Decrypt pgp message using only public key - node.js

Question

In the following use case, It shows that using the private key when encrypting a message is optional but when decrypting the message, they make the private key mandatory!
How can I encrypted and decrypt a message using only one key, the public key?
const openpgp = require('openpgp'); // use as CommonJS, AMD, ES6 module or via window.openpgp
(async () => {
// put keys in backtick (``) to avoid errors caused by spaces or tabs
const publicKeyArmored = `-----BEGIN PGP PUBLIC KEY BLOCK-----
...
-----END PGP PUBLIC KEY BLOCK-----`;
const privateKeyArmored = `-----BEGIN PGP PRIVATE KEY BLOCK-----
...
-----END PGP PRIVATE KEY BLOCK-----`; // encrypted private key
const passphrase = `yourPassphrase`; // what the private key is encrypted with
const publicKey = await openpgp.readKey({ armoredKey: publicKeyArmored });
const privateKey = await openpgp.decryptKey({
privateKey: await openpgp.readPrivateKey({ armoredKey: privateKeyArmored }),
passphrase
});
const encrypted = await openpgp.encrypt({
message: await openpgp.createMessage({ text: 'Hello, World!' }), // input as Message object
encryptionKeys: publicKey,
signingKeys: privateKey // optional
});
console.log(encrypted); // '-----BEGIN PGP MESSAGE ... END PGP MESSAGE-----'
const message = await openpgp.readMessage({
armoredMessage: encrypted // parse armored message
});
const { data: decrypted, signatures } = await openpgp.decrypt({
message,
verificationKeys: publicKey, // optional
decryptionKeys: privateKey
});
console.log(decrypted); // 'Hello, World!'
// check signature validity (signed messages only)
try {
await signatures[0].verified; // throws on invalid signature
console.log('Signature is valid');
} catch (e) {
throw new Error('Signature could not be verified: ' + e.message);
}
})();