Hybrid RC4 and Affine Ciphers to Secure Short Message Service on Android

Nowadays, short message service is one of the most popular communication systems between individuals using cellular phones. Every SMS entering a person cellular phone should be considered as a private matter, therefore its privacy as well a security are very important and be kept secret. One popular technique in securing the confidentiality of a message is cryptography whereby a plain message is transformed into an encrypted text. In this paper, we propose a hybrid of two symmetric cryptography algorithms in order to secure messages to be sent on an Android device. Algorithm to encrypt and to decrypt the message is RC4 cipher, while the algorithm to encrypt and decrypt the RC4 key is Affine cipher. The sender firstly encrypts the message to be sent and then encrypts the key. Both the cipher text and the cypher key are sent to the receiver. The receiver should decrypt the cipher key before the ciphertext could be decrypted. An Android based application has been built to implement the proposed method and the result obtained showed that the two algorithms provide a consistent encryption result where the length of the message before being encrypted is always the same as the message obtained after decryption.