Simple Implementation of SQL Server 2008 Encryption

Question

I am looking at using some of SQL Server 2008's encryption.

I have read the following article on MSDN:

• http://msdn.microsoft.com/en-us/library/ms179331.aspx

Q1a: Is the master key password created per DB instance?

IF NOT EXISTS 
    (SELECT * FROM sys.symmetric_keys WHERE symmetric_key_id = 101)
    CREATE MASTER KEY ENCRYPTION BY 
    PASSWORD = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
GO

Q1b: If I create a master key password on Server1 and encrypt a column on a table. When I backup that DB (.bak) will I be able to restore the DB to another Server2 or will I have to create a master key on Server2 with the same password?

Q2: Do I need to backup the certificate, master key or symmetric key if I want to restore to another server?

CREATE CERTIFICATE HumanResources037
   WITH SUBJECT = 'Employee Social Security Numbers';
GO

CREATE SYMMETRIC KEY SSN_Key_01
    WITH ALGORITHM = AES_256
    ENCRYPTION BY CERTIFICATE HumanResources037;
GO

Q3: When should the symmetric key be opened? Should it be done before each select and then closed? i.e. opened at the start of a stored procedure and closed afterwards.

-- Open the symmetric key with which to encrypt the data.
OPEN SYMMETRIC KEY SSN_Key_01
   DECRYPTION BY CERTIFICATE HumanResources037;

Q4: Should I worry about who can open and close the symmetric key? I use SQL Server 2008 for ASP.NET web applications. The DB's are hosted on Windows Server 2008 VPS's. I generally create one DB user of type SysAdmin and supply the connection details from the web.config file. I would be the only person with access to the VPS so I do not need to factor in other developers accessing the DB. However, if the VPS/Windows security was breached and an attacker gained access to the VPS and/or DB. Could they compromise my DB and information?

Answer 1

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.