Each letter is swapped with its opposite in the alphabet. For example, A ↔ Z, B ↔ Y, C ↔ X
So CAT becomes XZG. Spaces, special characters and punctuation stay the same.
A Caesar cipher is a secret way of writing messages by shifting letters in the alphabet. You pick a number, like 2, and move every letter forward by that many places.
For example, with a shift of 2: A → C, B → D, Z → B (it goes back to the start)
So CAT becomes ECV. Spaces, special characters and punctuation stay the same.
A Vigenère cipher is a way of hiding a message using a secret word instead of just one number.
Each letter in the secret word tells you how much to shift a letter in the message.
The secret word repeats until it matches the length of the message.
For example, if the message is LOVE and the key is ACS: A means shift by 1, C means shift by 3 and S means shift by 19.
L → M (Shift 1), O → R (Shift 3), V → O (Shift 19), E → F (Shift 1)