This is a logging system built using blockchain and programming language Solidity

Secure Logging System with Blockchain

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract SecureLogging {

    address private owner;
    mapping (uint => Log) private logs;
    uint private logCount;
    
    struct Log {
        uint timestamp;
        string message;
        string tag;
        string identity;
    }

    constructor() {
        owner = msg.sender;
        logCount = 0;
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "Only the contract owner can call this function.");
        _;
    }

    function addLog(string memory _message,string memory _tag,string memory _identity) public onlyOwner returns (uint) {
        logCount++;
        logs[logCount] = Log(block.timestamp, _message,_tag,_identity);
        return logCount;
    }

    function getLog(uint _logId) public view returns (uint, string memory,string memory,string memory) {
        require(_logId > 0 && _logId <= logCount, "Invalid log ID.");
        return (logs[_logId].timestamp, logs[_logId].message,logs[_logId].tag,logs[_logId].identity);
    }

    function getLogs() public view returns (Log[] memory) {
        Log[] memory allLogs = new Log[](logCount);

        for (uint i = 0; i < logCount; i++) {
            allLogs[i] = logs[i];
        }

        return allLogs;
    }

    function searchLogs(uint timestamp, string memory message, string memory tag, string memory identity) public view returns (Log[] memory) {
        Log[] memory matchingLogs = new Log[](logCount);
        uint matchingLogsIndex = 0;

        for (uint i = 0; i < logCount; i++) {
            if (logs[i].timestamp == timestamp || bytes(message).length == 0 || keccak256(bytes(logs[i].message)) == keccak256(bytes(message)) || bytes(tag).length == 0 || keccak256(bytes(logs[i].tag)) == keccak256(bytes(tag)) ||  keccak256(bytes(logs[i].identity)) == keccak256(bytes(identity))) {
                matchingLogs[matchingLogsIndex] = logs[i];
                matchingLogsIndex++;
            }
        }

        Log[] memory result = new Log[](matchingLogsIndex);
        for (uint i = 0; i < matchingLogsIndex; i++) {
            result[i] = matchingLogs[i];
        }

        return result;
    }

    function getLatestLog() public view returns (uint, string memory,string memory,string memory) {
        require(logCount > 0, "No logs found.");
        return (logs[logCount].timestamp, logs[logCount].message,logs[logCount].tag,logs[logCount].identity);
    }
}