Already in the antiquity invisible ink and encrypted writing techniques were known. For example it is possible to write invisibly with lemon juice. The secret message will become visible in brown writing, if one holds the paper under a candle. However, only inaugurated persons know about the existence of the secret message. Such techniques are called in summary"Steganography ", which is Greek and means something like "hiding writing".
From the times of ancient Egypt until the Middle Ages papyrus as well as pergament were very expensive writing materials. Therefore one scraped off, washed off or with new layers glued over old scrolls in order to reuse it afterwards. Such overwritten documents are well-known in the professional world as"Palimpsest". However, one could use this technique also for transmitting secret messages. If the original text had not been completely removed, one can make it visible again with the help of modern techniques like the fluorescence photography. As an important technique of palaeographie, works of Euripides and Cicero could be recovered in this way for future generations.
Inspired by this application the term "Palimpsest" was also introduced in post modern literature interpretation. Here one tries to capture at the same time all conceivable meanings of a word or of a formulation in its context. Thus various interpretations can be unfolded.
In former times it seemed to become unnecessary to hide secret information when applying cryptographic methods. However, with the development of computer technology and crypto analysis more cryptographic procedures could be cracked very efficiently by means of modern electronics. Even the first computers were already used for decyphering secret messages. At that time the allies of world war 2 decrypted the code of the German encryption machine Enigma.
One of today's best encryption mechanisms is public key cryptography. Here all users generate themselves a public and a secret key. The public key can and will mostly be really published or will at least be passed on to friends. Everything that was encrypted with the public key, can be decrypted with the secret key and in reverse. It is however very time consuming (several millions of years) to calculate from the public key and an encrypted message possible secret keys. Two partners exchange a secret message as follows: The sender encrypts the message with his secret key and additionally with the public key of the recipient. The recipient decrypts this message then with his secret key and with the public key of the sender.
A weakness of other encryption techniques lies in the exchange of the key at least if there is only a single secret key: Whoever gets hold of the key, can decrypt the message very easily. Furthermore it is assumed that most secret services nevertheless know practicable methods to calculate a secret key from a public key. An indication for this is that the US authorities succeeded to determine the origin of an anonymized blackmailer e-mail. The message had passed through several anonymous remailers whose input and output messages are encrypted in different ways (by means of PGP). In order to be able to retrace the message, it must have been possible to crack the encryption.
For that reason one turns today to use keys that are as long as possible combined with cascades of several protective mechanisms. In this array of components hiding information has become a very important member: With the explosion of the amount of data travelling over the internet, packets are mostly examined automatically whether they contain explosive contents. Only a very small percentage will be examined exactly by humans if there is a reason for suspicion. If explosive information is hidden inconspicuously in harmless text, no suspicion will arise. Even for persistent aggressors this is a maximal impediment: They don't know: (a) Whether information is hidden (b) With which method and which options information was hidden and (c) which additional encryption and data compression methods were applied.
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