function login() {
    var username = document.getElementById ( "form-username" ).value;
    var password = document.getElementById ( "form-password" ).value;
    password = SHA512 ( password );
    var xhr = new XMLHttpRequest ( );
    xhr.open ( "POST", 'index.php', true );
    xhr.onreadystatechange = function ( ) {
        if ( xhr.readyState == XMLHttpRequest.DONE && xhr.status == 200 ) {
            if (xhr.status == 200) {
                var response = xhr.responseText;
                response = JSON.parse ( response );
                if ( response [ 'status' ] == 200 ) {
                    window.location.href = "/";
                }
                else {
                    document.getElementById ( "responsetext" ).innerHTML = response [ 'error_message' ];
                }
            }
        else {
                console.log ( 'Server communication error: ' + xhr.statusText );
            }
        }
    }
    data = new FormData ( );
    data.append('username', username);
    data.append('password', password);
    xhr.send( data );
}