Add encryption

[vjeux]

This PR implements the following scheme:

Upload:

• The client generates a random secret (eg: 20 characters in base 64, we’ll need to figure out the right size).
• The client uses symmetric encryption to encode the json scene with the secret. (Encryption algorithm to be determined)
• The client uploads the encrypted data to the server (but not the secret)
• The server replies back the id
• The client generates a url that contains the id in the query param and the secret after the # (so it doesn’t get sent to the excalidraw.com server, but is available from locally ran javascript code). We may want to also put the id after the hash too.

Download:

• The client reads the id from the url
• The client queries the server for the id and receives the encrypted data
• The client reads the secret from the url
• The client decodes the json scene from the encrypted data and secret
• The client renders the scene

Fixes #610

[vercel]

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[lipis]

The backend supports storing plain text data in version 2: https://json.excalidraw.com

[dwelle]

Ad gzip: We could manually compress before encrypting (with some caveats).

[vjeux]

@lipis it seems like v2 is not whitelisted for CORS :(

Access to fetch at 'https://json.excalidraw.com/api/v2/post/' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
data.ts:130 POST https://json.excalidraw.com/api/v2/post/ net::ERR_FAILED

[vjeux]

@lipis if you want to test it out, I updated the PR.

Repro:

• Go to https://excalidraw-re7oki8g8.now.sh/
• draw a rectangle
• open the export menu
• click export
• See that it throws on const json = await response.json();
• If you change it to .text(), you'll see that the text response is empty
• On the network tool, the response says it's 200 OK and the response length is 91
  
• But can't show the response:
  

[lipis]

I got a response, by removing these two lines from the commit suggestions.

[vjeux]

The entire flow is working!

You can try with those two scenes:

• #json=5725470830100480,i21D-alTzlc_yTrPjRFVrg
• ?id=5746640992337920

[lipis]

The list of recently loaded scenes is broken though.. also when there is only one entry it doesn't reload it.

[vjeux]

Oops, I didn't test the recent list, thanks, I'll look into it

[vjeux]

I can't reproduce, it seems to be loading the scenes correctly for me.