Overview

Implementation of the game Hangman in Zokrates using zero-knowledge proofs. ZKPs are used to prove that a letter exists in a secret word without revealing the word itself.

Deployed on Goerli, Scroll Testnet, Polygon, and can be accessed at https://hangman-zk.netlify.app/

The repo contains:

• Zokrates program
• Solidity contract
• React front-end

Documentation related to the contract and the UI is in the respective sub-folders.

Zokrates Program

The game is implemented in one Zokrates program that accepts:

• A secret word as a private input (an array of ASCII characters with a size of 16)
• A sha-256 hash of the secret word as a public input
• A symbol(letter) that we want to check

The output is a mask that indicates positions of the symbol within the word.

def main(private u8[16] word, u32[8] hash, u8 symbol) -> bool[16]

Example

Let's say the secret word is ethereum.

ASCII representation in decimal (padded with zeros to 16 characters) is:

101 116 104 101 114 101 117 109 0 0 0 0 0 0 0 0

Therefore the private input is:

[101, 116, 104, 101, 114, 101, 117, 109, 0, 0, 0, 0, 0, 0, 0, 0]

sha-256 hash of this input is:

e02e50cf081ce920c2ca5032927f13a07701045f609060b2f9fb9065c4c893c2

Converted to u32[8] it's:

[3761131727, 136112416, 3268038706, 2457801632, 1996555359, 1620074674, 4194013285, 3301479362]

Letter that we want to check is e which is 101

The output of the program will be [true, false, false, true, false, true, false, false, false, false, false, false, false, false, false, false]

Run the program

zokrates compile -i hangman.zok
# Compiling hangman.zok
 
# Compiled code written to 'out'
# Number of constraints: 26519

zokrates setup
# Performing setup...
# Verification key written to 'verification.key'
# Proving key written to 'proving.key'
# Setup completed

zokrates compute-witness -a 65 74 68 65 72 65 75 6d 00 00 00 00 00 00 00 00 3761131727 136112416 3268038706 2457801632 1996555359 1620074674 4194013285 3301479362 101
# Computing witness...
# Witness file written to 'witness'

zokrates generate-proof
# Generating proof...
# Proof written to 'proof.json'

zokrates verify
# Performing verification...
# PASSED

TODO

• Introduce seed to prevent brute-forcing
• Limit to lowercase letters only
• Event listening for game actions
• Increase wallet support
• Test coverage
• Optimize circuit