Secure Software Distribution

TL;DR

This guide shows you how to build software in a way that makes it much harder for malware to get into your releases and safer for users. We’ll cover building reproducible builds, using code signing, and checking dependencies.

1. Set up a Clean Build Environment

The first step is making sure the place where you build your software isn’t already infected or compromised. This means using dedicated machines or containers.

• Virtual Machines (VMs): Use VMs like VirtualBox or VMware. Keep them separate from your everyday work computer.
• Containers: Docker is a popular choice. It lets you create isolated environments that are easy to recreate.

Regularly rebuild these environments from scratch.

2. Reproducible Builds

Reproducible builds mean that given the same source code and build instructions, anyone should be able to produce *exactly* the same binary output. This is crucial for verifying integrity.

• Version Control: Use Git (or similar) to track all your source code changes.
• Build Scripts: Write detailed build scripts that specify every step of the process. Avoid relying on implicit settings or temporary files.
• Dependency Management: Pin specific versions of all dependencies (see section 4).
• Timestamps: Remove timestamps from the build output if possible. Some build tools include them by default, which breaks reproducibility.

Tools like Reproducible Builds can help you check if your builds are truly reproducible.

3. Code Signing

Code signing adds a digital signature to your software, verifying that it comes from you and hasn’t been tampered with after signing.

• Get a Certificate: You’ll need a code signing certificate from a trusted Certificate Authority (CA).
• Signing Process: Use tools specific to your platform. For example:
  • Windows: SignTool (part of the Windows SDK)
  • macOS: codesign
  • Linux: gpg or similar signing utilities
• Example (SignTool):

  signtool sign /f "your_certificate.pfx" /p "your_password" /t http://timestamp.digicert.com your_executable.exe

Store your signing key securely (e.g., using a Hardware Security Module – HSM).

4. Dependency Checking

Your software relies on other libraries and packages. These dependencies can introduce vulnerabilities.

• Dependency Management Tools: Use tools like:
  • npm (Node.js): npm audit
  • pip (Python): pip check or use a tool like Safety
  • Maven/Gradle (Java): Dependency Check plugin
• Pin Versions: Specify exact versions of all dependencies in your project file. Avoid using ranges that allow automatic updates.

  # Example package-lock.json (Node.js)
  "dependencies": {
    "lodash": "4.17.21"
  }

• Regular Scanning: Automate dependency scanning as part of your build process.

5. Static Analysis

Static analysis tools examine your code without running it, looking for potential security flaws.

• Tools: Examples include SonarQube, Coverity, and linters specific to your programming language.
• Integration: Integrate static analysis into your CI/CD pipeline.

6. Secure Distribution Channels

Even with secure builds, you need a safe way to deliver the software.

• HTTPS: Always use HTTPS for downloads.
• Content Delivery Networks (CDNs): CDNs can help distribute your software quickly and reliably.
• Checksums/Hashes: Provide checksums (SHA256, SHA512) of the downloaded files so users can verify their integrity.