Myagmarbayar Nergui et al.

information so that they can be separated without detectable damaging either file. In this chapter, we explain digital watermarking, a data-hiding technique that can embed patient diagnosis information within retinal fundus images. Additionally, we explain our digital watermarking and compression techniques, cryptographic protocols, and error correcting codes in this chapter. We have compared the performances of different types of error control codes (ECCs) with that of our technique. Watermarking helps to secure patient records from being compromised and assures that the information will remain intact even in noisy channels.

11.1. Related Studies

In hospitals and healthcare centers, today, patient diagnosis information and retinal fundus images constitute a large component of the data-generated information. The volume of data processed and stored in hospital and healthcare centers is increasing as the usage of electronic diagnostic instruments increases. Patient diagnosis must be coordinated with retinal fundus images so that patient and image information can be securely and accurately maintained. It is common for hospital and clinic admission staff to record patient information in text files. The patient diagnosis file is then modified during each examination. The diagnosis (text) information and its corresponding retinal fundus images are preserved under an admission number or name. The current practice is to store the retinal fundus images individually in a standard file format. In this format, a lost file identification tag can make it difficult to match the image to the correct patient. Furthermore, patient records stored as standard files are not protected. Unauthorized staff may gain access to the information. Therefore, the highest priority is to secure the information. Digital imaging and communication (DICOM) is the main file format for the medical images transmission in clinic centers.1,2 The DICOM image format employs a header containing key words, attributes, and details of the image, ensuring that the patient image is not erroneously distinguished from patient data. Almost hospitals and healthcare centers have data-management systems that are grounded on the DICOM standard.1,2 The DICOM standard has some disadvantages. For example, it does not protect patient diagnosis information from being viewed by unauthorized

320

Reliable Transmission of Retinal Fundus Images

users, nor does it defend information integrity as it passes through imperfect transmission mediums or is stored in imperfect media. We have proposed a method for solving these problems using the DICOM standard in this chapter. We suggest using cryptographic protocols (to prevent security attacks) and using ECCs to preserve data security.

Digital watermarking3−10 can be used to conceal statistic data into image. For intellectual property authentication and copyright protection, many applications use the digital watermarking techniques to protect rights. Applying this technique, text data, such as patient diagnosis history, can be concealed in the image.11−14 In order to disguise it, the information is embedded into a cover data, most likely unobjectionable audio, image, or video. The media is then protected against distortion potentially caused by watermarking. Hence, digital watermarking techniques can be applied to conceal patient text data into image data, creating a complex data comprising both image and embedded text data that is hidden in the image. Cryptographic algorithms and ECCs process this complex block of data to secure the information.

Two types of domain methods, spatial domains and frequency domains, are watermarking techniques. The first watermarking method, spatial domains, is composed of least significant bit (LSB) insertion.4,5,7,8 This method works that the LSB of each pixel value of the image data is substituted by one bit of the ASCII character set presenting the data to be embedded into image, so that it is placed into the LSB of the consecutive pixels in the image. It is possible to embed 192 kb text information data into a 256 × 256 digital color image. Since difference between original image and embedded image is so small, and is only one bit, the human eye cannot detect the distortion between the watermarked (embedded) image and the original image. In the second watermarking method, frequency domains,5,7,9 the watermarking is performed in the transform domain. The redundant frequency content, which is related to the image, is substituted with characters from the text file. The substitution allows images to be watermarked using the discrete Fourier transforms (DFT), the discrete cosine transforms (DCT), or the discrete wavelet transforms (DWT).14 Because diagnosis data often comprise confidential and private data, a cryptographic algorithm, advanced encryption standard (AES),15,16 is applied

321

Myagmarbayar Nergui et al.

to protect patient data. This algorithm is robust and safe against attacks by hackers.

Compression techniques are used to compress data so that it requires less bandwidth during a data transmission and less disk space for storage. Using compression schemes for communication equipment, such as modems, bridges, and routers, improves the throughput over standard channels. Image compression is a kind of data compression technique applied to digital images to cut down the redundancy of the image data for storing or transmitting in an efficient form.

Information passing through a communication channel can be affected due to nonqualities in the transmission medium and receiver system.A transmitted signal reduces in strength, as it passes through a physical channel. This strength reduction is caused by energy absorption and spreads during image transmission, introducing noise as it does so. In satellite communication terminology, this phenomenon is called “free space loss” and can exceed 200 dB on a 14/12-GHz frequency geosynchronous satellite channel.17 The fading fluctuates with the geometry of the surrounding environments, the speed of the vehicle, and the terrain in mobile radio applications. A radio signal transmitted can be detoured by surrounding environments, such as buildings, mountains, and plants onto one of several paths. Various types of the signals may reflect to each other positively or negatively, as these signals may have different wavelengths and different degrees of fading, depending on the location of the receiver. An active receiver will receive a series of energy peaks and troughs, which reflect the frequency of occurrence. The number of peaks and troughs depends on agent speed. These types of communication channels are called “multi-path fading channels.” Therefore, we conclude that transmission channels are not perfect and can often receive corrupted data.

In 1948, Shannon18 argued that the right pre-coding of information could decrease errors made by a noisy channel to an expected level without sacrificing information, as long as the information does not exceed the channel capacity. ECC was issued to determine the veracity of this theorem. Since the 1950s, ECC has been used to defend the information integrity in communication channels or in storage. ECC has become a substantial component of the telecommunications revolution, and has improved computer systems, the internet, digital recordings, and space exploration.19 All CDs,

322

Reliable Transmission of Retinal Fundus Images

CD-ROMs, hard disks, and DVDs employ ECCs to defend the data encoded in them. ECC techniques are employed in the data link layer to enhance the reliability of the transmission channel.

The patient data is encrypted (applying the AES) and hidden in the retinal fundus image. The complex block of data composing patient text data and image information is encoded by using ECC algorithms. We have employed Huffman codes, Bose-Chaudhuri-Hocquenghem (BCH) codes, Reed-Solomon (RS) codes, convolutional codes, and Turbo codes in our experiment. For measuring the improvement supplied by this system of coded data transmission, we calculated the bit error rate (BER) and the symbol error rate (SER) at the receiving system, for the error control coded and uncoded transmission pass through an AWGN channel by employing Monte-Carlo techniques. We obtained and plotted the BER values against the various levels of SNR. The coding gain (CG), a quantity of the decrease in the SNR required to accomplish a particular BER compared to uncoded case, is determined from this plot. Hence, the CG measures the improvement of information integrity supplied by using ECC.

The chapter is organized as follows. Section 11.2 is outlined the encryption algorithm employed for enhancing protection of data and the digital watermarking technique employed for data concealing characteristics. In Sec. 11.3, we outlined compression techniques. In Sec. 11.4, we explained the prominent characteristics of the ECC and interleavers techniques employed in this study. In Sec. 11.5, we showed our simulation results that measure the efficiency of these systems. In Sec. 11.6, we discussed our experiment. Finally, we presented our conclusions in Sec. 11.7.

Figure 11.1a, b shows the proposed transmitter and receiver block diagrams.

11.2. Encryption and Watermarking of Text Information

11.2.1. Encryption

For storage, information is encrypted and hidden in image data used for digital watermarking to improve the security. John Daemen and Vincent Rijmen designed an algorithm that can withstand attacks and hacking

323

Myagmarbayar Nergui et al.

(a)

(b)

Fig. 11.1. (a) Proposed transmitter block diagram and (b) proposed receiver block diagram.

attempts, the AES or the Rijndeal algorithm, and used it to encrypt text data.14−16

During the encryption process, data changes from plain text to cipher text, which is unintelligible to the human eye. Applying digital watermarking techniques, encrypted data is hidden into image data, and created watermarked image data. The watermarked image data is encoded with a suitable ECC algorithm. The secure image is passed through the communication channel. The received data (corrupted by noise) is corrected by ECC algorithms, and then patient data is extracted from the watermarked image and recovered into its original form using decryption process of AES. Three types of AES are capable of applying cryptographic keys of 128-, 192-, and 256-bits length to encrypt and decrypt data into 128-bits blocks.

For AES, the Rijndeal algorithm uses the following operating combinations: security, performance, efficiency, implementation, and flexibility. This algorithm has a quick setup time, and its agility is well suited for our purposes. The Rijndeal algorithm requires low memory, making it well suited for conditions in which space is limited. The amount of security that the Rijndeal algorithm offers is contingent on the computational power and time required to decrypt the cipher. In this chapter, we have applied the AES algorithm with a cryptographic 256-bit key to encrypt and decrypt our patient information.

324

Reliable Transmission of Retinal Fundus Images

Fig. 11.2. (a) Original patient diagnosis data and (b) encrypted patient data.

We show the original patient text and the encrypted patient text, respectively in Figs. 11.2a and 11.2b.

11.2.2. Digital Watermarking Technique

During digital watermarking, data is hidden within a digital signal, or cover data, which may be audio, video, or pictorial. Whenever the watermarked signal is copied, the hidden data is also conveyed to the copied file.

Digital watermarking can be divided by visible or invisible. For visible watermarking, the embedded data is seeable in the watermarked video or pictorial record. Generally, the embedded data can be any logo or text data that recognizes the proprietor of the cover data. For invisible watermarking, the embedded data is inserted into an audio stream or a pictorial or videographic record, but cannot be detected by human eyes. Invisible watermarking is applied in intellectual property right protection systems, such as steganography, which is the writing of hidden messages and an application of digital watermarking.3 Digital watermarking hides message data in cover data without distorting the information. This watermarking technique can compress storage or transmit text information, even in image and video records. We have applied LSB insertion watermarking to hide patient information in retinal fundus images.

We show an original retinal fundus image in Fig. 11.3a, and a watermarked (embedded) retinal fundus image in Fig. 11.3b. No distortion in the

325