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rblampain

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backup data on dvd using encryption

Linux Back up questions. (Fedora C2)

1) Is there available software to back up data from a raid 1 to DVD using encryption?

2) I read adds from an Internet Backup ISP offering a compression of 10/1 using blowfish algorithm, how can I reach maximum compression and what ratio would that be?
 
3)  Could I obtain the result wanted in 1) and 2) using software from the distro and a script or commands?

4) How can I find the speed of DVD writing (figuring out how long the backups will take)?

Your advices are most welcome.
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Barthax
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Sorry, forgot about the compression... Blowfish is a cipher - it encrypts the data in a 1:1 fashion - there's no compression at all.  It sounds like they are describing something they have no knowledge of.

Compression ratios themselves are completely subject to the data being compressed.  No algorithm can guarantee any level of ratio.  A 10/1 compression ratio is farsical given real-world data, unless the compressor can guarantee the type of data to be compressed.  Let me give you an example: I can guarantee a compression ratio of 100:1 IF you let me veta any data that does not have a single byte repeated at least 200 times.

Alternatively, it can be mathematically proven to be impossible to guarantee anything other than 1:1.  If you take a single byte to be the end result of the compressed data, then you can only expand that to represent 2^8 possibilities - full stop.  There's no "oh, if you can expand it to that point then you can expand it again...".  Nope, 2^8 = 256 different possibilities (i.e., one byte's worth = 1:1).  Compression works based on the assumption that the data being compressed can be described in a fashion that can be logically reversed.  Not only does that description have an amount of overhead, but a compressed set of bytes cannot be expanded to cover all possible outcomes of data for a given length.

In all cases, it is best to compress data before encrypting it.
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Thank you for the detailed explanations.