Cryptography is the science of protecting information by changing it into a form that others cannot read. It keeps data safe when it is stored or sent across the internet.
Plaintext, ciphertext and keys
The original readable message is called plaintext. After encryption it becomes scrambled ciphertext. Turning the ciphertext back into readable text is decryption. Both steps use a key — a secret value that controls how the message is scrambled and unscrambled.
Key idea
plaintext + key → encryption → ciphertext, and ciphertext + key → decryption → plaintext. Without the correct key, the message stays unreadable.
The Caesar cipher
The Caesar cipher is a simple method that shifts each letter forward by a fixed number of places. With a shift of 3, A becomes D, B becomes E, and so on. When you reach the end of the alphabet you wrap around, so X becomes A, Y becomes B and Z becomes C. Here the shift number is the key.
Example
Encrypt the word CAT with a shift of 3. C→F, A→D, T→W, giving the ciphertext FDW. To decrypt, shift each letter back by 3 to recover CAT.
The Caesar cipher is easy to break because there are only 25 possible shifts, so an attacker can simply try each one until readable text appears. Modern systems use far stronger methods with very long keys, but the same three ideas still apply: real data security always depends on encryption, decryption and a secret key working together to keep private information safe.