This paper describes important related work towards the SMS Security and its implementation of Combined Algorithm (Digital Signature and SHA). The paper begins by describing the GSM security architecture which are applicable to almost every cellular network management. Then we describe the SIM Card security, RSA Digital Signature Algorithm, Secure Hash Algorithm. After describing the theoretical background we introduces the Problem statement and assumption and discusses the existing security keys and also discusses their flaws. At last in this paper we gives the proposed security keys and its security features and provides the role of Key Distribution Center (KDC) in the proposed work and also explains why RSA-1024 is used in the proposed work and also discusses how it can be effectively implemented using Binary method and Blakley's Method We generate a Digital Signature for short messages using RSA-1024 to be used for secure transactions through mobile devices. These digitally signed documents can be used for various business transactions using GSM cellular network to preserve their authenticity, integrity and confidentiality. We assume that the entire current GSM cellular standard will provide a backbone to support the communication once digital signature has been generated.