  Security of multimedia data is a major concern due to its widespread
transmission over various communication channels. Hence design and study of
good image encryption schemes has become a major research topic. During the
last few decades, there has been a increasing in chaos-based cryptography. This
paper proposes an attack on a recently proposed chaos based image encryption
scheme. The cryptosystem under study proceed by first shuffling the original
image to disturb the arrangement of pixels by applying a chaotic map several
times. Second, a keystream is generated using Chen's chaotic system to mix it
with the shuffled pixels to finally obtain the cipher image. A chosen
ciphertext attack can be done to recover the system without any knowledge of
the key. It simply demands two pairs of plaintext-ciphertext to completely
break the cryptosystem.
