Simple code to encrypt/decrypt with shared key

enc.js

function ab2str(buf) {
  return String.fromCharCode.apply(null, new Uint16Array(buf));
}

function str2ab(str) {
  var buf = new ArrayBuffer(str.length * 2); // 2 bytes for each char
  var bufView = new Uint16Array(buf);
  for (var i = 0, strLen = str.length; i < strLen; i++) {
    bufView[i] = str.charCodeAt(i);
  }
  return buf;
}

async function generateKey() {
  const key = await window.crypto.subtle.generateKey({
    name: 'AES-GCM',
    length: 256
  }, true, ['encrypt', 'decrypt']);
  const a = await crypto.subtle.exportKey('jwk', key);
  console.log(btoa(JSON.stringify(a)));
  return a;
}

/* generateKey(); */


async function getKeyFromString(keyStr) {
  const key = JSON.parse(atob(keyStr))
  return await crypto.subtle.importKey('jwk', key, {
    name: 'AES-GCM',
    length: 256
  }, true, ['encrypt', 'decrypt']);
}

async function encrypt(text, key) {
  const vector = crypto.getRandomValues(new Uint8Array(16));
  const data = new TextEncoder('UTF-8').encode(text);
  const enc = await crypto.subtle.encrypt({
    name: 'AES-GCM',
    iv: vector
  }, key, data);
  const cipherText = new Uint8Array(enc);
  return encodeURIComponent(btoa(ab2str(vector) + ab2str(cipherText)));
}

async function decrypt(data, key) {
  const binaryData = atob(decodeURIComponent(data));
  const vector = new Uint8Array(new Uint16Array(str2ab(binaryData.slice(0, 16))));
  const buffer = new Uint8Array(new Uint16Array(str2ab(binaryData.slice(16))));
  const decoded = await crypto.subtle.decrypt({
    name: 'AES-GCM',
    iv: vector
  }, key, buffer);
  return new TextDecoder("UTF-8").decode(decoded);
}

const keyStr = 'eyJhbGciOiJBMjU2R0NNIiwiZXh0Ijp0cnVlLCJrIjoiQ3RDTjVXOGY4elcyN1BwTWRtaGJJVkl0ajVJWERtU3hwYzRNeWlKSGpISSIsImtleV9vcHMiOlsiZW5jcnlwdCIsImRlY3J5cHQiXSwia3R5Ijoib2N0In0=';

getKeyFromString(keyStr).then((key) => {
  encrypt('rodrigo', key)
    .then(async (data) => console.log(await decrypt(data, key)));
})