Hiding values from calculation (zkSnarks?)

Question

In reference to a calculation of a hidden stored value.

For example: Account A & Account B Both have hidden stored values.

Accounts A cannot know B's hidden stored value, and vice versa.

Is it possible for A to interact with B's value, without knowledge of B's hidden value.

Could a smart contract be written to allow A to interact with B's value without knowing (in this case compare and change the value?)

Im assuming this icovered by zkSnarks?

Answer 1

Short answer is Yes, but depends on what you really need. The following contract stores the age of the person and without revealing it, can "certify" if the person is a teen or an adult. You can also increase the age without knowing it.

pragma solidity ^0.4.25;

contract ZK {
    uint private age;

    constructor (uint _age) public {
        age = _age;
    }

    function isAdult() public view returns (bool) {
        return (age >= 18);
    }

    function isTeen() public view returns (bool) {
        return (age >= 13 && age <= 19);
    }

    function addYears (uint _years) public {
        age += _years;
    }
}

Of course the age variable is not really "hidden", but, as you said, is "stored" on the blockchain. From the point of view of a smart contract it is hidden...