java pdf 签名 修改,使用Java中的自定义数字签名来签名PDF

要对PDF进行数字签名，可以使用Java中的Bouncy Castle库。以下是签名PDF的一般步骤：

1. 从文件中读取要签名的PDF。

File inFile = new File("example.pdf");
byte[] inputBytes = Files.readAllBytes(inFile.toPath());

2. 初始化签名密钥库，并加载签名密钥。

KeyStore ks = KeyStore.getInstance("PKCS12");
char[] password = "password".toCharArray();
ks.load(new FileInputStream("keystore.p12"), password);
String alias = ks.aliases().nextElement();
PrivateKey privateKey = (PrivateKey) ks.getKey(alias, password);
Certificate[] chain = ks.getCertificateChain(alias);

3. 初始化PDF签名器。

PdfReader reader = new PdfReader(inputBytes);
ByteArrayOutputStream outputBytes = new ByteArrayOutputStream();
PdfStamper stamper = PdfStamper.createSignature(reader, outputBytes, '\0', null, true);
PdfSignatureAppearance appearance = stamper.getSignatureAppearance();
appearance.setReason("I am the author");
appearance.setLocation("Worldwide");
appearance.setVisibleSignature(new Rectangle(36, 748, 144, 780), 1, "signature");

4. 初始化数字签名。

BouncyCastleProvider provider = new BouncyCastleProvider();
Security.addProvider(provider);
PdfSignature dic = new PdfSignature(PdfName.ADOBE_PPKLITE, PdfName.ADBE_PKCS7_DETACHED);
dic.setReason(appearance.getReason());
dic.setLocation(appearance.getLocation());
dic.setContact(appearance.getContact());
dic.setDate(new PdfDate(appearance.getSignDate()));
appearance.setCryptoDictionary(dic);
PdfPKCS7 sgn = new PdfPKCS7(privateKey, chain, null, "SHA256", provider, false);
byte[] hash = DigestAlgorithms.digest(inputBytes, "SHA256");
Calendar cal = Calendar.getInstance();
byte[] signature = sgn.getAuthenticatedAttributeBytes(hash, cal, null, null, CryptoStandard.CMS);
sgn.update(hash, 0, hash.length);
byte[] encodedSig = sgn.getEncodedPKCS7(hash, cal, null, null, null, CryptoStandard.CMS);

5. 将数字签名附加到PDF中。

PdfDictionary dic2 = new PdfDictionary();
dic2.put(PdfName.CONTENTS, new PdfString(encodedSig).setHexWriting(true));
appearance.setCryptoDictionary(dic2);
appearance.preClose();

6. 输出签名后的PDF。

stamper.close();
Files.write(new File("signed.pdf").toPath(), outputBytes.toByteArray());

这些步骤提供了一个概述，但是实现数字签名需要更多的细节和错误处理。