Introduction - If you have any usage issues, please Google them yourself
The number of changing pixel rate (NPCR) and the
unified averaged changed intensity (UACI) are two most common
quantities used to uate the strength of image encryption
algorithms/ciphers with respect to differential attacks.
Conventionally, a high NPCR/UACI score is usually interpreted
as a high resistance to differential attacks. However, it is not clear
how high NPCR/UACI is such that the image cipher indeed has a
high security level. In this paper, we approach this problem by
establishing a mathematical model for ideally encrypted images
and then derive expectations and variances of NPCR and UACI
under this model. Further, these theoretical values are used to
form statistical hypothesis NPCR and UACI tests.