[sllug-members]: Encrypted backup tapes

[Lamont Peterson]

On Thursday 11 January 2007 07:49am, Andrew GIlmore wrote:
> On Wed, 2007-01-10 at 10:02 -0700, Lamont Peterson wrote:
> > Recently, I ran this on my notebook (/secure/ is an encrypted
> > partition):
> >
> > tar -cf - /secure/ | gzip -9 | gpg -c >
> > /media/usb/backup/secure.tar.gz.gpg
>
> My experience with a small tar file of text files files was that the
> size of the tar file gzipped and then encrypted was the same size as the
> tarfile just encrypted.

In my case, it went from 2.6GB to 1.6GB.  But I also wanted to compression 
because the USB hard drive was still the bottleneck for me (I only have 1 
USB1.1 port on this notebook).

> At the time it made sense to me because the 
> encryption routine needs to get the highest entropy possible, and
> encrypting plain text starts with low entropy.

Exactly.  Though, I usually explain it like this:  Compression is an important 
building block for real-world cryptosystems, as compression significantly 
reduces redundancy.  By compressing data before encrypting it, we get a much 
more difficult to attack ciphertext.

> Does this hold true for larger files?

It's even more important for large file than for "average sized ones", just as 
it is more important for tiny files than for average sized ones.
-- 
Lamont Peterson <lamont at gurulabs.com>
Senior Instructor
Guru Labs, L.C. [ http://www.GuruLabs.com/ ]

NOTE:  All messages from this email address should be digitally signed with my
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