Which type of encryption uses the exact same secret key to both encrypt plaintext into ciphertext and decrypt ciphertext back into plaintext?

### Why this is right The lesson explicitly identifies the initial file-encryption exercise as symmetric encryption. Symmetric encryption uses one shared secret key for encryption and decryption. ### The concept Encryption converts readable plaintext into ciphertext. In symmetric encryption, the same secret key is used to encrypt and decrypt, so the parties must protect and share that key. Asymmetric encryption uses a public and private key pair. Hybrid encryption uses symmetric encryption for the file and asymmetric encryption to protect the symmetric key. ### The other options - **B. Asymmetric encryption using a public key only**: public-key encryption is a different method and requires the matching private key for decryption. - **C. Digital signing using a private key**: signing checks authenticity and integrity; it is not the file-encryption exercise. - **D. Hybrid encryption using two keys**: hybrid encryption combines a symmetric file key with asymmetric protection of that key. ### Terms - **Symmetric encryption**: Encryption and decryption using the same shared secret key. - **Plaintext**: Readable, unencrypted data. - **Ciphertext**: Encrypted data. - **Hybrid encryption**: A combination of symmetric and asymmetric encryption. ### Remember If one shared secret key handles both directions, the method is symmetric.