[Cryptography] Rubber-hose resistance?

[Ray Dillinger]

On 12/20/2017 02:27 PM, Jerry Leichter wrote:
>> 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?  

Many filesystems, on catching sight of a bunch of logical sectors all
filled with the same content, will happily store all of them on the
same physical space.

For several reasons this is especially true with zero; the filesystem
may be keeping a list of sector ranges that are zero, and just update
one of the ranges to include the sector you wrote, or add a new range if
neither of the adjacent sectors are already part of an existing range.

Obviously, when this happens the information is still there on the
physical media, and it doesn't take a national lab level attack to get
it out; it takes somebody poking the zero-sectors list.

Random bits are incompressible, and therefore no file manager is going
to discover that it can double them up or compress them to avoid the
need to overwrite the stuff you intend should be in fact overwritten.

				Bear

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