Due to the rapid growth of computer networks, critical highly confidential information shared across these networks. Accordingly, securing such information from unauthorized intruders has become a vital issue in the field of information technology. In this paper, we present a new algorithm for encrypting and decrypting English plain text based on the well-known Caesar's algorithm and a special type of functions called One to One function. The proposed algorithm is referred to as the One to One function algorithm (OtO). The OtO belongs to a symmetric key concept where the same key is used in both encryption and decryption processes. In the OtO algorithm, the triplet ( K, a, b ) represent the private keys. To speed up the proposed OtO computation, the value of K is calculated based on Fibonacci sequence, on eigenvalues, Leslie matrices and Markov chain. This private key K only knew to the transmitter and receiver and considered one of the private keys used in the encryption process. When the message arrives at the receiver, it uses the inverse function of the proposed one to one function that used at the transmitter. The proposed OtO algorithm is conducted using MATLAB and its efficiency is checked in terms of encrypted time, decrypted time, and Avalanche Effect. We think that the obtained results are acceptable compared to famous algorithms DES, 3DES, AES and RSA.