Implementing encryption/decryption in Typescript using Web Crypto API

I'm having a security requirement not to send data as plain-text to the server and the credentials shouldn't be visible in the network tab of the browser. I could have used hashing but the catch is we already have old stored passwords with hash and salting applied. Now, not to disturb the old database, I was thinking to simply encrypt the data and decrypt it on server side (node).

Henceforth, I'm trying to use Web Crypto API to achieve the desired outcome. Below is a simple script I was trying to write before exporting the logic to both client and server environments :

function base64ToArrayBuffer(base64): ArrayBuffer {
    return Uint8Array.from(atob(base64), c => c.charCodeAt(0))
}

function ArrayBufferToBase64(buffer): string {
    return btoa(String.fromCharCode(...new Uint8Array(buffer)));
}

async function encryptString(privateKey: string, publicKey: string, data: string) {

    let params: AesGcmParams = {
        name: 'AES-GCM',
        iv: base64ToArrayBuffer(publicKey)
    }

    let keyImported: CryptoKey = await window.crypto.subtle.importKey("raw", base64ToArrayBuffer(privateKey), "AES-GCM", true, ["encrypt", "decrypt"]);
    let dataInstance: ArrayBuffer = new TextEncoder().encode(data);
    return Promise.resolve(await window.crypto.subtle.encrypt(params, keyImported, dataInstance));
}

async function decryptString(privateKey: string, publicKey: string, data: string) {

    let params: AesGcmParams = {
        name: 'AES-GCM',
        iv: base64ToArrayBuffer(publicKey)
    }

    let keyImported: CryptoKey = await window.crypto.subtle.importKey("raw", base64ToArrayBuffer(privateKey), "AES-GCM", true, ["encrypt", "decrypt"]);
    let dataInstance: ArrayBuffer = new TextEncoder().encode(data);
    return Promise.resolve(await window.crypto.subtle.decrypt(params, keyImported, dataInstance));
}

// Key for encryption/decryption process
let PRIVATE_KEY: string = 'Fdz2z54vAwIjq0tginHyzgyL62q+f4sBlMi5IOOXJ7c=';
let PUBLIC_KEY: string = btoa(String.fromCharCode(...window.crypto.getRandomValues(new Uint8Array(12))));

// Password to be encrypted before being sent to server
let password: string = "Test@123";

(async () => {

    let encrypted_data: string = ArrayBufferToBase64(await encryptString(PRIVATE_KEY, PUBLIC_KEY, password));
    console.log(encrypted_data);

    let decrypted_data: string = ArrayBufferToBase64(await decryptString(PRIVATE_KEY, PUBLIC_KEY, encrypted_data));
    console.log(decrypted_data);

})();

I'm able to encrypt the string stored by the password variable but not able to decrypt it.
Getting following error message :

Promise {<pending>}
VM98:33 FSWTBSgdQNVgYOelgRpU5JZm/4G0TRynIyr2jg==
VM98:36 Uncaught (in promise) Error
await (async)
(anonymous) @ VM98:36

Also, attaching screenshot for details not pasted under logs:

Need help to understand the root cause of this error alongwith the fix I can add to the script to successfully encrypt/decrypt some data.

Incase if someone's aware of a library that I can easily use on both client/server side, that would also be appreciated as an answer.

Below is the script that worked for me as per inputs provided from the community:

    function base64ToArrayBuffer(base64: string): ArrayBuffer {
        return Uint8Array.from(atob(base64), c => c.charCodeAt(0))
    }
    
    function ArrayBufferToBase64(buffer: ArrayBuffer): string {
        return btoa(String.fromCharCode(...new Uint8Array(buffer)));
    }
    
    async function encryptString(privateKey: string, publicKey: string, data: string) {
    
        let params: AesGcmParams = {
            name: 'AES-GCM',
            iv: base64ToArrayBuffer(publicKey)
        }
    
        let keyImported: CryptoKey = await window.crypto.subtle.importKey("raw", base64ToArrayBuffer(privateKey), "AES-GCM", true, ["encrypt", "decrypt"]);
        let dataInstance: ArrayBuffer = new TextEncoder().encode(data);
        return window.crypto.subtle.encrypt(params, keyImported, dataInstance);
    }
    
    async function decryptString(privateKey: string, publicKey: string, data: string) {
    
        let params: AesGcmParams = {
            name: 'AES-GCM',
            iv: base64ToArrayBuffer(publicKey)
        }
    
        let keyImported: CryptoKey = await window.crypto.subtle.importKey("raw", base64ToArrayBuffer(privateKey), "AES-GCM", true, ["encrypt", "decrypt"]);
        return window.crypto.subtle.decrypt(params, keyImported, base64ToArrayBuffer(data));
    }
    
    // Key for encryption/decryption process
    let PRIVATE_KEY: string = 'Fdz2z54vAwIjq0tginHyzgyL62q+f4sBlMi5IOOXJ7c=';
    let PUBLIC_KEY: string = btoa(String.fromCharCode(...window.crypto.getRandomValues(new Uint8Array(12))));
    
    // Password to be encrypted before being sent to server
    let password: string = "Test@123";
    
    (async () => {
    
        let encrypted_data: string = ArrayBufferToBase64(await encryptString(PRIVATE_KEY, PUBLIC_KEY, password));
        console.log(encrypted_data);
    
        let decrypted_data: any = new TextDecoder().decode(await decryptString(PRIVATE_KEY, PUBLIC_KEY, encrypted_data));
        console.log(decrypted_data);
    
    })();