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srm - secure remove (secure_deletion toolkit)


srm [-d] [-f] [-l] [-l] [-r] [-v] [-z] files


srm is designed to delete data on mediums in a secure manner which can not be recovered by
thieves, law enforcement or other threats. The wipe algorythm is based on the paper
"Secure Deletion of Data from Magnetic and Solid-State Memory" presented at the 6th Usenix
Security Symposium by Peter Gutmann, one of the leading civilian cryptographers.

The secure data deletion process of srm goes like this:

* 1 pass with 0xff

* 5 random passes. /dev/urandom is used for a secure RNG if available.

* 27 passes with special values defined by Peter Gutmann.

* 5 random passes. /dev/urandom is used for a secure RNG if available.

* Rename the file to a random value

* Truncate the file

As an additional measure of security, the file is opened in O_SYNC mode and after each
pass an fsync() call is done. srm writes 32k blocks for the purpose of speed, filling
buffers of disk caches to force them to flush and overwriting old data which belonged to
the file.


-d ignore the two special dot files . and .. on the commandline. (so you can execute
it like "srm -d .* *")

-f fast (and insecure mode): no /dev/urandom, no synchronize mode.

-l lessens the security. Only two passes are written: one mode with 0xff and a final
mode random values.

-l -l for a second time lessons the security even more: only one random pass is

-r recursive mode, deletes all subdirectories.

-v verbose mode

-z wipes the last write with zeros instead of random data


NFS Beware of NFS. You can't ensure you really completely wiped your data from the
remote disks.

Raid Raid Systems use stripped disks and have got large caches. It's hard to wipe them.

swap, /tmp, etc.
Some of your data might have a temporary (deleted) copy somewhere on the disk. You
should use sfill which comes with the secure_deletion package to ensure to wipe
also the free diskspace. However, If already a small file acquired a block with
your precious data, no tool known to me can help you here. For a secure deletion of
the swap space sswap is available.

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