The Vigenère square
This is a very secure system which remained uncracked for many (over 300) years!
The system is essentially 26 Caesar Cyphers arranged into a square each with a different offset on each row.
The encoder then uses a key like "SERVER", this is then used along with the text to be encoded to create the final text.
Text to endcode: NICEDAYFORAWALK
repeat the key until it's the same length as the text: SERVERSERVERSER
then read across the top for the repeated key, then down the side for the text to encode.
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Result: FMTZHRQJFMENSPB
To decode, for each letter, use the letter from the key to find the column, then lookup the corresponding letter, then read off the letter on the left.
More info can be found here: Wikipedia