How to Obtain the Digital Signature From a Signed PDF Document?

[Vincent Ycasas]

Question:

How can I extract the digital signature from a PDF that has been already signed?

Answer:

To obtain the certificate, you will need to locate the signature form field and get its value. For example:

PDFDoc doc = ...
if (doc.HasSignatures()) {
    Field sigField = doc.GetField("My SignatureField's Name"); // you will need to know the name of the signature field.
    Obj value = sigField.GetValue();
    // value is the signature dictionary and has the following:
    //   1. Type
    //   2. Filter
    //   3. ByteRange
    //   4. Contents
    // The Contents entry in the signature dictionary will contain the signature envelope which will hold the public certificate which can be used to verify the signature.
}

[Ryan]

Question:

How do I get the date, location, name, etc. of the signature?

Answer:

While the digital signature certificate defined in the Contents entry, mentioned above, may contain this information, the PDF can also.

The following code will iterate all the digital signatures in a PDFDoc object, and print out the string metadata for that signature. The switch statement also includes the other possible entries, though these other ones are non-text. See PDF32000_2008 PDF 1.7, Section 12.8.1 Digital Signatures General, Table 252 "Entries in a signature dictionary" for details of these other entries.

if(!doc.hasSignatures()) return;

FieldIterator fitr = doc.getFieldIterator();
while(fitr.hasNext())
{
    Field field=(Field)(fitr.next());
    int type = field.getType();
    if(type != Field.e_signature) continue;
    Obj value = field.getValue();
    if(value == null || !value.isDict()) continue;

    System.out.println("========================================");
    System.out.println(field.getName());
    for (DictIterator itr = value.getDictIterator(); itr.hasNext(); itr.next())
    {
        String k = itr.key().getName();
        switch(k)
        {
            // case "Filter":
            // case "SubFilter":
            // case "Contents":
            // case "Cert":
            // case "Reference":
            case "ByteRange":
                for (int i = 0; i < itr.Value().Size(); i += 2 )
                {
                    int offset = (int)itr.value().getAt(i).getNumber();
                    int length_in_bytes = (int)itr.value().getAt(i + 1).getNumber();
                    System.out.println("ByteRange " + offset + "-" + (offset + length_in_bytes - 1));
                }
                break;
            case "Changes":
                Obj v = itr.value();
                if(v.isArray() && v.size() == 3)
                {
                    int num_pages_altered = (int)v.getAt(0).getNumber();
                    int num_fields_altered = (int)v.getAt(1).getNumber();
                    int num_fields_filled = (int)v.getAt(2).getNumber();
                    System.out.println(num_pages_altered + " Pages Altered\n" + num_fields_altered + " Fields Altered\n" + num_fields_filled + " Fields Filled in");
                }
                break;
            case "Name":
            case "M":
            case "Location":
            case "Reason":
            case "ContectInfo":
                System.out.println(k + " : " + itr.value().getAsPDFText());
                break;
            case "R":
            case "V":
                System.out.println(k + " : " + itr.value().getNumber());
                break;
            // case "Prop_Build":
            // case "Prop_AuthTime":
            // case "Prop_AuthType":
        }
    }
    System.out.println("========================================");
}