How to store private encrypted user data in the database, but make them available to other chosen users?

Question

firstly, I apologize if my question sounds little confusing, I will try my best to describe my scenario as detailed as possible:

I have website where user can input

Answer 1

This can be solved using public-key cryptography:

1. Generate a public/private key pair for each user; and only ever decrypt the private key temporarily with the user's password.
2. For each data item, randomly choose a (symmetric) key S and encrypt the data d with it. Store S(d).
3. Encrypt S with the the public key P⁺_u of the user you want to grant access. Initially, that's the user u whose data you're storing.
4. Store P⁺_u(S) permanently. Forget all other keys.

Now, when a user u wants to share the data with the user x, do the following:

1. Decrypt the user's private key P^-_u with the user's password.
2. Using that private key, decrypt the stored data: P^-_u(P⁺_u(S)) = S.
3. Encrypt S with the public key of the user you want to share the information with.
4. Store the resulting P⁺_x(S) permanently. Forget all other keys.

Now, when any user x wants to access the data, perform the following process:

1. Decrypt the user's private key P^-_x with the user's password.
2. Find P⁺_x(S). (If it's not stored, that means nobody shared the data with the poor user x).
3. Using the private key, decrypt the stored data: P^-_x(P⁺_x(S)) = S.
4. Using S, decrypt the stored encrypted S(d): S(S(d)) = d.