Figure 1: Symmetric Encryption

1. Sue (aka Sender) selects a key, and then uses that key to encrypt the plaintext to produce the ciphertext.
   
   
2. Sue gives both the key and the ciphertext to you (aka Receiver). (Not together, obviously, or anyone could intercept the delivery and use the key to decrypt the ciphertext.)
   
   
3. You use that same key to decrypt the ciphertext to regenerate the plaintext.