Verifying digital signatures in PDFs involves looking into the document's integrity and making sure the signer's private key—which matches the public key in their certificate—was used to establish the signature. Numerous programming frameworks can be used to automate this operation.
Steps to Verify a PDF Signature:
1. Extract the Signature:
- Identify the signature field in the PDF.
- Extract the signature data (the actual signed hash) and the signer's certificate.
2. Validate the Signer's Certificate:
- Check the certificate's validity (not expired, not revoked).
- Ensure the certificate is trusted (part of your trusted certificate store or chained to a trusted root).
3. Recompute the Document's Hash:
- Using the same algorithm as the signature, compute a new hash of the PDF content (excluding the signature itself).
4. Verify the Signature:
- Use the recomputed hash, the extracted signature, and the signer's public key (from the certificate) to verify the signature.
- If the verification succeeds, the signature is authentic and the document hasn’t been tampered with.
Libraries for Verifying PDF Signatures in Python:
- PyPDF2: For handling PDFs, though it might not directly support signature verification.
- pyHanko: A more specialized library for working with PDF signatures in Python.
- cryptography: For the cryptographic verification aspects. You might need to manually extract signature data using PyPDF2 and then verify it with cryptography.
from pyhanko.sign import signer, verify
with open('path/to/your/pdf.pdf', 'rb') as doc:
result = verify_pdf_signature(doc)
if result[0]: # result is a tuple where the first element indicates success
print("Signature verified successfully")
else:
print("Signature verification failed")
