Web Cryptography API + ECDH + AES

index.js

(async () => {
  // Generate Keys
  const pro1 = new Protector();
  await pro1.generateKey();
  const pub1 = pro1.getPublicKey();

  const pro2 = new Protector();
  await pro2.generateKey();
  const pub2 = pro2.getPublicKey();

  // Exchange Keys
  await pro1.setRemotePublicKey(pub2);
  await pro2.setRemotePublicKey(pub1);

  // Let`s Encrypt
  const crypted = await pro1.encrypt('Hello World');
  const descrypted = await pro2.decrypt(crypted);
})();

Protector.js

class Protector {
  ab2str(buffer) {
    return new TextDecoder().decode(buffer);
  }

  str2ab(text) {
    return new TextEncoder().encode(text);
  }

  generateIv() {
    return crypto.getRandomValues(new Uint8Array(16));
  }

  /**
   * @see https://github.com/mdn/dom-examples/blob/master/web-crypto/derive-bits/ecdh.js
   */
  async generateKey() {
    this.key = await window.crypto.subtle.generateKey(
      { name: 'ECDH', namedCurve: 'P-256' },
      false,
      ['deriveBits']
    );
  }

  async encrypt(plaintext) {
    const counter = this.generateIv();
    const buffer = await crypto.subtle.decrypt({
      name: 'aes-ctr',
      counter: counter,
      length: 128
    }, this.importedKey, this.str2ab(plaintext));
    return { buffer, counter };
  }

  async decrypt(data) {
    const buffer = await crypto.subtle.decrypt({
      name: 'aes-ctr',
      counter: data.counter,
      length: 128
    }, this.importedKey, data.buffer);
    return this.ab2str(buffer);
  }

  getPublicKey() {
    return {publicKey: this.key.publicKey};
  }

  async setRemotePublicKey(key) {
    this.clientKey = key;
    
    this.sharedSecret = await window.crypto.subtle.deriveBits(
      { name: 'ECDH', namedCurve: 'P-256', public: this.clientKey.publicKey },
      this.key.privateKey,
      256
    );

    this.importedKey = await crypto.subtle.importKey(
      'raw',
      this.sharedSecret,
      'aes-ctr',
      false,
      ['encrypt', 'decrypt']
    );
  }
}