*4.6. Information Entropy*

Information entropy reflects the uncertainty of an image [30]. The larger the entropy, the greater the uncertainty. The entropy of an image was calculated according to Equation (19)

$$H = -\sum\_{i=0}^{L} p(i) \log\_2 p(i) \tag{19}$$

where *L* is the number of pixel grey levels, and *p*(*i*) is the probability that the grey value *i* appears. From Table 4, the information entropy of the encrypted images approaches the ideal value of eight, which indicates that the encrypted images have considerable uncertainty.

**Table 4.** The information entropy of different images.

