  This paper analyzes the security of an image encryption algorithm proposed by
Ye and Huang [\textit{IEEE MultiMedia}, vol. 23, pp. 64-71, 2016]. The Ye-Huang
algorithm uses electrocardiography (ECG) signals to generate the initial key
for a chaotic system and applies an autoblocking method to divide a plain image
into blocks of certain sizes suitable for subsequent encryption. The designers
claimed that the proposed algorithm is "strong and flexible enough for
practical applications". In this paper, we perform a thorough analysis of their
algorithm from the view point of modern cryptography. We find it is vulnerable
to the known plaintext attack: based on one pair of a known plain-image and its
corresponding cipher-image, an adversary is able to derive a mask image, which
can be used as an equivalent secret key to successfully decrypt other
cipher-images encrypted under the same key with a non-negligible probability of
1/256. Using this as a typical counterexample, we summarize security defects in
the design of the Ye-Huang algorithm. The lessons are generally applicable to
many other image encryption schemes.
