ECC Encryption in WebCrypto is hard to implement

[Zhaoxun]

Hi I am on a project to receive an ECDH pubkey from backend in order to encrypt messages so that only the true backend can decrypted, for I have to use a proxy site to pass clients' message to my actual server. I have checked pages like https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/generateKey & /importKey & /exportKey &etc.

First thing first, generate a public key:

curve = { name: 'ECDH', namedCurve: 'P-256' };
//k = await crypto.subtle.generateKey(curve, true, ['deriveKey']);
//pub = new Uint8Array( await crypto.subtle.exportKey('raw', k.publicKey) );
crypto.subtle.generateKey(curve, true, ["deriveKey"] ).
 then( key=>crypto.subtle.exportKey('raw', key.publicKey) ).
  then( raw=> pub = new Uint8Array(raw) )

I do not know why the await syntax worked in browser console but failed when embedded in a web page.
Anyway, since the public key pub is produced, let me try to import it:

crypto.subtle.importKey( "raw", pub, curve, "true",['deriveKey'] ).
  then(key => console.log(key))

Then there is a DOM Exception:Cannot create a key using the specified key usages. I cannot use ['encrypt'] either. So does it mean that encryption through an ECC public key is Not Supported?