When sending messages using symmetric encryption, which of the following must happen FIRST?

Exchanging the encryption key must happen after an encryption method has been agreed upon, as the key's properties and usage are determined by the chosen algorithm.

Establishing digital signatures provides authenticity and integrity, which is a separate cryptographic goal from the confidentiality offered by symmetric encryption and is not a prerequisite for it.

Before any data can be encrypted or decrypted using symmetric keys, both communicating parties must first agree on the specific symmetric encryption algorithm (the 'method') they will employ, such as AES or Triple DES. This agreement is foundational because the chosen algorithm dictates the properties of the shared secret key and the precise steps for transformation, making it a prerequisite for all subsequent cryptographic operations.

Installing digital certificates is primarily associated with public key infrastructure (PKI) and asymmetric encryption for identity verification or secure key exchange, not as the direct first step for symmetric encryption itself.