The software employs symmetric key (private key) encryption, similar to AES. This means the same passphrase is used to encrypt and decrypt the message.
The pre-shared key (PSK) must be distributed among all parties involved prior to encryption, and a secure channel must be used. When sharing your PSK, do not use any supported services as they are inherently insecure. The exception is ProtonMail, it may be used to convey your PSK.