Please help me create this decryption function for XChaCha20-Poly1305

[2820402]

I am developing a service in Next.js that handles both file encryption and decryption using xchacha20-poly1305. While I have successfully implemented the encryption code, I am facing challenges with the decryption code. Could you please provide guidance on the most suitable decryption code for this encryption function? Additionally, I was also taking password as input from user

I am utilizing service workers to encrypt files in the browser, ensuring that it does not impact the main thread.

const [file, setFile] = useState();
const [password, setPassword] = useState();
navigator.serviceWorker.ready.then((reg) => {
    if (!reg || !reg.active) {
        setIsEncrypting(false);
        toast.error('Service worker is not ready or its not supported in your browser');
        return;
    }
    reg.active.postMessage({
        cmd: 'encryptFile',
            file,
            password
    });
});

I am using libsodium-wrappers-sumo library foe encryption

service-worker.js

self.addEventListener('install', (event) =>
	event.waitUntil(self.skipWaiting())
);

self.addEventListener('activate', (event) =>
	event.waitUntil(self.clients.claim())
);

const _sodium = require('libsodium-wrappers-sumo');
const STATIC_SIGNATURE = 'Encrypted By XXXXXXX';

(async () => {
	await _sodium.ready;
	const sodium = _sodium;

	addEventListener('message', async (e) => {
		switch (e.data.cmd) {
			case 'encryptFile':
				const startTime = performance.now();
				const { encryptedBlob, encryptedFileName } = await encryptFile(
					e.data.file,
					e.data.password
				);

				e.source.postMessage({
					reply: 'encryptionFinished',
					encryptedBlob,
					encryptedFileName,
				});
				break;
		}
	});

	const encryptFile = async (file, password) => {
        // Generate encryption key
        const salt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
        const key = sodium.crypto_pwhash(
            sodium.crypto_secretstream_xchacha20poly1305_KEYBYTES,
            sodium.from_string(password),
            salt,
            sodium.crypto_pwhash_OPSLIMIT_INTERACTIVE,
            sodium.crypto_pwhash_MEMLIMIT_INTERACTIVE,
            sodium.crypto_pwhash_ALG_ARGON2ID13
        );

        // Initialize encryption
        const { state, header } =
            sodium.crypto_secretstream_xchacha20poly1305_init_push(key);

        // Create a stream controller for chunked processing
        const streamController = new TransformStream();
        const writer = streamController.writable.getWriter();

        // Write signature, salt, and header to the stream
        const signature = sodium.from_string(STATIC_SIGNATURE);
        writer.write(signature);
        writer.write(salt);
        writer.write(header);

        // Encrypt file in chunks
        const chunkSize = 64 * 1024 * 1024;
        const reader = file.stream().getReader();

        while (true) {
            const { done, value } = await reader.read();

            if (done) {
                // Finalize encryption and close the stream
                const encryptedChunk =
                    sodium.crypto_secretstream_xchacha20poly1305_push(
                        state,
                        new Uint8Array(0),
                        null,
                        sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL
                    );
                writer.write(encryptedChunk);
                writer.close();

                // Get the encrypted file blob
                const encryptedBlob = await new Response(
                    streamController.readable
                ).blob();

                // send the encrypted file with the original filename + '.enc'
                const encryptedFileName = `${file.name}.enc`;
                return { encryptedBlob, encryptedFileName };
            }

            // Use chunkSize to control the size of each chunk
            for (let i = 0; i < value.length; i += chunkSize) {
                const chunk = value.slice(i, i + chunkSize);
                const encryptedChunk =
                    sodium.crypto_secretstream_xchacha20poly1305_push(
                        state,
                        new Uint8Array(chunk),
                        null,
                        sodium.crypto_secretstream_xchacha20poly1305_TAG_MESSAGE
                    );
                writer.write(encryptedChunk);
            }
        }
    };

})();

Now, I need a decryptFile function to first check the signature. It should verify if the file is encrypted by the same platform. After that, it should check whether the password provided by the user is correct to decrypt the file. Following that, the decoding process will occur, decrypting the file chunk by chunk. Finally, the function should return the decryptedBlob similar to how I implemented it in the encryptFile function, using a const chunkSize = 64 * 1024 * 1024; and also change the file name to .enc to non .enc

self.addEventListener('install', (event) =>
	event.waitUntil(self.skipWaiting())
);

self.addEventListener('activate', (event) =>
	event.waitUntil(self.clients.claim())
);

const _sodium = require('libsodium-wrappers-sumo');
const STATIC_SIGNATURE = 'Encrypted By XXXXXXX';

(async () => {
	await _sodium.ready;
	const sodium = _sodium;

	addEventListener('message', async (e) => {
		switch (e.data.cmd) {
			case 'encryptFile':
				...
				break;
			case 'decryptFile':
				const {decryptedBlob, decryptedFileName} = await decryptFile(e.data.encFile, e.data.password);
				e.source.postMessage({
                    reply: 'decryptionFinished',
                    decryptedBlob,
                    decryptedFileName
                });
                break;
		}
	});

	const encryptFile = async (file, password) => {
        ...
    };

	const decryptFile = async (encFile, password) => {
	    ... // help me to write this function
    };

})();

I am new to web cryptography and am currently reading the documentation for libsodium. However, I can't seem to find a solution. Please help me write the decryptFile function.

Also, if you could suggest any changes to the current code, that would be most welcome. Please provide recommendations for better performance and reliability.