Steganography
The steganography (also: Steganography ) is the art or science of hidden storage or transmission of information in a carrier medium (container). The word can be attributed to the ancient Greek ingredients στεγανός Steganos, covered ' and γράφειν gráphein, write ' lead back, that literally means " covered writing " or " secret writing". The modified medium is referred to as steganogram.
- 4.1 Historical
- 4.2 Technical steganography
- 4.3 Linguistic Steganography 4.3.1 Semagramm
- 4.3.2 Open Code 4.3.2.1 Masked secret writing, jargon - code
- 4.3.2.2 Camouflaged secret writing
- 5.1 Literature
- 5.2 See also
- 5.3 External links
Objectives of steganography
The use of steganography has secrecy and confidentiality to the destination. Information is hidden so that a third party does not suspect when considering the carrier medium. From this it also ensures that the hidden information becomes known to third parties, ie confidentiality is guaranteed ( as in cryptography ).
The classification of steganography is usually performed in two possible ways: either it is considered as a sub- chapter of cryptography, or as an independent science. For the latter, is the fact that the objective of cryptography ( secrecy ) does not coincide with the objective of steganography ( confidential confidentiality by concealing the secrecy ). In practice, cryptography and steganography are often combined, as for example, have ciphertexts for steganography interesting statistical characteristics.
Demarcation
The operating principle of steganography based on the fact that an outsider does not recognize the existence of steganographierten information. Characterized steganography differs from the cryptography in which an outsider namely by the existence of the white information, however, is not able, due to the encryption, to understand the content.
In the steganography is used as the scenario usually sending messages from a sender to a receiver. The data storage can be mapped to it; in which case it is communication with yourself (sender = receiver). This special case but is usually neglected.
Very similar to steganography are imperceptible digital watermarks, but their objective is different. Steganography aims to secure confidentiality, while digital watermarks are designed for robustness ( the carrier you destroy the watermark, there is thus so degraded that it is no longer usable).
Security
A steganographic method is valid if and only considered safe when can drag onto a medium third persons any conclusions on the application of the method, if present in a medium non-obvious information is hidden. Another, but subordinated security feature is that embedded information even with knowledge of the existence of which can not be read by third parties. With the uncovering of steganographic contamination and the analysis of steganographic methods, the steganalysis employed ( analogous to cryptanalysis in cryptography ).
Encrypts to the message before embedding, so bring the out of hiding the message content, a further advantage: An encrypted message is usually indistinguishable from random data. In this it resembles background noise, such as is found in images, video or audio data. So it is statistically advantageous to encrypt a message before embedding.
Kerckhoffs ' principle in steganography
Kerckhoffs ' principle states that the security of a system should not depend on the secrecy of the algorithms, but only on the secrecy of a key. For safe procedure is thus only the knowledge of the correct key for the recognition of importance.
The consideration of the kerckhoffsschen principle in Steganography is historically considered of secondary importance, since it was first necessary to establish the Nichtdetektierbarkeit to human senses. Therefore, especially older Steganographiealgorithmen are inherently uncertain as they are open source available.
In steganography, a higher effort must be as in cryptography to meet Kerckhoffs ' principle and not to miss at the same time, the main objective of steganography, preserving the imperceptibility and Nichtdetektierbarkeit.
Symmetric steganography
Similar to the symmetric cryptography is based symmetric steganography that sender and receiver have a message ahead of the covert communication exchanged a secret key. Both know in what way and where a message is hidden.
Asymmetric steganography
The asymmetric steganography (including public-key steganography ) is based - such as asymmetric cryptography - solely on the fact that every potential recipient of a covert message is a (possibly authentic ) public key available, which is used to hide a message. With this public key, the message is encrypted and embedded. The message can be read only by the recipient, for the specially equipped with a private key. The sender can not decrypt the message. Where it is the algorithm used by Kerckhoffs ' principle, so he is not even able to find out whether in a medium hides a message. The only exception is that it compares the carrier medium directly to the steganogram.
Types of Steganography
Historical
A more time-consuming method is known from ancient times: There a slave 's head was shaved and tattooed a message on the scalp. Once the hair had grown back, the slave was sent to the receiver.
Well-known examples are wax tablets which normally contain carved into wax messages. In contrast, the secret messages in the wood were carved under it, poured the wax over it and provided with another message.
Other historical examples are not difficult to recognize or watermarks in paper or banknotes.
Friedrich L. Bauer describes a soldier in the war, who in his letters home, in each case by the first letter after the salutation tells his parents a letter of his whereabouts Tunis. He ignored the ( randomly different ) Running times of the individual letters. When his parents ask some time later, where's lying Nutsi that flies itself unobtrusive method.
There are many classical methods of steganography, including:
- The "invisible " secret ink (such as lemon juice)
- A double bottom in packets or envelopes
- Hollow heels of shoes and the like
- The microdot
- Secret writing with light: Stenographia
- Embedding a message in another below the threshold of perception.
Technical steganography
Examples are:
- The use of microfilm is known from earlier thrillers, which partly an A4 page can be hidden on the size of a typewriter point. Such a point ( specified in the technical terminology " micrate " or microdot ) is easy to hide.
- Hidden Tattoo antiquity covered by this term.
- Marking of copies or printouts by machine identification code.
Linguistic steganography
Spammimic is a program that encrypts a brief input message in innocuous-looking text that resembles spam.
Nice text converts a binary file into pseudo- natural text. To this end, the program uses context-free grammars. The program includes a dictionary and writing styles. The dictionary contains English words that are classified into five grammatical types ( article, noun, verb, adjective, preposition ). The style determines the syntactic rules for different types of records. A simple sentence has, for example, the construction of ART- SUBST -VERB - ART- SUBST.
For the transformation, the encoder selects a style. The input bits are used as pointers to the words in the different classes of the dictionary. The decoding is based on a simple reverse codebook search.
Example of Principle: Suppose the dictionary contains four words in the ART class ( with the binary indices 00-11 ) and 32 words in SUBST ( with the binary indices 00000-11111 ). The input is the bit sequence 0,101,110th The first two bits of the input (01 ) are replaced by the second word in ART. The next word corresponds to the 15th word in SUBST.
Semagramm
A subclass of linguistic steganography is the Semagramm. This information is transmitted by small details in an innocuous message in itself, a picture or a drawing.
In a word, the sign of a secret message can be masked by choosing different fonts, such as the coded characters in the original form of the Bacon cipher. However, these small differences are clearly visible to the untrained eye. Less conspicuous, for example, the use of presence or smears, small Tintenpatzern, seemingly hanging Typewriter types and the like.
Besides Textsemagrammen can be in pictures hide messages. Thus, the length of blades of grass on a creek could be a Morse code, the number and arrangement of the clouds in a seemingly child hand drawn landscape stand for a letter. The shipping a box of watches can be a Semagramm. The arrangement and hand position could contain important information.
Open code
Ornate to develop their own secret language. For example, secret characters are sprinkled in a specific pattern. The advantage of this method is that it, unlike a Semagramm, can not easily be identified by third parties as a secret message.
Masked secret writing, jargon - code
A masked secret writing is a kind of secret language. Certain phrases, words or signs will be assigned a special meaning, it must be previously agreed between the parties. Some expressions such secret languages have also been held as jargon way into everyday language, is to think:
- " Coal ", " gravel " → Money
- "Hole ", " ports " → prison
- "Stuff" → Drugs
- " Rat " traitor →
Unfair Card Players can enter by show of hands whether, with whom and what they want to play. Also possible are notes by phrases or words that begin with "H", which could indicate that "heart" is to be played. The individual is such a code, the more inconspicuous it is. However, the created or spoken text may appear slightly artificial and bloated.
Masked ciphers are vulnerable to censorship: A censor, the same content, for example, by using synonyms rewrite texts, can destroy the secret contents unknowingly.
Camouflaged secret writing
Secret messages as to hide in a text that they do not interfere with the normal flow of text, can be expensive. The secret characters are in a specific pattern in the text unremarkable in itself, as for example, could be a letter of the mystery word, every other character after a comma. When Wester Linck or "one, one, one " code the secret message is encoded by the number of syllables of the text words.
Templates that are placed over a text and leave only shine through the relevant secret words, hot Cardan grille. This is usually characterized by awkward sentence structure and peculiar choice of words.
- See also: Bible code.