University College Dublin
ElGamal encryption, by its efficiency, is one of the most used schemes in cryptographic applications. However, the original el-gamal scheme is only provably secure against passive attacks. Damgard proposed a slight modification of elgamal encryption scheme (named damgard's elgamal scheme) that provides security against non-adaptive chosen ciphertext attacks under knowledge of exponent assumption. Recently, the CCA1-security of damgard's elgamal scheme has been proven under more standard assumptions. In this paper, the authors study the open problem of CCA2-security of damgard's elgamal. By employing a data encapsulation mechanism, they prove that the resulted hybrid damgard's elgamal encryption is secure against adaptive chosen ciphertext attacks.