This is an exercise to do in your head $$ in both senses of the phrase! We are combining the

concepts of steganography and some concepts from cryptography.

Consider that Bob wants to pass a message to Alice; Alice and Bob don$$t know each other or

where each other lives, but Alice has received a message, purportedly sent by Bob, that told

her to check a photo at a particular website each day and to see if the value at offset $2671677$

has changed from $0$xFE to $0$xFF $$ that is a $1$bit change. If that value has changed, then Alice

is to go to a store and buy a bottle of coke, take the coke bottle and place it beside a park

bench in the centre of town and leave.

Without speculating on the meaning of the actions following her discovery that the bit has

changed, propose a mechanism to detect the single bit change on a random and unknown

image on the internet. Consider all options legally available to you...and think laterally!