Does the Code Signature Check Out?

About this interactive

Code signing attaches a certificate and a signature to software so your computer can check it before installing. The publisher signs the code with its own private key; the certificate authority signs the publisher's certificate, not the code. (The video says the certificate authority signs the code; it does not.) The check rests on public key infrastructure: the operating system ships with root certificates of certificate authorities it trusts, and the publisher's certificate must chain back to one of them, directly or through an intermediate CA. The signature also covers the file's fingerprint, so a file changed after signing no longer matches. Certificates carry an expiry date. On its own, that would make signed code untrusted the day the certificate expires. A timestamp authority fixes this: it stamps the signature with the time it was made, so code signed while the certificate was valid stays trusted afterwards. Code that was never timestamped loses its trust at expiry. One limit, straight from the lesson: a certificate does not make code legitimate. Anyone can buy one, so a signature that checks out tells you who signed the code and that it is unchanged, not that it is safe.

What you'll learn

Decide whether an installer's code signature checks out, from its certificate chain, its expiry and timestamp, and whether the file changed after signing
Explain that a signature that checks out proves who signed the code and that it is unchanged, not that the code is safe

Aligned to

CompTIA Security+

4.1 Given a scenario, apply common security techniques to computing resources.

1.4 Explain the importance of using appropriate cryptographic solutions.

Key terms

Code Signing: The process of applying a digital signature to software using a certificate and private key so that users can verify the authenticity and integrity of downloaded programs.
Timestamp Authority: A trusted third party that issues cryptographic timestamps to signed code, ensuring the code remains trusted even after its signing certificate expires.
Root Certificate: The self-signed certificate at the top of a PKI hierarchy that serves as the ultimate anchor of trust for all subordinate certificates.
Certificate Authority: A trusted entity that issues digital certificates used to verify the identity of individuals, organizations, or devices.
Digital Signature: A cryptographic mechanism used to verify the authenticity and integrity of a digital message or document.

Topics

Interactive Streak Sort

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