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ElGamal encryption, by its efficiency, is one of the most used schemes in cryptographic applications. However, the original ElGamal scheme is only provably secure against passive attacks. Damgård proposed a slight modification of ElGamal encryption scheme (named Damgård's ElGamal scheme) that provides security against non-adaptive chosen ciphertext attacks under a knowledge-of-exponent assumption. Recently, the CCA1-security of Damgård's ElGamal scheme has been proven under more standard assumptions. In this paper, we study the open problem of CCA2-security of Damgård's ElGamal. By employing a data encapsulation mechanism, we prove that the resulted hybrid Damgård's ElGamal Encryption is secure against adaptive chosen ciphertext attacks. The down side is that the proof of security is based on a knowledge-of-exponent assumption. In terms of efficiency, this scheme is more efficient (e.g. one exponentiation less in encryption) than Kurosawa-Desmedt scheme, the most efficient scheme in the standard model so far.