###### Abstract  
Over the years, there has been a significant surge in the popularity of instant messaging applications (IMAs). However, the message notification functionality in IMAs exhibits certain limitations. Some IMAs fail to alert users about new messages after their phone restarts unless they unlock the phone. This is a consequence of end-to-end encryption (E2 EE) and the app not knowing the message is in the queue until the app decrypts it. This approach using E2 EE is used to prevent offline attacks, as the key is unavailable to decrypt the notification messages. In this paper, we introduce a novel design and implementation of a message notification protocol for IMAs based on the Signal protocol. The proposed protocol aims to securely display notifications on a locked device and ensures that cryptographic keys are stored in a location that is isolated from the user's device to prevent offline attacks. This approach enhances the security of private key storage, safeguarding private keys against various external threats. The innovative design strengthens the off-site key management system, rendering it resilient against offline attacks and mitigating the risk of key compromise. Additionally, the proposed protocol is highly efficient, requiring no specialized hardware for implementation. It offers confidentiality of cryptographic keys and protection against offline attacks, further enhancing the overall security of the system. We evaluate the protocol's effectiveness by analyzing multiple independent implementations that pass a suite of formal tests via ProVerif. c 202420202414202  
problems are proven to be solved [3; 4; 5]. The asymmetric key algorithm resolves the issue of key distribution. However, due to the inherent complexity of asymmetric key operations, their practical use is limited when encrypting large amounts of data [6]. On the other hand, symmetric key algorithms perform the encryption and decryption process in constant time for the input size [7; 8]. Therefore, symmetric key algorithms are widely used to provide confidentiality services. Nevertheless, key distribution between the sender and recipient is the main challenge in a symmetric key cryptosystem. In such schemes, the same key is used for both encryption and decryption. Consequently, both parties must share the key through a secure channel before communication. Therefore, secure key management and distribution are crucial aspects of ensuring secure communication. The importance of key management systems (KMSs) has attracted researchers to design various models that have been presented in the literature [9; 10; 11; 12]. In 1999, [PERSON] et al. designed an IBM secure coprocessor enriched with various security features and tamper-resistant hardware modules that handle cryptographic operations, safe booting, and secure software with extreme caution [13]. The suggested protocol provides physical protection to cryptographic keys; however, it is impractical for use in portable electronic devices due to its high cost, inconvenient design, and the large amount of resources it consumes. The concept of a remote server connected through a network to conduct private key activities was recommended by [PERSON], [PERSON], [PERSON], and [PERSON] [14; 15]. In such a scheme, an attacker would need to breach the server's security and the user's password protection to gain access to the private key. According to [PERSON], this approach can be utilized to handle the signature verification process. However, [PERSON] and [PERSON] reiterated in their study that the server assists the device with executing the private key. In practical terms, securing the user's private key in software is more feasible than deploying tamper-resistant hardware. In both studies, security relies on users trusting the remote server. However, a thorough examination of their studies revealed vulnerabilities to impersonation. For instance, the software smart card was susceptible to impersonation attacks in various real-world scenarios.