Private-key cryptographic schemes work like that :
Alice and Bob have a private key.
Alice has a message for Bob.
Alice encrypts the message with a key and finally gets the cipher text.
Alice sends the cipher text to Bob.
Bob receives the cipher text from Alice.
Bob decrypts the cipher text with the (same) key and reads the original message.
For Caesar's Cipher the key is the number of shifts (to the left in the presentation).
Caesar's Cipher is rather easy to be broken, either by testing different shift values or applying frequency analysis on the ciphertext (cryptogram).
The following interactive tabula recta, is more suitable to be used in Vigenere Cipher (polyalphabetic cipher), but it also works for Caesar's Cipher, since we use only one alphabet (we set the horizontal ruler on a single alphabet/raw).
Start counting shifts from 0 ( i.e. 3rd shift should take you to alphabet/raw starting with letter D).
Shifts to the right could be also counted from the bottom of the table, starting from 1 (i.e. 3 shifts to the right should take you to alphabet/raw ending with letter W).
