Side Attacks on Stegosystems Executing Message Encryption Previous to Embedding

There are introduced two new steganalytic methods not depending on the statistics of the cover objects, namely side attacks stegosystems. The first one assumes that the plaintext, encrypted before embedding, is partly known by the attacker. In this case, the stegosystems detection is based on the calculation of mutual information between message and extracted encrypted data. For this calculation, a notion of the k-nearest neighbor distance is applied. The second method is applied to HUGO, one of the most efficient steganographic algorithms. In this case the stegosystems detection is based on a verification of the NIST tests to the extracted encrypted messages. Moreover, we show that the problem to find a submatrix of the embedding matrix determining a trellis code structure in the HUGO algorithm provides a search of the stegokey by the proposed method.