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Gfsecret - Secret sharing tools
Gfsecret is a set of tools to facilitate secret sharing according to the Adi Shamir’s secret sharing scheme.
Two tools are provided: gfsec-split will split a file into several shares, and gfsec-use will reconstruct the original file from some of the shares.
Both tools use the concept of a share URI, which is a way of describing a share with a URI-like syntax. The gfsec-split program uses them to know where to dispatch the generated shares, and gfsec-use uses them to know where to search for shares in order to reconstruct the original file.
A share URI has the following form:
The scheme part indicates the method to use to access the share, and
file (to access a share on the local filesystem),
access a share on an external volume identified by its label),
(to access a share on an external volume identified by its UUID), or
mtp (to access a share on a MTP-compliant device identified by its
The authority part identifies the storage device. It's the volume's
label when using the
label:// scheme, its UUID when using the
uuid:// scheme, and the device's serial number when using the
scheme. In the
file:// scheme, that part should be empty.
The path part is the pathname to the share file on the device.
Finally, the parameters part, which is optional, may specify options
key=value pairs. Currently, valid options are
specifies a SHA2-256 hash that the share should match, and
which indicates that the corresponding share is not actually a share but
contains the whole secret.
The gfsec-split program allows to split a file into M shares, dispatch them at distinct locations (including external storage devices), then delete the original file.
The program takes the path to the file to split as its first positional argument. The remaining positional arguments are URIs describing the shares to create using the URI syntax described above.
Consider the following example:
gfsec-split -n 2 \ /home/alice/mysecret \ file:///home/alice/.local/share/gfsecret/mysecret \ label://USBSTICK/mysecret \ mtp://RF2GB6X704P/Documents/mysecret
This will split the file
/home/alice/mysecret into three shares: one
in Alice's home directory on the local filesystem, one on a USB storage
device with the label
USBSTICK, and one on a MTP-compliant device with
the serial number
RF2GB6X704P. Two shares will be needed to
reconstruct the original file, which means in this case that one of the
two removable devices will have to be present.
Note that the shares need not to have the same basename than the original file.
Once the file is splitted, gfsec-split will delete the original file
(unless you used the
-k option), and will create a configuration file
$XDG_CONFIG_HOME/gfsecret/mysecret (or at any location specified by
-c option) which could later be used by gfsec-use.
If you move the shares around after they have been generated, you should take care of updating that configuration file, otherwise gfsec-use will be unable to fetch the shares.
As a convenience, before splitting you may call gfsec-split with the
-l option to list the available external volumes and their
identifiers. Alternatively, the
-i option may be used to both list
the available volumes and select those to use in an interactive menu.
The gfsec-use program allows to temporarily reconstruct a splitted file from shares that have been dispersed on several external devices.
The program needs a configuration file (by default,
$XDG_CONFIG_HOME/gfsecret/default.conf) which describes the shared
secret. If the original file has been splitted using the gfsec-split
program, the configuration file will have been automatically generated.
For example, here is the configuration file that could have been generated by the example above:
OUTFILE=/home/alice/mysecret MINSHARES=2 URI=file:///home/alice/.local/share/gfsecret/mysecret.024 URI=label://MYSTICK/mysecret.070 URI=mtp://RF2GB6X704P/Documents/mysecret.139
With such a configuration, gfse-use will attempt to reconstruct the
/home/alice/mysecret, using at least two of the three available
shares: one available on the local filesystem, one on a USB storage
device with the label
MYSTICK, and one on a MTP-compliant device with
the serial number
RF2GB6X704P. Gfsec-use will automatically detect
which devices are currently connected and will fetch from them the
Once the secret file has been reconstructed (if enough shares are available), gfsec-use will spawn a new shell (or any other program specified on its command line). When the shell (or the user-specified command) terminates, the reconstructed secret file will be automatically deleted.
Gfsecret depends on the following libraries at compile-time:
- libgfshare, which implements the secret sharing scheme proper (mandatory);
- libgcrypt, for the SHA-256 implementation and random number generation (mandatory);
- GIO, to access shares stored on external volumes (typically USB storage) (optional);
- libmtp, to access shares stored on MTP-compliant devices (optional).
Gfsecret is distributed under the terms of the GNU General Public License, version 3 or higher. The full license is included in the COPYING file of the source distribution.