Chapter 1: Making the Kernel

In this chapter, you’ll learn to create and initialize the kernel, a hidden value inside the Walnut, and increment it by implementing a shake function. Estimated time: ~10 minutes.

Defining the kernel

The kernel is the hidden number inside the Walnut. Using Seismic’s suint256 type, the kernel is shielded on-chain. Open up packages/contracts/Walnut.sol and define the kernel as a state variable and initialize it in the constructor:

// SPDX-License-Identifier: MIT License
pragma solidity ^0.8.13;

contract Walnut {
    suint256 kernel; // The shielded kernel (number inside the Walnut)

    // Constructor to initialize the kernel
    constructor(suint256 _kernel) {
        kernel = _kernel;
    }
}

Add a shake function

Next, let’s implement a function to increment the kernel. The shake function takes an suint256 parameter, _numShakes which specifies the amount to increment the kernel by.

function shake(suint256 _numShakes) public {
    kernel += _numShakes; // Increment the kernel value using the shielded parameter.
    emit Shake(msg.sender); // Log the shake event.
}

What's happening here?

Since shake takes in an stype as one of its parameters, it is key that no information about this parameter (in this case, the number of shakes) is leaked at any time during the function call. This means that the value of _numShakes is known only to the function caller and is encrypted on-chain.

The function also updates a state variable (kernel ) and hence constitutes a state transition, which makes a call to this function a shielded write.