I spent some time Googling this about 6 months ago. Lots of tutorials on Crypto-js, not so many (and almost zero "here's a super simple implementation") for WebCrypto API. I can understand if this is a hobby project why you'd lean into one rather than the other, I probably would have done the same.
Not going to pretend that I know what the most of the stuff mean, or if it is even safe enough, but I've followed the MDN articles and put together this TypeScript snippet [1]. Maybe somebody could comment on it?
Also sorry for the long link. Is there any accepted way to post a shorted URL?
If you call "encrypt" more than once in that code, you'll leak the authentication key. Every invocation of GCM encryption needs a unique nonce. Cryptography nerds will chastise you for using a random nonce (there theoretically isn't enough room in the GCM nonce space to safely encrypt large numbers of message with random nonces), but the alternative (using a counter) is even more hazardous. This problem motivates a lot of people to use other AEADs like XChapoly, which has an extended nonce space that safely admits random nonces. Isn't cryptography fun?
It's still considered safe to call AES-GCM with a single key and a randomly generated nonce 2^32 times [1], most practical systems don't come anywhere near this limit. And there's AES-GCM-SIV that solves nonce reuse (mostly), though it's not available in the Web Cryptography API at the moment.
I don't personally care, and probably wouldn't even sev:info a random GCM nonce in an assessment, but I would also choose extended-nonce Chapoly in preference to GCM in part because of this issue.
Oh yes! Thank you for the feedback.
I've added a new version where the `iv` and the `salt` is random.
Maybe a followup question: Because you need both the `iv` and `salt` to decrypt the message is it ok in an E2E scenario to send all three: `iv`, `salt` and the encrypted message?
I didn't look to see what "salt" means in your design, but the idiom for using GCM in message encryption is to send ciphertexts that take the form `nonce || ciphertext`, and to decrypt by reading the nonce off the front of the message.
I don't know what "secure" means. Is their implementation of OFB correct? Probably. But using OFB mode is itself a problem. From what I can see, crypto-js implements no authenticated modes, and exposes all sorts of crufty old things nobody should be using. The parent comment suggesting WebCrypto is correct in this case. Avoid crypto-js.
