  Steganography is a technique to hide the presence of secret communication.
When one of the communication elements is under the influence of the enemy, it
can be used. The main measure to evaluate steganography methods in a certain
capacity is security. Therefore, in a certain capacity, reducing the amount of
changes in the cover media, creates a higher embedding efficiency and thus more
security of an steganography method. Mostly, security and capacity are in
conflict with each other, the increase of one lead to the decrease of the
other. The presence of a single criterion that represents security and capacity
at the same time be useful in comparing steganography methods. EMD and the
relevant methods are a group of steganography techniques, which optimize the
amount of changes resulting from embedding (security). The present paper is
aimed to provide an evaluation criterion for this group of steganography
methods. In this study, after a general review and comparison of EMD-based
steganography techniques, we present a method to compare them exactly, from the
perspective of embedding efficiency. First, a formula is presented to determine
the value of embedding efficiency, which indicates the effect of one or more
changes on one or more pixels. The results demonstrate that the proposed
embedding efficiency formula shows the performance of the methods better when
several changes are made on a pixel compared to the existing criteria. In the
second step, we have obtained an upper bound, which determines the best
efficiency for each certain capacity. Finally, based on the introduced bound,
another evaluation criterion for a better comparison of the methods is
presented.
