I known the post from here and here regarding RSA padding. I still don't understand why simple addition of the message with an random number and then encrypting the result with plain RSA isn't secure.
Here's a more precise description of the scheme:
Suppose the message is $m$. Then suppose we draw a random integer $r$ and compute $m'=m+r\bmod n$ and feed $m'$ to the RSA function.
At a later time I will recover the encrypted message from the server and I'll decrypt it myself. I known that RSA isn't best scheme for this application. Is the proposed padding secure in this context?