Abstract
In cipher algorithms, the encryption and decryption are based on the same key. There are some limitations in cipher algorithms, for example in polyalphabetic substitution cipher the key size must be equal to plaintext otherwise it will be repeated and if the key is known then encryption becomes useless. This paper aims to improve the said limitations by designing of Triple key security algorithm (TKS) in which the key is modified on polyalphabetic substitution cipher to maintain the size of the key and plaintext. Each plaintext character is substituted by an alternative message. The mode of substitution is transformed cyclically which depends on the current position of the modified communication. Three keys are used in the encryption and decryption process on 8 or 16 rounds with the Exclusively-OR (XOR) of the 1st key. This study also identifies a single-key attack on multiple rounds block cipher in mobile communications and applied the proposed technique to prevent the attack. By utilization of the TKS algorithm, the decryption is illustrated, and security is analyzed in detail with mathematical examples.