"Lock" 签名字段中的字典是签名后签名损坏的原因

Question

在 PDFBox 2.x 中，我将 /Lock 字典放入签名字段：

import org.apache.pdfbox.cos.COSDictionary;
import org.apache.pdfbox.cos.COSName;
import org.apache.pdfbox.pdmodel.interactive.form.PDAcroForm;
import org.apache.pdfbox.pdmodel.interactive.form.PDSignatureField;

public class SigningUtils {
    public static final COSName COS_NAME_LOCK = COSName.getPDFName("Lock");
    public static final COSName COS_NAME_ACTION = COSName.getPDFName("Action");
    public static final COSName COS_NAME_ALL = COSName.getPDFName("All");
    public static final COSName COS_NAME_SIG_FIELD_LOCK = COSName.getPDFName("SigFieldLock");

    public static void setLock(PDSignatureField pdSignatureField, PDAcroForm acroForm) {
        COSDictionary lockDict = new COSDictionary();
        lockDict.setItem(COS_NAME_ACTION, COS_NAME_ALL);
        lockDict.setItem(COSName.TYPE, COS_NAME_SIG_FIELD_LOCK);
        pdSignatureField.getCOSObject().setItem(COS_NAME_LOCK, lockDict);
    }
}

那我在签名区签名：

PDSignature signature = findExistingSignature(document, signatureFieldName); //This is some method to find signature field and create PDSignature dictionary

signature.setFilter(PDSignature.FILTER_ADOBE_PPKLITE);
signature.setSubFilter(PDSignature.SUBFILTER_ADBE_PKCS7_DETACHED);

signature.setName("blablabla");
signature.setLocation("blablabla");
signature.setReason("blablabla");
signature.setSignDate(Calendar.getInstance());
document.addSignature(signature, this);

一切看起来都很好，除了当我在 Adobe Acrobat 中打开签名的文档时，它抱怨文档的内容被更改。如果我不添加 /Lock 字典，一切都很好。

有人知道哪里出了问题吗？

Answer 1

问题是 PDFBox 签名没有考虑 Lock 字典。

根据 ISO 32000-1（以及类似的 ISO 32000-2）：

12.8.2.4 FieldMDP

The FieldMDP transform method shall be used to detect changes to the values of a list of form fields. The entries in its transform parameters dictionary are listed in Table 256.

[...]

The author can also specify that after a specific recipient has signed the document, any modifications to specific form fields shall invalidate that recipient’s signature. There shall be a separate signature field for each designated recipient, each having an associated signature field lock dictionary (see Table 233) specifying the form fields that shall be locked for that user.

When the recipient signs the field, the signature, signature reference, and transform parameters dictionaries shall be created. The Action and Fields entries in the transform parameters dictionary shall be copied from the corresponding fields in the signature field lock dictionary.

因此，签名 Lock 字典的预期处理包括添加匹配 FieldMDP 转换数据到签名字段值。 PDFBox 签名默认不这样做。

您可以在签名时手动执行此操作：

PDSignatureField signatureField = FIND_YOUR_SIGNATURE_FIELD_TO_SIGN;
PDSignature signature = new PDSignature();
signatureField.setValue(signature);

COSBase lock = signatureField.getCOSObject().getDictionaryObject(COSName.getPDFName("Lock"));
if (lock instanceof COSDictionary)
{
    COSDictionary lockDict = (COSDictionary) lock;
    COSDictionary transformParams = new COSDictionary(lockDict);
    transformParams.setItem(COSName.TYPE, COSName.getPDFName("TransformParams"));
    transformParams.setItem(COSName.V, COSName.getPDFName("1.2"));
    transformParams.setDirect(true);
    COSDictionary sigRef = new COSDictionary();
    sigRef.setItem(COSName.TYPE, COSName.getPDFName("SigRef"));
    sigRef.setItem(COSName.getPDFName("TransformParams"), transformParams);
    sigRef.setItem(COSName.getPDFName("TransformMethod"), COSName.getPDFName("FieldMDP"));
    sigRef.setItem(COSName.getPDFName("Data"), document.getDocumentCatalog());
    sigRef.setDirect(true);
    COSArray referenceArray = new COSArray();
    referenceArray.add(sigRef);
    signature.getCOSObject().setItem(COSName.getPDFName("Reference"), referenceArray);
}

signature.setFilter(PDSignature.FILTER_ADOBE_PPKLITE);
signature.setSubFilter(PDSignature.SUBFILTER_ADBE_PKCS7_DETACHED);
signature.setName("blablabla");
signature.setLocation("blablabla");
signature.setReason("blablabla");
signature.setSignDate(Calendar.getInstance());
document.addSignature(signature [, ...]);

(CreateSignature 辅助方法 signExistingFieldWithLock)

关于签名Lock词典中的P条目评论中讨论：该条目已在Adobe ISO补充文件中引入32000，扩展级别 3。

"Lock" 签名字段中的字典是签名后签名损坏的原因

"Lock" dictionary in signature field is the reason of broken signature after signing

java

pdf

pdfbox

12.8.2.4 FieldMDP