Cryptography, which derives from the Greek word kryptos meaning hidden, is both an art and a science.

Codes in History:

Codes and ciphers have been used since ancient times.

• In 405 BC the Greek general Lysander of Sparta was sent a coded message written on the inside of a servant's belt. When Lysander wound the belt around a wooden baton the message was revealed. The message warned Lysander that Persia was about to go to war against him. He immediately set sail and defeated the Persians. The Greeks also invented a code which changed letters into numbers. A is written as 11, B is 12, and so on. So WAR would read 52 11 42. A form of this code was still being used two thousand years later during the First World War.

• The Roman Emperor Julius Caesar invented his own simple code. The Caesar Substitution cipher, and all like it were called "simple substitution" (to the cryptologist, "monalphabetic substitution") because throughout the message, each character was always replaced by the identical cipher character. Caesar moved each letter of the alphabet along three places, so that A became D, B became E and so on. His famous phrase VENI, VIDI, VICI ("I came, I saw, I conquered") would have read YHQL YLGL YLFL. After the fall of the Roman Empire codes were not used much until the sixteenth century. Then Italian and French scholars began to make up very complicated codes. The science of code-breaking - CRYPTANALYSIS - had begun.

• In Elizabethan England Mary Queen of Scots sent coded messages to her supporters who where plotting to murder Queen Elizabeth I. The messages were intercepted by the head of Elizabeth's secret service, Sir Francis Walsingham. He deciphered them and discovered the plot. Mary was executed for treason in 1587.

• The invention of Morse Code in 1838 by the American, Samuel Morse, created an entirely new way of sending messages quickly over long distances. Morse is not actually a code but an electro-magnetic telegraph system that uses signals of 'dots' and 'dashes' to represent letters of the alphabet. If a message in Morse was to be kept secret, it had to be put into code before it was sent.

By the end of the First World War, armies and navies sent many of their important orders by telegraph or wireless using Morse. British and German code-breakers tried to intercept and decode each other's messages.

Cryptography

The field of study that cryptography is a part of is called cryptology. Cryptography: is the making of a cipher system and the breaking of a cipher system and is called cryptanalysis. The main thrust of cryptography uses two general scrambling techniques:

1. Transposition: where each symbol maintains its identity but changes its position and substitution, where each symbol maintains its position but changes its identity.

2. Modern cryptography also consists of Asymmetrical Techniques: where for a given encryption key the corresponding decryption key is different. Unfortunately, there does not exist a mathematical proof for security, thus it is currently impossible to achieve absolute security. Proof of security is usually reduced to some other problem known to be extremely difficult to solve, such as the factorization of large numbers.

There exist three fundamental parts to the encryption/decryption process: the message, a key, and an algorithm. Currently, as per Kerckhoff's Principle, the algorithm can be disclosed to the public, but keeping the key secret during actual encryption and decryption of a message is the most significant aspect to effective cryptography. One of the challenges of maintaining secure communications is key management. As the number of users and messages increase the more keys are used, thus increasing the chance of making mistakes or exposing patterns and vulnerabilities. Security requires four fundamental services integrity, confidentiality, authentication and non-repudiation.

Security attacks can be either passive, such as eavesdropping or active, such as modifying a message, and exist in four general categories interruption, interception, modification and fabrication.

Cryptography requires problems that are computationally hard so that scrambled messages are hard to break without the correct key. Checking whether or not a key is correct is easily verifiable, which might suggest that cryptographic problems are class NP problems. However, cryptographic strength does not relate to NP-complete problems because NP-complete problems deal with difficulty in the worst case, which means that the problem may be easily solvable in all other cases. Complexity for cryptography requires analyzing best case solutions, such that a candidate for a cipher system would be difficult in the best case to solve, thus not being concerned with worst case complexity. Hence, a cipher system requires that average case complexity needs to be difficult, to achieve a cipher system that is practically unbreakable.

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Enigma Cipher Machine Homepage: Jamie Brownell

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