A public parameter PK is key information used in a secret search system 1000 having: a transmission device 100 generating and sending an encryption tag which is an encrypted keyword; a server 200 receiving and storing the encryption tag and conducting a secret search in response to a request for the secret search; and a reception device 300 generating a trapdoor which corresponds to a digital signature of the keyword and as well data requesting the secret search, sending the trapdoor to the server 200, and receiving a search result. The public parameter PK includes a true public parameter PP and a protection key PK'. The transmission device 100 and the reception device 300 require both the public parameter PP and the protection key PK'; however, the server 200 does not require the protection key PK'. Accordingly, the key generation device 300 generates separately the true public parameter PP and the protection key PK' included in the public parameter PK.
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