enabling comments on an internally shared file

[Super-Baleine]

ref. muonium/server#13

Enabling comments on an internally shared file

1. generate LEK (social encryption key)
2. encrypt it with the user's public key, such as: LEK_enc=AsymEnc(PKB_u, LEK)
3. send it to the server

Allowing a user to comment on a file

1. user gets his PKB (public key)
2. user decrypts LEK with his PKB
3. user gets PKB of the user he wants to comment on the file with
4. user encrypts LEK under the user's PKB
5. user sends LEK_enc