What is Cosign?
The OpenSSF (Open Source Software Security Foundation) provides lots of useful tooling to improve supply chain security for open source projects. One of these tools is the sigstore project which “allows developers to securely sign software artifacts” and the Cosign tooling specifically to sign OCI (container) images.
Signing the Fluent Bit container images with Cosign ensures that users can verify that the image they are using is the official one built and provided by the Fluent Bit project. Full details on verifying the signature can be found in the documentation.
This follows a similar approach to the GPG signing we (and package developers in general) have provided for native Linux packages (RPM/DEB) as well as the repository metadata itself which is required for various more secure platforms (e.g. FIPS compliance).
We encourage anyone using OSS to get projects added to the OpenSSF if they can contribute in that way.
Another useful project provided by the OpenSSF is the gitsign tooling. Gitsign simplifies signed commit handling and helps prevent some of the possible attacks with standard GPG keys. We would also encourage anyone using Git to adopt it. For further details please look at the official documentation and this useful background post.