Is there some means by which I could derive symmetric keys using bitcoin public keys or HD Wallet public key which I have shared with a second user.

Is it possible in bitcoin to start a smart contract preloaded with some amount of money. And if some other user is ready to pay to the smart contract a certain amount of money, the smart contract should release the preloaded amount of money in the smart contract to the user who paid to it ?

Is there some way in which User A who could load a transaction onto the block chain and if some other User B who is willing to pay User A, without User A being online could release the loaded transaction. Something like a smart contract ?

Lets say if A and B wants to exchange some bitcoin transactions between each other. But they both dont know each other before. A list his address some where and when B sees it, B will issue a bitcoin transaction to A.

A might not be online when B has issued this transaction. But B should receive back a transaction from A, if B has payed to A's address. How's it possible ?

How do I formulate a Transaction with OP_RETURN to a particular address using Bitcoinj library.

What is the Best way to share the Master Public Key and Master Chain code generated using BIP32 in Bicoins with a user that I don't trust ?

I just want the second user to monitor my wallet and not spend coins. What will be the best way to share my Master Keys ?

Is there some kind of implementation in bitcoin to communicate between nodes, which is similar to the Whisper Communication in Ethereum ?

https://github.com/ethereum/wiki/wiki/Whisper

Is there some way in which I could get the public key of a Bitcoin user ?

Lets say If I have transaction from a Bitcoin user. Is there some means by which I could get the public key of a particular Bitcoin address ?

How does https://slock.it/ guys does it ? From where do they get the public key from ?

https://www.youtube.com/watch?v=-Ht23KXic1k

I have User A who generates BIP32 Master Public and Private Keys. It shares the Master Public Key with User B.

Using the Master Public Key User B generates Address of A and Issues bitcoins from Address of B to Address of A.

Will the transaction be received in wallet of User A, if User A knows the index used by User B. ?

The main idea is that User A don't want to share its receiving address with User B. Instead of that User A must find out itself from the Master Public Key.

Is it possible to share my extended public key to an another wallet using the Bitcoin network ?

Is there any issue in sharing my master public key with a second wallet ?

I am trying to built an application that uses the BIP-32. But I am not sure on how to use it.

I have a passphrase with which I generate the Derived Private Key and Derived Public Key. If I share my Derived Public Key with a second wallet will the wallet be able to generate others Derived Public Key from it ? Is it possible ?

And since I have the original Derived Private Key with me, can I see what is happening in the second wallet ?

Also if I loose both the Derived Private Key and Derived Public Key, will I be able to recover my wallets using only the Passphrase ?

I am trying to develop an application using bitcoinj library. I want to get the bitcoin transaction details.

The details, that I want to extract from a transaction includes the following.

1) Who send me the transaction (Address and Public Key of the sender).

2) The amount that I received.

3) When was the transaction issued.

4) Get the data stored in the transaction in OP_RETURN.

Thanks a lot. At last I found this one https://accounts.blockcypher.com/testnet-faucet this was the only one which was working.
I tested on some coins and send them back. Thanks anyways for the help. 

I am running the bitcoinj wallettemplate on testnet3. I have this address miAKWcmebJ7TYyZCYUx54djHsatVGdNZA8 , I tried out some websites which sends me testnet coins (Faucet). But I am not receiving any. ? What I am I doing wrong ?

I am working with the testnet. I downloaded the bitcoin-core application on Linux and I am running the version Bitcoin Core version v0.12.1.0-g9779e1e (64-bit) I was able to download the bitcoin block, recently its no longer syncing with the bitcoin network. Its showing 3 days behind and 0 active connection to the bitcoin network. How can I fix the issue. I want to connect to the testnet. I tried to change the network settings in the application. Still its not working.

Thanks again for the detailed answer. I understand how it works now.

Thanks 

I am quite new to Bitcoins. I have a doubt.

Lets say I person A, have 1 Bitcoin with me, I want to send .5 BTC to person B and .5 BTC to person C in a single transaction. Is it possible to do within one single transaction ?

Later on if Person C sends the same Bitcoin to Person B, will it be possible for person B to verify that, the Bitcoin which it now received from Person B and the one which it already received from Person A, were from the same source ?