Adobe PDF Signature Authentication

TL;DR

Verifying Adobe .pdf electronic signatures involves checking digital certificates and signature validity within the document itself or using Adobe Acrobat/Reader. This guide covers both methods, ensuring you can trust signed PDFs.

Checking Signatures in Adobe Acrobat/Reader

1. Open the PDF: Open the signed .pdf file in Adobe Acrobat Reader (free) or Acrobat Pro (paid).
2. Signature Panel: Look for a notification bar at the top of the document indicating a signature. Alternatively, go to View > Show/Hide > Navigation Panes > Signatures. This opens the Signatures panel on the left side.
3. Examine Signature Details: Select the signature in the list. The details pane will show information about the signer, date and time of signing, and the validity status.
4. Validate Identity:
   • Trusted Certificates: A green checkmark indicates a trusted certificate authority (CA) issued the digital ID used for signing. This is the most secure scenario.
   • Unknown/Untrusted Certificates: If the certificate isn’t recognized, Acrobat will display a warning. You may need to contact the signer to verify their identity or add their CA to your trusted identities (use caution!).
5. Signature Properties: Right-click on the signature and select Properties. This provides detailed technical information about the signature, including hashing algorithms used.
   • Check for Alterations: Look for any indications of document modifications after signing. Acrobat will highlight changes if detected.

Verifying Signatures Without Adobe Software (Limited)

Without Adobe software, full verification is difficult. However, you can check basic signature presence and some metadata.

1. Open in a PDF Viewer: Open the .pdf file in any standard PDF viewer (e.g., Chrome’s built-in viewer).
2. Look for Signature Field: Check if there is a visible signature field or a digital signature icon.
3. Document Properties: Go to File > Properties (or similar, depending on the viewer). Look for a ‘Security’ tab.
   • Digital Signatures Section: If present, this section will list any signatures in the document.
   • Signature Validity: The properties may show basic validity information (e.g., valid/invalid), but detailed checks are usually unavailable without Adobe software.

Advanced Verification with OpenSSL (Technical Users)

OpenSSL can be used to verify the digital signature, but requires technical expertise.

1. Extract Signature: Use a PDF parsing tool or library to extract the digital signature from the .pdf file. This is complex and beyond the scope of this guide.
2. Verify with OpenSSL: Once extracted, you can use OpenSSL to verify the signature against the signer’s certificate.

   openssl dgst -sha256 -verify public_key.pem -signature signature.sig document.pdf

   Replace public_key.pem with the signer’s public key file, signature.sig with the extracted signature file, and document.pdf with the original PDF.

Important Considerations

• Certificate Revocation: Always check if the signer’s certificate has been revoked by a Certificate Authority (CA). Adobe Acrobat/Reader usually performs this check automatically.
• Document Tampering: Verify that the document hasn’t been altered after signing. Look for any visual inconsistencies or warnings in Acrobat.
• Trust Chain: Ensure the certificate chain is complete and trusted, leading back to a root CA that your system recognizes.
• cyber security best practice: Be cautious of PDFs from unknown sources. Always verify signatures before taking action based on their content.