To attack steganographic schemes in graphics interchange format (GIF) images

a steganalytic algorithm was proposed based on sum of distances (SOD) of sub-block. The distance was computed as an absolute value of the difference between one index and other one in a sub-block of an image. The difference between two indices may be positive or negative so that an SOD was further classified into two kinds. Considering the sign of algebraic sum of positive and negative SODs in a sub-block

the two SODs were differentiated as primary and secondary SODs respectively. 32 statistics were computed using these quantities in an image and a 56-dimensional feature vector was obtained. A steganalytic algorithm was designed based on the feature vector and Support vector machine. Experimental results show that the presented algorithm outperforms traditional methods. The number of different indices in a sub-block was utilized to improve the extracting feature vectors techrique.