The Relationship Between a Key and Its Ciphertext

Symmetric-key cryptography uses the same key for both encryption and decryption. The original, unencrypted message is called the plaintext. This lesson uses a simple scheme that shifts each letter forward by the key amount, so key 3 turns H into K. Decrypting with the same key used to encrypt restores the plaintext, but decrypting with a different key does not.