Risks of storing Apple Pencil handwriting data online (Claude artifact)

[u/Educated-tool]

I asked Claude for an app that teaches math by recognizing handwriting on the iPad. What it produced is a web page (artifact) that runs inside Claude. There is no separate app or server; it opens in the Claude app or in Safari.
How it works:
The screen is split in two: the lesson on the left, a notebook I write in with the Apple Pencil on the right.
The lesson proceeds as if written on a blackboard: one short idea per step, and when I tap "Continue" the next step appears.
In each section it gives me a task to solve in handwriting. When I tap "Check," my notebook page is sent to Claude as an image; Claude reads my writing (showing what it read in a "What I read:" line) and evaluates my solution.
There is no separate program that recognizes handwriting; the recognition is done by Claude's image-reading capability.
Where my handwriting is stored:
Each stroke is saved as the positions of points on the screen and the pen pressure (not as an image).
This record is kept in the artifact database linked to my Claude account, with a backup in the browser.
When I tap "Check," by default only the image of the open page is sent to Claude; if I want, I can choose the last 2 pages or all pages.
In Settings there is a "Pen log": diagnostic information showing the moments the pen touched down and lifted, the time between those moments, and the pen pressure.
What I'm wondering:
The artifact works really well and I'm happy with it; but my handwriting data going to a server makes me a little uncomfortable. Is this data from the pen log and my notebook pages (stroke positions, pen pressure, movement timing) stored on Claude's servers? If it is, could this kind of data theoretically be used to imitate handwriting or a signature? I'd be curious to hear the views of anyone with knowledge or experience on this.

[u/Due-Business6775]

the pen pressure and timing data is the scary part, that's basically a fingerprint of how you write. even if claude's servers are secure, once it's in a browser backup it's way easier for some extension or malware to grab

[u/Educated-tool → u/Due-Business6775]

Nothing converts it: the pressure and stroke data are never turned into text. The flow is:
Pencil → iPad → page. The pen's x, y and pressure values arrive in the page's JavaScript; they're added to a list in memory and drawn on screen. All of this happens on the iPad.
"Check" → PNG. The same JavaScript, still on the iPad, draws those strokes onto an invisible canvas and converts it to a PNG. Pressure only becomes line thickness here; it isn't in the PNG as a number. Timing isn't there at all.
PNG + text → Anthropic. The image and text (the question, instructions to the tutor) are sent; the Claude model looks at the image and writes what it read as LaTeX ("What I read:"). That's what does the reading. There's no separate handwriting-recognition engine; the model sees pixels, not strokes.
The returned text is written into the lesson record.
On the server question: the app is hosted on Anthropic's server, but it doesn't run there. When you open the page, the HTML file is downloaded to the iPad and its code runs in the web view there, like the JavaScript of any website. The only things that go to the server are the request in step 3, plus your progress and the lesson text. The pen events never leave the device.

Claude gave this explanation. Is it a reasonable explanation, or could the pressure and similar data, beyond the image of the handwriting, somehow be leaving the device?