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Casper the Friendly Javascript Ghost 👻

Travis CI

A Javascript implementation of Vlad Zamfir's Casper the Friendly Ghost CBC consensus protocol.

The paper is based primarily from the CasperTFG paper by Vlad Zamfir and also draws inspiration from the current Python implementation by Nate Rush, Danny Ryan, Vlad Zamfir, Karl Floersch and others.

Note: this project is still in very early stages. It is likely faulty and is subject to significant change. This should only be the case for the next few weeks (i.e., until mid-April 2018).

Casper: GHOST or finality gadget?

There are two distinct projects, both sharing the name "Casper".

  1. Casper Correct-by-Construction (aka Casper CBC, "Vlad's Casper")
  2. Casper the Friendly Finality Gadget (aka Casper FFG, "Vitalik's Casper)

This project is implementing (1), Casper CBC. This project seeks to simulate a network of peers coming to consensus across a loosely-asynchronous network whilst tolerating some faulty behaviour. This project is not (at this stage) concerned with any economic incentive mechanisms (i.e., it does not implement staking).

For more detail, see Jon Choi's "Ethereum Casper 101" (the "A Tale of Two Caspers" section):


The motivation for creating a new implemenation can be seen in the following:

  • Browser compatibility: running Casper sims in the browser should hopefully improve accessibility for those who wish to understand how Casper TFG work.
  • Diversity: mutliple implementations will hopefully allow for a wider range of perspectives.


This codebase has been developed in node v9.8.0. It uses some ES6 syntax and therefore some older versions of node will be incompatible.


For first time usage, clone the repo and run npm install in the repo directory.


The command-line script is casper.js and it has a fully featured argument parser; you can run ./casper.js -h (or node casper.js -h) to view help.

Random Binary Conensus Simulator

Currenly the only available command-line function is $ ./casper.js random which will run a binary consensus simulation where the following conditions are randomised:

  • The initial estimate of the validators. I.e., if they choose to start with a 0 or 1.
  • The senders and recipients of messages for each round.

In this binary simulation, there is no concept of "rounds". I.e., validators will not wait for some minimum threshold of votes before they issue a new estimate to the network. This means that it is possible for all validators to reach consensus on a value which was not average of all validator starting points. E.g., there exists a pattern of messages so that validators with starting points (0, 1, 1) can come to consensus on the 0 value.

The random sim will output a JSON object to console with the following properties:

  • initialConfig: The starting values and weights for each validator. I.e., what their states were before the consensus process started.
  • decisions: The estimates and associated safety ratios for each validator, as of simulation completion.
  • majorityFlip: true if the majority of validators decided upon the average of all staring positions.
  • messageLogLength: the number of messages which were sent whilst forming consensus.


The following example is running a simulation with the following attributes:

  • -n 3: Three validators will form consensus.
  • -s 0.5: The simulation will end once at least half (0.5) of validators consider themselves safe with half (0.5) the other validators. We do not wait for all validators to get to target safety because this can take a very long time with random message propagation.
$ ./casper.js random -n 3
  "intialConfig": [
      "name": "0",
      "weight": 100,
      "startingPoint": 1
      "name": "1",
      "weight": 100,
      "startingPoint": 0
      "name": "2",
      "weight": 100,
      "startingPoint": 0
  "decisions": {
    "0": {
      "estimate": 0,
      "safe": true,
      "safety": 1
    "1": {
      "estimate": 0,
      "safe": true,
      "safety": 0.6666666666666666
    "2": {
      "estimate": 0,
      "safe": true,
      "safety": 1
  "majorityFlip": false,
  "messageLogLength": 6

My laptop can comfortably run ./casper.js random -n 100 in about 5 seconds.


Tests are written in Mocha, run them with $ npm run test.


To reduce the processing burden of equivocation detection, some new requirements for message formation were added:

  • A validator must include a message from themself in the justifications of each message they send (unless the message is an "initial message" which does not provide any justification).
  • A validator must not include two messages from the same validator in their justification. This only applies to the first level of justifications, not the justifications of justifications (and their justifications, and so on).

Any validator which defies these rules will be flagged as Byzantine.


  • Binary Consensus
    • Estimator
    • Byzantine Fault Detection
    • Safety Oracle
    • Command-line simulator
    • Shared database for faster sims
  • Blockchain Consensus
  • Full code coverage for tests
  • Linting
  • Graphical simulation (to be implemented as a separate project)


JS implementation of Vlad Zamfir's CBC Casper TFG





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