Chapter 1

Steganography is the art of hiding a message. Cryptography is about making a message unreadable by a third party.

Cesar's shift cipher used to shift the alphabet. It was used and refined during the first millennium under the principle of mono alphabetic substitution: each letter is replaced by another symbol.

Between 800 and 1200, the Arab civilisation is prosperous and allow for the development of frequency analysis, which is then used to break the mono alphabetic substitution.

Chapter 2

In the end of the XVI, Vigenere propose the Vigenere cipher. This cipher is strong enough to resist frequency analysis, but its use is more involved than a simple mono alphabetic substitution so it remains less frequently used.

At the same time, black chambers, means to eavesdrop and decrypt messages, start to grow in multiple countries. Decrypting messages is becoming industrialized. With the arrival of the telegraph in the XIX, having a strong cipher becomes more pressing. This pushes for the use of the Vigenere cipher, which remains uncracked at that time.

Around 1854, Babbage broke the Vigenere cipher by finding repetitions to discover the key length and considering every nth letter and applying frequency analysis to it.

Cryptanalysts have now the advantage over cryptographers.

Chapter 3

By the beginning of the XX, the radio communication arrived and thrills the military, but makes necessary a reliable encryption mechanism.

During the first WW, the Allies are eavesdropping and decrypting foreign messages, giving them valuable informations.

After the war, when the German learned about the British deciphering their messages, they understand the necessity of improving the cipher, and turn to the enigma machine.

Fearing the invasion of Germans, the Poles soon put resources into deciphering messages. Around 1930, Rejewski find a way to get the day key and decrypt German messages. But by the beginning of 1939, Germans add new scramblers to enigma, leaving the Poles clueless. Before being invaded, they invite the French and British to explain their technique. Then, at Bletchley park, the British take over the task of breaking enigma. They use cribs, patterns known in the plaintext message, to backtrack the key. That plus Turing machine allow them to decrypt enigma's messages.

Chapter 4

Chapter 5 - The language Barrier

Machine cyphers are very efficient but they require a lot of logistic. In some situations it was imposible for the military to use them. Instead they came with the idea of using the langage of native americans: the Navajo. For the 2nd WW, they trained Navajo and sipatched them on troops. The langage was so alien for the japanese that they were and remained unable to decipher anthing.

Amongst the best craptanalysts are the archealists trying to undestand long forgotten langages. Deciphering heiorglyphs was one of this challenge. It was tackled by identifying some partfs of the text that looked like god names and extrapolating from there. Anpother one is the decipherment of Linear B, a greek inspired script.

Chapter 6

On 15 May 1973, America's National Bureau of Standards formally requested proposals for a standard encryption system. The more established cypher algo, from a german emergro, Horst Feistel, working at IBM, was officially adopted as DES on 23 November 1976.

The problem that arrised from that was to distribute keys. Companies were paying high logistical costs to have courriers exchange keys.

In 1974, diffie met Hellman and they both started a collaboration to tackle the key distibution problem. In 1976 they came with a protocol to exchange a key without revealing it to someone who would eavesdrop the exchange.

Diffie also came up with the idea of asymetric cryptography but had no mathematical function to do it. so he wrote a paper describing the idea and explaining the constraints. In April 1977, Rivest, Shamir and Adleman found such a function, providing RSA, the first asymetric cryptography algorithm.

After the British declassified documents, the world discovered that James Ellis came up with the same idea and Clifford Cock found the mathematical function.

Chapter 7

Phil Zimmermann developped PGP as a way to promote encryption to the general public. It raised concerns in the gouvernement who saw encryption as an amunition and thus saw Phil as giving US weapons to the rest of the world. It raised a debate of liberty versus security (the spying of NSA and wiretaping of the police would be much harder), and after 3 yers nothing wnent further againt Phil.

Chapter 8

By being able to do all possible computation at the same time, a quantum computer could theoretically brak RSa and the other cryptographic algorithms that we use today.

In the 1960s, forgot his name came up with the idea of quantum money. Nobody showed interest in his idea but Benett. Bennett then talked to Brassard and together in 1984 they came up with a protocol for quantum cryptography which is in theory unbreakable. The idea is to transmit polarized photons to the recipient. If the recipient knows the orientation of the photon he can get the correct value. Without knowing the orientation, because of the "principe d'incertitude d'Heisenberg" it is impossible to get the correct value, thus making a secure communication.