function validateClaim(address _lostAddress, bytes32 _nonce) private view returns (bool){
        PreClaim memory preClaim = preClaims[msg.sender];
        require(preClaim.msghash != 0);
        require(preClaim.timestamp + preClaimPeriod <= block.timestamp);
        require(preClaim.timestamp + 2*preClaimPeriod >= block.timestamp);
        return preClaim.msghash == keccak256(abi.encodePacked(_nonce, msg.sender, _lostAddress));
    }