TLS & File Signing: Should You Do Both?

TL;DR

Yes, you should consider signing files even when transporting them securely with TLS (like HTTPS). TLS protects data in transit, but doesn’t guarantee the file hasn’t been altered after delivery or that it originates from a trusted source. File signing adds an extra layer of verification.

Why Both?

Think of TLS as a secure postman – they get your letter safely delivered, but don’t check if the contents were changed during the journey or who actually wrote the letter. File signing is like adding a wax seal with your unique crest to prove authenticity and integrity.

Step-by-Step Guide

1. Understand TLS Limitations: TLS encrypts communication between two points. It protects against eavesdropping and tampering during transmission. However:
   • It doesn’t protect the file on either end (before upload or after download).
   • A compromised server could still serve a malicious file, even with TLS enabled.
   • TLS certificates verify the *server’s* identity, not necessarily the file itself.
2. What File Signing Does: Digital signatures use cryptography to create a unique ‘fingerprint’ of the file.
   • Integrity Check: If the file is altered in any way after signing, the signature will be invalid.
   • Authentication: The signature proves that the file came from the person or organisation who owns the corresponding private key.
   • Non-Repudiation: The signer can’t easily deny having signed the file.
3. Choose a Signing Method: Several options exist:
   • Code Signing Certificates: For executables and scripts (e.g., .exe, .dll, .msi). These are issued by Certificate Authorities (CAs) like DigiCert or Sectigo.
   • GPG/PGP: A free and open-source option for signing various file types.
   • Hash-Based Message Authentication Code (HMAC): Useful when both parties share a secret key. Less common for public distribution.
4. Using GPG/PGP to Sign a File: This is a practical example.
   1. Generate a Key Pair (if you don’t have one):

      gpg --gen-key

      Follow the prompts. Keep your private key very secure!

   2. Sign the File:

      gpg --sign filename.txt

      This creates a file named filename.txt.gpg (or similar, depending on options).

   3. Verify the Signature:

      gpg --verify filename.txt.gpg

      GPG will tell you if the signature is valid and who signed it.

5. Using Code Signing Certificates (Briefly):
   • You’ll need to purchase a certificate from a CA.
   • Tools like Microsoft Authenticode or similar signing utilities are used to embed the signature into the file.
   • The operating system will then verify the signature when the file is executed.
6. Implementation Considerations:
   • Automate Signing: Integrate signing into your build or deployment process.
   • Key Management: Protect your private keys! Use Hardware Security Modules (HSMs) if possible.
   • Revocation: Have a plan for revoking compromised certificates/keys.
   • User Education: If users need to verify signatures manually, provide clear instructions.

In Summary

TLS is essential for secure transport, but file signing adds a vital layer of trust and integrity. Combining both provides stronger cyber security than relying on either one alone.