[Pkg-cryptsetup-devel] What is the benefit of SSL/GnuPG encrypting keys instead of XOR?

[David Härdeman]

Hi,

I've just joined the list because I'm interested in adding LUKS 
cryptoroot support to the Debian installer (partman-crypto that is).

So far I've been working on other components which are also necessary 
for this. Most importantly, adding luks cryptoroot support to 
initramfs-tools and klibc so that the initramfs can recognize and setup 
a luks encrypted root (see bug #348147).

I've followed the "Status of partman-crypto" thread with great interest, 
but I can't really understand what the benefit is of SSL/GnuPG 
encrypting keys which are stored on for instance removable media. 

Presumably it allows for a kind of two-factor authentication (knowing 
passphrase plus having possession of removable media with encrypted 
key being necessary to access harddrive).

However, exactly the same benefit can be acheived by simply XOR:ing the 
passphrase and the key stored on the media together. The main advantage 
being that no extra binaries are needed on the initrd/initramfs/etc 
(GnuPG seems like a quite heavy requirement in early userspace).

Additionally, should the media with the key be lost or copied without 
the knowledge of the user, it is possible for the person who has 
gotten hold of the encrypted keyfile to attempt a brute-force attack as 
GnuPG use also means that it is possible to know when the correct 
keyphrase has been found.

In contrast, if the keyfile is simply XOR:ed with the passphrase, there 
is no way of brute-force attacking the key unless the attacker also has 
access to something which to test the result upon (i.e. the encrypted 
harddrive).

Am I missing something here?

Regards,
David Härdeman
david at 2gen.com