This is an example of how to use Web Crypto API to encrypt/decrypt using AES algorithm.
The encrypt function takes a plain text and a passphrase and returns a base64 encoded cipher text encrypted with AES-GCM. The passphrase turns to a sha256 hash to be used as the key of encryption. You may want to use PBKDF2 instead.
The decrypt function takes a base64 encoded cipher text and a passphrase and returns the plain text decrypted by AES-GCM.