Negative correlation in image cryptography I am working on an algorithm to encrypt images and I am getting a negative correlation between the encrypted image and the original image. Ideally correlation should be zero but what should I infer from a negative value?
A negative correlation simply implies that your encrypted image resembles the negative of the original image more than one would expect if the two images were completely unrelated.

Of course, the measured correlation between any two random sets of data is rarely if ever exactly zero, even if the values are actually completely independent.  However, if the correlation appears consistently and/or is large enough (in absolute magnitude) to be considered statistically significant, it indicates that your encryption scheme leaks information about the original image, and is therefore not secure.

(Also, note that zero correlation between the original and the encrypted image is not, by itself, sufficient to demonstrate security.  For example, an encryption scheme that simply inverted the colors of every second pixel in the image would achieve very close to a zero correlation for most typical images, but would be trivially easy for anyone to decrypt.)