the problem is where should the client store encryption keys?if one has multiple client on multiple devices,how one does manage all of them?(that's just a question,thanks).
A key generated from a username/passphrase will work fine. Generally you'd use a key derivation function (eg PBKDF2) which turns your auth information into a 32-byte (256 bit) cryptographic "master key." Assuming you can remember your passphrase, then you can easily re-generate your key when you "log in" from any machine.