TIKA-4756: Detect unsigned signature fields in PDFs with AcroForm

Adds PDF.HAS_SIGNATURE_FIELDS metadata property (pdf:hasSignatureFields)
to distinguish PDFs that contain /FT /Sig AcroForm fields from those
with actual applied digital signatures (TikaCoreProperties.HAS_SIGNATURE).

Previously, PDFs with unsigned signature fields were not detectable,
making it impossible for downstream applications (e.g. PDF/A converters)
to identify documents with signature infrastructure that should not be
re-processed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: srujana-kuntumalla

tika-core/src/main/java/org/apache/tika/metadata/PDF.java

@@ -151,6 +151,13 @@ public interface PDF {
      */
     Property HAS_ACROFORM_FIELDS = Property.internalBoolean(PDF_PREFIX + "hasAcroFormFields");
 
+    /**
+     * Has > 0 AcroForm signature fields (/FT /Sig), regardless of whether a signature
+     * has been applied. Use {@link TikaCoreProperties#HAS_SIGNATURE} to check for
+     * an actual applied digital signature.
+     */
+    Property HAS_SIGNATURE_FIELDS = Property.internalBoolean(PDF_PREFIX + "hasSignatureFields");
+
     Property HAS_MARKED_CONTENT = Property.internalBoolean(PDF_PREFIX + "hasMarkedContent");
 
     /**

tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-pdf-module/src/main/java/org/apache/tika/parser/pdf/PDFParser.java

@@ -53,6 +53,7 @@
 import org.apache.pdfbox.pdmodel.fixup.processor.AcroFormDefaultsProcessor;
 import org.apache.pdfbox.pdmodel.interactive.digitalsignature.PDSignature;
 import org.apache.pdfbox.pdmodel.interactive.form.PDAcroForm;
+import org.apache.pdfbox.pdmodel.interactive.form.PDSignatureField;
 import org.xml.sax.ContentHandler;
 import org.xml.sax.SAXException;
 
@@ -398,8 +399,14 @@ private void checkAccessPermissions(PDFParserConfig.AccessCheckMode mode, Metada
     }
 
     private void extractSignatures(PDDocument pdfDocument, Metadata metadata) {
+        List<PDSignatureField> signatureFields = pdfDocument.getSignatureFields();
+        if (!signatureFields.isEmpty()) {
+            metadata.set(PDF.HAS_SIGNATURE_FIELDS, true);
+        }
+
         boolean hasSignature = false;
-        for (PDSignature signature : pdfDocument.getSignatureDictionaries()) {
+        for (PDSignatureField sigField : signatureFields) {
+            PDSignature signature = sigField.getSignature();
             if (signature == null) {
                 continue;
             }
@@ -414,11 +421,10 @@ private void extractSignatures(PDDocument pdfDocument, Metadata metadata) {
             PDMetadataExtractor.addNotNull(signature.getLocation(), metadata, TikaCoreProperties.SIGNATURE_LOCATION);
             PDMetadataExtractor.addNotNull(signature.getReason(), metadata, TikaCoreProperties.SIGNATURE_REASON);
             hasSignature = true;
-
         }
 
         if (hasSignature) {
-            metadata.set(TikaCoreProperties.HAS_SIGNATURE, hasSignature);
+            metadata.set(TikaCoreProperties.HAS_SIGNATURE, true);
         }
     }
 

tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-pdf-module/src/test/java/org/apache/tika/parser/pdf/PDFParserTest.java

@@ -629,9 +629,19 @@ public void testSignatureInAcroForm() throws Exception {
         assertEquals("true", m.get(PDF.HAS_XMP));
         assertEquals("true", m.get(PDF.HAS_ACROFORM_FIELDS));
         assertEquals("false", m.get(PDF.HAS_XFA));
+        assertEquals("true", m.get(PDF.HAS_SIGNATURE_FIELDS));
+        assertNull(m.get(TikaCoreProperties.HAS_SIGNATURE));
         assertContains("<li>aTextField: TIKA-1226</li>", result.xml);
     }
 
+    //TIKA-4756
+    @Test
+    public void testUnsignedSignatureField() throws Exception {
+        Metadata m = getXML("testPDF_unsigned_sig_field.pdf").metadata;
+        assertEquals("true", m.get(PDF.HAS_SIGNATURE_FIELDS));
+        assertNull(m.get(TikaCoreProperties.HAS_SIGNATURE));
+    }
+
     @Test
     public void testSingleCloseDoc() throws Exception {
         //TIKA-1341