IV re-use and multiple calls to `encrypt`

[fabrice102]

If encrypt may be called several times for the same key, the IV should not be derived from the the key:

WebCrypto2Example/index.js

Line 23 in bf474ee

const iv = derivation.slice(keylen);

Instead, it should be generated uniformly at random and prepended to the ciphertext. Re-using IV in AES-CBC modes allows to know if the same data has been encrypted twice or not. See https://security.stackexchange.com/a/1097