how can a signature for a document on a blockchain be revoked?

Question

I am trying to do document verification through blockchain. The way I'm trying to do this is by digitally signing a document, and storing its hash on the blockchain. Now I'm trying to create a feature for revoking a document. How can this be achieved? For example: if Bob has been issued a driving licence by the government which has put his licence on a public blockchain, but is caught drunk driving and the government decides to revoke his licence. what would be a way to achieve this functionality so that when someone queries his licence they see the signature as invalid? or is there a different method I can use to achieve the same functionality?

Answer 1

As I see it you can simply store some extra data along with the hash. At its simplest it could be a boolean representing whether the original document is still valid.

Here's a very simplified example:

pragma solidity ^0.5.5;

contract Revoke {

    mapping(string => bool) validities;

    function store(string memory hash) public {
        validities[hash] = true;
    }

    function revoke(string memory hash) public {
        validities[hash] = false;
    }

    function isValid(string memory hash) public view returns (bool) {
        return validities[hash];
    }
}