| THE MOST IMPORTANT POINTS AT A GLANCE Not just images: The visual appearance of a signature on a PDF has no legal significance—what matters is the cryptographic certificate in the background. The three pillars of verification: During validation, the identity of the signer, the authenticity of the document (integrity), and the validity of the certificate are always verified. The EUTL factor: A qualified electronic signature (QES) is only 100% legally recognized throughout Europe if the Trust Service Provider (TSP) is listed on the EU’s official Trust List (EUTL). The U.S. competitive disadvantage: Many U.S. tools fail to pass European verification tools (such as the government-run RTR Accreditation Service) because they do not sufficiently anchor certificate chains. |
Introduction: Why “Verification” Is the True Foundation of Digital Processes
In many companies, digitally signing contracts has long been standard practice. But while a tremendous amount of effort is invested in the signing process, one fundamental question often goes unanswered in day-to-day B2B operations: How do you verify the documents that are returned to your company once they’ve been signed?
Companies exchange sensitive contracts on a daily basis—from employment contracts in HR to supplier agreements and high-volume financial transactions. Anyone who assumes that a scanned signature or a simple visual placeholder image in a PDF is legally valid is taking on a massive compliance and liability risk. Only systematic, cryptographic verification gives you legally binding assurance that your contracting party is indeed who they claim to be and that the contract text has not been tampered with afterward.
The Technical Anatomy: What Happens When You Verify a Digital Signature?
When you verify a digital signature (for example, using sproof Validator, Acrobat Reader, government validation services such as Rundfunk und Telekom Regulierungs-GmbH in Austria, or specialized software), a three-step process takes place in the background:
1. Verifying Document Integrity (Hash Comparison)
During the signing process, the entire file contents are compressed by an algorithm into a unique digital fingerprint—known as a hash value. The verification tool recalculates this hash value when the file is opened. If the current hash value matches exactly the value encrypted at the time of signing, it is certain that the document has not been altered by a single character since it was signed.
| sproof Insight vs. the U.S. market leader: sproof sign directly and legally embeds each signature cryptographically in the PDF at the time of signing. U.S. providers such as DocuSign often place signatures on the document during the process merely as visual images and only add the final collective seal during the final download. This makes it difficult for verification tools to trace exactly when specific changes to form fields were made. |
2. Validation of the Digital Certificate (Identity)
Every advanced (FES) or qualified electronic signature (QES) is inextricably linked to a digital certificate. This certificate serves as a digital ID. The verification tool extracts the certificate’s metadata to retrieve the signer’s name, verified email address, and the issuer (Trust Service Provider, or TSP for short).
3. Comparison with the European Union Trusted List (EUTL)
The eIDAS Regulation ensures maximum legal validity within the European Union. Highly regulated industries require a qualified electronic signature (QES) as a standard, since only this is legally equivalent to a handwritten signature 100 percent of the time. When verifying a QES, the software checks the issuing TSP against the European Commission’s official Trusted List (EUTL). If the provider is not accredited on that list, the signature loses its status as a QES.
Why Some Signatures Are Flagged as “Invalid” in European Audit Tools
A common occurrence in European legal departments: A contract signed using a U.S. tool is uploaded to the official verification service of RTR (Austria) and is promptly classified as “invalid” or “technically unverifiable.”
There is a reason for this: a lack of EUTL accreditation. Many global hyperscalers use their own certificate chains for their standard signatures; while these are marked as “green” by commercial lists (such as the Adobe Approved Trust List—AATL), they do not meet the strict regulatory criteria of European government oversight authorities.
Step by Step: How to Verify a Digital Signature in Practice
For companies, there are primarily three proven methods for validating signatures in an audit-proof manner:
- Method 1: Automated Validation via Software & API (For Enterprises) Large organizations cannot manually verify incoming documents. This is where modules like sproof Validate come into play. Via a REST API, contracts are automatically scanned upon receipt, cryptographically verified, and archived in your document management system (DMS) with an immutable audit trail.
- Method 2: Validation Directly in the sproof sign Platform If you manage documents in your central sproof sign dashboard, all it takes is a single click. Simply hover your mouse over the signature card in the document editor. The tool immediately displays an overlay showing whether the signature is valid, which security standard (EES, FES, QES) applies, and which trust service provider issued the certificate.
- Method 3: Manual Verification Using Adobe Acrobat Reader Thanks to the global PAdES standard, LTV-enabled (Long-Term Validation) signatures can also be verified offline in Adobe Reader. To do this, Adobe uses the embedded revocation data (CRL/OCSP) to confirm the historical validity of the signature over decades.
Put an End to Legal Uncertainties in Document Management
Automate your verification processes and protect your company from compliance violations. With sproof Sign and sproof Validate, you’re choosing a 100% European solution—with no ties to the U.S., fully GDPR-compliant, and ISO 27001-certified.
Schedule a no-obligation demo now or try sproof sign for free
FAQs on the topic
-
+–What is the difference between a visual signature and a digital certificate?The image of the signature on the PDF serves solely as visual information for humans. The only element that is… show more
The image of the signature on the PDF serves solely as visual information for humans. The only element that is legally and technically binding is the digital certificate, which is cryptographically embedded in the machine-readable data of the PDF document. A mere image without a certificate has little evidentiary value as an electronic signature.
-
+–Can a digital signature be verified without an Internet connection?Yes, provided that the signature supports the LTV (Long-Term Validation) standard. For LTV-enabled signatures, all necessary certificate revocation information is… show more
Yes, provided that the signature supports the LTV (Long-Term Validation) standard. For LTV-enabled signatures, all necessary certificate revocation information is embedded directly into the PDF at the time of signing.
-
+–What does it mean when a verification tool displays the message: “Issuer’s identity unknown”?This message usually appears when the document has been signed using a tool whose root certificate is not included in… show more
This message usually appears when the document has been signed using a tool whose root certificate is not included in the EU’s EUTL list or in the local Windows/Adobe certificate store. Although the document’s integrity may remain intact, its status as a legally valid “Qualified Electronic Signature” (QES) is thereby invalidated.
-
+–What happens if a document is slightly modified after it has been signed (for example, by adding a page number)?As soon as a PDF document is altered even slightly after a digital signature has been applied, the cryptographic chain… show more
As soon as a PDF document is altered even slightly after a digital signature has been applied, the cryptographic chain of seals is broken. Any professional verification tool will immediately report that the document has been tampered with and that the signature is therefore invalid. With sproof, because it is deeply embedded in the PDF, it is possible to precisely trace whether any changes were made in violation of the rules and, if so, which ones.
-
+–How long is a digital signature valid and verifiable?Standard signatures often can no longer be validated correctly after the underlying certificate expires (usually after 2 to 3 years)…. show more
Standard signatures often can no longer be validated correctly after the underlying certificate expires (usually after 2 to 3 years). sproof sign therefore consistently uses the LTV (Long-Term Validation) standard. In this process, revocation information (such as CRL or OCSP lists) is embedded directly into the document at the time of signing. This allows the validity of the signature to be verified in an audit-proof manner—even decades later—and independently of the issuer.
-
+–Is it possible to verify multiple digital signatures on a single document independently of one another?Yes. When multiple people sign a document one after another (sequential workflow), the PDF contains a history of what are… show more
Yes. When multiple people sign a document one after another (sequential workflow), the PDF contains a history of what are known as revision states. A validation tool (e.g., sproof Validate) validates each signature individually against the state of the document that existed at the time of each signature.
-
+–Why is the audit trail so important for compliance managers?The audit trail provides a complete, admissible-in-court history of the entire signature process. It documents when and by whom the… show more
The audit trail provides a complete, admissible-in-court history of the entire signature process. It documents when and by whom the document was viewed, approved, and signed, including the verification method used (e.g., two-factor authentication or qualified eID). This log serves as primary evidence during audits or legal disputes that the signature was properly created.
