private String calculateCheckSum(NodeRef node) throws Exception {

		MessageDigest md = MessageDigest.getInstance("SHA-256");
		ContentReader reader = this.serviceRegistry.getContentService().getReader(node, ContentModel.PROP_CONTENT);
		InputStream is = reader.getContentInputStream();
		DigestInputStream dis = new DigestInputStream(is, md);
		byte[] digest = md.digest();
		return digest.toString().toLowerCase();

	}