Of all the endless variety of phenomena which nature presents to our senses, there is none that fills our minds with greater wonder than that inconceivably complex movement which, in its entirety, we designate as human life; Its mysterious origin is veiled in the forever impenetrable mist of the past, its character is rendered incomprehensible by its infinite intricacy, and its destination is hidden in the unfathomable depths of the future. Whence does it come?
Useful Terminology Much of the terminology of cryptography can be linked back to the time when only written messages were being encrypted and the same terminology is still used regardless of whether it is being applied to a written message or a stream of binary code between two computers.
CODE An unvarying rule for replacing a piece of information with another object, not necessarily of the same sort e. Ciphers, as in the case of codes, also replace a piece of information an element of the plaintext that may consist of a letter or word or string of symbols with another object.
The difference is that the replacement is made according to a rule defined by a secret key known only to the transmitter and legitimate receiver s in the expectation that an outsider, ignorant of the key, will not be able to undo the replacement and retrieve the original plaintext. Early Cryptographic Systems It seems reasonable to assume that people have tried to conceal information in written form since writing was developed and examples survive in stone inscriptions and papyruses showing that many ancient civilisations including the Egyptians, Hebrews and Assyrians all developed cryptographic systems.
The first recorded use of cryptography for correspondence was by the Spartans who as early as BC employed a cipher device called a "scytale" to send secret communications between military commanders. The scytale consisted of a tapered baton around which was wrapped a piece of parchment inscribed with the message.
Once unwrapped the parchment appeared to contain an incomprehensible set of letters, however when wrapped around another baton of identical size the original text appears.
The Greeks were therefore the inventors of the first transposition cipher and in the fourth century BC the earliest treatise on the subject was written by a Greek, Aeneas Tacticus, as part of a work entitled On the Defence of Fortifications.
Another Greek, Polybius later devised a means of encoding letters into pairs of symbols using a device known as the Polybius checkerboard which contains many elements common to later encryption systems. In addition to the Greeks there are similar examples of primitive substitution or transposition ciphers in use by other civilisations including the Romans.
The Polybius checkerboard consists of a five by five grid containing all the letters of the alphabet. Each letter is converted into two numbers, the first is the row in which the letter can be found and the second is the column. Hence the letter A becomes 11, the letter B 12 and so forth.
The Arabs were the first people to clearly understand the principles of cryptography and to elucidate the beginning of cryptanalysis. They devised and used both substitution and transposition ciphers and discovered the use of letter frequency distributions in cryptanalysis. He also gave explicit instructions on how to cryptanalyze ciphertext using letter frequency counts including examples illustrating the technique.
European cryptography dates from the Middle Ages during which it was developed by the Papal and Italian city states. The earliest ciphers involved only vowel substitution leaving the consonants unchanged.
Circa the first European manual on cryptography, consisting of a compilation of ciphers, was produced by Gabriele de Lavinde of Parma, who served Pope Clement VII. This manual contains a set of keys for correspondents and uses symbols for letters and nulls with several two character code equivalents for words and names.
The first brief code vocabularies, called nomenclators, were expanded gradually and for several centuries were the mainstay of diplomatic communications for nearly all European governments. By large codes were in common use for diplomatic communications and cipher systems had become a rarity for this application however cipher systems prevailed for military communications except for high-command communications because of the difficulty of protecting codebooks from capture or compromise.
While the Union cryptanalysts solved most of the intercepted Confederate ciphers, the Confederacy in desperation, sometimes published Union ciphers in newspapers, appealing for help from readers in cryptanalysing them.
Cryptography During The Two World Wars During the first world war both sides employed cipher systems almost exclusively for tactical communications while code systems were still used mainly for high-command and diplomatic communications.
Although field cipher systems such as the U.
Signal Corps cipher disk lacked sophistication some complicated cipher systems were used for high-level communications by the end of the war. In the s the maturing of mechanical and electromechanical technology came together with the needs of telegraphy and radio to bring about a revolution in cryptodevices - the development of rotor cipher machines.
The concept of the rotor had been anticipated in the older mechanical cipher disks however it was an American, Edward Hebern, who recognised that by hardwiring a monoalphabetic substitution in the connections from the contacts on one side of an electrical rotor to those on the other side and cascading a collection of such rotors, polyalphabetic substitutions of almost any complexity could be produced.
From and continuing through the next decade, Hebern constructed a series of steadily improving rotor machines that were evaluated by the U. At almost the same time as Hebern was inventing the rotor cipher machine in the United States, European engineers such as Hugo Koch Netherlands and Arthur Scherbius Germany independently discovered the rotor concept and designed the precursors to the most famous cipher machine in history - the German Enigma machine which was used during World War 2.
The Japanese cipher machines of World War 2 have an interesting history linking them to both the Hebern and the Enigma machines. After Herbert Yardley, an American cryptographer who organised and directed the U.
With this in mind, it purchased the rotor machines of Hebern and the commercial Enigmas, as well as several other contemporary machines, for study.
However, drawing on experience gained from cryptanalysing the ciphers produced by the Hebern rotor machines the U. Cryptography In The Modern Age After World War 2 the electronics that had been developed in support of radar were adapted to cryptomachines.
The first electrical cryptomachines were little more than rotor machines where the rotors had been replaced by electronic substitutions. The only advantage of these electronic rotor machines was their speed of operation and they inherited the inherent weaknesses of the mechanical rotor machines.
There is little information available regarding the secret cipher machines of the s and it is likely that this subject will remain the shrouded in rumour until the relevant information is de-classified. The era of computers and electronics has meant an unprecedented freedom for cipher designers to use elaborate designs which would be far too prone to error if handled by pencil and paper, or far to expensive to implement in the form of an electromechanical cipher machine.
Cryptographic systems are generally grouped according to three facts about them: The mathematical operation that changes the plaintext into the ciphertext using the encryption key. Whether a block or a stream cipher is produced.
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