Learn in the textbookApplying Encryption and Digital Signatures
DefinitionA signing key (署名鍵) is the key used to create a digital signature: the private key (秘密鍵) that only the signer holds.
ExplanationSince only the owner holds the signing key, a signature made with it is treated as the owner's own. Conversely, if the signing key leaks to someone else, that person can impersonate the owner and create signatures.
AnalogyIt is like a registered personal seal that only its owner keeps.
Memory hookA signature is something you write with your own hand, so the key behind it is one only you hold: the private key. The word itself points the way.