A Robust Method of Encryption and Steganography Using ElGamal and Spread Spectrum Technique Based on MP3 Audio File Priadhana Edi Kresnha Department of Informatics Engineering University of Muhammadiyah Jakarta Central Jakarta 10510 priadhana.edi@informatika.ftumj.ac.id Aini Mukaromah Faculty of Computer Science University of Mercu Buana Kebon Jeruk, West Jakarta 11650 ainialmukaromah@yahoo.co.id Abstract— Security is one of the most important things to be considered when exchanging message, particularly when the message exchanged is top secret message. In this research, a tool is developed to encrypt the message and hide it into a digital object. Encryption method used is ElGamal Encryption, and the object where the message hidden is mp3 audio file. The part in mp3 file used for attaching the message is homogenous frame. This process is enhanced with spread spectrum method and pseudo noise modulation to make the encrypted message more randomized and harder to decrypt. The result shows that the sound quality of the original file and stego-file are almost the same. The noise produced by the message is measured by calculating Error rate and PSNR (Peak Signal to Noise Ratio) between original file and stego-file. Keywords— Encryption; Mp3 file; Spread Spectrum; Steganography; Stego-Object; I. INTRODUCTION Security is one of the most important things to be considered when exchanging message, particularly when the message exchanged is top secret message. The importance of concealing the message has been considered since ancient age which makes the theory of cryptography becomes one of the oldest theories that human made. One of the oldest cryptography methods which have ever been made is Caesar cipher. It is also known as shift cipher, Caesar’s code, or Caesar shift. It is one of the simplest and most widely known encryption techniques. It works by shifting the order of the alphabet by some fixed number. Other advance shifting method was then discovered. The newer one uses key to shift the order of alphabet. The example of this one is vigenere encryption. Along with encryption, steganography as a method to conceal the message has been emerged as well. Differ from encryption which converts the message into unreadable form, steganography tries to hide a message into an object. The oldest steganography method was hiding the message on a courier head. Before writing the message on the courier head, his hair had to be cut. The message was then written on his bald head. After writing the message, the courier had to be quarantined for approximately three months, while waiting until the hair grew up again. The courier would be sent to the field commander to convey the message from the supreme leader. Later on the courier had to be killed to prevent the message uncovered by unauthorized person. In this research, both encryption and steganography are used to conceal the message. Encryption is used so that no unauthorized person can read the message, and steganography is used so that no unauthorized person realizes that the object being sent contains a message, thus an effort to break encryption codes or do cryptanalysis action can be avoided. This way, exchanging message can be more secure. There are several studies on cryptography and steganography technique, particularly digital steganography. The most used digital steganography technique is Least Significant Bit (LSB). This technique manipulates bits which give insignificant changes when they are modified. It has been generally known that computer processes data in digital form which consists of bit 0 and 1 only. The bits are combined to represent a single value of data. I.e. a character is represented by the combination of 8 bits (1 byte). Therefore a character can be valued from 0 to 255. In digital file, particularly multimedia file, a single value data can be represented by various numbers of bits. I.e. jpeg file, each pixel of every channel (Red, Green, and Blue) is represented by 8 bits. By changing the least significant bit, the modification cannot be seen clearly. For example, if the red value of the pixel is 255, after the least significant bit has been changed, it becomes 254. For human, this changed will not affect the sense of the picture. But if the modified bit is the most significant bit, the value will decrease into 128 and this will drastically change the sense of the picture. This kind of technique has been conducted by [1,2,3]. LSB is able to be implemented in audio file as well. In [1] MPEG formatted Audio becomes the cover file to conceal the message. It uses two-steps algorithm to embed watermark bits