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Encrypting and Decrypting SQL Server Stored Procedures, Views and User-Defined Functions


By:   |   Read Comments (11)   |   Related Tips: 1 | 2 | More > Encryption

Problem

You work in a shop that puts business or application logic in the SQL Server using stored procedures, views and functions to return values to the calling applications or perform tasks. This is not unusual in companies that use the SQL Server layer to perform business tasks, such as finance operations, or incorporate application functionality into the programmability layer. You wish to preserve secrecy on some procedures, views or functions in order to maintain security.

Solution

SQL Server stored procedures, views and functions are able to use the WITH ENCRYPTION option to disguise the contents of a particular procedure or function from discovery. The contents are not able to be scripted using conventional means in SQL Server Management Studio; nor do the definitions appear in the definition column of sys.sql_modules. This allows the cautious DBA to keep stored procedures and functions securely in source control and protecting the intellectual property contained therein. This tip will focus on encrypting and decrypting a user-defined function.

Encrypting a UDF

To encrypt a user-defined function, simply add WITH ENCRYPTION to the CREATE FUNCTION statement (after the RETURNS element). Throughout this tip, I will be building an encrypted UDF (and decrypting it) to demonstrate the principle.

First, create the UDF. Here's mine - it's a simple module that accepts an input string and returns the encrypted varbinary hash value. There are two vital pieces of information here that MUST NOT be given away to the user of the function - the encryption standard, and the salt.

CREATE FUNCTION dbo.getHash ( @inputString VARCHAR(20) )
RETURNS VARBINARY(8000)
AS BEGIN
    DECLARE @salt VARCHAR(32)    
 DECLARE @outputHash VARBINARY(8000)
 SET @salt = '9CE08BE9AB824EEF8ABDF4EBCC8ADB19'
 SET @outputHash = HASHBYTES('SHA2_256', (@inputString + @salt))
RETURN @outputHash
END
GO

In this example the salt is fixed and an attacker, given the encryption standard (SHA-256) and the salt, could be able to decrypt the hash into plaintext. We can view the definition of a function by finding it in SQL Server Management Studio, right-clicking and scripting out the function:


The attacker can also use sp_helptext, or query sys.sql_modules if he/she knows the function name:

SELECT definition
FROM sys.sql_modules 
WHERE definition LIKE ('%getHash%')

There is some protection built in; by using role-based security or sensibly allowing the least required privileges to users, the attack surface can be lessened as the VIEW DEFINITION permission is required (or ownership of the function) in SQL Server 2005 upwards. Note in earlier versions, this permission is not required. The user may also have the permission granted implicitly by holding other permissions on the object. (See 'Metadata Visibility Configuration' at http://msdn.microsoft.com/en-GB/library/ms187113.aspx for more detail).

We can amend the function definition like so:

ALTER FUNCTION dbo.getHash ( @inputString VARCHAR(20) )
RETURNS VARBINARY(8000) WITH ENCRYPTION 
-- rest of function here

Note that WITH ENCRYPTION occurs after RETURNS, not before. With stored procedures, the WITH ENCRYPTION option occurs immediately after the CREATE PROCEDURE x ( @somevar) statement.

With our encrypted function we can attempt to script it out in SQL Server Management Studio again, or look at sys.sql_modules. Here's what we get:



Querying sys.sql_modules definition column for this function returns NULL. Executing sp_helptext returns the error:

The text for object 'dbo.getHash' is encrypted.

Note the UDF is exactly as effective as it was before - we can still call it and return a value:



I'm using an undocumented stored procedure here called fn_varbintohex to convert the VARBINARY output from my function into a hexadecimal format, for portability between applications and clarity - it's not directly relevant to this example. Normally the VARBINARY output of HASHBYTES is passed directly to the calling application.


Decrypting a Function

Firstly, open a Dedicated Administrator Connection (DAC) to SQL Server. Using SQL Server Management Studio, this is easily done by prefixing admin: to the connection string upon connection of a query window. Note that the DAC can only be used if you are logged onto the server and using a client on that server, and if you hold the sysadmin role. You can also get to it by using SQLCMD with the -A option. Note: DAC won't work unless you're using TCP/IP; you'll get this rather cryptic error (in both SQLCMD and SSMS):

Sqlcmd: Error: Microsoft SQL Server Native Client 11.0 : Client unable to establish 
connection because an error was encountered during handshakes before login.  Common 
causes include client attempting to connect to an unsupported version of SQL Server, 
server too busy to accept new connections or a resource limitation (memory or 
maximum allowed connections) on the server.

If you can't access the server directly for whatever reason, you can enable remote administrative connections (a remote DAC) as follows. You'll still need to use TCP/IP:

exec sp_configure 'show advanced options', 1
RECONFIGURE WITH OVERRIDE
exec sp_configure 'remote admin connections', 1
RECONFIGURE WITH OVERRIDE

The procedure for decryption comes in three steps. First, get the encrypted value of the procedure definition from sys.sysobjvalues (via the DAC connection). Secondly, get the encrypted value of a 'blank' procedure, where the definition is filled in by '-'. Thirdly, get the plaintext blank procedure statement(unencrypted). Now XOR them together (XOR is the simplest of decryption procedures and forms the basis of many algorithms including MD5) as shown below to retrieve the output of the procedure. You'll find my take on this algorithm in the code example below:

-- Connect using the DAC then execute the below

SET NOCOUNT ON
GO

ALTER PROCEDURE dbo.TestDecryption WITH ENCRYPTION AS
BEGIN
 PRINT 'This text is going to be decrypted'
END 
GO

DECLARE @encrypted NVARCHAR(MAX)
SET @encrypted = ( 
 SELECT imageval 
 FROM sys.sysobjvalues
 WHERE OBJECT_NAME(objid) = 'TestDecryption' )
DECLARE @encryptedLength INT
SET @encryptedLength = DATALENGTH(@encrypted) / 2

DECLARE @procedureHeader NVARCHAR(MAX)
SET @procedureHeader = N'ALTER PROCEDURE dbo.TestDecryption WITH ENCRYPTION AS '
SET @procedureHeader = @procedureHeader + REPLICATE(N'-',(@encryptedLength - LEN(@procedureHeader)))
EXEC sp_executesql @procedureHeader
DECLARE @blankEncrypted NVARCHAR(MAX)
SET @blankEncrypted = ( 
 SELECT imageval 
 FROM sys.sysobjvalues
 WHERE OBJECT_NAME(objid) = 'TestDecryption' )

SET @procedureHeader = N'CREATE PROCEDURE dbo.TestDecryption WITH ENCRYPTION AS '
SET @procedureHeader = @procedureHeader + REPLICATE(N'-',(@encryptedLength - LEN(@procedureHeader)))

DECLARE @cnt SMALLINT
DECLARE @decryptedChar NCHAR(1)
DECLARE @decryptedMessage NVARCHAR(MAX)
SET @decryptedMessage = ''
SET @cnt = 1
WHILE @cnt <> @encryptedLength
BEGIN
  SET @decryptedChar = 
      NCHAR(
        UNICODE(SUBSTRING(
           @encrypted, @cnt, 1)) ^
        UNICODE(SUBSTRING(
           @procedureHeader, @cnt, 1)) ^
        UNICODE(SUBSTRING(
           @blankEncrypted, @cnt, 1))
     )
  SET @decryptedMessage = @decryptedMessage + @decryptedChar
 SET @cnt = @cnt + 1
END
SELECT @decryptedMessage

If you still cannot access via DAC or prefer a code-based approach, then you can use one of a number of freeware third-party .NET-based decryption software packages to do this for you. The blog Sqljunkieshare also purports to have a method of doing this (code untested) that looks viable - you can find the link here:

http://sqljunkieshare.com/2012/03/07/decrypting-encrypted-stored-procedures-views-functions-in-sql-server-20052008-r2/ (use at your own risk)

Next Steps


Last Update:






About the author
MSSQLTips author Derek Colley Derek Colley is a UK-based DBA and BI Developer with more than a decade of experience working with SQL Server, Oracle and MySQL.

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Thursday, April 27, 2017 - 10:21:09 AM - Greg Robidoux Back To Top

Hi Brenda,

take a look at this tip for information on the DAC - https://www.mssqltips.com/sqlservertip/1801/enable-sql-server-dedicated-administrator-connection/

Thanks
Greg


Thursday, April 27, 2017 - 9:36:41 AM - brenda Back To Top

 How do you do this?

Firstly, open a Dedicated Administrator Connection (DAC) to SQL Server. Using SQL Server Management Studio, this is easily done by prefixing admin: to the connection string upon connection of a query window. Note that the DAC can only be used if you are logged onto the server and using a client on that server, and if you hold the sysadmin role. You can also get to it by using SQLCMD with the -A option. Note: DAC won't work unless you're using TCP/IP; you'll get this rather cryptic error (in both SQLCMD and SSMS):


Tuesday, July 19, 2016 - 7:14:29 AM - Mizanur Rahman Back To Top

 Dear Author,

 

i have some inportant stored procedure code in my sql server database 2012. i need very badly to lock the code or encryted those code.Even i cal also not to able to view the code even not SA user. how i can do this. please assist me.

Remember, if i do encryption the code, then very easy to decryption the code, i dont want this.

Regards

Mizan

 


Wednesday, August 05, 2015 - 12:49:03 PM - JC Back To Top

Thank you for the great tips.

Have one question regarding the decryption though.

I came acrossed few stored procedures that were encrypted by previous developers that were no longer here by adding 'with encryption' in the store procedures.

Is there a way to decrypt the stored procedures at all? (without any keys or some sort).

 

Thank you.

 


Saturday, July 25, 2015 - 8:00:19 PM - Peter Back To Top

Script for decrypting function works great!  However, can you modify the code to ROLLBACK the transaction so that the stored procedure is reverted back to normal?  Otherwise after running the code I'm left with

CREATE PROCEDURE dbo.procedure WITH ENCRYPTION AS ----------------

I can just copy the decrypted code and paste it again to ALTER it to get it back but it's an extra step.

Thanks!


Saturday, July 05, 2014 - 4:05:12 AM - satya thakur Back To Top

 

THANKS FOR HELPING USER LIKE US.............


Wednesday, March 12, 2014 - 9:58:52 AM - Jason Back To Top

Brilliant. I was using Theo Ekelmans' sp since many years ago. It worked well.


Friday, July 12, 2013 - 12:17:00 AM - suryam raju Back To Top

Dear sir / Madam,

 

                  Good Morning...

   I have some doubts and please clarify me as early as possible why because working as a database developer in a   private limited company.

Questions: 

1. What is the Use of co - related subqueries.

2. Can i Write one trigger on all datbase tables, If yes how can i do that.

3. Use of with check option in creating views.

 


Tuesday, June 04, 2013 - 10:07:50 AM - Derek Colley Back To Top

*** TIP UPDATED *** I have written a correction to the original tip as per the comments above.  Apologies for any confusion and thanks to James for pointing out the omissions.


Thursday, May 30, 2013 - 9:51:53 AM - Derek Colley Back To Top

@James Lean - Thanks for your comments.  I've re-reviewed what I've written about the DAC and I'm a bit confused to be honest, as I cannot now replicate the last steps in the article above to read the decrypted data.  I also get NULL values, same as you.  Looking elsewhere,  I can see details of the decryption algorithm used (XORs character-by-character of the cryptotext to get the plaintext) via the DAC but no evidence the DAC itself is the key.

If this is the case then the final points I made about using the DAC are incorrect and I apologise for this (I'm also baffled as to how I got the plaintext out in my screenshot!).  I would highly recommend the StackOverflow link above for details of the algorithm required for decryption.

 


Thursday, May 30, 2013 - 5:33:36 AM - James Lean Back To Top

I may be missing something, but I can't get this to work just by using the DAC to view the encrypted definition.  I just get a NULL definition, same as if I use a normal connection (I've tried this on SQL2005 and 2012 just in case the behaviour changed).

To be honest, I would be a bit worried if it *was* this easy to view an encrypted definition.  I know it is fairly trivial to use third-party tools anyway, but at least you have to go to a little bit of effort that way ;-)


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