Simple, Efficient, Highly Secure, and Multiple Purposed Method on Data Cryptography

In recent times, the process of circulating digital data through various social media has increased, and a lot of this data, whether it is short text messages or text files of varying size, or digital images of various types (color, gray and binary) and multiple sizes, or audio files of various types (mono and stereo) and various sizes require protection and to a high degree because Enable intruders to penetrate it or enable unauthorized parties to understand the content of this data for the following reasons [1-5]:

-    Digital data may be confidential or of a private nature, and any unauthorized person or entity must be prevented from accessing and understanding it.

-    Digital data, especially digital images, may contain confidential or private data.

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Figure 1. Data cryptography

One of the popular methods used to protect data is data cryptography, which means encryption and decryption. Encryption process is used to destroy the data so as to become un understandable, while decryption process is used to recover the original data from the encrypted one as shown in Figure 1.

Data cryptography usually applied by using a private (secret) key which is known by the sender and receiver, this key can be used in implementing a set of logical and arithmetic operation to generate the encrypted and decrypted data.

The quality of the encrypted and decrypted data can be measured by mean square error (MSE) and/or peak signal to noise ratio (PSNR). The value of MSE must be very high and the PSNR value must be very low using the encrypted data (this means fully destruction of the original data), while MSE must be closed to zero and PSNR must be closed to infinite (or very high) for the decrypted data (this means fully data recovery), MSE and PSNR can be calculated using Eqns. (1) and (2).

$M S R_{S R}==\frac{1}{N} \sum_{j=0}^{n-1}[S(j)-R(j)]^{2}, N=\mathrm{n}$      (1)

$P S N R_{\mathrm{sk}}=10^{*} \log _{10} \frac{\left(M A X_{\mathrm{j}}\right)^{2}}{M S E_{\mathrm{sR}}}$      (2)

Any method of encryption and decryption is considered a good method if it achieves the following:

- The private key must be secure and difficult to penetrate; this will increase the protection degree.

- It must be efficient by maximizing the method throughput (number of bytes encrypted/decrypted in a second).

- The method must give the necessary value of MSE and PSNR after executing the encryption and decryption phases.

- The method must be simple and easy to implement.

- The method must be easily used to encrypt/decrypt any type of data (texts, images, speeches).

Color digital images are now available everywhere and can be obtained easily and at no cost. The color image is a huge data store that can easily be employed to generate a private secret key if the specified image is kept secret between the sender and receiver.

The digital image is processed in easy ways and it can be converted into a special type that fits with the audio files or changed its size to match the size of the data to be encrypted or decrypted.

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Figure 2. Proposed method of data cryptography

The proposed method uses a digital color image as an image-key (see Figure 2); this image must be kept in secret. The image-key must be agreed upon by the sender and the receiver, so that it will be saved without the process of sending it, and it can be replaced easily by another image any time and when needed without modifying the method of data cryptography.

One of the most important advantages of using a digital color image as a key image lies in the following:

- Ease of obtaining the digital image at no cost.

- Multiple digital image acquisition sources.

- Ease of digital image processing.

- Possibility to resize the image to obtain vectors with a specified length.

- The possibility of converting the values in the digital image from range 0 to 255 to range -1 to 1 to suit the data to be processed.

The proposed method can be used to encrypt-decrypt any kind of data (short message (SM), text file, digital image, and speech file), there is no limitation on the input data size, here the data size can be varied and the image-key can be resized to match the input data size. The image-key pixel value falls in the range 0 to 255, so it is necessary to convert the image-key to NTSC image with pixel value range from -1 to 1, the NTSC image value here will be closed to the speech file values, this can be done applying Eq. (3).

$\left[\begin{array}{l}Y \\ I \\ Q\end{array}\right]=\left[\begin{array}{rrr}0.299 & 0.587 & 0.114 \\ 0.596 & -0.274 & -0.322 \\ 0.211 & -0.523 & 0.312\end{array}\right]\left[\begin{array}{l}R \\ G \\ B\end{array}\right]$     (3)

Any color image can be resized to get an array with a specified length. This can be done by applying image resizing. Figures 3, 4 and 5 illustrate an example of image-key resizing and converting to NTSC image applying Eq. (3).

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Figure 3. Private key generation for message encryption

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Figure 4. Private key generation for image encryption

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Figure 5. Private key generation for speech encryption

The proposed method can be used to encrypt and data applying the following steps:

1) Get the image-key.

2) Select the data to be encrypted.

3) Get the data size.

4) If the data is speech file skip to step 8

5) Resize the image-key to match the data size

6) Apply XORing to get the encrypted data

7) Exit

8) Convert the image-key to NTSC image.

9) Resize the NTSC image to match the speech file size

10)  Add the resized image to the speech file to get the encrypted data.

The decryption phase can be implemented applying the following steps:

1) Get the image-key.

2) Select the encrypted data size.

3) Get the encrypted data size.

4) If the data is speech file skip to step 8

5) Resize the image-key to match the data size

6) Apply XORing to get the encrypted data

7) Exit

8) Convert the image-key to NTSC image.

9) Resize the NTSC image to match the speech file size

10) Subtract the resized image from the encrypted speech file to get the decrypted data.

From the obtained results shown in table we can see that the proposed method has better performance by increasing the short messages encryption process throughput as shown in Figure 8 and the proposed method has a significant speedup comparing with other related method, the speedup can be calculated using Eq. (4).

Speedup $_{1,2}=\frac{\text { Encryptiontime }_{2}}{\text { Encryptiontime }_{1}}$     (4)

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Figure 8. Methods throughputs

Increasing the data size (using text files) will rapidly increase the encryption time for the related method and slowly for the proposed method as shown in Figure 9. Taking the results shown in Table 3 we can calculate the methods speedup, the results of calculations are shown in Table 7.

Using the proposed method to encrypt-decrypt images and speeches also improves the method performance by decreasing the encryption time-decryption times.

The proposed method satisfied the requirements of good method of data cryptography by providing good value for the quality parameters MSE and PSNR (see Table 8).

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Figure 9. Encryption times comparisons