java - 使用 iText 库在 pdf 中插入隐藏摘要

Question

我搜索一种使用 iText 库 (Java) 将摘要(字节数组或字符串)插入 PDF 文件的方法。我使用以下方法从字符串创建摘要:

private String crypt(double x, ByteArrayOutputStream baos) throws UnsupportedEncodingException, NoSuchAlgorithmException{
    MessageDigest md = MessageDigest.getInstance("SHA-256");
    md.update(String.valueOf(x).getBytes("UTF-8"));
    md.update(String.valueOf(baos).getBytes("UTF-8"));
    byte[] digest = md.digest();

    StringBuffer sb = new StringBuffer();
    for(byte d:digest){
        sb.append(Integer.toHexString(0xFF & d));
    }
    return sb.toString();
}

摘要不应在 PDF 中看到，但必须将其提取出来进行比较。

Answer 1

此类私有(private)数据可以存储在PieceInfo字典中:

A page-piece dictionary (PDF 1.3) may be used to hold private conforming product data. The data may be associated with a page or form XObject by means of the optional PieceInfo entry in the page object (see Table 30) or form dictionary (see Table 95). Beginning with PDF 1.4, private data may also be associated with the PDF document by means of the PieceInfo entry in the document catalogue (see Table 28).

(section 14.5 of ISO 32000-1)

就您而言，文档目录中的PieceInfo似乎最合适。

使用 iText，您可以在其中存储数据并使用下面的 DocumentPieceInfo 帮助器类再次检索它们:

存储文档PieceInfo数据

PdfName appName = new PdfName("MYAPP");
PdfName dataName = new PdfName("Hash");

DocumentPieceInfo dpi = new DocumentPieceInfo();

PdfReader reader = new PdfReader(...);
dpi.addPieceInfo(reader, appName, dataName, new PdfString(data));

PdfStamper stamper = new PdfStamper(reader, ...);
stamper.close();

检索文档PieceInfo数据

PdfName appName = new PdfName("MYAPP");
PdfName dataName = new PdfName("Hash");

DocumentPieceInfo dpi = new DocumentPieceInfo();

PdfReader reader = new PdfReader("target/test-outputs/test-with-piece-info.pdf");
PdfObject myData = dpi.getPieceInfo(reader, appName, dataName);

DocumentPieceInfo 辅助类

public class DocumentPieceInfo
{
    static PdfName PIECE_INFO = new PdfName("PieceInfo");
    static PdfName LAST_MODIFIED = new PdfName("LastModified");
    static PdfName PRIVATE = new PdfName("Private");

    void addPieceInfo(PdfReader reader, PdfName app, PdfName name, PdfObject value)
    {
        PdfDictionary catalog = reader.getCatalog();
        PdfDictionary pieceInfo = catalog.getAsDict(PIECE_INFO);
        if (pieceInfo == null)
        {
            pieceInfo = new PdfDictionary();
            catalog.put(PIECE_INFO, pieceInfo);
        }

        PdfDictionary appData = pieceInfo.getAsDict(app);
        if (appData == null)
        {
            appData = new PdfDictionary();
            pieceInfo.put(app, appData);
        }

        PdfDictionary privateData = appData.getAsDict(PRIVATE);
        if (privateData == null)
        {
            privateData = new PdfDictionary();
            appData.put(PRIVATE, privateData);
        }

        appData.put(LAST_MODIFIED, new PdfDate());
        privateData.put(name, value);
    }

    PdfObject getPieceInfo(PdfReader reader, PdfName app, PdfName name)
    {
        PdfDictionary catalog = reader.getCatalog();

        PdfDictionary pieceInfo = catalog.getAsDict(PIECE_INFO);
        if (pieceInfo == null)
            return null;

        PdfDictionary appData = pieceInfo.getAsDict(app);
        if (appData == null)
            return null;

        PdfDictionary privateData = appData.getAsDict(PRIVATE);
        if (privateData == null)
            return null;

        return privateData.get(name);
    }
}

此类假定Private值是一个字典，其中存储了私有(private)数据。不过，它可能是任何东西。要处理其他程序生成的私有(private)数据，您可能需要一些变化。