In this paper, it is proved that Lin et al.'s scheme that tried to strengthen the Hill cipher against the known-plaintext attack has several security flaws and is vulnerable to the chosen-ciphertext attack. This paper also introduces a secure and efficient symmetric cryptosystem based on affine transformation. The proposed cryptosystem includes an encryption algorithm that is an improved variant of the Affine Hill cipher, and two cryptographic protocols that are introduced for the proposed cryptosystem. Copyright © 2009 John Wiley & Sons, Ltd.