Steganography is the art and science of writing hidden messages in such a way that no-one apart from the sender and intended recipient even realizes there is a hidden message. By contrast, cryptography obscures the meaning of a message, but it does not conceal the fact that there is a message. Today, the term steganography includes the concealment of digital information within computer files. For example, the sender might start with an ordinary-looking image file, then adjust the color of every 100th pixel to correspond to a letter in the alphabet—a change so subtle that someone who isn't actively looking for it is unlikely to notice it.
Generally, a steganographic message will appear to be something else: a picture, an article, a shopping list, or some other message. This apparent message is the covertext. For instance, a message may be hidden by using invisible ink between the visible lines of innocuous documents.
The advantage of steganography over cryptography alone is that messages do not attract attention to themselves, to messengers, or to recipients. An unhidden coded message, no matter how unbreakable it is, will arouse suspicion and may in itself be incriminating, as in countries where encryption is illegal.[2] Often, steganography and cryptography are used together to ensure security of the covert message.
Electronic communications can include steganographic coding inside of a transport layer, such as a file, or a protocol, such as UDP. Media files are ideal for steganographic transmission because of their large size, and are usually compressed either losslessly, as with FLAC or PNG, or lossily, as with JPEG images, MPEG video, or MP3 audio.
A steganographic message (the plaintext) is often first encrypted by some traditional means, producing a ciphertext. Then, a covertext is modified in some way to contain the ciphertext, resulting in stegotext. For example, the letter size, spacing, typeface, or other characteristics of a covertext can be manipulated to carry the hidden message; only the recipient (who must know the technique used) can recover the message and then decrypt it.
Applications
Usage in modern printers
Steganography is used by some modern printers, including HP and Xerox brand color laser printers. Tiny yellow dots are added to each page. The dots are barely visible and contain encoded printer serial numbers, as well as date and time stamps.
Example from modern practice
Image of a tree.By removing all but the last 2 bits of each color component, an almost completely black image results. Making the resulting image 85 times brighter results in the image below.
Image extracted from above image.
The larger the cover message is (in data content terms—number of bits) relative to the hidden message, the easier it is to hide the latter.
For this reason, digital pictures (which contain large amounts of data) are used to hide messages on the Internet and on other communication media. It is not clear how commonly this is actually done. For example: a 24-bit bitmap will have 8 bits representing each of the three color values (red, green, and blue) at each pixel. If we consider just the blue there will be 28 different values of blue. The difference between 11111111 and 11111110 in the value for blue intensity is likely to be undetectable by the human eye. Therefore, the least significant bit can be used (more or less undetectably) for something else other than color information. If we do it with the green and the red as well we can get one letter of ASCII text for every three pixels.
Stated somewhat more formally, the objective for making steganographic encoding difficult to detect is to ensure that the changes to the carrier (the original signal) due to the injection of the payload (the signal to covertly embed) are visually (and ideally, statistically) negligible; that is to say, the changes are indistinguishable from the noise floor of the carrier.
Generally, a steganographic message will appear to be something else: a picture, an article, a shopping list, or some other message. This apparent message is the covertext. For instance, a message may be hidden by using invisible ink between the visible lines of innocuous documents.
The advantage of steganography over cryptography alone is that messages do not attract attention to themselves, to messengers, or to recipients. An unhidden coded message, no matter how unbreakable it is, will arouse suspicion and may in itself be incriminating, as in countries where encryption is illegal.[2] Often, steganography and cryptography are used together to ensure security of the covert message.
Electronic communications can include steganographic coding inside of a transport layer, such as a file, or a protocol, such as UDP. Media files are ideal for steganographic transmission because of their large size, and are usually compressed either losslessly, as with FLAC or PNG, or lossily, as with JPEG images, MPEG video, or MP3 audio.
A steganographic message (the plaintext) is often first encrypted by some traditional means, producing a ciphertext. Then, a covertext is modified in some way to contain the ciphertext, resulting in stegotext. For example, the letter size, spacing, typeface, or other characteristics of a covertext can be manipulated to carry the hidden message; only the recipient (who must know the technique used) can recover the message and then decrypt it.
Applications
Usage in modern printers
Steganography is used by some modern printers, including HP and Xerox brand color laser printers. Tiny yellow dots are added to each page. The dots are barely visible and contain encoded printer serial numbers, as well as date and time stamps.
Example from modern practice
Image of a tree.By removing all but the last 2 bits of each color component, an almost completely black image results. Making the resulting image 85 times brighter results in the image below.
Image extracted from above image.
The larger the cover message is (in data content terms—number of bits) relative to the hidden message, the easier it is to hide the latter.
For this reason, digital pictures (which contain large amounts of data) are used to hide messages on the Internet and on other communication media. It is not clear how commonly this is actually done. For example: a 24-bit bitmap will have 8 bits representing each of the three color values (red, green, and blue) at each pixel. If we consider just the blue there will be 28 different values of blue. The difference between 11111111 and 11111110 in the value for blue intensity is likely to be undetectable by the human eye. Therefore, the least significant bit can be used (more or less undetectably) for something else other than color information. If we do it with the green and the red as well we can get one letter of ASCII text for every three pixels.
Stated somewhat more formally, the objective for making steganographic encoding difficult to detect is to ensure that the changes to the carrier (the original signal) due to the injection of the payload (the signal to covertly embed) are visually (and ideally, statistically) negligible; that is to say, the changes are indistinguishable from the noise floor of the carrier.
