对pdf文档进行数字签名

写了个项目，要求对DF文档进行数字签名，核心代码如下：

public static void sign() throws Exception{
		// Security.addProvider(new
		// org.bouncycastle.jce.provider.BouncyCastleProvider());

		KeyStore ks = KeyStore.getInstance("pkcs12");
		ks.load(new FileInputStream(cert_path), cert_pwd.toCharArray());
		String alias = (String)ks.aliases().nextElement();
		PrivateKey key = (PrivateKey)ks.getKey(alias, cert_pwd.toCharArray());
		Certificate[] chain = ks.getCertificateChain(alias);
		PdfReader reader = new PdfReader("F:\\feiqing.pdf");
		FileOutputStream fout = new FileOutputStream("F:\\signed.pdf");
		PdfStamper stp = PdfStamper.createSignature(reader, fout, '\0');
		// 最后一个参数为true，允许对同一文档多次签名
   		//stp = PdfStamper.createSignature(reader, fout, '\0', null, true);
		
		PdfSignatureAppearance sap = stp.getSignatureAppearance();
		sap.setCrypto(key, chain, null, PdfSignatureAppearance.WINCER_SIGNED);
		sap.setReason("I'm the author");
		sap.setLocation("Ma");
		// comment next line to have an invisible signature
		sap.setVisibleSignature(new Rectangle(100, 100, 200, 200), 1, null);
				
		stp.close();
	}