Local mode

Summary

Local mode of the client application provides two commands, distribute and reconstruct. Distribute command reads a given file and computes n shares over m groups with a data threshold k and a group threshold l. It means totally n shares will be made from a secret file, and you must have at least k shares from at least l different groups to reconstruct the secret.

Reconstruct command does the reconstruction process; i.e. it reads at least k share files and reconstruct the secret file.

Distribute command

$ cgss local distribute <file> <group threshold> <data threshold> <allocation>

This command reads a secret file <file> and makes share files. Allocation takes a comma separated share assignment information. For example, you want to allocate two shares to the first group, three shares to the second one, and one share to the last group, the allocation value is 2,3,1.

The produced share files has the original filename as the prefix, and the j-th share for the i-th group has suffix .i.j.xz.

This command also takes an optional flag --chunk to specify the byte size of each chunk. The given secret file is divided to chunks based on this size and distributed in shares.

Reconstruct command

$ cgss local reconstruct <file>...

This command reconstructs a secret from a list of share files. It produces a file based on the given share’s file name by removing the above suffix. For example, if the names of share files are sample.txt.1.1.xz, sample.txt.2.1.xz, …, then the default file name of the reconstructed secret will be sample.txt.

You can use --output flag to use another file name.

Tutorial

Suppose secret.dat is a secret file and distributing it allocations share to three groups so that each group has two shares with both group and data threshold are 3, the following commands creates totally six shares:

$ cgss local distribute secret.dat 3 3 2,2,2

The above command creates a set of secrets, secret.dat.1.1.xz, secret.dat.1.2.xz, secret.dat.2.1.xz, …, secret.dat.3.2.xz. We can store each share file into a different storage in order to prevent information leakage, and now we can delete the secret file secret.dat.

To reconstruct the secret from shares, we must to collect at least 3 share files from each group. Suppose we have secret.dat.1.1.xz, secret.dat.2.1.xz, and secret.dat.3.1.xz.

$ cgss local reconstruct secret.dat.1.1.xz secret.dat.2.1.xz secret.dat.3.1.xz

The above command reconstructs the secret and stores it as secret.dat.

Installation

If you’re a Homebrew user, you can install the client application by

$ brew tap itslab-kyushu/cgss
$ brew install cgss

Compiled binaries for some platforms are available on Github. To use these binaries, after downloading a binary to your environment, decompress and put it in a directory included in your $PATH.

You can also compile the client application by yourself. To compile it, you first download the code base:

$ git clone https://github.com/itslab-kyushu/cgss $GOPATH/src/itslab-kyushu/cgss

Then, build the client application cgss:

$ cd $GOPATH/src/itslab-kyushu/cgss/client
$ go get -d -t -v .
$ go build -o cgss

To build the command, Go > 1.7.4 is required.