Adding contracts

Contracts/DigitalSignature.sol

+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+contract DigitalSignature {
+    address public admin;
+    mapping(address => bool) public whitelisted;
+
+    constructor() {
+        admin = msg.sender;
+    }
+
+    struct DigitalSignatureOwnership {
+        address owner;
+        string hash;
+        string digitalSignature;
+    }
+
+    mapping(string => DigitalSignatureOwnership) public digitalSignatures;
+
+    modifier onlyAdmin() {
+        require(msg.sender == admin, "Caller is not the admin");
+        _;
+    }
+
+    modifier onlyWhitelisted() {
+        require(whitelisted[msg.sender] == true, "Not whitelised");
+        _;
+    }
+
+    function addWhitelist(address trustworthy) public onlyAdmin {
+        whitelisted[trustworthy] = true;
+    }
+
+    function addOrUpdateDigitalSignature(
+        string memory _ref,
+        string memory _hash,
+        string memory _digitalSignature
+    ) public onlyWhitelisted {
+        DigitalSignatureOwnership memory digSigCurrent = digitalSignatures[_ref];
+        if (digSigCurrent.owner == address(0)) {
+            digSigCurrent.owner = msg.sender;
+            digSigCurrent.hash = _hash;
+        } else {
+            require(digSigCurrent.owner == msg.sender, "Not owner");
+        }
+
+        require(keccak256(bytes(digSigCurrent.hash)) == keccak256(bytes(_hash)), "Different hash");
+        digitalSignatures[_ref].digitalSignature = _digitalSignature;
+    }
+
+    function getDigitalSignature(string memory _ref) public view returns (string memory) {
+        return digitalSignatures[_ref].digitalSignature;
+    }
+}
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