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Bug#748063: debian-installer: Please offer luks serpent xts 512b



Cyril Brulebois <kibi@debian.org> (2014-05-13):
> Hallo Thomas,
> 
> and thanks for your report.
> 
> Thomas Renard <cybaer42@web.de> (2014-05-13):
> > Package: debian-installer
> > Version: testing
> > Severity: wishlist
> > 
> > Dear Maintainer,
> > 
> > according to
> > http://www.reddit.com/r/crypto/comments/235i58/linux_cryptosetup_strong_crypto_settings/
> > 
> > serpent-512b-xts seems to be the fastest software crypt algorithm for
> > luks. But the debian installer menu only offers 256b-xts. It would be
> > nice to have a 512b option.
> 
> I know nothing about the prerequisites on the crypto side, but maybe the
> installer only needs partman-crypto to be patched, this way:
> | --- a/ciphers/dm-crypt/serpent/keysize
> | +++ b/ciphers/dm-crypt/serpent/keysize
> | @@ -1 +1 @@
> | -128 192 256
> | +128 192 256 512

So I've hacked the d-i bits, and I verified that the modified image
indeed proposes serpent/512/xts as an option (even though I didn't test
without my patch; I'm assuming you did).

Now, trying to use that doesn't work, given the kernel module is unhappy
about the parameter being passed.

Looking at the kernel sources (v3.15-rc5-77-g14186fe), I see:
| crypto/serpent_generic.c:       .cia_max_keysize        =       SERPENT_MAX_KEY_SIZE,
| crypto/serpent_generic.c:       .cia_max_keysize        =       SERPENT_MAX_KEY_SIZE,
| include/crypto/serpent.h:#define SERPENT_MAX_KEY_SIZE            32

which assuming max key size is expressed in bytes, would match a maximum
key size of 256 bits.

But then I'm probably missing something obvious. And as I said, I don't
know anything in the crypto area to being with.

Mraw,
KiBi.

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