我在 Stackoverflow 上检查了其他类似问题，但它不适用于我的情况。

情况：我正在开发一个需要签署 pdf 文档的应用程序。签名密钥由另一家公司持有，假设它是 CompanyA。

我做了以下步骤：

1. 准备好要签名的 pdf 文档。
2. 创建了一个临时 pdf 文件，在原始 pdf 中添加了一个空签名。
3. 阅读 Temp pdf 以获取消息摘要。 （将其编码为 base64）
4. 将消息摘要（Base64 编码）发送给 CompanyA 以进行签名。
5. 从 CompanyA 获取签名摘要（base64 编码）。
6. 进行 base64 解码。并将结果嵌入到 Temp pdf 中以获得最终签名的 pdf。

一切顺利，我可以获得最终签名的 pdf。但是当我在 Adob​​e Reader 中打开它时，它说“自应用签名以来，文档已被更改或损坏”。

我使用这个 getHashBase64Str2Sign 来获取消息摘要（在 base64 中）。该方法调用emptySignature()方法创建签名为空的Temp文件，然后调用getSignatureHash()方法读取Temp文件获取消息摘要。

public static String getHashBase64Str2Sign() {
    try {
        // Add BC provider
        BouncyCastleProvider providerBC = new BouncyCastleProvider();
        Security.addProvider(providerBC);

        // Create parent path of dest pdf file,if not exist
        File file = new File(DEST).getParentFile();
        if (!file.exists()) {
            file.mkdirs();
        }


        CertificateFactory factory = CertificateFactory.getInstance("X.509");
        Certificate[] chain = new Certificate[1];
        try (InputStream certIs = new FileInputStream(CERT)) {
            chain[0] = factory.generateCertificate(certIs);
        }

        // Get byte[] hash
        DeferredSigning app = new DeferredSigning();

        app.emptySignature(SRC,TEMP,"sig",chain);

        byte[] sh = app.getSignatureHash(TEMP,"SHA256",chain);

        // Encode byte[] hash to base64 String and return
        return Base64.getEncoder().encodeToString(sh);
    } catch (IOException | GeneralSecurityException e) {
        e.printStackTrace();
        return null;
    }
}

    

private void emptySignature(String src,String dest,String fieldname,Certificate[] chain)
        throws IOException,GeneralSecurityException {
    PdfReader reader = new PdfReader(src);
    PdfSigner signer = new PdfSigner(reader,new FileOutputStream(dest),new StampingProperties());
    PdfSignatureAppearance appearance = signer.getSignatureAppearance();
    appearance.setPageRect(new Rectangle(100,500,200,100));
    appearance.setPageNumber(1);
    appearance.setCertificate(chain[0]);
    appearance.setReason("For test");
    appearance.setLocation("HKSAR");
    signer.setfieldName(fieldname);

    /*
     * ExternalBlankSignatureContainer constructor will create the PdfDictionary for
     * the signature information and will insert the /Filter and /SubFilter values
     * into this dictionary. It will leave just a blank placeholder for the
     * signature that is to be inserted later.
     */
    IExternalSignatureContainer external = new ExternalBlankSignatureContainer(PdfName.Adobe_PPKLite,PdfName.Adbe_pkcs7_detached);
    

    // Sign the document using an external container.
    // 8192 is the size of the empty signature placeholder.
    signer.signExternalContainer(external,100000);
}

private byte[] getSignatureHash(String src,String hashAlgorithm,GeneralSecurityException {
    InputStream is = new FileInputStream(src);
    // Get the hash
    BouncyCastleDigest digest = new BouncyCastleDigest();
    Pdfpkcs7 sgn = new Pdfpkcs7(null,chain,hashAlgorithm,null,digest,false);

    byte hash[] = DigestAlgorithms.digest(is,digest.getMessageDigest(sgn.getHashAlgorithm()));
    return sgn.getauthenticatedAttributeBytes(hash,PdfSigner.CryptoStandard.CMS,null);
}

private void createSignature(String src,String fieldName,byte[] sig)
        throws IOException,GeneralSecurityException {
    PdfReader reader = new PdfReader(src);
    try (FileOutputStream os = new FileOutputStream(dest)) {
        PdfSigner signer = new PdfSigner(reader,os,new StampingProperties());

        IExternalSignatureContainer external = new MyExternalSignatureContainer(sig);

        // Signs a PDF where space was already reserved. The field must cover the whole
        // document.
        PdfSigner.signDeferred(signer.getDocument(),fieldName,external);
    }
}

然后，消息摘要被发送到 CompanyA 进行签名。从 CompanyA 获得签名摘要（base64 编码）后，我调用 embedSignedHashToPdf() 方法来获取签名的 pdf 文档。

public static void embedSignedHashToPdf(String signedHash) {
    try {
        byte[] sig = Base64.getDecoder().decode(signedHash);
        // Get byte[] hash
        DeferredSigning app = new DeferredSigning();
        app.createSignature(TEMP,DEST,sig);
    } catch (IOException | GeneralSecurityException e) {
        e.printStackTrace();
    }
}

class MyExternalSignatureContainer implements IExternalSignatureContainer {

    protected byte[] sig;

    public MyExternalSignatureContainer(byte[] sig) {
        this.sig = sig;
    }

    @Override
    public void modifySigningDictionary(PdfDictionary signDic) {

    }

    @Override
    public byte[] sign(InputStream arg0) throws GeneralSecurityException {
        return sig;
    }

}

最后，我可以得到签名的pdf文档，但在Adobe Reader中显示错误，如下所示：

请检查原始 pdf、临时 pdf 和最终签名的 pdf 文件如下：

Original pdf - helloworld.pdf

Temp pdf - helloworld_empty_signed.pdf

Final pdf - helloworld_signed_ok.pdf