When we try to hide a message, make it illegible or incomprehensible by unauthorized persons but accessible to authenticated, we can say that we are working on "cryptography".
Cryptology consists of two parts:
The cryptography: Encryption of clear messages.
The cryptanalysis: Deciphering cryptograms (encrypted messages).
Cryptography is born for military and diplomatic reasons.
The first coded document has been found in Iraq. Indeed, a ciphertext
appearing on a clay tablet was engraved by a Babylonian potter who wanted conceal his recipe for more success.
After adding a few consonants and recognized words to the spelling visionary, archaeologists have been able to decipher the formula
manufacturing of varnishes potter jealously guarded the exclusivity.
The Greeks have renovated the cryptography putting a new system available to their powerful army. To understand the system you
must take a stick with well-defined diameter (scytale) used key for encoding, then take a ribbon and wrap the stick with parallel turns.
The message will be written directly on the wound strip, line by line along the length of the stick!
Once done, pull the tape and your message is encoded Hop. To decode it you must use the same key used for encryption,
that is, a stick of the same diameter as that used for coding!
It was not a good system because, to send a coded message from Greece to Rome it was sent with the proper and clear key. It was enough that the stick is stolen by spies so an unauthorized person is able to read the secret message!
Waiting the second century to see a real change in the method; Polybius the Romans improved transmission system that was designed by Kléoxénos and Démokleïtos .
Polybius invented a system of encryption square, it was called "Polybius square". The idea was to put all the letters of the alphabet in a 5x5 table. To encode a letter you had to replace it with its coordinates in the table.
One of the first mathematical algorithms was made by Julius Caesar in order to communicate his orders and relationships with his lieutenants.
The principle is very basic: It is a shift system which involves replacing a letter by the next letter or letter after the 3rd or 4th and so on in the order
of the alphabet. There are many variations to this system.
Exemple : Plaintext = "Bonjour" Ciphertext = "CP OKPV S"By a simple brute attack anyone should be able to break the code, so I can not not advise you to use it to encrypt your messages!
We have seen that the encryption can never be a sure way against attacks. You should never say "My code can never be broken," , avoid the guilt of the Germans because we know now that what is impossible to break today becomes a breeze 2 years later! However there is only one algorithm proved mathematically secure, the Vernam cipher that we will see later.