What is steganography?
Imagine if you could hide the presence of a secret information in plain sight! Turns out you can actually do so with steganography!
The word “Steganography” has been derived from the Greek words “steganos”, meaning “covered, concealed, or protected,” and “graphein” meaning “writing or drawing”. It can be defined as the process of concealing any information, whether a message, image, audio or video within another message, image, audio or video. In other words, it is the practice of hiding data by embedding it within other, seemingly harmless or non-suspicious data.
History of Steganography
The technique of steganography is not new but rather dates back to 440 B.C., where an ancient Greek ruler shaved the head of the slaves, then tattooed the message on the scalp and waited for the hair to grow back. The recipient of the message had to shave the head in order to read the message and used the same technique to send a reply. Invisible inks made through various
sources such as milk, fruit juice, urine, etc., to be uncovered mostly through the application of heat or light, was another form of steganography employed during the American revolutionary war.
However, today, in the digital age, steganography is performed either by adding secret bits to audio or image files or replacing the original with the secret bits.
How is it different from cryptography?
While steganography and cryptography both have the same end goal, that is, to protect the messages or information from third parties, yet there is a distinct difference in their mechanisms to achieve that goal.
Cryptography changes the plaintext or information into ciphertext, which makes the messages unreadable and useless to third parties. Therefore, although the messages may be intercepted, yet it will remain inaccessible to unauthorised individuals because of encryption. On the contrary,
steganography does not change the format of the messages but mainly focuses on hiding the very existence of that message.
Thus, while cryptography protects information by making it inaccessible to third parties, steganography protects information by hiding its presence.
Important Terms
Payload: It is the information which is to be concealed or hidden.
Carrier or Cover Medium: It is the medium which is used for concealing the secret message. In other words, it is the original file within which the payload will be hidden and it can be any image file, audio file, or text file.
Stego-medium: The combination of the payload or embedded message and the cover medium is referred to as the Stego-medium.
Stegokey: It is the additional secret data which may be needed in the information hiding process such as a password and which is also needed to extract the embedded message again in the final destination.
Different Types of Steganography
Depending on the type of the carrier or the cover medium, that is, whether an image, audio, video or text file, steganography can be divided into different types such as text steganography, image steganography, audio steganography, etc.
Text steganography conceals information within text files through various ways such as modifying the linguistic or semantic structure; or changing the format of the original text by modifying the spaces between words, lines and paragraphs, etc. For example, two extra spaces between words could imply the binary digit “1”, whereas a single extra space could be defined as the binary digit “0”.
When an image file is used as the cover medium, it is known as Image steganography. There are a number of techniques employed to hide secret information within an image file but one of the most common technique is known as the Least Significant Bit (LSB). Every byte of an image pixel is made up of 8 bits.
However, not all the 8 bits are equally necessary to define if a pixel of an image is red or blue. In this technique, the last bit of each image pixel (least significant) is overwritten with the bit from the secret information. As it involves the least significant bit, it only alters the original file slightly.
Similarly, Audio steganography conceals information within harmless cover audio files. It is generally a more complicated process than image steganography. However, the classic technique of LSB used in image steganography can also be applied here, that is, the algorithm replaces the least significant bit of each byte in the original audio file with one bit from the secret message.
The significance of these bits is so minor when compared to the whole, that altering these bits could produce almost similar to the same result. Another technique is to introduce noise into the audio signal in order to embed the secret data, known as the spread spectrum technique.
In the same way, when the secret data is embedded into a video file, it is known as Video Steganography. As a video is made up of several frames or pictures, large amount of data can be hidden either by embedding the secret data directly into the compressed video or embedding the secret data in an uncompressed raw video and compressing it later.
Steganalysis
In simple words, the detection of steganographic content or the hidden/concealed message is known as steganalysis. The techniques applicable are also dependant on factors such as whether the original file used for the cover medium and the cover medium containing the embedded
message are both available, or whether the stego tool or algorithm used for the process is known.
Accordingly, there are various steganalytic approaches. Visual inspection of the original image or audio file used as the carrier medium can be performed and compared with the carrier image containing the secret data. If there are enough inconsistencies between the two, it might be enough to label the file a suspect. Structural detection of differences in size, date/time, content, hash value, etc., can be done.
Similarly, as steganographic processes generally alter the typical statistics of the cover medium, statistical analysis such as changes in the pattern of the pixels or histogram analysis can also be performed to determine possible deviations from the expected norm.
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