S

Image Encryption Using Pixel Scrambling and AES Algorithm:

A critical comparison of different modes of AES

PRESENTED BY:MAMTA DAS (B112068)

SANDEEP PARIDA (B112036)CHANDRASEKHAR SATAPATHY (B112014)

CRYPTOGRAPHY

Practice and study of hiding information.

Art and Science of converting a plain intelligible data into an unintelligible data and again retransforming that message into its original form.

It provides CONFIDENTIALITY, INTEGRITY and ACCURACY.

INTRODUCTION

A digital image is defined as a matrix. Each element of the matrix represents the

intensity values of the pixels of an image. The cryptographic approaches that work

quite well on text data, do not give the same performance on multimedia data.

Need to develop algorithms to encrypt multimedia data.

Fails to securely encrypt digital images that have pixels similar to each other.

Has a lot of redundancy and the adjacent pixels are having close similarity among them.

Advanced Encryption Standard (AES) is a well-known algorithm to encrypt the digital data.

The cause for this can be attributed to the fact that the multimedia data is very large in size.

An attempt has been made to augment the strengths of AES algorithm for image data encryption.

A preprocessing step has been introduced In this step the contents of the image are

divided into blocks containing some pixels. Then these blocks are scrambled and

randomly passed to the different modes of AES for encryption.

AES ALGORITHM

Based on a design principle known as a substitution-permutation network.

It has a fixed block size of 128 bits, and a key size of 128, 192, or 256 bits.

The key size used for an AES cipher specifies the number of repetitions of transformation rounds that convert the input, called the plaintext, into the final output, called the ciphertext.

BLOCK DIAGRAM

BYTE SUBSTITUTION LAYER

It can be viewed as a row of 16 parallel S-Boxes, each with 8 input and output bits.

In the layer, each state byte Ai is replaced, i.e., substituted, by another byte Bi:

S(Ai) = Bi. The S-Box substitution is a bijective mapping,

i.e., each of the 28 = 256 possible input elements is one-to-one mapped to one output element.

Table: AES S-Box: Substitution values in hexadecimal notation for input byte (xy)

DIFFUSION LAYER

It consists of two sublayers, the ShiftRows transformation layer and the MixColumn transformation layer.

The diffusion layer performs a linear operation on state matrices A and B, i.e.,

DIFF(A)+ DIFF(B) = DIFF(A + B).

SHIFT ROWS SUBLAYER

The ShiftRows transformation cyclically shifts the second row of the state matrix by three bytes to the right, the third row by two bytes to the right and the fourth row by one byte to the right.

The purpose of the ShiftRows transformation is to increase the diffusion properties of AES.

Input of the ShiftRows Layer

The Output is the new State

MIX COLUMN SUBLAYER

It is a linear transformation which mixes each column of the state matrix.

We denote the 16-byte input state by B and the 16-byte output state by C

C = MixColumn(B),

where B is the state after the ShiftRows operation.

For Eg:

The second column of output bytes (C4,C5,C6,C7) is computed by multiplying the four input bytes (B4,B9,B14,B3) by the same constant matrix, and so on.

KEY ADDITION LAYER

The two inputs to the Key Addition layer are the current 16-byte state matrix and a sub key which also consists of 16 bytes.

The two inputs are combined through a bitwise XOR operation

S

MODES OF AES

Initialization Vector(IV) : Block of bits that are used by several modes to randomize the encryption.

Initialization vector is never reused under the same key.

An initialization vector has different security requirements than a key, so the IV usually does not need to be secret.

Padding: A block cipher works on units of a fixed size (known as a block size), but messages come in a variety of lengths.

So some modes require that the final block be padded before encryption.

The simplest is to add null bytes to the plaintext to bring its length up to a multiple of the block size.

ECB (Electronic Code Book)

Simplest of the encryption modes. Message is divided into blocks, and each

block is encrypted separately. In ECB identical plaintext blocks are

encrypted into identical ciphertext blocks; thus, it does not hide data patterns well.

Fig:ECB encryption and decryption block diagram

CBC (Cipher Block Chaining)

Each block of plaintext is XORed with the previous ciphertext block before being encrypted.

To make each message unique, an initialization vector must be used in the first block

Ciphertext stealing: A one-bit change in a plaintext or IV affects all following ciphertext blocks.

Fig: CBC encryption and decryption block diagram

CFB (Cipher Feedback)

A close relative of CBC. It makes a block cipher into a self-

synchronizing stream cipher. Operation is very similar to CBC; in

particular, CFB decryption is almost identical to CBC encryption performed in reverse.

Fig: CFB encryption and decryption block diagram

OFB (Ouput Feedback)

This mode makes a block cipher into a synchronous stream cipher.

It generates keystream blocks, which are then XORed with the plaintext blocks to get the ciphertext.

This property allows many error correcting codes to function normally even when applied before encryption.

Fig: OFB encryption and decryption block diagram

CTR (Counter)

Counter mode turns a block cipher into a stream cipher.

It generates the next key stream block by encrypting successive values of a "counter". The counter can be any function which produces a sequence which is guaranteed not to repeat for a long time.

It does not suffer from the short-cycle problem that can affect OFB.

Fig: CTR encryption and decryption block diagram

S

PARAMETERS USED FOR

SECURITY ANALYSIS

SSIM (Structural Similarity Index)

It is a method for predicting the perceived quality of digital images.

SSIM is used for measuring the similarity between two images.

SSIM is a full reference metric; in other words, the measurement or prediction of image quality is based on an initial uncompressed or distortion-free image as reference.

The SSIM index is calculated on various windows of an image. The measure between two windows x and y of common size N * N is:

MSE (Mean Square Error)

MSE measures the average of the squares of the errors or deviations i.e., the difference between the estimator and what is estimated.

The difference occurs because of randomness or because the estimator doesn’t account for information that could produce a more accurate estimate.

Quality is based on the apparent differences between a degraded image and the original, unmodified image.

A value greater than one implies less similarity and will continue to grow as the average difference between pixel intensities increases as well.

S

PROPOSED CRYPTOSYSTEM

Goals Of The Proposed System

Decrease the similarity among the neighboring pixels of the encripted image.

Compare the similarity of the original image with ciphered image (only AES) and with the results obtained from our proposed algorithm.

Design Of The Proposed System

The image when provided as the input is divided into a number of blocks consisting of pixels.

Inside each block, pixels are rotated according to the algorithm.

The scrambled image is obtained.

Scrambling Procedure

Crypto Cipher module is used in Python to implement the AES algorithm.

The scrambled image from the previous step is fed as input to the AES.

The processed output image seems to have less structural similarity and more mean square error as compared to when we received output using AES only.

Encryption Procedure

S

RESULT AND ANALYSIS

Simulation Methodology

The scrambling and encryption has been done in Python software package.

For analysis the following image has been selected.

Result After Using Only Scrambling

Original Image Scrambled Image

Results After Using Proposed System To

Different Modes Of AES

ECB Mode of Encryption

Original Image CBC Mode

CFB Mode OFB Mode CTR mode

Security Analysis

Though the images that are generated after encryption seems to be the same, there are subtle differences which can only be measured by using advanced image matching techniques.

To analyze the difference between image encrypted using only AES and our proposed cryptosystem, we measure the SSIM(Structural Similarity Index) and MSE(Mean Square Error) values between the original and the encrypted image(of each mode)

SSIM gives the degree of similarity between images while MSE gives the degree of dissimilarity. Thus we could define two contrasting parameters to support our cause.

Mode Of AES SSIM Value using only AESSSIM values using proposed system

ECB 0.019907587 0.016413107

CBC 0.011467977 0.012191389

CFB 0.012845624 0.011730625

OFB 0.011932028 0.011735661

CTR 0.010901383 0.009560976

SSIM Values

ECB CBC CFB OFB CTR0

0.005

0.01

0.015

0.02

0.025

SSIM Value us-ing only AESSSIM values us-ing proposed system

Column graph for SSIM Values

Mode of AES MSE value using only AESMSE value using proposed system

ECB 57537.13911 57661.60757

CBC 61200.18457 61387.75557

CFB 61440.41335 61549.76558

OFB 61284.56683 61366.94688

CTR 61595.48264 61653.10451

MSE Values

ECB CBC CFB OFB CTR55000

56000

57000

58000

59000

60000

61000

62000

MSE value using only AESMSE value using proposed system

Column graph for MSE values

CONCLUSION

A simple and a robust method has been proposed using a combination of block based random pixel scrambling and encryption technique.

Results show that the SSIM value has decreased and MSE value has increased when the proposed algorithm was applied to the image..

Also it was noted that Counter(CTR) method was the most efficient image encryption technique among all the modes of AES.

REFERENCES

Reza Shakerian, Morisen Rahmarn, “ A New Modified Version of Advanced Encryption Standard Based Algorithm for Image Encryption” International Conference on Electronics & Information Engineering (ICEIE 2010), (VI 141-VI 145) ,IEEE,2010

Jui-Cheng Yen, Jiun-In Guo, “A New Chaotic Image Encryption Algorithm ”, International Conference on Computer Security, Department of Electronics Engineering National Lien-Ho College of Technology and Commerce, Miaoli, Taiwan, Republic of China, pp 124-128,2007.

THANK YOU