What Is Code Signing?

Code signing is the process of digitally signing software or code artifacts to prove the code’s origin and guarantee that it has not been altered since it was signed.

At its core, code signing uses cryptographic techniques (specifically, public key infrastructure or PKI) to:

• Attest authenticity: Is this file created by the claimed publisher?
• Ensure integrity: Has the file been modified, infected, or corrupted after signing?
• Enable non-repudiation: The signer cannot later deny authorship, as their cryptographic private key was used.

When a piece of signed code, be it an executable (.exe), mobile app, script, driver, or update package, is distributed, users (and their devices) can verify the digital signature against a trusted certificate authority (CA). If the code has been tampered with, or the publisher is unknown or untrusted, the system can block the code, warn the user, or log an incident.

How Does Code Signing Work? 

The mechanics of code signing are rooted in asymmetric cryptography, using a unique pair of keys: one private (kept secret by the publisher) and one public (shared with everyone).

1. Certificate and Key Generation:

• The software publisher or organization obtains a code signing certificate from a trusted Certificate Authority (CA), like DigiCert, GlobalSign, or Sectigo.
• The CA verifies the publisher’s identity and binds it to the public/private key pair.

2. Signing the Code:

• Using code signing tools (such as Microsoft SignTool, OpenSSL, Jarsigner, or platform-specific tools), the publisher generates a cryptographic hash of the software artifact. 
• This hash is then encrypted with the publisher’s private signing key, creating the “signature.”
• The signature, along with the public key (and potentially the CA’s certificate chain), is attached to the code artifact.

3. Distribution:

• The signed code, now a package, binary, firmware, script, or container image, is distributed to users, endpoints, or devices via download, update channels, or physical media.

4. Verification at Installation or Execution:

• When the code is received, the recipient’s system or security software decrypts the signature using the included public key (or retrieves a certificate chain from a CA).
• It compares the decrypted hash with a fresh hash of the file:
  
  • If they match, the code is genuine, unaltered, and from a trusted source.
  • If they don’t, the code is either corrupted or maliciously modified.

5. User or System Action:

• Most operating systems, browsers, and app stores will flag unsigned or invalidly signed code, often blocking or warning users before execution.
• Valid signatures, especially those chained to well-known CAs, enable seamless, trusted execution and may unlock advanced system privileges.

Why Is Code Signing So Important? (Benefits & Risk Reduction)

1. Provenance: Trust the Source

Malicious actors routinely disguise malware as legitimate software, tricking users into installing ransomware, backdoors, or spyware. Code signing provides cryptographic proof of the real author. In a supply chain, this provenance is essential: only code signed by known, trusted entities enters production.

2. Integrity: Guard Against Tampering

Even if code begins its life as legitimate, a man-in-the-middle attack, malware-injection via a compromised build server, or an insider threat can alter files in transit. Code signing breaks this attack chain; a mismatch in the hash immediately exposes unauthorized changes.

3. User and System Trust: Fewer Roadblocks, More Security

Unsigned executables, drivers, apps, and browser extensions trip antivirus and security prompts, frustrating end-users and raising support burdens. Signed code builds trust, clears security hurdles, and improves user confidence, often essential for adoption in enterprise environments.

4. Compliance, Legal, and Commercial Requirements

Many platforms, app stores, and regulatory frameworks now require code to be signed:

• Microsoft Windows: Drivers, UWP apps, and PowerShell scripts must be signed to run without warnings.
• Apple macOS/iOS: All apps must be signed for distribution; unsigned code is blocked.
• Android: APKs must be signed to install.
• Web Browsers: Browser extensions/plugins need signatures from the Chrome Web Store or Mozilla Add-ons.
• PCI DSS, HIPAA, and FDA: Industries with strict data security requirements often mandate code signing for any software affecting protected systems.

Neglecting code signing can lead to product rejection, compliance failures, or regulatory action.

5. Incident Response and Forensics

When incidents occur, code signing enables investigation teams to verify the chain of custody and isolate whether code was altered post-release. Unsigned and thus unauditable code increases business and legal risk.

Common Use Cases

While code signing’s role in OS installers and app stores is well known, its scope is far wider:

1. Operating Systems and Application Installers

All modern desktop and server operating systems require, or strongly recommend, that executable installers, system utilities, and drivers be code-signed. Unsigned code is likely to be blocked, trigger warnings, or generate errors under modern security policies.

2. Software Updates and Patch Delivery

Automatic updaters and patch management tools rely on code signatures to prove each update is authentic and unmodified, protecting users from malicious “update” attacks or poisoned patches.

3. Mobile Apps

Android and iOS mandate code signing not just for initial release, but also for subsequent updates, beta channels, and enterprise deployments.

4. Scripts and Macros

Scripts, including PowerShell, VBScript, and even Office macros, can be signed to manage trust within enterprises and minimize the risk of macro-based malware attacks.

5. Browser Extensions and Plugins

All major browsers require code/signed extensions, from Chrome Web Store, Firefox Add-ons, etc., to protect users from malicious plugins and ensure publisher traceability.

6. Containers, Cloud Artifacts, and DevSecOps

With the shift to cloud-native and infrastructure-as-code practices, container images, Helm charts, GitOps sources, and even IaC templates are now routinely signed as part of secure DevOps pipelines, using tools like Cosign or Notary.

7. Firmware and Embedded Devices

IoT and hardware devices use code signing to prevent malicious firmware upgrades (think: smart TVs, routers, medical devices), enabling devices to reject unsigned/unauthorized updates.