Steganography Using LSB Substitution and Pixel Value Differencing

Khodaei and Faez’s LSB+PVD Technique

This technique was proposed by Khodaei & Faez (2012). The embedding procedure is mentioned in the following steps.

• Step 1: The image is raster scanned and divided into 1×3 size non-overlapping blocks, Figure 1. The pixels are designated as , , and .

Figure 1.

A pixel block

• Step 2: In k-bit LSB substitution is applied, where k is chosen from the list {5, 6, 7, 8}. The new value of is . Let L be the decimal value of the k LSBs of and S be the decimal value of k-data bits hidden in . The value can be further optimized using Eq. (1), where d=L-S.

  
  (1)

• Step 3: Using .the following two differences are calculated as in Eq. (2).

  
  ,
  
  (2)

• Step 4: The Table 1 is range table for variant-1 and Table 2 is range table for variant 2.

Range	=[0,7]	=[8,15]	=[16,31]	=[32,63]	=[64,255]
Embedding length	3	3	3	4	4

Range	=[0,7]	=[8,15]	=[16,31]	=[32,63]	=[64, 255]
Embedding length	3	3	4	5	6

• Step 5: The value falls into a range whose embedding length is and lower bound is . Similarly, the value falls into a range whose embedding length is and lower bound is .

• Step 6: From the secret binary data stream bits of data is taken and converted to a decimal value . Similarly, from the secret binary data stream bits of data is taken and converted to a decimal value . Two new difference values and are computed as in Eq. (3). + , + (3)

• Step 7: Using and two new values for namely, and .are calculated. Similarly, using and two new values for namely, and are calculated using Eq. (4). = - , = + , = - , = + (4)

• Step 8: Now the stego value of , say is calculated using and . Similarly, the stego value of, say is calculated using and . This is done using Eq. (5) and Eq. (6).

  
  (5)
  
  (6)

Hence the stego block is represented in Figure 2.

Figure 2.

The stego-block

The extraction of secret embedded data can be done in the following manner. Suppose the stego-block is as shown in Figure 2 from which we have to extract the hidden data. The following steps are used.

• Step 1: From the center pixel , k-LSBs are extracted. The value k is one of the values in the list {5, 6, 7, 8} which was chosen during embedding.

• Step 2: Using and , the difference value is computed. Similarly, using and , the difference value is computed as in Eq. (7).

  
  ,
  
  (7)

• Step 3: The difference belongs to a range whose lower bound is and embedding length is . Similarly, the difference belongs to a range whose lower bound is and embedding length is . Refer the range table.

• Step 4: As in Eq. (8), using and , calculate the decimal value and convert to binary bits. Similarly using and , calculate the decimal value and convert to binary bits. = , = (8)