[Cryptography] Rubber-hose resistance?

[Jerry Leichter]

> Before you leave to return home, you need to delete all the confidential
> information from the laptop.  To ensure no trace of it remains on the
> disk, the simplest safe way I can imagine is this:
> 
> $ cat /dev/urandom >tmp1
> # Now wait until you run out of disk space.
This of course may not do what you think on an SSD - which it probably is these days.

> $ gpg --print-md sha256 tmp1
> 
> # Now wait for gpg to hash the many gigabytes of random data....
> 
> $ rm tmp1
> 
> # And thus reclaim your disk space.
What attack will filling the disk with random bits stop that simply zeroing the disk won't stop?  If you're worried about national-lab level attacks ... who really knows what they are?  Some of the attacks against physical disks - like reading off the edge of the track - may well work just as well against random erasure as against zeroing.  And at that level, SSD attacks are significant - in fact, many much lower-level labs could carry them out - and nothing you can do from the host can really protect you.

And using gpg checksumming as a replacement for 'sync' ... doesn't really work:  If the data is in memory buffers waiting to be written, gpg will (under the covers, inside the implementation for read()) get it from there.

                                                        -- Jerry