The mobile instant messaging applications have become a famous wireless service in the world allowing a user to communicate with anyone in anywhere. Consequently, the manipulation of the user data creates a pressing need for securing the information and the associated technologies. The protection of these communications has become a paramount objective. The concept of cryptography has become a basic concept in communication and the exchange of messages within companies. This paper presents a hybrid security system allowing messages to be exchanged within a company in a secure manner. Our goal is to provide end-to-end encryption through the combination of asymmetric cryptography, signature, and integration of IMEI parameter. The confidentiality of the messages is ensured by the RSA algorithm, while the integrity of these messages and the sender authentication are ensured by the digital signature, that is carried out with the SHA-2 hash function.