Implement digital signature detection

src/ocrmypdf/_pipeline.py

@@ -27,6 +27,7 @@ from ocrmypdf._jobcontext import PageContext, PdfContext
 from ocrmypdf._version import PROGRAM_NAME
 from ocrmypdf._version import __version__ as VERSION
 from ocrmypdf.exceptions import (
+    DigitalSignatureError,
     DpiError,
     EncryptedPdfError,
     InputFileError,
@@ -197,6 +198,8 @@ def validate_pdfinfo_options(context: PdfContext) -> None:
             "Designer and can only be read by Adobe Acrobat or Adobe Reader."
         )
     if pdfinfo.has_acroform:
+        if pdfinfo.has_signature:
+            raise DigitalSignatureError()
         if options.redo_ocr:
             raise InputFileError(
                 "This PDF has a user fillable form. --redo-ocr is not "

src/ocrmypdf/exceptions.py

@@ -108,6 +108,18 @@ class EncryptedPdfError(ExitCodeException):
     )
 
 
+class DigitalSignatureError(ExitCodeException):
+    """PDF has a digital signature."""
+
+    exit_code = ExitCode.input_file
+    message = dedent(
+        """\
+        Input PDF has a digital signature. OCR would alter the document,
+        invalidating the signature.
+        """
+    )
+
+
 class TesseractConfigError(ExitCodeException):
     """Tesseract config can't be parsed."""
 

tests/test_acroform.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 
 import logging
 
+import pikepdf
 import pytest
 
 import ocrmypdf
@@ -31,3 +32,17 @@ def test_acroform_message(acroform, caplog, outpdf):
     check_ocrmypdf(acroform, outpdf, '--plugin', 'tests/plugins/tesseract_noop.py')
     assert 'fillable form' in caplog.text
     assert '--force-ocr' in caplog.text
+
+
+@pytest.fixture
+def digitally_signed(acroform, outdir):
+    out = outdir / 'acroform_signed.pdf'
+    with pikepdf.open(acroform) as pdf:
+        pdf.Root.AcroForm.SigFlags = 3
+        pdf.save(out)
+    yield out
+
+
+def test_digital_signature(digitally_signed, no_outpdf):
+    with pytest.raises(ocrmypdf.exceptions.DigitalSignatureError):
+        check_ocrmypdf(digitally_signed, no_outpdf)