Timestamping – Why Digital Signatures Need Timestamps?

Timestamping can permanently preserve the code signing certificate. Using a timestamping server, the software gets the current date impressed upon it as it is downloaded.

When you digitally sign code, it proves that the software is authentic and hasn’t been altered. But, if your code signing certificate expires and there’s no timestamp, the signature becomes invalid.

Code signing timestamping helps software developers, IT professionals, and businesses keep their signed applications, drivers, executables, and software programs trusted over time. It embeds a secure, third party-issued timestamp into your signature, so your software remains valid long after the certificate’s lifecycle ends.

How Code Signing Time Stamping Maintains Software Trust Over Time?

Timestamping in code signing attaches a verifiable time to the digital signature, confirming when the software was signed. This preserves the originality of the code, supports software authenticity, and prevents it from being flagged as untrusted due to certificate expiration or revocation.

When someone runs signed software, the operating system checks its digital signature to confirm it hasn’t been altered. Without timestamping, an expired certificate can trigger an unknown publisher warning, or worse, block the installation entirely. Timestamping avoids this by letting the system recognize that the software was signed when the certificate was still valid. This not only keeps it trusted long after the certificate expires but also enables Long-Term Validation (LTV), allowing the software to pass security checks without issues.

How Time Stamping Verifies Code Signing Authenticity?

Timestamping records the exact time the code was signed, helping users and systems verify its authenticity and safety. The process follows these steps:

1. Hashing Process in Code Signing

   Using a secure hash algorithm like SHA-256, a cryptographic hash of the software’s code is generated. This hash serves as a unique fingerprint of the code that assures that even the smallest modification will produce a different hash.

2. Requesting a Time Stamp from Trusted TSA

   The generated hash is sent to a Time Stamping Authority, an independent trusted entity responsible for issuing secure time stamps. The actual software is not sent—only the hash—to preserve confidentiality.

3. Time-stamp Token Generation for Software Verification

   The TSA appends the current date and time to the received hash. This information is then digitally signed using the TSA’s private key, creating a time stamp token.

4. Returning the Time-stamp Token for Validation

   The TSA sends the time stamp token back to the client application. This token confirms when the software was signed and provides cryptographic proof that the signing occurred before the X.509 certificate expired.

5. Integration of Time Stamp in Digital Signature Authentication

   The software’s digital signature is embedded with the time stamp which make sures that whenever the software is verified in future, it can prove that it was signed at a time when the certificate was still valid.

Later, when a user installs or verifies the software, the system checks the time stamp and confirms that the signature was valid at the time of signing, allowing the software to remain trusted indefinitely—even if the certificate has since expired or been revoked.

Why Time Stamping Matters in Code Signing Certificates?

Time stamping adds a layer of reliability to signed applications by recording exactly when they were signed. This helps in distinguishing legitimate software from tampered or outdated versions, keeping it recognizable and valid even if the certificate expires or gets revoked.

1. Prevents Expired Certificates from Invalidating Software

   Software without a time stamp may be flagged as untrusted by major operating systems once the certificate expires leading to loss of user trust.

2. Allows Trust Even If a Certificate Is Revoked

   If a certificate gets revoked, timestamping guarantees that signatures applied before the revocation remain valid.

3. Meets Security Compliance Standards

   Many industries and regulatory compliance require timestamping to verify when code was signed for software distribution.

4. Enhances User Confidence

   Users and enterprises are more likely to install software that remains verifiable over time.

Technology Behind Digital Code Signing Time Stamping

Timestamping in code signing relies on Public Key Infrastructure (PKI) and trusted entities to create a verifiable record of when a software was signed. This process makes sure that a digital signature remains valid even after a certificate expires or is revoked.

1. Public Key Infrastructure (PKI)

   PKI forms the backbone of code signing timestamping. It consists of:

   • Certificate Authorities (CAs): These are trusted entities that issue digital certificates to authenticate software publishers.
   • Asymmetric Cryptography: It is a system that uses public and private key pairs to make sure that signatures can be verified but not forged.
   • Secure Hash Algorithms (SHA): Cryptographic hash functions (like SHA-256 and SHA-512) generate a unique fingerprint of the software that prevents unauthorized modifications.

2. Time Stamping Authority (TSA)

   TSA is an independent entity that provides cryptographic proof of when a signature was applied.

   • It uses a time-stamping server to receive the hash of the signed software. The Sectigo timestamp server and DigiCert timestamp server are widely used for secure timestamping.
   • Appends a cryptographic time stamp that can confirm the exact signing time.
   • Signs the timestamp token with its private key verifying authenticity and integrity.

3. Verification and Signing Process

   • When a software is signed its cryptographic hash is generated and sent to a TSA for timestamping.
   • Leading Certificate Authorities (CAs) like DigiCert and Sectigo provide secure timestamping servers, allowing software publishers to add timestamps to their signed software.
   • The TSA then returns a timestamp token which is attached to the signature.
   • During verification this timestamp token is checked against the TSA’s records to confirm its authenticity.
   • Even if the original signing certificate expires or is revoked, the time stamped signature remains valid as long as it was signed before the event.

This PKI-based cryptographic system guarantees that software remains verifiable, even if the original signing certificate expires or the software is distributed over time.

What is Time Stamping Server for Software Signing Certificate?

A timestamping server is an online service provided by a Time Stamping Authority that generates cryptographically signed timestamps which operate on RFC 3161-compliant protocols, providing industry-wide compatibility and security. They play a crucial role in PKI by providing immutable and tamper-proof evidence of when a digital signature was applied.

When a software publisher signs code, they can configure their signing process to automatically contact a TSA’s time stamp server. The software’s cryptographic hash is sent to the TSA, which then issues a timestamp token containing the hash, the exact signing time, and a digital signature from the TSA’s private key.

The timestamp token is then embedded into the signed software package. Since these tokens are independently verifiable using the TSA’s public key, they provide irrefutable proof that the signature was valid at the time of signing, even if the signing certificate expires or is revoked. Using a trusted timestamp server establishes long-term trust in software authenticity without requiring manual intervention.

Best Practices for Code Signing Time Stamping

To improve the effectiveness of code signing timestamping, consider the following best practices:

1. Every signed executable should include a time-stamp to preserve long-term validity.
2. Use reputable certificate providers with secure, highly available timestamping services. A trusted TSA delivers cryptographic proof of signing time, preventing issues with expired or revoked certificates.
3. Using a robust hash function like SHA-256 assures long term data integrity which prevents quantum computing attacks that can computationally calculate private key for the hash function. Older algorithms like SHA-1 are highly vulnerable to attacks where in time-stamps can be forged or altered.
4. While timestamping preserves trust, renewing certificates before expiration maintains seamless security. A well-maintained signing environment reduces risks associated with outdated or compromised credentials.
5. Using a hardware security module (HSM) you can prevent unauthorized access to your code signing credentials. This strengthens the integrity of the timestamping process by avoiding the risk of security breaches.

Best Options to Get Trusted Code Signing Certificate

You should choose code singing certificate from trusted certificate authority who are permitting you to timestamp your signed code.

Conclusion

Without timestamping, code signing loses its long-term value. Expired and revoked certificates can lead to installation failures, security warnings and majorly loss of user trust. Using a trusted code signing certificate and a time-stamp server guarantees that developers can confirm their software remains verifiable and trusted for ages without getting expired.

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