zkRelay

zkRelay facilitates a chain-relay from Bitcoin to Ethereum through zkSNARKS.

The detailed concept of zkRelay can be found in the respective research paper published at IEEE Security & Privacy on the Blockchain 2020: Preprint at Cryptology ePrint Archive / IEEE Conference Proceedings

The implementation is based on ZoKrates and performs off-chain Bitcoin header chain validations, while only the resulting proof is submitted to the target ledger.

The workflow of zkRelay is seperated into three steps, ZoKrates code generation, a one-time compilation and setup step and many-time validation.

As a prerequisite, ZoKrates needs to be installed in version 0.5.1 for both steps.

Setup zkRelay-CLI and env

Our cli requires you to use python version 3.

Before you install the required dependencies, we recommend to set up a venv:

$ python3 -m venv venv
$ . venv/bin/activate

To install the required python dependencies and setup the CLI, run:

$ pip3 install -r python-requirements

Now, you can use the cli by executing (in the venv):

$ zkRelay

In $PROJECT_DIR/conf/zkRelay-cli.toml you can find the configuration file for the cli.

Generate ZoKrates code

As the ZoKrates code is static for each distinct batch size, we provide a script to generate the corresponding ZoKrates code for a given batch size n:

$ zkRelay generate-files n

Compilation and Setup

In this step, the off-chain program is compiled, the setup for generatung proving and verification keys are executed and the smart contract verifier is generated. zkRelay integrates these three tasks in a single execution step, the setup:

$ zkRelay setup

The resulting zkRelay contract batch_verifier.sol has to be deployed using a tool of your choice. It references the generated verification contract verifier.sol which has to be available during deployment.

Off-chain validation

As a prerequisite, the zkRelay-cli needs a connection to a Bitcoin client. You have two options:
- Update the default values in the configuration file located under $PROJECT_DIR/conf/zkRelay-cli.toml -> bitcoin_client .
  - Possible config parameters:
    
    Parameter Description
    
    host Host of BC client
    
    port Port of BC client
    
    user Username for access to the BC client
    
    pwd Password for access to the BC client
- Pass your overriding parameters directly to the CLI when executing the following command (see zkRelay validate -h for parameters)
To validate a batch, run the following cmd, where n corresponds to the block number of the first block in the batch:

Parameter	Description
host	Host of BC client
port	Port of BC client
user	Username for access to the BC client
pwd	Password for access to the BC client

$ zkRelay validate n

The script retrieves the blocks from the Bitcoin client, formats them and uses ZoKrates to perform the off-chain validation. Thereafter it creates a proof for the given batch.
The resulting witness and proof are stored in the outputs folder
To retrieve a proof format that can be easily submitted, use ZoKrates' print command (the target proof has to be moved to the same directory and name proof.json):

$ zokrates print-proof --format [remix, json]

Create inclusion proofs for intermediary blocks

zkRelay provides a mechanism for adding intermediary blocks of previously submitted batches to the relay contract. For this purpose, a Merkle tree is generated during the off-chain block validation. The resulting Merkle root is stored within the contract. To submit an intermediary block to the contract, the Merkle root is generated within a ZoKrates program. The correctness of the off-chain execution is again validated by the relay contract. Only if the execution was correct and the computed Merkle root is equivalent with the Merkle root stored for a given batch, the submitted block header is stored. Thereafter, it can be used for securely to derive SPVs and so forth.

As the inclusion proof is conducted within a ZoKrates program, a setup is required before off-chain execusions are possible. The process is similar to the setup of the off-chain block validation program described previously.

Setup

$ zkRelay setup-merkle-proof

The respective files are stored in the mk_tree_validation folder.

Generation of inclusion proofs

To generate an inclusion proof for a block with block number n, execute the following cmd:

$ zkRelay create-merkle-proof n

The proof is stoed in ./mk_tree_validation/proof.json
To retrieve a proof format that can be easily submitted, use ZoKrates' print command from within the mk_tree_validation folder:

$ zokrates print-proof --format [remix, json]

petscheit / zkRelay