I recently started to learn how to use ZoKrates. The default installation only supports the proving scheme G16, which is malleable, that is, an attacker can easily generate another valid proof if he/she sees a valid proof.

It took me quite some time to figure out how to properly install ZoKrates on my Mac to enable the other two proving schemes and I feel like it is worth sharing with others.

1. Download ZoKrates using git clone https://github.com/Zokrates/ZoKrates.git

2. Modify $zokrates_home/zokrates_core/build.rs as follows:

   let libsnark = cmake::Config::new(libsnark_source_path)
           .define("WITH_SUPERCOP", "OFF")
           .define("WITH_PROCPS", "OFF")
           .define("CURVE", "ALT_BN128")
           .define("USE_PT_COMPRESSION", "OFF")
           .define("MONTGOMERY_OUTPUT", "ON")
           .define("BINARY_OUTPUT", "ON")
           .build();       

3. Make sure that openssl has been installed. Otherwise, install via brew install openssl.

4. Set export PKG_CONFIG_PATH=$(brew --prefix openssl)/lib/pkgconfig

5. Set export WITH_LIBSNARK=1

6. Run build_release.sh and the compiled executable zokrates can be found in $zokrates_home/target/release/

As a simple example, I want to prove my knowledge of two secret numbers whose sum equals 10. To do that using ZoKrates, we first write file simple_add.code as:

def main(private field a, private field b) -> (field):
    a+b == 10
    return 1

Suppose we want to use proving scheme GM17 and the secret numbers are 3 and 7, we do:

zokrates setup -s gm17
zokrates export-verifier -s gm17
zokrates compute-witness -a 3 7 
zokrates generate-proof -s gm17

Within the current folder, we can see the exported contract verifier.sol that implements the verification function and the proof proof.json.