Key exposure is very harmful to a cryptographic system. To decrease the loss from the deputy signing key vulnerability in identity-based proxy signature systems, we propose the method of key protected deputy signature (IBKPDS) using the method of parallel key insulation. The proposed IBKPDS is based on identities and is shown to be secure with the cryptographic proof. In the proof, there is no random oracle. In an IBKPPS crypto-system, a user stores his short-lived deputy signing key by himself and saves two long-lived keys in two heavily guarded boxes respectively. The derived IBKPDS cryptographic system is heavily key-separated. A thief who wants to obtain crucial information can not corrupt the IBKPDS when he get only one long-lived key. In addition, the user can change the short-lived deputy signing keys frequently at low risk.
Key Protected Deputy Signature Scheme against the Deputy Signing Key Exposure