Signatures in macOS: Identity, Integrity, and the Mach-O Format

#developer #mac

Unlike Linux or older versions of Windows, macOS doesn't just check permissions (can I read/write this?); it checks Identity and Integrity at the kernel level before a single instruction is executed.

The Three Pillars of a Signature

A macOS code signature isn't just a digital "stamp." It is a multi-layered security contract embedded directly into the Mach-O (the Mac executable format) binary.

1. The Seal (Integrity)

When a developer signs a binary, the codesign utility creates a cryptographic hash (a unique fingerprint) for every "page" of the executable code.

The Check: As the program loads into memory, the macOS kernel constantly re-calculates these hashes.
The "Kill": If even one bit of the binary is modified (by a hex editor, a virus, or a corrupted download), the hashes no longer match the signature. The kernel doesn't show a popup; it simply terminates the process with a SIGKILL.

2. The Identity (Trust)

The signature identifies who created the code. There are three tiers of trust:

Apple-Signed: Core OS components. High privilege.
Developer ID: Signed by a registered Apple Developer ($99/year). These can be "Notarized" (pre-scanned by Apple's servers for malware).
Ad-hoc Signatures (-): A signature without a verified certificate. It says "I am identifying myself as this specific version of the code, but I have no verified ID." On Intel Macs, this is the bare minimum required for many system-level features to work.

3. The Entitlements (The Permission Slip)

This is the most powerful part of the signature. Entitlements are an embedded XML "dictionary" that tells the system what the program is allowed to do.

Hardened Runtime: Introduced in Big Sur, this requires binaries to explicitly "ask" for permissions (like using the microphone or loading a third-party plugin) within their signature. If the code tries to perform an action not listed in its entitlements, the kernel shuts it down.

How to Inspect a Signature

You can look "inside" the signature of any binary on your Mac using the codesign tool:

codesign -dvv /path/to/binary

Executable: The path to the file.
Identifier: The unique string (like com.google.chrome) for the app.
CodeDirectory: The hash version and "flags" (like hardened).
Authority: Who signed it (e.g., Developer ID Application: Apple Inc.).

Why This Matters

For developers, the signature system is the reason why "it works on my machine" often fails when sharing a binary. Without a valid signature and the correct entitlements, your code isn't just "untrusted" to the macOS kernel, it is effectively broken.

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