On Wed, 26 Feb 2014 09:39:02 +0100
Ben RUBSON <ben.rub...@gmail.com> wrote:
| 2014-02-25 1:36 GMT+01:00 Anthony Thyssen <a.thys...@griffith.edu.au>:
|
| > On Mon, 24 Feb 2014 14:25:40 +0100 Ben RUBSON <ben.rub...@gmail.com>
| > wrote:
| > |
| > | To be sure that all IOs of one file will be aligned, does the encryption
| > of
| > | 4K of data give exactly 4K of encrypted data ?
| > |
| > encryption happens in 'blocks of data'. If the data encrypted in
| > is less a block in size it increases to that block size.
| >
|
| In this situation, we are sure that all IOs will be aligned.
|
| However, is the following situation possible ?
| Can we have the size of an encrypted block of data bigger than the size of
| the block of data itself ?
| In other words, can we have :
| size(encrypt($block_of_data)) > size($block_of_data)
|
| This could lead into a misalignment somewhere in the file, or EncFS could
| have to read 2 disk blocks to serve 1 data block.
|
| Thank you,
|
| Best regards,
|
| Ben
The encryption block size is very small by comparison to that of disk block
size.
A typical encryption used today is aes-256 That is 256 bit encryption blocks
or 32 bytes per encrypted block. Compare that to disk blocks of 4Kbytes.
I/O alignment is no problem, and is actually very common. It is only headers
that cause problems, and headers are not that big either, which is why you can
pack a lot of per block information into a single 4K header block for
each file, so as to preserve I/O block alignment.
Anthony Thyssen ( System Programmer ) <a.thys...@griffith.edu.au>
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