Symmetric-Key Encryption
Symmetric-key algorithms use the same key for encrypting the plaintext into ciphertext and for decrypting the ciphertext back into plaintext. As one can imagine, a critical part of this mechanism is that the encryption key must remain secret. Despite this, it must somehow be shared between (at least) two agents who would like to exchange information together securely.
If this key were to become public, anyone with the key could decrypt the information that was exchanged between the intended parties. This makes symmetric-key encryption entirely dependent on continued maintenance of the key's secrecy.
One might think this just shifts the problem to confidentiality of the key rather than solving it, but it still has its use case. For example, when performance and faster speed is valued more.
Some example algorithms are:
Relevant Note(s): Encryption Asymmetric-Key Encryption Diffie–Hellman Key Exchange