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Nowadays, instant messaging (IM) represents the most used messaging service for both home and corporate users. IM is used in many different application fields. Thus, it is an effective and easy means of network-based communication. Unfortunately, this technology does not provide built-in support for security features. This paper proposes a new and secure instant messaging protocol for a centralized communication group. This protocol utilizes the ElGamal cryptosystem, RSA algorithm, and Chinese Remainder Theorem (CRT.) CRT takes the responsibility of renewing the users' private keys. The intractability of the new algorithm depends on the integer factorization problem (IFP) and the discrete logarithm problem (DLP.) We show that our proposed scheme satisfies expected security requirements.