[clue] bizarre block size Was Related To: secure erase techniques?

Raymond DeRoo rderoo at deroo.net
Tue May 31 09:30:24 MDT 2011


Folks--

A few have asked me why I choose the options to dd that I provided. Since enough asked I thought a reply to the list would be in order, for those not overly interested in techno mumbo jumbo please make judicious use of your DELETE key.


Let my start by saying there was an error in the line I provided, it should have used 383 for both values and not 387. So question still remains "why"?

Depend upon your distro a default block sizing of either 512 or 1024 will used by when non is provided. Additionally most applications and file system also allocate space on byte boundaries. Remembering that the goal here is to scramble the data, and to help reduce the "magnetic ghost image", I choose a non-standard block size. So why the value of 383 specifically? Well it changes each time I need to do this, but I nearly always choose a value that is a prime number. Now the value being prime does not, in and of itself, lead to any greater amount of security, but having the data aligned on non-standard boundaries does make the work of recovery specialist all the more difficult. 

To the question when not providing a block size "Is my method wrong." the answer is simply "No". It makes it a *bit* easier to recover, but nothing which I would ever loose sleep over.

.r


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