zkInterface, a standard tool for zero-knowledge interoperability

zkInterface is a standard tool for zero-knowledge interoperability between different ZK DSLs, gadget libraries, and proving systems. The zkInterface project was born in the ZKProof community.

See a live demo.

See the specification and the serialization format.

See the Rust implementation:

Integration instructions:

• Dalek Bulletproofs
• Bellman
• libSNARK
• Gadgets

Introduction

zkInterface is specification and associated tools for enabling interoperability between implementations of general-purpose zero-knowledge proof systems. It aims to facilitate interoperability between zero knowledge proof implementations, at the level of the low-constraint systems that represent the statements to be proven. Such constraint systems are generated by frontends (e.g., by compilation from higher-level specifications), and are consumed by cryptographic backends which generate and verify the proofs. The goal is to enable decoupling of frontends from backends, allowing application writers to choose the frontend most convenient for their functional and development needs and combine it with the backend that best matches their performance and security needs.

The standard specifies the protocol for communicating constraint systems, for communicating variable assignments (for production of proofs), and for constructing constraint systems out of smaller building blocks (gadgets). These are specified using language-agnostic calling conventions and formats, to enable interoperability between different authors, frameworks and languages.

A simple special case is monolithic representation of a whole constraint system and its variable assignments. However, there are a need for more richer and more nuanced forms of interoperability:

• Precisely-specified statement semantics, variable representation and variable mapping
• Witness reduction, from high-level witnesses to variable assignments
• Gadgets interoperability, allowing components of constraint systems to be packaged in reusable and interoperable form
• Procedural interoperability, allowing execution of complex code to facilitate the above

Current Status

Implementations

Frontends:

	Circuit Type	Export Circuits	Import Circuits
ZoKrates	R1CS	Yes	No
Libsnark	R1CS	Yes	No
Mir r1cs	R1CS	Yes	No
PySNARK	R1CS	Yes	No
Bellman	R1CS	Yes	Yes

Backends:

	Proving System
Bellman	Groth16
Dalek	Bulletproofs
Libsnark	BCTV14a

See also the WebAssembly modules used to build the live demo.

See the ecosystem/ folder for a collection of instructions to install and connect multiple systems.

Repository content

zkInterface.pdf	The interface specification
zkinterface.fbs	The gadget interface definition using FlatBuffers
examples/	An example circuit in binary and JSON
rust/	Cargo package zkinterface
rust/src/zkinterface_generated.rs	Generated Rust code
rust/src/reading.rs	Rust helpers to read messages
rust/src/writing.rs	Rust helpers to write messages
rust/src/cpp_gadget.rs	Rust helpers to interact with C++
rust/src/examples.rs	Example messages for a simple test circuit
rust/src/gadget_call.rs	Example gadget call in Rust
cpp/zkinterface_generated.h	Generated C++ code
cpp/gadget_example.cpp	Example gadget in C++
js/	NPM package zkinterface
js/zkinterface_generated.js	Generated JavaScript code
build.rs	Generate Rust and C++ code from zkinterface.fbs, and compile the C++ example
cpp/libsnark_integration.hpp	Libsnark support
cpp/libsnark_example.cpp	Libsnark gadget example

Testing

In the rust directory, run unit tests:

cargo test

The following commands will generate and print a file containing the messages Circuit, R1CSConstraints, and Witness for a toy circuit in rust/src/examples.rs:

cargo run example  > example.zkif
cargo run explain  < example.zkif

Generated code

Generated C++ and Rust code is included.

For other languages, install the FlatBuffers code generator (flatc). One way is to compile it with the following:

git clone https://github.com/google/flatbuffers.git
cd flatbuffers
cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release
make

Then run: flatc --LANGUAGE zkinterface.fbs

How to contribute

• In a frontend, implement a feature to export the circuits or gadgets to zkInterface format.
• In a proving system, support loading circuits from zkInterface buffers or files.

See the implementation guide section in the spec above for more details, and check out the existing implementations below.

Specification

The zkInterface specification document providers further context on the above, and defines the communication protocol and calling convention between frontends and backends:

• The logical content of messages being exchange.
• The serialization format of the messages (which is based on FlatBuffers and may be used in-memory, saved or streamed).
• A protocol for building a constraint system from gadget composition.
• Technical recommendations for implementation.

Integration examples

Dalek Bulletproofs

Install

git clone https://github.com/QED-it/bulletproofs.git
cd bulletproofs
cargo install --features yoloproofs --path .

Prove

zkif example - | zkif_bulletproofs prove

Verify

zkif example - | zkif_bulletproofs verify

Bellman

Install

git clone https://github.com/QED-it/zkinterface-bellman.git
cd zkinterface-bellman
cargo install --path .

Validate / Print

zkif example - | zkif_bellman print

Prove

zkif example - | zkif_bellman prove

Verify

(not implemented)

libSNARK

Install

export ZKINTERFACE_LIBSNARK_PATH=$PWD/dist
PATH=$PATH:$ZKINTERFACE_LIBSNARK_PATH/bin

git clone https://github.com/QED-it/zkinterface.git
cd zkinterface/cpp

./deps.sh

cmake -DCMAKE_INSTALL_PREFIX=$ZKINTERFACE_LIBSNARK_PATH . && make install

Setup

zkif example - | zkif_snark setup

Prove

zkif example - | zkif_snark prove

Verify

zkif example - | zkif_snark verify

Gadgets in Rust

[dependencies]
    zkinterface = { version = "1.1.3" }
    zkinterface-libsnark = { version = "1.1.3" }

See examples in libsnark-rust/src/test.rs

Gadgets in C / Foreign Function Interface

CFLAGS="-I $ZKINTERFACE_LIBSNARK_PATH/include -L $ZKINTERFACE_LIBSNARK_PATH/lib -l ff -l gmp -l zkif_gadgetlib"

See API in gadgetlib.h

See FFI call in Rust libsnark-rust/src/gadgetlib.rs

Limitations

This first revision focuses on non-interactive proof systems (NIZKs) for general statements (i.e., NP relations) represented in the R1CS/QAP-style constraint system representation. Extension to other forms of constraint systems is planned.

The associated code is experimental.

See the specification document for more information about limitations and scope.