HD wallets as described <a href="https://en.bitcoin.it/wiki/Deterministic_wallet">here</a> are built based on a master seed and a master public key as follows:

    PK.b = PK.a + sG
    SK.b = SK.a + s

 1. I just got a little confused about this. Is `s` the mnemonic seed that
    user knows? which generates the private keys. 
 2. Secondly a service
    provider only needs to know `sG` to calculate the user's next address.
    Am I right? 
 3. And lastly more importantly I am just wondering whether it is possible to
    calculate the master private key (seed) if an attacker or service provider gets one child's private key address?