در مورد امنیت این سرویس ها:
even though the crypto code may run on the client, all of these services are server-based at heart. If the server turns evil or is given a gagged subpoena, you have no way to know that the code they are sending to your client (never mind what code they publish on their open-source site) isn't pulling a fast one on you. They could be sending you code that does not encrypt securely, or that sniffs your passwords, or all of the above. And you'd never know
Deep down, they are compromising user security because of their need to make money
They use servers because by keeping your data (encrypted, no doubt), or your private key, or whatever, they'll be able to charge you at some point, especially if you want more storage or more features such as time-limited decryption à la Dmail or Virtru
http://qr.ae/ROn5i8