Question

I am trying to write a stored procedure to decrypt some data encrypted by a Symmetric Key that’s encrypted with an Asymmetric Key with a password.

OPEN SYMMETRIC KEY requires a string literal for the password so I had to do a work around with EXEC sp_executesql. Is there a better way to do this?

DECLARE @open nvarchar(200), @close nvarchar(200)
SET @open = 'OPEN SYMMETRIC KEY skey DECRYPTION BY ASYMMETRIC KEY akey WITH PASSWORD = ' + quotename(@password,'''') + ';';
SET @close = 'CLOSE SYMMETRIC KEY skey;';

EXEC sp_executesql @open

SELECT [TransactionID],Convert(varchar(max),DECRYPTBYKEY([EncryptedText])) as DecryptedText FROM [dbo].[TestTable];

EXEC sp_executesql @close

If you execute it with the wrong password it throws up the following errors:

Msg 15466, Level 16, State 1, Line 1
An error occurred during decryption.
Msg 15315, Level 16, State 1, Line 1
The key 'skey' is not open. Please open the key before using it.

Should I just wrap EXEC sp_executesql @open in a TRY ... CATCH and return NULL or is there a (more) elegant way of handling this?

EDIT: What’s the best way to handle someone calling this procedure with an incorrect password?

Answer 1

EXAMPLE of using keys, check your data and code:

CREATE ASYMMETRIC KEY akey WITH ALGORITHM = RSA_2048 
ENCRYPTION BY PASSWORD = 'aaa123'
GO
CREATE SYMMETRIC KEY skey WITH ALGORITHM = AES_256 ENCRYPTION BY ASYMMETRIC KEY akey
GO

DECLARE @t TABLE(plain VARCHAR(100), ciphered VARBINARY(MAX), unciphered VARCHAR(100))

INSERT @t(plain)
VALUES('11111'), ('22222'), ('33333')

OPEN SYMMETRIC KEY skey DECRYPTION BY ASYMMETRIC KEY akey WITH PASSWORD = 'aaa123'

UPDATE @t SET Ciphered = ENCRYPTBYKEY(KEY_GUID('skey'), plain)


UPDATE @t SET unciphered = CAST(DECRYPTBYKEY(ciphered) AS VARCHAR)

SELECT * FROM @t

CLOSE SYMMETRIC KEY skey
DROP SYMMETRIC KEY skey
DROP ASYMMETRIC KEY akey

For proper logging of wrong password decryption attempts try the following example, play with variable @password:

CREATE ASYMMETRIC KEY akey WITH ALGORITHM = RSA_2048 
ENCRYPTION BY PASSWORD = 'aaa123'
GO
CREATE SYMMETRIC KEY skey WITH ALGORITHM = AES_256 ENCRYPTION BY ASYMMETRIC KEY akey
GO

DECLARE @t TABLE(plain VARCHAR(100), ciphered VARBINARY(MAX), unciphered VARCHAR(100))

INSERT @t(plain)
VALUES('11111'), ('22222'), ('33333')

OPEN SYMMETRIC KEY skey DECRYPTION BY ASYMMETRIC KEY akey WITH PASSWORD = 'aaa123'

UPDATE @t SET Ciphered = ENCRYPTBYKEY(KEY_GUID('skey'), plain)
CLOSE SYMMETRIC KEY skey

DECLARE @open nvarchar(200), @close nvarchar(200), @password VARCHAR(20) = 'aaa123x'
SET @open = 'OPEN SYMMETRIC KEY skey DECRYPTION BY ASYMMETRIC KEY akey WITH PASSWORD = ' + quotename(@password,'''') + ';';
SET @close = 'CLOSE SYMMETRIC KEY skey;';
BEGIN TRY 
  EXEC sp_executesql @open
  UPDATE @t SET unciphered = CAST(DECRYPTBYKEY(ciphered) AS VARCHAR)
  SELECT * FROM @t
  EXEC sp_executesql @close
END TRY BEGIN CATCH 
  SELECT 'Do whatever you want to do here with this caller. Suspicious caller: '+SUSER_SNAME()+', at: '+CAST(GETDATE() AS VARCHAR)
END CATCH

DROP SYMMETRIC KEY skey
DROP ASYMMETRIC KEY akey