Mobile Ad Hoc network (MANET) is a connection of mobile nodes that are joined together to communicate and share information using a wireless link.Some of the MANET in use include mobile smart phones, laptops, personal digital assistant (PDAs), among others.However, MANET has been known for the major challenge of being vulnerable to malicious attacks within the network. One of the techniques which have been used by several research works is the cryptographic approach using advanced encryption technique (AES). AES has been found suitable in the MANET domain because it does not take much space in mobile nodes which are known for their limited space resources. But one of the challenges facing AES which has not been given much attention is the optimal generation of its secret keys. So, therefore, this research work presents a symmetric cryptography technique by developing a model for the optimal generation of secret keys in AES using the linear function mayfly AES (LFM-AES) algorithm. The developed model was simulated in MATLAB 2020 programming environment. LFM-AES was compared with mayfly-AES, particle swarm optimization AES (PSO-AES) using encryption time, computational time, encryption throughput, and mean square error. The simulation results showed that LFM-AES has lower encryption, computational, mean square error, and higher encryption throughput. Keywords-- MANET, Data Security, Key Management, LFM-AES, Mayfly-AES, PSO-AES, AES