bk-create-nonce.php

<?php
echo '<input type="hidden" name="bk-ajax-nonce" id="bk-ajax-nonce" value="' . wp_create_nonce( 'bk-ajax-nonce' ) . '" />';
?>
<script type="text/javascript">
	jQuery( document ).ready( function ( $ ) {
		var data = {
			action: 'my_action',
			security: $( '#bk-ajax-nonce' ).val()
		};
		$.post( ajaxurl, data, function ( response ) {
			alert( "Response: " + response );
		} );
	} );
</script>