codesigning

Code signing is the process of digitally signing software to verify its origin and to ensure it has not been altered since it was signed. It relies on public key cryptography and digital certificates to provide authenticity, integrity, and non-repudiation.

In a typical workflow, a developer uses a private signing key to create a signature for the software by hashing the code and encrypting the hash with the private key. The resulting signature, along with a signing certificate that binds the public key to the signer’s identity, is embedded in or attached to the artifact. The signing certificate is issued by a trusted certificate authority (CA) and may form part of a larger PKI trust chain.

On distribution or installation, the platform or application verifies the signature by decrypting it with the public key from the signer's certificate, checking the certificate chain to a trusted root, and confirming that the code hash matches the content. Timestamping can be used to prove that a signature remained valid even after the certificate expired. If verification succeeds, the software is presented as trusted or allowed to install; if not, users may receive warnings or blocks.

Code signing certificates must be protected, as the signing key controls the trustworthiness of signed code. Practices include secure storage (often hardware security modules or secure enclaves), key rotation, and revocation mechanisms (CRLs or OCSP) when keys are compromised.

Many platforms implement their own signing schemes, such as Windows Authenticode, macOS and iOS codesigning, and Android APK signing, all relying on similar PKI-based verification.