Images Encryption Method using Steganographic LSB Method, AES and RSA algorithm

1 Introduction

The images are considered as one of the most widely used form of information, the internet revolution and the massive use of information technology facilitate communication and thus make more fragile the information. The exchange of digital data posed a security problem, so encryption become more important.

Based on the keys we can classify cryptography in two branches known as symmetric and asymmetric. The best known symmetric algorithms are AES (Advanced Encryption Standard), DES (Data Encryption Standard) and 3-DES [9]. These techniques are economical and comparatively secure. The biggest problem with these techniques is the exchange and storage of the secret key.

The second branch is Asymmetric (public) key cryptosystem [1], it uses the same algorithm for encryption and decryption with a pair of keys, public and private, computationally is impossible to derive the private key from the public key. Asymmetric systems such as RSA (Rivest, Shamir and Adleman) [1] requires the use of large numbers (greater than 512 bits) which is inappropriate for encrypting images [2, 8]. This branch of cryptography has of major interest, it removes problem of transfer of the key. But it can not grab the place of symmetric encryption algorithm because its computation time is comparatively long. For a large amount of data such as image, it is not preferable to use asymmetric encryption, for example, the RSA is 1500 times slower than symmetric DES algorithm [10, 11].

The steganography can also be a solution to increase amount of security. Steganography is a technique that imperceptibly hides secret data into cover media, such that an Oscar will not be able to extract the secret data. In literature one can find many efficient methods of steganography. Among all renown methods, LSB (Least Significant Bit) substitution, which embeds secret data by replacing k LSBs of a pixel with k secret bits directly 12].

In this paper we propose a method based on AES, RSA and LSB method. We encrypt the image using AES, then, the secret key is encrypted using RSA and it is hidden in the ciphred image using LSB technique. The major advantages of our approach it is eliminates the problem of key transmission. Presented approach is more efficient in terms of computation cost compared with schemes that use asymmetric encryption. We believe that projected approach is more secure due the strength of RSA, AES and LSB methods.

In Section 2 we will present an introduction to the cryptography and AES algorithm. Then, in Section 3, we wil give a brief description of the RSA algorithm. Section 4 consist of LSB steganographic method. After that, we describe propose approach in Section 5 and provide some experimental analyzes in Section 6. Finally, we give our conclusion in Section 7.

2 Advanced Encryption Standard (AES)

Advanced Encryption Standard (AES) which is adopted by the U.S. government in 2001, is now used in the Wifi security, compression tools. This algorithm has been developed by two Belgian researchers, Joan Daemen and Vincent Rijmen to replace the DES and the 3DES algorithms [5]. The AES algorithm is easy to implement, it consumes less memory and it uses keys of 128, 192 or 256 bits. The required number of rounds (i.e., linear and non-linear transformations), depend on the key size [4], see Table 1.

AES has four steps such as Byte sub, shift row, mixed column and add round key (see Figure 1) [3, 4]. The only nonlinear step which is responsible to create confusion in the data is byte sub, the remaining steps are nonlinear. In other words we can say that in shift row, mixed column and add round key we are only applying permutation operation for the sake to diffusion.

Fig. 1

Algorithm 1 : AES Encryption Algorithm

1. SubBytes: A non linear transformation of the bytes. Each byte is replaced with a matrix called S-box using a substation table (s-box), which is constructed by multiplicative inverse and affine transformation.

2. ShiftRows: It is a simple cyclic shift of each row of the matrix to the left.

3. MixColumns: Is a multiplication of matrix data by an other matrix to reorder the columns.

4. AddRoundKey: It is a simple XOR between the working state and the roundkey.

The AES algorithm can support the following methods of encryption : ECB, CBC, OFB, CFB and CTR [4].

3 RSA Algorithm (Rivest, Shamir and Adleman):

The first solid system have been invented public key. It uses the same algorithm for encryption and decryption with a pair of keys, public and private (see Figure 2). Published in 1977 by Ron Rivest, Adi Shamir and Leonard Adleman of the Massachusetts Institute of Technology (MIT), the RSA is based on the difficulty of factoring large numbers.

Fig. 2

Asymmetric encryption/decryption

In RSA we uses the same algorithm for encryption and decryption, we need pair of keys, public and private key (see Figure 2). We have three principal steps [18]:

1. Key generation :

   1. Manufacturing large prime numbers p and q (+ − 100 digits).

   2. Given an integer n = pq, it is very difficult to find the factors p and q from n.

   3. Private key : (p, q), large prime numbers ”p” and ‘”q”.

   4. Public key: (n, e), n = pq and an integer ”e” prime with (p − 1)(q − 1).

   If ”M” the plaintext and ”C” the ciphertext.

2. Encryption :

   C=Me mod [n](1)

3. Decryption : Is based on the inverse function :

   M=Cd mod [n](2)

   where e.d = 1 mod [(p − 1)(q − 1)]

4 Hiding Methods in Image Steganography

Steganography is the science to hide secret information in other data, long before the invention of the computer. One can find in literature several different techniques of steganography [7, 12]. LSB is the best known method. LSB is to change the least significant bit of the cover media [6, 12, 13].

5 Our approach

Asymmetric encryption is inappropriate for images because the computation time is long, but it is more secure than symmetric encryption because it removes the exchange of the secret key, also it is mathematically infeasible to know the private key from the public key. The speed of symmetric encryption is better then asymmetric encryption but less secure since it requires secret key sharing.

To take advantage of the speed of symmetric encryption and security of asymmetric encryption and steganographic methods. We propose an algorithm which combines AES, RSA and LSB method. First of all we encrypt the plaintext image using AES and a secret key k generated randomly (see Figure 3). Then this secret key k is encrypted using RSA, finally the ciphered key k′ is hidden in the ciphertext image using LSB method. As showed in Figure 3, the proposed approach eliminates sharing key during the encryption process.

Fig. 3

Our approach

Initially, we have encrypted the original image using a symmetric algorithm. In our case we have used advance encryption standard (AES).

Then, the AES secret key is encrypted by using an asymmetrical RSA algorithm.

At the end, we will hide the encrypted secret key in the cipher image using least significance bits (LSB) technique.

In propose scheme we have encrypted the plain image using AES and the secret key is enciphered by RSA asymmetric algorithm. The strength of our technique is based on the plus points of RSA and AES. Because for huge data it is not economical to use RSA as DES is 1500 time faster than RSA [10, 11].

6 Experimental Results

We analyze propose algorithm on several gray-scale images of different sizes in order to evaluate its strength. We have chosen three images which are presented in Figures 4, 5 and 6 with dimensions of 256 × 256 and 512 × 512 respectively. For encryption we applied stream ciphers using AES algorithm in OFB mode (Output Feedback Block) with a 128 bits key length. The key k has been encrypted with the RSA algorithm. Then using the LSB method, the key k′ is hidden into the ciphered image (Figure 4.b.). After extracting the secret key and decrypting (Figure 4.c.) we get the decrypted image (Figure 4.d.).

Fig. 4

Lung cancer: (a) Original image, (b) Ciphered image, (c) Stego-image, (d) Decrypted image.

Fig. 5

Skull: (a) Original image, (b) Ciphered image, (c) Stego-image, (d) Decrypted image.

Fig. 6

Hand: (a) Original image, (b) Ciphered image, (c) Stego-image, (d) Decrypted image.

7 Conclusion

In this paper, we introduced a new method of image encryption. Our approach is based on symmetric, asymmetric encryption and steganography. In the beginning, we use symmetric encryption for the encryption of image, then we make use of asymmetric algorithm for the security of the key. Furthermore we hide our encrypted key in the cipher image using LSB steganographic method.

The outcomes of the security analyzes can be seen in Tables 2, 3, 4, 5, 6 and 7 and one can examine that propose algorithm is invulnerable against renown attacks. So we can make use of it for secure and economical image encryption.