Answer: Shift ciphers
One of the simplest cryptographic ciphers in existence was used by Julius Caesar over 2,000 years ago with great success. In fact, the method is so closely associated with it that, in addition to the name "shift cipher," it is often referred to by names that refer to Caesar, such as "Caesar's cipher" or "Caesar shift."
The basis of The Shift cipher simply shifts the letter value down a fixed number of positions in the alphabet. The most basic shift encoding using the modern English alphabet is simply swapping one letter with its nearest front neighbor (ABC thus becomes BCD). I've been working well for Caesar for several reasons. First, the idea of text encryption / encoding was largely unknown at the time (the most common method of securing communications was to use a language the interceptor would not know). Second, his enemies were mostly illiterate. There is no record that his cipher was ever broken. The idea of frequency analysis for cracking coded text was proposed in the 9th century by the Arab mathematician Al-Kindi.