My plan is to develop a mini-wallet that can be used for the sole purpose of notarized transactions. The idea is that buyer, seller and notary would use this mini-wallet to create three new public private key pairs. Then they would use some feature of this wallet to securely exchange their public keys. The risk here is that somebody tries to be an impostor (seller faking being the notary as well...etc). The buyer would then create a multi signature address and transfer the bitcoins into it. The transaction would then be broadcast to the network. The seller and notary could then verify that there is indeed a multisigaddress with the correct redeemScript. Finally two of the three will decide to release the funds into some random address. Either of the three will get their mini-wallet populated with the original amount of funds and if they are wise they would put those funds into a more secure wallet.

My problem is that for this wallet to make sense it should not require a local blockchain. So it would either work like Electrum or it would have SPV functionality like bitcoinj.

I was actually giving a look at bitcoinj but this tidbit under Limitations put me off:

P2SH (pay to script hash) is not supported by the wallet. Such transactions will be ignored. Use regular outputs if you want your payments to be noticed by bitcoinj wallets. P2SH is processed correctly by full verification mode.

I would like to know if my vision is possible using bitcoinj from the man himself if you please.

Is there such a thing if that is at all possible?

Can anyone answer me this?

Do you need to download the whole blockchain to make use of advanced scripting features of Bitcoin? I'm talking about m-of-n addresses mostly.

I don't know if it is a functionality specific to Bitcoin or not but in case it isn't, which other altcoins have working implementations of m-of-n transactions and which could later support it, in theory?

I am trying to brainstorm ways of preventing crackers from gaining access to my website
and changing my bitcoin address to one of theirs. Of course all of this is just in the
eventuality that my website gets compromised. I like to be prepared.

I am thinking of creating a new public-private key pair and signing up with a CA in
the same way I would do if I was setting up https functionality. Then I would create a list of
bitcoin addresses, and encrypt all of them with my new private key and uploading them to
the server. The users can retrieve the certificate from the CA, get my public key, and decrypt
the address. How is this different from normal SSL/HTTPS? The difference is that private key
will not be hosted on the server.

Can you comment on this strategy, especially if it is fatally flawed somehow?

As for user functionality, I'm thinking of writing a userscript to do the decrypting and
verifying if there isn't already a viable add-on for Firefox and Chrome.