A signing subkey is a secondary cryptographic key used for a specific signing role, separate from a main or primary key. In GPG and similar systems, the main key may identify the owner, while the subkey handles day-to-day signing of releases, updates, or artifacts. Because it can be rotated or revoked independently, a subkey gives teams more control over key management.
This matters in cyber security because signed software is trusted only if the signing key remains under control. If a signing subkey is exposed in a repository, stolen from a build system, or copied from a developer machine, defenders may revoke it and issue a replacement to preserve trust in future releases. Attackers target signing keys because they can make malicious software look legitimate; defenders use subkey separation to limit damage and speed recovery when a secret is compromised.