Code signing

Code signing is the process of digitally signing executables and scripts to confirm the software author and guarantee that the code has not been altered or corrupted since it was signed. The process employs the use of a cryptographic hash to validate authenticity and integrity.

Code signing can provide several valuable features. The most common use of code signing is to provide security when deploying; in some programming languages, it can also be used to help prevent namespace conflicts. Almost every code signing implementation will provide some sort of digital signature mechanism to verify the identity of the author or build system, and a checksum to verify that the object has not been modified. It can also be used to provide versioning information about an object or to store other meta data about an object.

The efficacy of code signing as an authentication mechanism for software depends on the security of underpinning signing keys. As with other public key infrastructure (PKI) technologies, the integrity of the system relies on publishers securing their private keys against unauthorized access. Keys stored in software on general-purpose computers are susceptible to compromise. Therefore it is more secure, and best practice, to store keys in secure, tamper-proof, cryptographic hardware devices known as hardware security modules or HSMs.

Many code signing implementations will provide a way to sign the code using a system involving a pair of keys, one public and one private, similar to the process employed by SSL or SSH. For example, in the case of .NET, the developer uses a private key to sign their libraries or executables each time they build. This key will be unique to a developer or group or sometimes per application or object. The developer can either generate this key on their own or obtain one from a trusted certificate authority (CA).

Code signing is particularly valuable in distributed environments, where the source of a given piece of code may not be immediately evident - for example Java applets, ActiveX controls and other active web and browser scripting code. Another important usage is to safely provide updates and patches to existing software.Windows, Mac OS X, and most Linux distributions provide updates using code signing to ensure that it is not possible for others to maliciously distribute code via the patch system. It allows the receiving operating system to verify that the update is legitimate, even if the update was delivered by third parties or physical media (disks).

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