 wrote:
>>>>> Are there any plans or is there already support for full
>>>>> disk encryption without the need for a boot partition?
>>>> No, the userland tool that sets up the decryption of the root partition,
>>>> as well as the kernel and modules need to be somewhere that is not
>>>> encrypted - otherwise the boot loader would need to support the disk
>>>> encryption.
>>> Seems like OpenBSD 5.2's bootloader can do that.
>>> Any idea how they did it?
>>  I didn't say that it's impossible, I just stated what would be
>> required. There are no plans to do any such thing in DragonFly BSD, as
>> there is pretty much no point. Doing it in any other way than with the
>> separate /boot partition overcomplicates everything by an order of
>> magnitude (since, for example, the setup cannot occur in userland
>> anymore) for no real benefit.
> Sorry that I wasn't precise. I meant to say I find it hard to believe that
> OpenBSD extended the bootloader to do decryption.
> Your stated reasons make a lot of sense and I will try to find out
> how it works in OpenBSD because I'm curious.

I digged around a bit this morning out of interest. It seems a good
chunk of cryptography was added to libstand, which, afaict, is what the
bootloader uses - pbkdf2, AES, AES-XTS, SHA1, HMAC-SHA1.

Those components are indicative of disk encryption - pbkdf2 will derive
a key from your passphrase, AES-XTS is the nowadays preferred block
cipher (and mode) for disk encryption, HMAC-SHA1 provides authenticated
message integrity.

That said, I haven't found much else in the bootloader code except some
rudimentary softraid support. From a short look I'd say pieces are still
missing (or maybe I've just missed them), but there is definitely some
work on extending the OpenBSD bootloader to support full disk encryption.

That said and going back to DragonFly - our encryption scheme is much
more flexible than OpenBSD's. The in-kernel part is almost completely
generic, and userland tools provide the knowledge about the header for
LUKS (cryptsetup) and TrueCrypt (tcplay), for example. We also support a
large number of block ciphers, IV generation schemes, etc, as required
by LUKS and tcplay.

Supporting everything in the bootloader would be a lot of work, and I
simply don't see the benefit. There is nothing wrong with having an
unencrypted /boot partition - and you could in principle use an external
/boot partition on, say, a USB stick.


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