对pdf文档进展数字签名

写了个项目，要求对DF文档进行数字签名，核心代码如下：

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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();	}

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