How to Add PDF Security Signatures in Python

Aspose.PDF FOSS for Python includes a signature-compromise detector that inspects an already-signed PDF for a specific tampering pattern: meaningful bytes appended after a signature’s signed byte range, a risk unique to incrementally-updated PDF files. This guide adds that check to a Python workflow using the SignaturesCompromiseDetector and CompromiseCheckResult classes. Creating or applying new digital signatures is a separate part of the library and is not covered here. The library is installed from PyPI with pip.

Step-by-Step Guide

Step 1: Install the Package

Install the Aspose.PDF FOSS package from PyPI:

pip install aspose-pdf-foss-for-python

Verify the installation by importing SignaturesCompromiseDetector and printing a confirmation message:

from aspose_pdf import SignaturesCompromiseDetector
print("aspose-pdf-foss-for-python is ready.")

Step 2: Import Required Classes

from aspose_pdf import CompromiseCheckResult, SignaturesCompromiseDetector

Step 3: Prepare an Object Exposing a Signatures List

SignaturesCompromiseDetector does not parse a PDF itself — its constructor accepts any object that exposes a signatures attribute: a list of signature entries, each carrying valid, byte_range, and reference_data. PdfSignature — a plain, directly constructible class — matches that shape, so it is useful for seeing how the check behaves before wiring it to your own document object:

from types import SimpleNamespace
from aspose_pdf import PdfSignature

with open("signed.pdf", "rb") as handle:
    pdf_bytes = handle.read()

signature = PdfSignature(
    name="Signature1",
    contents=b"...",  # PKCS#7 signed-data blob from the signature dictionary
    byte_range=[0, 1024, 1040, len(pdf_bytes) - 1040],
    reference_data=pdf_bytes,
)

signed_document = SimpleNamespace(signatures=[signature])

Swap signed_document for your own document object once it exposes a signatures list in the same shape — any object with that attribute works.

Step 4: Run the Compromise Check

from aspose_pdf import SignaturesCompromiseDetector

detector = SignaturesCompromiseDetector(signed_document)
result = detector.check()

print(result.compromised)                # bool
print(result.has_compromised_signatures)  # bool -- same value as compromised
print(result.signatures_coverage)         # int -- number of signatures inspected
print(result.reasons)                     # list[str] -- human-readable findings

check() walks every signature in signed_document.signatures, looking for unsigned content appended after each signature’s signed byte range. A CompromiseCheckResult summarizes the outcome: compromised and has_compromised_signatures report the same boolean under two names, signatures_coverage reports how many signatures were examined, and reasons lists a human-readable explanation for each issue found.

Step 5: Handle a Document With No Signatures

An empty or missing signatures list is not an error condition — it is reported the same way as a document with nothing to check:

from types import SimpleNamespace
from aspose_pdf import SignaturesCompromiseDetector

unsigned_document = SimpleNamespace(signatures=[])
detector = SignaturesCompromiseDetector(unsigned_document)
result = detector.check()

print(result.compromised)  # False
print(result.reasons)      # ["unsigned document"]

Passing SignaturesCompromiseDetector(None) — the constructor’s default — behaves the same way.

Common Issues and Fixes

compromised is False for a PDF you know was edited after signing

SignaturesCompromiseDetector looks specifically for unsigned bytes appended after a signature’s signed byte range — a pattern typical of naive incremental-update tampering. It does not perform full cryptographic signature verification. For that, call validate() on the individual PdfSignature object.

compromised and has_compromised_signatures seem redundant

They are the same value: compromised is a computed property that returns has_compromised_signatures. Use whichever name reads better in your code.

No exception is raised when the document has no signatures attribute at all

SignaturesCompromiseDetector treats a document with no signatures attribute, or document=None, the same as an unsigned document — check() returns a result with has_compromised_signatures=False and reasons=["unsigned document"] rather than raising.

A malformed PdfSignature doesn’t get flagged

check() skips any signature whose byte_range is not a 4-element list, or whose reference_data is not bytes/bytearray, rather than raising or reporting it as compromised. A signature built with the wrong shape is silently excluded from the check, not flagged.

Confusing this detector with signature creation

SignaturesCompromiseDetector and CompromiseCheckResult only inspect signatures that already exist on a document — they have no method to create, apply, or embed a new signature.

Frequently Asked Questions

What exactly counts as “compromised” here?

Meaningful, non-whitespace bytes appended to the PDF after a signature’s signed byte range — with two exceptions the detector already accounts for: a later signature or timestamp that itself covers those bytes, and an incremental update that only adds validation material (such as a /DSS) without layering new content like annotations.

Does this verify the cryptographic validity of the signature itself?

No. SignaturesCompromiseDetector checks for tampering patterns around the signed byte range. Cryptographic validity is a separate check, available through PdfSignature.validate().

What does `signatures_coverage` tell me?

The number of signatures the check actually examined on the document passed in — useful for confirming the detector saw the signatures you expected before trusting a clean result.

Can I check a document with multiple signatures at once?

Yes. check() iterates every entry in signed_document.signatures and returns one aggregate CompromiseCheckResult covering all of them.

Do I need to construct `PdfSignature` objects myself in normal use?

No — Step 3’s PdfSignature construction is for exploring the detector’s behavior directly. In a real workflow, pass whatever object your document-loading code already produces, as long as it exposes a signatures list of PdfSignature-shaped entries.