Modelling and Cryptanalysis of Symmetric Key-Based Cryptosystem with Fibonacci Sequences for Key Variability Concept Shaligram Prajapat International Institute of Professional Studies Devi Ahilya University, Indore India shaligram.prajapat@iips.edu.in Abstract. The study of a cryptosystem is core of every information system. The aim of this paper is to model a symmetric key based system to transmit or receive information of large file size. From designer perspective it is essential to investi- gate the strength , weekness and characteristics of modeled cryptosystem. This paper first establishes Fibo-Q-based cryptosystem and later on analyses it from hackers or cryptanalysts perspective. During analysis, various situations have been posed and their solution has been investigated using Stochastic and Mar- kovian Schemes. The Fibo-Q matrix has been used instead of the traditional XOR operator. To analyze them cryptic mining concepts are also discussed. Keywords: Symmetric key Cryptography, key size, Cryptosystem, Cassini For- mula. 1 Introduction Cryptography is a core operation for securing information assets and all communica- tions over network (Figure 1). On the top of other security mechanism such as author- isation, authentication, Intrusion Detection/Prevention mechanism and firewall are im- plemented (Figure 1). The success of a Cryptosystem not only depends on the strength of the cryptic algorithm or the security of the protocol but also depends on the key used by the algorithms. If the key gets compromised, or the opponent finds the keys, then the security of the system is gone. The prime care desired by the conventional crypto- system is the proper use of keys. The alternative AVK approach is there to avoid this issue. This work focuses on the formulation and development of a symmetric cryp- tosystem that is based on the AVK model. Further, answer research questions like, how will the key be varied? Have been discussed together with, How Encryption and De- cryption of plain text would take place? What is the probable strength of the developed system under different scenarios in detail?