On 11/09/15 12:26, jstacey wrote:
> I recently enabled encryption in my bacula-fd.conf with these
> entries:
>
> PKI Signatures = Yes PKI Encryption = Yes PKI Keypair =
> "/etc/bacula/client.pem" PKI Master Key = "/etc/bacula/master.cert"
>
> The encryption works but now my LTO4 tapes can only store around
> 812MB instead of the usual 1.2 -> 1.4 TB Is this normal? I might
> enable gzip software compression but this is going to take a chunk
> out of my CPU. I read that it is also possible to enable hardware
> encryption on the drive. Maybe this method would allow me to keep
> using hardware compression? Thanks
This is inherent in how encryption and compression work. Any good
encryption process should emit ciphertext that contains as little as
possible redundancy, because redundancy opens up possible methods to
attack the ciphertext. However, compression *relies upon* redundancy.
With no redundancy, compression cannot work. So ciphertext is generally
incompressible. On the other hand, eliminating as much redundancy as
possible from cleartext before encrypting it effectively strengthens the
encryption.
So, if you are going to both compress AND encrypt, FIRST compress, THEN
encrypt. If you encrypt first, you won't be able to compress the
result. So if you're going to use software encryption, and also want to
use compression, you must use software compression *before* the
encryption stage. You will not be able to employ hardware compression
on already-encrypted data. There should be very little to no redundancy
left to compress out.
--
Phil Stracchino
Babylon Communications
phils AT caerllewys DOT net
phil AT co.ordinate DOT org
Landline: 603.293.8485
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