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zkay: A Language for Private Smart Contracts on Ethereum

Zkay (pronounced as [zi: keɪ]) is a programming language which enables automatic compilation of intuitive data privacy specifications to Ethereum smart contracts leveraging (homomorphic) encryption and non-interactive zero-knowledge (NIZK) proofs. This repository provides a toolchain for compiling, deploying and using zkay contracts.

In addition to the instructions below, we refer to the following resources:

• The original research paper (zkay), which introduces the core concepts of zkay.
• The follow-up research paper (ZeeStar), which discusses how homomorphic encryption is introduced in zkay v0.3.
• The online documentation, which provides a tutorial, language reference and API documentation.

Further, the following documents describe the implementation details of zkay:

• The implementation of zkay v0.2 is discussed in a technical report.
• The implementation of zkay v0.3 is described in these notes.

Warning / Security Disclaimer

Zkay is a research project and its implementation should not be considered secure (e.g., it may contain bugs and has not undergone any security review)! Do not use zkay in a productive system or to process sensitive confidential data.

Prerequisites

Zkay requires python version 3.8 or later (we recommend using a separate conda environment). In addition, install the following dependencies using your system's package manager:

Debian/Ubuntu

sudo apt-get install default-jdk-headless git build-essential cmake libgmp-dev pkg-config libssl-dev libboost-dev libboost-program-options-dev

Arch Linux

sudo pacman -S --needed jdk-openjdk cmake pkgconf openssl gmp boost

Installation

Install zkay in editable mode as follows.

git clone https://github.com/eth-sri/zkay.git

# first, install the babygiant library (see zkay/babygiant-lib/README.md for troubleshooting)
cd zkay/babygiant-lib
bash ./install.sh
cd ..

# then, install zkay
pip install -e .

Using Docker

Alternatively, you can run zkay in a docker container using the provided Dockerfile in the install subdirectory:

make -C ./install run

Unit Tests

To run all unit tests, use:

python -m unittest discover --verbose zkay

Building the Docs

The documentation is hosted here. To build it locally, use the following commands (requires sphinx, sphinx-rtd-theme, and sphinx-autoapi):

cd docs
make html

The above commands create a tree of HTML files in _build/html. Developers with sufficient access rights can publish the documentation on GitHub Pages using the script publish_gh_pages.sh.

Usage

See the online documentation for a tutorial on how to use zkay. Below, we only show a summary of available commands.

Note: zkay supports tab completion in Bash shells via the argcomplete package. To enable this feature, argcomplete must be installed and activated on your system (see instructions).

Type-check Contracts

To type-check a zkay file test.zkay without compiling it, run:

zkay check test.zkay

Strip zkay Features from Contract

To strip zkay-specific features from test.zkay and output the resulting (location preserving) Solidity code, run:

zkay solify test.zkay

The transformed code is printed to stdout.

Deploy Library Contracts

Zkay requires a backend-dependent external public key infrastructure (PKI) contract and, depending on the proving scheme, additional library contracts to be deployed. These contracts can be compiled and deployed using the commands:

zkay deploy-pki <account>
zkay deploy-crypto-libs <account>

The account parameter specifies the wallet address from which the deployment transaction should be issued.

Note: The groth16 proving scheme (enabled by default) does not require additional libraries, in which case zkay deploy-crypto-libs is not required.

Note: The default eth-tester blockchain backend does not require manually deploying the PKI or library contracts.

Compile Contracts

To compile a zkay file test.zkay, run:

zkay compile [-o "<output_dir>"] test.zkay

This performs the following steps:

• Type checking (equivalent to zkay check)
• Compilation to Solidity
• NIZK proof circuit compilation and key generation
• Generation of the contract interface contract.py, which can be used to transparently interact with a deployed zkay contract

Exporting a Contract Package

To package a zkay contract that was previously compiled with output directory <compilation_output>, run:

zkay export [-o "<output_filename>"] "<compilation_output>"

This creates an archive containing the zkay code, the NIZK prover and verifier keys, and manifest file. The recommended file extension for the archive is *.zkp. This archive can be distributed to users of the contract.

Importing a Contract Package

To unpack and compile a contract package contract.zkp:

zkay import [-o "<unpack_directory>"] contract.zkp

Interacting with a Contract

Assume you have compiled a file test.zkay using zkay compile -o "output_dir" (or imported an archive contract.zkp using zkay import -o "output_dir" contract.zkp), you can open an interactive shell for deploying and interacting with the contract as follows:

zkay run output_dir
>>> ...

The python shell provides the following commands:

• help(): Get a list of all contract functions and their arguments
• user1, user2, ..., userN = create_dummy_accounts(N): Get addresses of pre-funded test accounts for experimentation (only supported in eth-tester and ganache backends)
• handle = deploy(*constructor_args, user: str): Issue a deployment transaction for the contract from the account user (address literal).
• handle = connect(contract_addr: str, user: str): Create a handle to interact with the deployed contract at address contract_addr from account user. Fails if remote contract does not match local files.
• handle.address: Get the address of the deployed contract corresponding to this handle
• handle.some_func(*args[, value: int]): The account which created handle issues a zkay transaction which calls the zkay contract function some_func with the given arguments. Encryption, transaction transformation and proof generation happen automatically. If the function is payable, the additional argument wei_amount can be used to set the wei amount to be transferred.
• handle.state.get_raw('varname', *indices): Retrieve the current raw value of state variable name[indices[0]][indices[1]][...].
• handle.state.get_plain('varname', *indices): Retrieve the current plaintext value (decrypted with @me key if necessary) of state variable name[indices[0]][indices[1]][...].

Update Solc to Latest Compatible Version

To download and install the latest compatible version of solc (requires internet connection):

zkay update-solc

Third-party Libraries

Zkay is based on various third-party source code and libraries:

• zkay-libsnark (fork of libsnark): Downloaded and built during setup of zkay.
• zkay-jsnark (fork of jsnark): Bundled in zkay/jsnark_interface/JsnarkCircuitBuilder.jar.
• pygments-lexer-solidity: See docs/custom_highlighting/zkay_lexer.py.
• solidity-BN256G2: See zkay/compiler/privacy/bn256g2.sol.
• solidity alt_bn128 pairing library: See zkay/compiler/privacy/library_contracts.py.
• solidity-antlr4: See zkay/solidity_parser/Solidity.g4.
• Bouncy Castle Crypto APIs for Java: Bundled in zkay/jsnark_interface/bcprov-jdk15on-1.64.jar.
• sapling_jubjub: See zkay/transaction/crypto/babyjubjub.py.
• arkworks rust libraries: Dependencies of babygiant-lib.

See LICENSE-3RD-PARTIES for license information on third-party source code and libraries.

CCS 2019 Evaluation

The contracts evaluated in the CCS 2019 paper can be found in the folder eval-ccs2019. The scenarios have been adapted to use zkay's new frontend. The original artifact (zkay version 0.1) evaluated in the CCS 2019 paper can be found under the tag ccs2019.

S&P 2022 Evaluation

The artifact evaluated in the S&P 2022 paper (ZeeStar) can be found under the tag sp2022. The example contracts and instructions on how to reproduce the results can be found in the folder eval-sp2022.

Citing this Work

You are encouraged to cite the following research paper if you use zkay for academic research.

@inproceedings{steffen2019zkay,
    author = {Steffen, Samuel and Bichsel, Benjamin and Gersbach, Mario and Melchior, Noa and Tsankov, Petar and Vechev, Martin},
    title = {Zkay: Specifying and Enforcing Data Privacy in Smart Contracts},
    year = {2019},
    isbn = {9781450367479},
    publisher = {Association for Computing Machinery},
    address = {New York, NY, USA},
    url = {https://doi.org/10.1145/3319535.3363222},
    doi = {10.1145/3319535.3363222},
    booktitle = {Proceedings of the 2019 ACM SIGSAC Conference on Computer and Communications Security},
    pages = {1759–1776},
    numpages = {18},
    location = {London, United Kingdom},
    series = {CCS ’19}
}

The following research paper presents how zkay is extended by homomorphic encryption (implemented in version 0.3 of zkay).

@inproceedings{steffen2022zeestar
    author = {Steffen, Samuel and Bichsel, Benjamin and Baumgartner, Roger and Vechev, Martin},
    title = {ZeeStar: Private Smart Contracts by Homomorphic Encryption and Zero-knowledge Proofs},
    year = {2022},
    booktitle={2022 IEEE Symposium on Security and Privacy (SP)}, 
}

The following technical report describes version 0.2 of zkay, which introduces many vital features such as real encryption.

@techreport{baumann2020zkay,
    title={zkay v0.2: Practical Data Privacy for Smart Contracts},
    author={Nick Baumann and Samuel Steffen and Benjamin Bichsel and Petar Tsankov and Martin Vechev},
    year={2020},
    eprint={2009.01020},
    url={https://arxiv.org/abs/2009.01020}
}