## 1 Introduction
Conventional agricultural methods are still being used by farmers, resulting in low crop and fruit yields. As a result, integrating IoT technologies can boost crop productivity. IoT is an essential part of the smart agriculture architecture. With the integration of IoT devices, the agriculture systems have been monitored 24/7. These IoT devices sense the surrounding environment and physical parameters to take sensed information. Such sensed information is then sent to the backend server via Internet or through sensor gateway, where date is stored for future use or even user can view that sensed data. The user can make decisions about what actions to do in such a monitored environment. However, it's not an easy task to integrate everyday devices with the Internet. Security is a big challenge faced by IoT, although each device has its unique set of characteristics and requirements. First of all, each person, device and system which is connected to the Internet must be identified. Intruders will acquire access to the network and perform security breaches if they do not have a valid identity. In case of smart agriculture systems, if information of sensors monitoring the agriculture parameters is leaked, lost or tampered with, it can render the products useless and damage their value significantly hence resulting in huge economic food loss. Therefore, developing a concrete framework for secure monitoring of smart agriculture with the adaption of IoT devices is an essential requirement of this era. Since IoT devices are designed to be compact and cost effective, because they have limited computing power and memory. Their resource constrained nature makes it difficult to secure them and has necessitated the development of lightweight application protocols and security suites. Instead of using standard TCP/IP protocols i.e., Hyper Text Transfer Protocol (HTTP) and Transport Layer Security (TLS), these devices use the IoT protocol stack, which is a lightweight version of the TCP/IP model specially designed for IoT devices. The CoAP has been standardized as lightweight application protocol for IoT systems. However, developing a low-overhead security solution for CoAP remains a challenge. Though some security protocols i.e., standard Datagram Transport Layer Security protocol (DTLS) exists. However, it is not suitable for IoT constrained devices and is expensive due to its computational overhead, complexity and lengthy cipher suites process. Therefore, this paper proposed a robust and lightweight secure authentication framework in CoAP using symmetric encryption i.e., Advanced Encryption System (AES) and HMAC-SHA-224. The main contribution of this paper is to design robust and tamper-resistant secure authentication framework for agriculture application using CoAP by ensuring data integrity and authenticity. The proposed CSFSA is implemented in Contiki NG and Cooja simulator and evaluated performances in terms of packet size, communication overhead and power consumption. The significance of proposed CSFSA is to make resilient against various cyber security attacks i.e., replay attacks, Denial of Service (DoS) attacks, resource exhaustion. The proposed CSFSA provides secure monitoring of various agriculture parameters remotely. The architecture of proposed CSFSA has three modules i.e., 6 LoWPAN based internal network having IoT devices deployed in agriculture field for monitoring and controlling, a border router which acts as bridge between Internal network and external network i.e., Internet and a web server that provides access to the sensed data as shown in Figure 1.  
The paper is structured as follows. Section 2 provides brief explanation of literature review that has been done in smart agriculture, IoT and CoAP security. Section 3 provides an in-depth overview of standards and protocols pertinent to our research work. Proposed methodology is presented in Section 4. Simulation and results evaluation is discussed in Section 5. Finally, concluding remarks are given in Section 6.