16th-century book-shaped French
cipher machine, with arms of Henri II
of France
Enciphered letter from Gabriel de
Luetz d'Aramon, French Ambassador
to the Ottoman Empire, after 1546,
with partial decipherment
letter of the key word is used. In the mid-19th century Charles
Babbage showed that the Vigenère cipher was vulnerable to Kasiski
examination, but this was first published about ten years later by
Friedrich Kasiski.
[28]
Although frequency analysis can be a powerful and general technique
against many ciphers, encryption has still often been effective in
practice, as many a would-be cryptanalyst was unaware of the
technique. Breaking a message without using frequency analysis
essentially required knowledge of the cipher used and perhaps of the
key involved, thus making espionage, bribery, burglary, defection,
etc., more attractive approaches to the cryptanalytically uninformed. It
was finally explicitly recognized in the 19th century that secrecy of a
cipher's algorithm is not a sensible nor practical safeguard of message
security; in fact, it was further realized that any adequate
cryptographic scheme (including ciphers) should remain secure even
if the adversary fully understands the cipher algorithm itself. Security
of the key used should alone be sufficient for a good cipher to
maintain confidentiality under an attack. This fundamental principle
was first explicitly stated in 1883 by Auguste Kerckhoffs and is
generally called Kerckhoffs's Principle; alternatively and more bluntly,
it was restated by Claude Shannon, the inventor of information theory
and the fundamentals of theoretical cryptography, as Shannon's
Maxim—'the enemy knows the system'.
Different physical devices and aids have been used to assist with
ciphers. One of the earliest may have been the scytale of ancient
Greece, a rod supposedly used by the Spartans as an aid for a
transposition cipher. In medieval times, other aids were invented such as the cipher grille, which was also used
for a kind of steganography. With the invention of polyalphabetic ciphers came more sophisticated aids such as
Alberti's own cipher disk, Johannes Trithemius' tabula recta scheme, and Thomas Jefferson's wheel cypher
(not publicly known, and reinvented independently by Bazeries around 1900). Many mechanical
encryption/decryption devices were invented early in the 20th century, and several patented, among them rotor
machines—famously including the Enigma machine used by the German government and military from the
late 1920s and during World War II.
[29]
The ciphers implemented by better quality examples of these machine
designs brought about a substantial increase in cryptanalytic difficulty after WWI.
[30]
Prior to the early 20th century, cryptography was mainly concerned with linguistic and lexicographic patterns.
Since then the emphasis has shifted, and cryptography now makes extensive use of mathematics, including
aspects of information theory, computational complexity, statistics, combinatorics, abstract algebra, number
theory, and finite mathematics generally. Cryptography is also a branch of engineering, but an unusual one
since it deals with active, intelligent, and malevolent opposition; other kinds of engineering (e.g., civil or
chemical engineering) need deal only with neutral natural forces. There is also active research examining the
relationship between cryptographic problems and quantum physics.
Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much
more complex ciphers. Furthermore, computers allowed for the encryption of any kind of data representable in
any binary format, unlike classical ciphers which only encrypted written language texts; this was new and
significant. Computer use has thus supplanted linguistic cryptography, both for cipher design and
cryptanalysis. Many computer ciphers can be characterized by their operation on binary bit sequences
Computer era
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