Steganalysis in the wide sense consists of first identifying suspicious objects and then further analysis during which
we try to identify the steganographic scheme used for embedding, recover the stego key, and finally extract the
hidden message. In this paper, we present a methodology for identifying the stego key in key-dependent
steganographic schemes. Previous approaches for stego key search were exhaustive searches looking for some
recognizable structure (e.g., header) in the extracted bit-stream. However, if the message is encrypted, the search
will become much more expensive because for each stego key, all possible encryption keys would have to be tested.
In this paper, we show that for a very wide range of steganographic schemes, the complexity of the stego key search
is determined only by the size of the stego key space and is independent of the encryption algorithm. The correct
stego key can be determined through an exhaustive stego key search by quantifying statistical properties of samples
along portions of the embedding path. The correct stego key is then identified by an outlier sample distribution.
Although the search methodology is applicable to virtually all steganographic schemes, in this paper we focus on
JPEG steganography. Search techniques for spatial steganographic techniques are treated in our upcoming paper.