## 1 Introduction  
Underwater Wireless Sensor Networks (UWSNs) have developed as a potentially useful technology for a variety of applications that take place underwater, including oceanographic monitoring, environmental monitoring, and underwater surveillance [1]. UWSNs consist of many sensor nodes that are deployed in the underwater environment to gather data and transmit it to the sink node as shown in Figure 1. However, UWSNs face several challenges such as the harsh underwater environment, limited communication range, and low data rates [2]. One of the critical challenges is to develop a secure and lightweight key management framework that ensures the security of the network and prevents unauthorized access [3, 4]. Key management is an essential component of any secure communication system, and it plays a critical role in UWSNs. Key management involves generating, distributing, and revoking cryptographic keys that are used to encrypt and decrypt data. A secure key management system guarantees the confidentiality, integrity, and availability of the data transmitted over the network and ensures that only authorized nodes may access it [5]. Key management is a fundamental component of any secure communication system, and it plays a critical role in UWSNs. Key management involves generating, distributing, and revoking cryptographic keys that are used to encrypt and decrypt data [6]. A secure key management system guarantees that only authorized nodes can access the network and that data communicated over the network is confidential. Several key management frameworks have been proposed for UWSNs, but most of them have limitations such as high processing overhead, limited scalability, and vulnerability to attacks [7]. Therefore, there is a need for a secure and lightweight key management framework that can address the challenges of UWSNs and ensure the security of the network. This study proposes a secure and lightweight key management framework for UWSNs that combines symmetric and asymmetric encryption, uses lightweight cryptography algorithms, and ensures scalability. The proposed framework includes a robust key generation algorithm, fault-tolerant key distribution mechanisms, a key revocation mechanism, an authentication mechanism, and a lightweight implementation. The proposed framework builds upon the existing research on key management frameworks for UWSNs. There are three distinct methods for handling keys in UWSNs; they are centralized, hierarchical, and distributed [8]. A network that uses centralized key management has one governing body that generates and distributes keys to all other nodes. Hierarchical key management divides the network into clusters, and a cluster head is responsible for generating and distributing keys to nodes within the cluster. Distributed key management involves nodes generating and distributing their keys. However, most of the existing key management frameworks for UWSNs have limitations. The limitations of existing frameworks include high processing overhead, limited scalability, and vulnerability to attacks such as node compromise and key compromise. To  
Figure 1: UWS Network Model. overcome the limitations of existing key management frameworks for UWSNs, several studies have proposed hybrid key management frameworks that combine symmetric and asymmetric encryption [9]. Hybrid key management frameworks provide better security in terms of symmetric and public key approaches [10]. The key objectives of this research are:  
* To develop a framework that encompasses keys generation, keys distribution, and key revocation approaches. * To develop a lightweight framework that reduces computation and communication overhead. * To enhance the security and scalability of UWSNs as compared to the existing state-of-the-art computationally expensive frameworks. The motivation behind this article is to propose a secure and lightweight key management framework for UWSNs that overcomes the limitations of existing frameworks.