新增于: v11.6.0
根据此 KeyObject 的类型，此属性是 'secret' 表示秘密（对称）密钥，'public' 表示公共（非对称）密钥或 'private' 表示私有（非对称）密钥。
Added in: v11.6.0
Depending on the type of this KeyObject, this property is either
'secret' for secret (symmetric) keys, 'public' for public (asymmetric) keys
or 'private' for private (asymmetric) keys.