The correct option is Symmetric key encryption.

Symmetric key encryption uses a single shared secret key that both participants use to encrypt and decrypt messages. Common symmetric algorithms such as AES perform both encryption and decryption with the same key and are typically used for bulk confidentiality because they are efficient. To operate securely the shared key must first be established or distributed by a secure channel or by using a key agreement protocol.

Diffie Hellman key agreement is a method to derive a shared secret between parties over an insecure channel but it is a key agreement protocol rather than an encryption scheme. It is often used to produce the symmetric key that will then be used by a symmetric cipher, but it does not itself perform message encryption and decryption.

Public Key Infrastructure is a framework for issuing and managing certificates and public keys and it supports asymmetric cryptography for authentication and key distribution. It is not a method where both participants use the same secret to encrypt and decrypt messages.

Asymmetric key cryptography uses a pair of keys where the public key is different from the private key and the encrypting and decrypting keys are not the same. That property means it does not meet the requirement of using the same secret for both encryption and decryption.

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