On Thu, Apr 02, 2009 at 06:11:16PM -0400, Victor Hugo dos Santos wrote:

> is true that bacula don't compress encrypted files or data ??
> and in positive case, why ???

Compression works by reducing redundancy in low-entropy data
(e.g. eliminating patterns in data with patterns.)  Good encryption
schemes produce output that is nearly indistinguishable from random,
high entropy data (e.g. removes any obvious patterns.)  So the output
of good encryption is mathematically uncompressable.  This is related
to the reason why you can't compress data that has already been
compressed.

Note that you *can* go the other way, i.e. you can encrypt compressed
data.  Encryption doesn't care whether the input is low entropy or
high entropy.

- Morty

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