Scalable collective authority

cothority, cryptography, decentralized, distributed-systems, secure, trust
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Navigation: DEDIS :: Cothority


The collective authority (cothority) project provides a framework for development, analysis, and deployment of decentralized, distributed (cryptographic) protocols. A given set of servers running these protocols is referred to as a collective authority or cothority. Individual servers are called cothority servers or conodes. The code in this repository allows you to access the services of a cothority and/or run your own conode. The cothority project is developed and maintained by the DEDIS lab at EPFL.

This is an overview of this README:

  • Documentation with links to different parts of the cothority
    • Topically ordered explains the different functional pieces from the cothority from a research point of view
    • Network ordered gives an overview from a networking point of view
    • Links collects links to other repositories, papers and webpages that are important to the cothority.
  • 1st steps giving two example apps ready to use
  • Participating on how to help us getting cothority even better

Don't forget that the cothority is part of a bigger environment.

Versioning and Roadmap

We started to work on a 6-monthly cycle, synched up with the semesters at EPFL. From March to August we keep changes on the master-branch non-API breaking. Master and the current version are kept in sync. In September, we allow API-breaking changes in master and only selectively update the stable branch. End of February we freeze the current development in a stable branch and start again a non-breaking period of 6 months.

The current master branch is stable till the end of August 2018. If you are starting new work with this repository, use instead. The source code for this stable branch is here.

During 2018, we hope to bring you the following new pieces:

  • Cross platform mobile application to interact with the pop service
  • ByzCoin implementation using key/value pair storage and protected by darcs

Other pieces we hope to achieve in 2018:

  • Distributed internet archival functionality
  • New status-website of the cothority
  • Catena integration for skipchains

And some projects are private for the moment, but we hope to make them public as soon as possible.

Release v3.1.0

The release introduces the notion of signature scheme for a given skipchain so that one can define which co-signing algorithm will be used to sign the forward links. This was necessary in the context of weaknesses in the BLS signature algorithm (see the paper). New skipchains will be created with BDN set as the signature scheme.

Because of a new scheme is default, that means that skipchains created after v3.1.0 won't work with older versions as they are not aware of the new scheme. However, existing skipchains will continue to operate normally. In summary, if you need to create skipchains after updating to v3.1.0, make sure every conode is at least using v3.0.1 aswell.

Release v3.2.0

A new field has been added to the DataHeader, Version, so that new features or upgrades can be coordinated between the conodes to only start using it when enough of them are up to date. The leader will propose a change of version when it detects that enough of the participants can reach a consensus. A successful increase of version is announced by an empty block that will act as a barrier between the previous and the new version. Its DataHeader data will contain the new version.

When creating a ledger, the default version is the most recent one and blocks are continously created with the previous block version until the leader proposes an upgrade. Note that the initial version is zero for backwards compatibility.

Another important change for this version is about how transactions are created as they need to include the ByzCoin version to use the correct hash function. The initial version of the hash was not taking the invoke command into account and it has been fixed for version one and higher. See below examples:


client := byzcoin.NewClient(id, roster)
tx := client.CreateTransaction(instr1, instr2)


ClientTransaction tx = new ClientTransaction(instrs, rpc.getProtocolVersion());


const tx = ClientTransaction.make(rpc.getProtocolVersion(), instr1, instr2);


The goal of the cothority is to collect projects that handle aspects of collective authorities to support scalable, self-organizing communities. In this document we present the apps that are directly runnable from the cothority, as well as links to the services and protocols used in the cothority.

A cothority is a set of conodes that work together to offer collective authority services. This is used to create distributed and decentralized services where no single point of failure can put the whole system in jeopardy. Conodes communicate with each other using protocols that are short-lived, specific tasks with an outcome that can be read by services. Each conode has a set of services that can be contacted through a well-defined API from the outside. The communication through the API is done with a homebrewn protobuf over websockets implementation.

Topically ordered

When looking at the cothority modules from a topical point of view, we can break it up as follows:

|                          |APPLICATIONS|                          |
|     Onchain-Secrets      +------------+     Proof of Personhood  |
|                                                                  |
|       ByzCoin                 Status            E-voting         |
|                        |BUILDING BLOCKS|                         |
| Consensus              +---------------+       Key Sharding      |
|  - Skipchain                                    - Re-encryption  |
|  - BFT         General Crypto    Messaging      - Decryption     |
|  - Collective   - Neff Shuffle    - Broadcast   - Distributed    |
|    Signing      - RandHerd        - Propagate     Key Generation |
|                 - RandHound                                      |
|                                                                  |


Applications in cothority use different libraries and are the highest level of abstraction in our framework.

Here you get a list of all applications in the cothority.

There is one very special application that is considered apart - it's the conode itself, which holds all protocols and services and can be run as a service on a server.

What a Conode can do for you

Building Blocks

Building blocks are grouped in terms of research units and don't have a special representation in code. Also, the General Crypto library is more of a collection of different protocols.

Here you get a list of all building blocks in the cothority.

Network Ordered

If we look at the cothority from a purely networking point of view, we can describe it as follows:

              |CLI, JavaScript, | Frontend        
              |Java             |                 
| Conode,     | Services        | Client to Conode
| Simulations |-----------------+                 
|             | Protocols       | Conode to Conode

Command Line Interfaces

Command line interfaces (CLI) are used to communicate with one or more conodes. All CLIs need to have one or more conodes installed. For the two CLIs in first steps, you can use the running conodes at EPFL/DEDIS. If you want to test the other CLIs, you might need to set up a small network (often 3 nodes is enough) of conodes on your local computer to test it.

Here you get a list of all available CLIs in the cothority.


Every app communicates with one or more services to interact with one or more conodes. This is a list of available services that can be contacted on a running conode. Some of the services are open to all, while others might require authentication through a PIN or a public key. Most of the apps and services have the same name, but some are not available as an app or have more than one app using it.

Here you get a list of all available services in the cothority.


Protocols are used to communicate between conodes and execute cryptographic exchanges. This can be to form a collective signature, create a consensus on a new state, or simply to propagate a new block from a skipchain. Some protocols are useful for different services, while others are very service-specific. Most of the protocols have a paper that is describing how the protocol should perform and that compares it to other protocols.

Here you get a list of all available protocols in the cothority.


Cothority grew up as a research instrument, so one of its advantages is to have a framework to create simulations and running them locally or on remote servers. Some of the protocols presented here do have the simulation code. Check it out here: Cothority Simulations.

First steps

If you're just curious how things work, you can check the status of our test network or create a collective signature using our running nodes:


To get the status of the conodes in the cothority, first install the status binary:

go install ./status

Now you can run it by giving the definition of the dedis-cothority on the command line:

status -g dedis-cothority.toml

Collective Signing

Another service available is fault-tolerant collective signing, or ftCoSi. It requests a collective signature from a set of conodes. The signature is created on a given input data. For installation, type:

go install ./blscosi/blscosi

Now you can create a file and have it signed by the cothority:

date > /tmp/my_file
blscosi sign -g dedis-cothority.toml /tmp/my_file | tee sig.json

And later somebody can verify the signature is correct by running the following command:

blscosi verify -g dedis-cothority.toml --signature sig.json /tmp/my_file

If everything is correct, it should print

[+] OK: Signature is valid.

Participating in the cothority

There are different ways to participate in the cothority project. A first step is to simply test the different CLI applications in this repository and tell us what were your difficulties or what you would like to use them for.

A next step is to set up your own conode and participate in the

Setting up your own conode

A conode is a server program that includes all protocols and services. It can be run on a public facing server, but for testing purposes it's also possible to set up a network on a machine and only let it be accessed locally.

  • conode is the cothority server, a special app that includes all services and protocols.
  • How to run a conode gives an overview of the environment where a conode can be run
  • DEDIS-cothority explains how to join the DEDIS-cothority


If you want to contribute to Cothority-ONet, please have a look at CONTRIBUTION for licensing details. Once you are OK with those, you can have a look at our coding-guidelines in Coding. In short, we use the github-issues to communicate and pull-requests to do code-review. Travis makes sure that everything goes smoothly. And we'd like to have good code-coverage.


The software in this repository is put under a dual-licensing scheme: In general all of the provided code is open source via GNU/AGPL 3.0, please see the LICENSE file for more details. If you would like to use Cothority in a way not allowed by the applicable license, please contact us to inquire about conditions to get a commercial license.


We are always happy to hear about your experiences with the cothority project. Feel free to contact us on our mailing list or by email.

Reporting security problems

This library is offered as-is, and without a guarantee. It will need an independent security review before it should be considered ready for use in security-critical applications. If you integrate Cothority into your application it is YOUR RESPONSIBILITY to arrange for that audit.

If you notice a possible security problem, please report it to

Who is using our code?

This is a list of people outside of DEDIS who is using our codebase for research or applied projects. If you have an interesting project that you would like to have listed here, please contact us at

  • Unlynx - A decentralized privacy-preserving data sharing tool
  • Medco - Privacy preserving medical data sharing
  • ByzGen - Tracking and secure storage of digital and hard assets
  • PDCi2b2 - Private Data Characterization for i2b2