Imperial War Museums website - How Alan Turing cracked the Enigma code: https://www.iwm.org.uk/search/global?query=enigma
This particular cipher was used during American Civil War and World War I to communicate sensitive messages. The letters of the alphabet are rearranged based on pre-determined key or rule. It could be that all the words in the message are written backwards, or every pair of letters is swapped. If the rearrangement rule is complex, it might seem very difficult to decipher, however, with modern algorithms on the computer, it can be easily deciphered. For example: “the yellow car belongs to him” can become “eht wolley rac sgnoleb ot mih” when written backwards.
This particular cipher has been named after Julius Caesar, since he used it himself. A cipher was present for each letter of the alphabet, for example ROT1 is one of the ciphers. To decode the message, the person has to be aware which cipher has been used. In G cipher, A becomes G, B becomes H and so on. In Y Cipher, A becomes Y and so on. This particular cipher is not very difficult to decipher and hence secret messages do not remain secret for long. This particular cipher has been used as the basis for creation of more complex ciphers.
During the time of Oliver Cromwell, John Thurloe was the Head of an Intelligence Network. His spies/agents used various codes and cyphers. This website provides some information about John Thurloe's agents and the different codes/cyphers they used.
This is a cipher familiar to many children. Its key is simple: each letter of the alphabet is replaced with the following letter, so A is replaced with B, B is replaced with C, and so on. “ROT1” literally means “rotate 1 letter forward through the alphabet.” The message “I know what you did last summer” would become “J lopx xibu zpv eje mbtu tvnnfs” and so on. This cipher is fun because it is easy to understand and use, but it is equally easy to decipher if they key is used in reverse. This cipher is not suitable for serious use but can be of great amusement for children. Try to decipher the message “XBT JU B DBU J TBX?”
Source: http://listverse.com/2012/03/13/10-codes-and-ciphers/ - 10 Codes and Cyphers
This is also referred to as Tic-Tac-Toe Cipher, and is fairly simple substitution cipher. The letters of the alphabet are replaced by fragments of a geometrical grid. It is very simple and hence has been used in children’s books of secret writing as well. Although its origin cannot be ascertained, it goes back to the 18 century. The grid and the dots are the core elements of the cipher. The alphabets are arranged in two grids, followed by two Xs. It has been used in various novels –The Lost Symbol by Dan Brown, games like Assasin’s Creed II, TV series – Sherlock, etc.