What is a Microsoft Code Signing Certificate and How Does It Work?

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Code signing is a critical step in software development regardless of what platform you’re working with. Microsoft is no different — anyone developing software using Microsoft .Net or any other language or platform needs to invest in a Microsoft code signing certificate to ensure their software is trusted.

Here’s the thing: most code signing certificates are platform-agnostic. These x.509 certificates can be used to sign a whole range of software types depending on how they’re configured and the format they’re issued in. So, a Microsoft code signing certificate is really just a code signing certificate that signs using Microsoft Authenticode.

How Do I Get a Microsoft Code Signing Certificate?

The process for obtaining one is simple: just buy it from a trusted certificate authority (CA) just like you would an SSL certificate or anything else. Easy.

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Bear in mind, however, that you’ll need to undergo validation first. When a CA issues a code signing certificate, they’re giving you immense power. Anything you sign will be trusted, which means the CA is required to do its due diligence before ever issuing a code signing certificate. Validation for code signing certificates come in two forms — there’s individual/organization validation, comparable to OV SSL, and there’s extended validation (EV) code signing, which requires a little extra vetting but provides a reputation boost with the Microsoft SmartScreen web filter.

Once the certificate has been issued, you can begin using its private key to sign your software. This is done right on your server using the Authenticode client.

How Does a Code Signing Certificate Work?

When you sign a piece of code with your Microsoft code signing certificate, you’re affixing a digital signature to the software to prove that the code hasn’t been modified since it was originally signed. Then the two of them are hashed together. Hashing is a cryptographic function where data of any length is mapped to a fixed-length output. This is a called a digest or hash value. Two different inputs can never create the same output. This is why hashing serves as a great checksum to ensure the integrity of the software is intact: even the smallest tweak will result in a different hash value.

When a client receives the software, their device will verify the authenticity of the signature and the integrity of the software before giving the go-ahead to the user.