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.