In symmetric key cryptography, what is primarily used to encrypt both plaintext and ciphertext?

In symmetric key cryptography, the primary mechanism for both encrypting plaintext and decrypting ciphertext is a shared secret key. This key is identical for both the sender and the receiver, which is why it's termed "symmetric."

The encryption process involves using the same shared secret key to transform plaintext into ciphertext, making the data unreadable to unauthorized parties. Conversely, the same key is used to decrypt the ciphertext back to its original plaintext form. The security of symmetric key cryptography heavily relies on the confidentiality of this shared secret key—if the key is compromised, so too is the security of the encrypted data.

Other options, such as public keys or private keys, are associated with asymmetric cryptography, where two keys are involved: one for encryption (public key) and another for decryption (private key). Hashing functions serve an entirely different purpose, as they are designed to produce a fixed-size output from variable-size input, making them useful for data integrity but not for encryption. Thus, the shared secret key is the correct choice as it fundamentally underpins symmetric key cryptography.