byzcoin

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ByzCoin Example

The files in this directory give an overview of how to use ByzCoin with your own code and writing your own contract. Before reading on, be sure that you read and understand the documentation found here: [https://github.com/dedis/cothority/tree/master/byzcoin/README.md](ByzCoin Documentation).

When writing a contract, you first have to think what you want to store in ByzCoin and how you can verify if the data is valid or not. Your contract will be called by Instructions inside ClientTransactions in two ways:

1. when a client requests a new instance he will send a Spawn to a darc that has the appropriate permissions and this darc will spawn your new contract
2. when a client sends an Instruction directly to the already existing instance

When the ClientTransaction arrives at the ByzCoin service, it will be queued, and then batched together with other transactions to form a new block. This block will be verified by all nodes, so every node will run all the ClientTransactions and verify that the output is correct. One very important caveat is that you should not use any random method in your contract, as this will be different on every node!

Key Value storage and update

This is a better version of the [https://github.com/dedis/cothority/tree/master/byzcoin/contracts/value.go](value contract). It holds a set of key/value pairs and lets you update, add and remove key/value pairs.

• Spawn takes all the arguments and stores them as key/value pairs in the instance
• Invoke:update goes over all arguments and either updates the key/value pair if the key already exists, or adds a new key/value pair, or deletes it, if the value is empty.

Both of these options are protected by the darc where the value will be stored. A typical use case is:

1. an admin creates a darc_user for the new user with the two rules:

• Spawn:keyValue
• Invoke:update

Java API

There is a java-api with all the necessary definitions to interact with ByzCoin and the new contract. The java files are stored here: [../external/java](java files)

For testing, it uses a docker image. Before running the java tests, you need to create the docker file:

cd (go env GOPATH)/github.com/dedis/cothority_template
make docker

Every time you change the go-code, you need to update the docker.

Files

The following files are in this directory:

• service.go only serves to register the contract with ByzCoin. If you want to give more power to your service, be sure to look at the [../service](service example).
• keyvalue.go defines the contract
• proto.go has the definitions that will be translated into protobuf

A word on ProtoBuf

Usually you start protobuf with a .proto file and then translate it to different languages. Because we're using the dedis/protobuf library, our definitions reside in the proto.go files and are translated using proto.awk into .proto files. These files are then used to create the java and javascript definitions.