在用户之间安全共享客户端加密的数据

Question

I'm making a service that will store personal data written about a given user, that should be viewable only by the author and the target. I would like to perform the encryption client-side and store only the resulting garbage on the server.

The internet tells me to use , this will work well for a single users's data.

Problem

For my service, other users are able to enter data that should be viewable only by them and the target user - how might this be accomplished with client side encryption?

Is it possible to do this purely client side?

Update

After considering this, I doubt it can be done purely client side. I have come up with the following possible method:

All traffic uses SSL connection

• Generate a RSA keys, encrypt the private key with AES (using a salted passphrase)
  • Salt generated in-browser
• Store the passphrase salt, public key, encrypted private key on the server

• All user data encrypted with their public key, decrypted by having them first decrypt their private key in-browser by entering their passphrase, then using the RSA private key to decrypt the data

• If the user wishes to share data with another, encrypt said data with the other user's public key

Is my suggested method secure ?

Example of what would be stored on server:

+----------+---------------------+---------------+
|Public Key|Encrypted Private Key|Passphrase Salt|
+----------+---------------------+---------------+

Answer 1

It would be much easier for you to deal with OpenPGP. OpenPGP uses public-key cryptography (RSA or Elgamal are used for encryption), so the scheme would be similar to what you describe but without the hassle of reinventing the wheel.

OpenPGP implementations exist for different languages including JavaScript (although using Javascript way is bad due to various security weaknesses that can lead to information disclosure). Yet your RSA scheme implemented in JavaScript would be equally vulnerable due to the same conceptual weaknesses.

One of benefits of OpenPGP is that users are either already acquainted with OpenPGP or can find plenty of information about it (which adds trust to your solution), and can already have their own keys. Also users can use any OpenPGP tool to generate keys and to open encrypted data.