Signing Archives: Best Formats

TL;DR

For reliably signing archives, use TAR with GPG or ZIP with digital signatures. TAR+GPG is generally preferred for Linux/Unix environments due to its robustness and widespread support. ZIP with signatures works well cross-platform (Windows, macOS).

1. Understanding the Requirements

When choosing an archive format for cryptographic signing, consider these points:

• Integrity: The archive must not be altered after signing.
• Authenticity: You need to verify who signed the archive.
• Standardisation: Use widely supported formats and tools.
• Platform Compatibility: Consider where the archive will be used (Linux, Windows, macOS).

2. Common Archive Formats & Signing Options

Here’s a breakdown of popular options:

• ZIP: Supports built-in digital signatures. Relatively easy to use on all platforms.
• TAR (Tape Archive): A standard format for bundling files, but doesn’t natively support signing. Usually combined with GPG or other signing tools.
• RAR: Less common for security purposes; signature verification can be problematic. Avoid if possible.

3. TAR + GPG (Recommended for Linux/Unix)

This is a robust and secure approach.

1. Create the Archive: Use the tar command to create a .tar archive.

   tar -czvf myarchive.tar.gz /path/to/files

2. Sign the Archive: Use GPG (GNU Privacy Guard) to sign the archive.

   gpg --sign myarchive.tar.gz

   This creates a myarchive.tar.gz.sig file containing the signature.

3. Verify the Signature:

   gpg --verify myarchive.tar.gz.sig myarchive.tar.gz

   Check that the output confirms a valid signature and trusted key.

4. ZIP with Digital Signatures (Good Cross-Platform Option)

ZIP archives can store digital signatures directly.

1. Create the Archive: Use a ZIP tool that supports signing (e.g., 7-Zip, built-in tools on some operating systems).
2. Sign the Archive: The exact process varies depending on the tool. Typically involves adding a digital certificate to the archive during creation.

   For example, using 7-Zip’s GUI, you can add a signature after creating the ZIP file.

3. Verify the Signature: Use a compatible ZIP tool to verify the signature. The tool will check if the archive has been altered and confirm the signer’s identity.

   Again, 7-Zip provides an easy way to verify signatures through its GUI.

5. Important Considerations

• Key Management: Securely store your GPG private key or digital certificate.
• Timestamping: Consider using a timestamp server when signing to prove the signature was valid at a specific time.
• Tool Choice: Use reputable tools for archiving and signing.