Steganography is the art of hiding messages or files in a way that prevents the detection of the existence of these hidden messages. It encompasses several techniques, including physical methods like invisible ink on paper and digital techniques like hiding text on multimedia files like images and music files. In the modern digital era, steganography has become a useful tool to evade detection and perusal of secret messages. With the advent of social media, it is very easy to encode a message or file onto an image and upload it online for the intended recipients to access, decode, and read or use. In this case of digital steganography of messages or files onto images, an important factor to consider is the effect of image compression on the hidden message. Since most social media and other online image posting websites run some sort of compression, cropping, and other image transformations on the uploaded images, understanding these techniques and their effect on the hidden text can help one choose the most suitable steganography technique to use for a particular use case.
TopIntroduction
Before getting into Steganography lets first get into Cryptography. What is Cryptography? It is a technique that converts a plain readable text into something which is either not readable or readable with the real message hidden. This technique can be found as far back as 1900 B.C. Ancient Egyptians used to scribe hieroglyphics in the inscription. Cryptography was being used from 500-600 BC with simple techniques like ATBASH, reversed alphabet, simple substitution. Today the methods of cryptography used are far more complicated than these simple techniques, as they are implemented by computers. As cryptography was evolving over time Steganography thrived independently.
The origin of Steganography was with the 2 Greek words ‘Steganos’ which means secret or cover and ‘grafia’ which means drawing or writing (Khosravi et al., 2011). It is basically a technique which is used to hide any sensitive information into other harmless messages, audio files or pictures. It is hard to tell just by looking at some pleasing image that some sensitive information is hidden within it. This is the most important aspect of steganography that differentiates it from cryptography. In cryptography, the presence of the message is known, only contents are unknown, but steganography also hides the existence of the message. Just by looking at some encrypted text, it is apparent that there is a message encoded in it that can be read if decryption is achieved. Given enough time and resources, this may be possible or even easy to do, but in case of steganography, the message is hidden completely, and its presence itself also may not be known (Robert Krenn, 2004).
The first technique for steganography was developed around 440 B.C in ancient Greece. The Greek ruler used to shave the head of slaves, tattoo the secret message on the scalp, wait till the hair grows back and then send the slave to deliver the message. The receiver then again shaves the head of the slave and the original message was visible on their scalp. The next Steganography technique used by Demaratus was that the message is carved in the wood of wax tablet and then covered with another wax layer. It continued to develop in the early 1600s.
Steganography was heavily used during the American Revolutionary wars. Both the forces used various forms of Invisible Ink. This special Ink was visible under light or heat. Another technique from these times is the Null cipher which embeds unencrypted messages within some text (Siper et al., 2005).
For example, consider the carrier text, “Fishing freshwater bends and saltwater coasts rewards anyone feeling stressed. Resourceful anglers usually find masterful leapers fun and admit swordfish rank overwhelming any day.” By extracting the third character from every word, the real encoded message is decoded as “Send Lawyers, Guns, and Money” (Johnson, 1995)
Here the carrier text does not make much sense, so it may arouse suspicion in readers that some hidden message may exist in it. Often when the carrier text also conveys some meaning and is not nonsensical gibberish, even the existence of the hidden message is not suspected. In the techniques that shall be discussed further, avoidance of detection of the hidden message or its existence via visual inspection will also be considered as a factor to compare them. Before that, some of the terminologies used for steganography must be discussed. The knowledge of the exact meaning of these can help understand the techniques used for steganography quicker.
The payload, or message, is the information that is to be hidden and covertly communicated. This is facilitated by encoding it onto the carrier or cover, which is a signal, textual data, image or other type of file that can hide the message on it. The resulting signal, textual data or file with the payload encoded in it is called the stego file or covert message. Terms like plaintext, key and ciphertext are also borrowed from cryptography when both techniques are used in conjunction.