Q1a: Is the master key password created per DB instance?
A1a: Assuming the question really means "Is the master key created per DB?"" then answer is Yes. Each DB has an different master key. there is also a thing called the service master key, which is per SQL Server instance.
Q1b: When I backup that DB (.bak) will I be able to restore the DB to
another Server2?
A1b: Yes. The master key in the restored database can be open using the original password. Then the Server2 service master key encryption can be added to the restored DB master key.
Q2: Do I need to backup the certificate, master key or symmetric key
if I want to restore to another server?
A2: No. All these objects are part of the database and they are restored along with the database. Specific needs to backup individual keys may arise from operational requirements (eg. key escrow).
Q3: When should the symmetric key be opened?
A3: Keys that require opening have to be opened in the session. Once opened, they stay open until explicitly closed or until the session disconnects. An 'open' key is 'open' only in the session that opened it.
Q4: Should I worry about who can open and close the symmetric key? ...
A4: Now this is the real question. You have two alternatives really, which correspond to two distinct scenarios:
Scenario A: when the service needs to access encrypted data without asking the user for passwords to the data. This is the vast majority of the cases. In this case the service needs to open the keys somehow. the solution is that the SQL Server uses the service master key to encrypt the database master key and the database master key is used to encrypt the certificate's private key and the private key is used to encrypt a symmetric key and the symmetric key encrypts the data. The SQL Server itself can reach any data following this chain, because it has access to the service master key. In this case the data is cryptographically protected only against accidental media loss: a lost laptop with the database on it, or an improperly disposed HDD with database of backup files on it etc. For all other threats, the data is not cryptographically protected, is only protected by the access control: since the SQL Server itself can decrypt the data w/o needing a password from the user, any user with sufficient privileges can access the data w/o knowing the password. In other words, a compromised ASP application may allow access to the encrypted data. As a note, scenarios in which the root of encryption is some secret stored on the ASP web application itself are just a (badly designed) variation on this and do not change anything.
Scenario B: when the service requires the user to provide a password. The password must come from the end user. In a web application, the user must type the password in a form in the browser, it gets sent over SSL channel to the ASP application, which passes it to the SQL Session (again on an SSL channel) and SQL can now decrypt the data using this password. This scenario is typical for multi-tenant applications in which tenants provide the data access password. In this scenarios the data is cryptographically secured against unauthorized access, because the SQL Server itself, nor the intermediate ASP web application, simply do not know the password. Even for a syadmin user with all privileges it would be impossible to read the data. Data can be moved at will and remains just as unscrutable, as it can be, again, only be read by the end-user that has knowledge of the password at the root of the encryption chain. Note that any 'shortcut' deviation in this scenario in which the password is 'saved' somewhere intermediately and not provided by the end-user this scenario degrades immediately to the first Scenario A.
A mandatory read for you: Encryption Hierarchy.
Best Answer
There are five types of keys in the heirarchy of any SQL Server column encryption. They are service master key, database master key, asymmetric key, certificate and symmetric key. SQL Server contains commands to back up the service master key, database master key and the certificate with the private key to separate password protected files. If you want to have something to preserve an asymmetric key, you can create one from a strong named key file (.snk) that already contains the key pair and save that. For the symmetric key, you will need to create it from a command, specify the key_source and identity_value and save those two values in your Vault. IF you create symmetric and asymmetric keys without saving the inputs, then you will not be able to export them. If you want to learn more about another method of securing the keys, I suggest reading about Extensible Key Management systems, which is a SQL Server enterprise edition feature. An EKM is a separate piece of hardware designed to hold the keys and perform the encryption and decryption functions.