How to Use the Command 'codesign' (with examples)

The codesign command is a powerful tool in the macOS ecosystem used for creating and managing code signatures. These signatures are essential for validating the authenticity and integrity of applications and other code executables. By signing code, developers can ensure that their application is recognized as trusted by macOS, thus allowing them to distribute their software while providing users with confidence in its security and integrity. This article will guide you through two primary use cases: signing an application with a certificate and verifying its signature.

Use Case 1: Sign an application with a certificate

Code:

codesign --sign "My Company Name" path/to/application_file.app

Motivation:

The primary motivation for signing an application is to assure users and the macOS operating system of the application’s authenticity and integrity. Unsigned applications or those with unverified signatures are often flagged as suspicious, leading to warnings during installation or execution. By using a certificate tied to “My Company Name,” developers demonstrate that the application is indeed the product of a trusted source, thereby enhancing user trust and facilitating smooth software rollouts.

Explanation:

• codesign: This is the command-line utility used to interact with code signatures, providing the means to sign or verify code on macOS.
• --sign "My Company Name": This flag specifies the action of signing a bundle. By providing the argument "My Company Name", you instruct codesign to use a digital certificate associated with “My Company Name.” This is crucial in associating the application with a legitimate developer identity, authenticated by a trusted Certificate Authority.
• path/to/application_file.app: This specifies the location of the application bundle you wish to sign. The path should accurately point to the .app directory of the application in question.

Example Output:

path/to/application_file.app: signed bundle with Mach-O thin (x86_64) [AppName]

This output indicates that the application located at the specified path has been successfully signed. The format of the file is specified, and the architecture (here, x86_64) is acknowledged. It confirms that the application will be recognized by macOS as signed by the entity “My Company Name.”

Use Case 2: Verify the certificate of an application

Code:

codesign --verify path/to/application_file.app

Motivation:

After signing an application, it’s crucial to verify that the signature is intact and properly applied. Verification ensures that the application has not been tampered with after signing, maintaining its integrity. This step is necessary not only for developers but also for system administrators and users who need to confirm the legitimacy and security of the applications they install or distribute. Ensuring the application is successfully authenticated by its signature helps avoid potential security vulnerabilities or system warnings during operation.

Explanation:

• codesign: This remains the primary utility to manage code signing operations, including the crucial step of verification.
• --verify: This flag signals that the codesign utility should perform a verification check on the application file. Its goal is to ensure that the digital signature is valid and that the code has not been altered since it was signed.
• path/to/application_file.app: As with signing, this defines the exact location of the application file you want to verify. It should point to the .app directory of the target application.

Example Output:

path/to/application_file.app: valid on disk
path/to/application_file.app: satisfies its Designated Requirement

This output indicates the application located at the specified path has passed the verification checks. The messages confirm that not only is the signature valid, but it also meets the designated requirements set out for the application, providing assurance of its integrity and origin.

Conclusion:

The codesign utility serves a critical role in the macOS ecosystem by allowing developers to sign their applications, ensuring integrity, validation, and authentication. By understanding these use cases, developers and IT professionals can safeguard their applications against unauthorized modifications, thereby bolstering user trust and facilitating smooth deployments. Whether signing new applications or verifying existing ones, codesign is an indispensable tool in the macOS software development and distribution process.