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.

Share URIs

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:

scheme://authority/path?parameters

The scheme part indicates the method to use to access the share, and can be file (to access a share on the local filesystem), label (to access a share on an external volume identified by its label), uuid (to access a share on an external volume identified by its UUID), mtp (to access a share on a MTP-compliant device identified by its serial number), or http or https (to access a share on a remote web server).

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, the device's serial number when using the mtp:// scheme, and the hostname when using the http(s):// 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 as key=value pairs. Currently, valid options are sha256, which specifies a SHA2-256 hash that the share should match, and share=no, which indicates that the corresponding share is not actually a share but contains the whole secret.

gfsec-split

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 split, gfsec-split will delete the original file (unless you used the -k option), and will create a configuration file in $XDG_CONFIG_HOME/gfsecret/mysecret (or at any location specified by the -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 single -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.

gfsec-use

The gfsec-use program allows to temporarily reconstruct a split 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 split 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 file /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 corresponding shares.

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.

Install

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).
• libcurl, to access shares stored on remote web servers (optional).

Copying

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.

Homepage and repository

The project is located at https://incenp.org/dvlpt/gfsecret.html. The latest source code is available at https://git.incenp.org/damien/gfsecret.