Function symm_encrypt_message 

pub async fn symm_encrypt_message(
    plain: Vec<u8>,
    private_key_for_signing: SignedSecretKey,
    shared_secret: &str,
    compress: bool,
) -> Result<String>

Symmetrically encrypt the message. This is used for broadcast channels and for version 2 of the Securejoin protocol. shared secret is the secret that will be used for symmetric encryption.