[H-GEN] File system encryption

[Paul Gearon]

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On Tue, 3 Jun 2003, Jason Parker-Burlingham wrote:

> Lance Edwards <lanceedwards at optushome.com.au> writes:
> > A quick setup for a loop encrypted FS, see the link below but change
> > xor to aes / aes128 / aes192 or aes256...
<snip/>
>
> Goodness!  Why on earth are they suggesting that people use XOR
> encryption?  It would be okay if they were using a one-time-pad or
> something[1] but I see no evidence of that.  XOR encryption is
> trivially broken.

If it's "standard" XOR obfuscation, then sure, you're right.  However,
since *no-one* could be stupid enough to use this (could they???) perhaps
they mean Output Feedback (OFB) mode or Counter (CTR) mode encryption?
Both types generate a non-repeating stream of bits [1] which you then XOR
with the plaintext.

[1] OK, it does repeat eventually[2].  So you need to change your key
before that happens.  ie. every message.

[2] "eventually" for CTR mode is when your counter wraps, which is
probably at 2^64 bits.  Plenty of space for most messages.  OFB has a
finite chance of wrapping sooner, which is catastrophic, because you then
end up with the XOR encryption Jason mentioned earlier.  ie. Don't use OFB
mode!

Regards,
Paul "Having fun with Ferguson and Schneier's new cryptography book" Gearon


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