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Public Key Encryption With Equality Test Supporting Flexible Designated Authorization in Cloud Storage

In cloud storage, public key encryption with equality test (PKEET) is suitable for testing whether two ciphertexts generated from different public keys contain the same message without decryption. Recently, (Y. J. Wang et al., 2019) have proposed a public key signcryption scheme with designated equality test (PKS-DET). In this scheme, since the public key of the tester is used in the encryption, the tester needs to use its secret key to test the ciphertexts. Without trapdoor, PKS-DET prevents the attacker from testing the user’s ciphertexts with the stolen trapdoor. Therefore, the privacy of the user can be protected. However, if the user in the PKS-DET wants to authorize the other tester to test his/her ciphertexts for some reason, the message needs to be encrypted again by the public key of the other tester. This will lead to higher computational complexity. Hence, the PKS-DET is not flexible to authorize multiple testers. In this article, we propose a construction of PKEET supporting flexible designated authorization (PKEET-FDA). The user in our PKEET-FDA can adaptively authorize multiple testers to test his/her ciphertexts, and each authorized tester must use its secret key to perform equality test for the ciphertexts. More importantly, the user does not need to encrypt the message repeatedly any more. We demonstrate the security of PKEET-FDA under two types of adversaries. Compared with the related efficient PKEET schemes, PKEET-FDA satisfies high efficiency from the point of view of the user. In terms of storage requirement, our construction is superior to the PKS-DET.