“至少一个签名无效”。使用itext从pdf中删除签名后出现错误消息 [英] "AT least one signature is invalid" error message after removing signature from pdf using itext

问题描述

我有一个带有多个签名的数字签名pdf。现在我想只删除其中一个签名。我正在使用itext。代码如下：

I have a digitally signed pdf with multiple signatures. Now I want to remove only one of the signatures. I am using itext for the same. The code is as follows:

PdfReader reader = new PdfReader(src_path);

PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest_path));


         {
                AcroFields.Item item =        stamper.getAcroFields().getFieldItem(fieldname);

                ClearSignatureDictionary(item.getMerged(0));
                ClearSignatureDictionary(item.getWidget(0));
                ClearSignatureDictionary(item.getValue(0));
            }
        }

 private static void ClearSignatureDictionary(PdfDictionary dic)
  {
    dic.remove(PdfName.AP);
    dic.remove(PdfName.AS);
    dic.remove(PdfName.V);
    dic.remove(PdfName.DV);
    dic.remove(PdfName.SV);
    dic.remove(PdfName.FF);
    dic.put(PdfName.F, new PdfNumber(4));
 }

但是当我用删除的签名打开文档时，它给了我以下内容Acrobat reader上的错误
至少有一个签​​名无效

But when I open the document with the removed signature, it gives me the following error on Acrobat reader "At least one of the signatures is invalid"

推荐答案

您无法从中删除密钥签名文档中的字典，并期望签名保持有效。您只能删除添加的最后一个签名。如果文档由多人签名，并且您想要删除第一个签名，则所有后续签名都将被破坏。

You can not remove keys from a dictionary in a signed document, and expect the signatures to remain valid. You can only remove the last signature that was added. If a document was signed by multiple people, and you want to remove the first signature, all subsequent signatures will be broken.

此图片解释了原因：

此图像显示每个新的数字签名都保持原始字节的完整性。每个新签名都会添加新字节。 Rev1表示具有1个数字签名的文档的字节。 Rev2表示具有2个数字签名的文档的字节。第二个数字签名完全标志着Rev1。如果您删除第一个签名，则第二个签名将无效。

This image shows that every new digital signature keeps the original bytes intact. With every new signature new bytes are added. Rev1 represents the bytes of a document that has 1 digital signature. Rev2 represents the bytes of a document that has 2 digital signatures. The second digital signatures signs Rev1 completely. If you'd remove the first signature, the second signature would become invalid.

数字签名是一种特殊类型的表单字段。使用iText，您可以获得PDF的签名表单字段的名称，如下所示：

A digital signature is a special type of form field. With iText, you can get the names of the signature form fields of a PDF like this:

PdfReader reader = new PdfReader(path);
AcroFields fields = reader.getAcroFields();
ArrayList<String> names = fields.getSignatureNames();

您只能删除涵盖整个文档的签名，例如，如果我们有sig1，sig2，sig3（已添加按此顺序），只有 fields.signatureCoversWholeDocument（sig3）将返回true。

You can only remove the signature that covers the whole document, for instance, if we have "sig1", "sig2", and "sig3" (added in that order), only fields.signatureCoversWholeDocument("sig3") will return true.

你可以得到像这样的修订总数： fields.getTotalRevisions（）以及这样的特定修订： fields.getRevision（sig1）（假设有一个名为的签名字段sig1）。

You can get the total number of revisions like this: fields.getTotalRevisions() and a specific revision like this: fields.getRevision("sig1") (provided that there's a signature field named "sig1").

假设图像代表你的文档，您必须删除1个签名，然后您只能通过删除在修订版3（Rev3）中添加的所有字节来删除第三个签名。使用iText，这意味着返回修订版2（Rev2）。该修订是使用签名字段 sig2 签署的。
您可以像这样提取此修订：

Suppose that the image represents your document, and you have to remove 1 signature, then you can only remove the third signature by removing all the bytes that were added in revision 3 (Rev3). With iText, that means going back to revision 2 (Rev2). That revision was signed using the signature field sig2. You can extract this revision like this:

FileOutputStream os = new FileOutputStream("revision2.pdf");
byte bb[] = new byte[1028];
InputStream ip = fields.extractRevision("sig2");
int n = 0;
while ((n = ip.read(bb)) > 0)
    os.write(bb, 0, n);
os.close();
ip.close();

文件 revision2.pdf 将是文件由sig1和sig2签名，没有创建时添加的字节sig3。

The file revision2.pdf will be the file signed by "sig1" and "sig2" without the bytes that were added when creating "sig3".

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