If you want a minimal setup that does similar job with all encryption done in browser and absolute no plaintext, password send back to server, i just wrote https://www.relaysecret.com earlier this week. It is opensource, with minimal footprint on aws (1lambda, 1 gateway, 1 s3), deploy using terraform and should be very cheap to run. It's a pet project so i limit the filesize to 30mb but you can change that easily if you roll your own. :)