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Early History
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 civilizations 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 400
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 appeared.
Scytale
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.
As a result of this by approximately 1412 al-Kalka-shandi could include
in his encyclopaedia Subh al-a’sha a respectable if elementary treatment
of several cryptographic systems. He also gave explicit instructions on
how to cryptanalyze cipher text 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 1379 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 nomenclatures, were expanded gradually and for
several centuries were the mainstay of diplomatic communications for
nearly all European governments. In 1470 Leon Battista Alberti described
the first cipher disk in Trattati in cifra and the Traicté de chiffres,
published in 1586 by Blaise de Vigernère contained a square table
commonly attributed to him as well as descriptions of the first
plaintext and cipher text autokey systems.
By 1860 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. During the US Civil War, the Federal Army
extensively used transposition ciphers. The Confederate Army primarily
used the Vigenère cipher and on occasional monoalphabetic substitution.
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.
World Wars
During World War I, 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.S. Signal Corps cipher disk lacked
sophistication, some complicated cipher systems were used for high-level
communications by the end of the war.
In the 1920's, the maturing of mechanical and electromechanical technology
came together with the needs of telegraphy and radio to bring about a
revolution in crypto devices - 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
how to implement them. Beginning in 1921 and
continuing through the next decade, Hebern constructed a series of
steadily improving rotor machines that were evaluated by the U.S. Navy.
It was undoubtedly this work which led to the United States’ superior
position in cryptology during the second world war. 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.
Enigma Machine
The Enigma was a
portable cipher machine used to encrypt
and decrypt secret messages. More
precisely, Enigma was a family of
related electro-mechanical rotor
machines, as there were a variety of
different models.
The Enigma
was used commercially from the early
1920's on, and was also adopted by the
military and governmental services of a
number of nations — most famously by
Nazi Germany before and during World War
II. The German military model, the
Wehrmacht Enigma, is the version
most commonly discussed. The machine has
gained notoriety because Allied
cryptologists were able to decrypt a
large number of messages that had been
enciphered on the machine before being
broadcast by radio. The intelligence
gained through this source — codenamed
ULTRA— was a significant aid to the
Allied war effort. Some historians have
suggested that the end of the European
war was hastened by up to a year or more
because of the decryption of German
ciphers.
Like other rotor machines,
the Enigma machine is a combination of mechanical and
electrical systems. The mechanical mechanism consists of
a keyboard; a set of rotating disks, called rotors,
arranged adjacently along a spindle; and a stepping
mechanism to turn one or more of the rotors with each
key press. The exact mechanism varies, but the most
common form is for the right-hand rotor to step once
with every key stroke, and occasionally the motion of neighboring rotors is triggered. The continual movement
of the rotors results in a different cryptographic
transformation after each key press.
Enigma encryption for two
consecutive letters — current is passed into set of rotors, around the
reflector, and back out through the rotors again. Note: The grey outlines represent other possible circuits within each rotor, which are
hard-wired to contacts on each rotor. Letter A encrypts
differently with consecutive key presses, first to G, and then
to C. This is because the right hand rotor has stepped, sending
the signal on a completely different route.
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