Code Signing

What it is

Code signing is a technology that allows recipients of software packages to verify that the packages have not been modified by an attacker after being signed.

Some nice features:

• The verification does not have to make any assumptions about the trustworthiness of the way the software packages were obtained. This makes it possible to trust packages distributed through third-party channels, without requiring HTTPS, even if the download site got hacked. The verification can be done off-line.
• Already deployed VLC instances can update themselves to newer versions with assurance that the new versions are genuine.
• Even though attackers can verify signatures, they cannot automatically forge signatures.
• Signature verification can be automated, such that when everything is going well, this security feature is invisible (read "user-friendly") to the end user.

Checksums such as CRC, MD5, SHA1 etc. require the verifier to trust that the way in which the checksums were obtained has preserved their authenticity.

As nicely and seamlessly as it works once the system is set up, the set-up phase itself carries some risk. If in the bootstrapping phase an attacker can trick a user to trust the attacker instead of VideoLAN, the attacker can from then on make the user accept tampered software packages as genuine. The security of the bootstrapping phase is not yet a solved problem for VideoLAN, but even so, it greatly reduces the window of opportunity for an attacker to impersonate VideoLAN to its users. When VLC is obtained from the repositories of a Linux distro or from an app store, the trust bootstrapping is done at the moment when the user enters that ecosystem. Code signing is still useful in proving to the maintainers of those ecosystems that a certain package is genuine.

TODO: explain what certificates are used for.

Preparations for code signing

As currently done

New certificates are generated once a year and reuse the key from the previous year (a new OpenPGP identity and new signatures with new expiration dates are added to the file with the old key).

Problem: Crypto best practices recommend signers to take advantage of the generation of a new certificate to also change the key.

Proposed improvement

Generate a new key pair every year, certify it for two years, use it for signing only during the first year, use it to sign next year's key.

Problem: The way the auto-update currently does signature verification has some problems with this.

Code signing in the release process

As currently done

Packages are signed at publication time.

Proposed improvement

Synchronise the validity of the signature with the support duration for the particular version of the software package. If VideoLAN wanted to ensure that even old packages can be verified now, it would have to ensure that those packages get re-signed with the currently valid key.

Problem: Yes, if attempting to do this manually.

Signature verification

In the auto-update code

As currently done

TODO: Let someone with knowledge of the code document it.

Proposed improvement

If an instance of VLC has not been updated while the signature made with the public key known by the instance was valid, the process should acknowledge that a new bootstrap of the trust is needed. Either a fresh install should be required or a warning should be displayed or there should be another mechanism in place for establishing trust in the authenticity of the new package. In order to prevent the situation from even occurring, the certificates should have a long enough life span.

Problem: The users cannot be kept secure without bothering them a little in this situation.

Manually

As currently done

The manual verification works best only if the user ensures that the certificate is fresh, such as getting it from a key server.

Problem: The concept of updating a certificate because of an extension of the validity period is unusual, because it is recommended against.

Proposed improvement

When the certificate of a code signing key has expired, the verifier stops putting any trust in that key and in signatures made with that key. But there is always a key that is valid now and if the package was worth the effort, VideoLAN has provided a signature with a currently valid key.