Theory and Practice…
And most of all the Vigenère cipher.
Information from wikipedia about the Vigenère cipher
The Vigenère cipher is named for Blaise de Vigenère. Although Giovan Battista Bellaso had invented the cipher earlier, Vigenère developed a stronger autokey cipher.
A reproduction of the Confederacy’s cipher disk used in the U.S. Civil War on display in the National Cryptologic Museum. The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers based on the letters of a keyword. It is a form of polyalphabetic substitution.
The Vigenère (French pronunciation: [viʒnɛːʁ]) cipher has been reinvented many times. The method was originally described by Giovan Battista Bellaso in his 1553 book La cifra del. Sig. Giovan Battista Bellaso; however, the scheme was later misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the „Vigenère cipher“.
Though the cipher is easy to understand and implement, for three centuries it resisted all attempts to break it; this earned it the description le chiffre indéchiffrable (French for ‚the indecipherable cipher‘). Many people have tried to implement encryption schemes that are essentially Vigenère ciphers. Friedrich Kasiski was the first to publish a general method of deciphering a Vigenère cipher in 1863.