ContextQMD
Back to Bentopdf

Validate Signature

docs/tools/validate-signature-pdf.md

2.8.45.6 KB
Original Source

Validate Signature

Upload a signed PDF and this tool extracts every digital signature, identifies the signer, checks certificate validity, and reports whether the document has been modified since signing. You can also provide a trusted certificate to verify the signature chain.

How It Works

  1. Upload a signed PDF file. Validation starts automatically.
  2. The tool displays a summary: number of signatures found, how many are valid.
  3. Each signature gets a detailed card showing signer name, issuer, dates, and status.
  4. Optionally upload a trusted certificate (.pem, .crt, .cer, .der) to verify the signature against a specific trust anchor. Re-validation runs automatically.

What Gets Checked

Signature Parsing

The tool extracts PKCS#7 signature objects from the PDF, decodes the ASN.1 structure, and pulls out the signer's X.509 certificate along with any certificate chain embedded in the signature.

Certificate Validity

  • Expiration: Is the certificate currently within its valid date range?
  • Self-signed detection: Is the certificate its own issuer?
  • Trust chain: When a trusted certificate is provided, does the signer's certificate chain back to it?

Cryptographic Verification

  • The tool reconstructs the bytes covered by the signature's /ByteRange, hashes them with the algorithm the signer declared, and compares the result against the messageDigest attribute inside the signature.
  • It then re-serializes the signed attributes as a DER SET and verifies the signature against the signer certificate's public key.
  • A signature is reported as valid only when all of these pass. If the PDF bytes were modified after signing, or the embedded signature does not match the signer's key, the status shows "Invalid — Cryptographic Verification Failed" with the specific reason.

Supported Signature Algorithms

AlgorithmVerification path
RSA (PKCS#1 v1.5)node-forge publicKey.verify, with Web Crypto fallback
RSA-PSS (RSASSA-PSS)Web Crypto verify({name: 'RSA-PSS', saltLength})
ECDSA P-256/P-384/P-521Web Crypto verify({name: 'ECDSA', hash}) after DER → IEEE P1363 signature conversion

If a signature uses an algorithm outside this list (for example Ed25519, SM2, or RSA with an unusual digest OID), the card shows "Unverified — Unsupported Signature Algorithm" in yellow, along with the specific OID or reason. This is a deliberate three-state distinction:

  • Valid — signature cryptographically verified against the signer's public key.
  • Invalid — verification ran and produced a negative result (bytes changed, key mismatch).
  • Unverified — the tool could not run verification for this algorithm. The certificate metadata is still shown, but you should treat the signature as "unknown" and verify it with Adobe Acrobat or openssl cms -verify.

Insecure Digest Algorithms

  • Signatures using MD5 or SHA-1 are rejected as invalid and flagged with "Insecure Digest" status. Both algorithms have published collision attacks, so a signature over a SHA-1 or MD5 hash offers no integrity guarantee.
  • SHA-224, SHA-256, SHA-384, and SHA-512 are all accepted.

Document Coverage

  • Full coverage: The signature covers the entire PDF file, meaning no bytes were added or changed after signing.
  • Partial coverage: The signature covers only part of the file. This can indicate modifications were made after signing (not necessarily malicious -- incremental saves produce partial coverage).

Signature Details

Each signature card shows:

  • Signed By: Common name, organization, and email from the signer's certificate
  • Issuer: The Certificate Authority that issued the signer's certificate
  • Signed On: The timestamp embedded in the signature
  • Valid From / Valid Until: The certificate's validity period
  • Reason: Why the document was signed (if provided)
  • Location: Where it was signed (if provided)
  • Coverage Status: Full or Partial
  • Trust Badge: Trusted or Not in trust chain (only when a custom certificate is provided)

Technical Details

Expand the technical details section for:

  • Serial number of the signer's certificate
  • Digest algorithm (SHA-256, SHA-512, etc.)
  • Signature algorithm (RSA with SHA-256, ECDSA with SHA-256, etc.)
  • Error messages for invalid signatures

Use Cases

  • Verifying a digitally signed contract before countersigning
  • Auditing signed documents to confirm they have not been tampered with
  • Checking whether a certificate has expired since the document was signed
  • Validating signatures against your organization's root certificate
  • Inspecting the signing details of government or legal documents

Tips

  • A self-signed certificate does not mean the signature is invalid -- it means the signer's identity cannot be verified through a trusted third party. This is common for internal documents.
  • Partial coverage does not always indicate tampering. Many PDF workflows add incremental updates (like a second signature) that create partial coverage.
  • Upload your organization's root or intermediate certificate as the trusted certificate to get trust chain verification.