Changes the properties of a symmetric key.
Applies to: SQL Server (SQL Server 2008 through current version).
ALTER SYMMETRIC KEY Key_name <alter_option> <alter_option> ::= ADD ENCRYPTION BY <encrypting_mechanism> [ , ... n ] | DROP ENCRYPTION BY <encrypting_mechanism> [ , ... n ] <encrypting_mechanism> ::= CERTIFICATE certificate_name | PASSWORD = 'password' | SYMMETRIC KEY Symmetric_Key_Name | ASYMMETRIC KEY Asym_Key_Name
When a symmetric key is encrypted with a password instead of with the public key of the database master key, the TRIPLE_DES encryption algorithm is used. Because of this, keys that are created with a strong encryption algorithm, such as AES, are themselves secured by a weaker algorithm.
To change the encryption of the symmetric key, use the ADD ENCRYPTION and DROP ENCRYPTION phrases. It is never possible for a key to be entirely without encryption. For this reason, the best practice is to add the new form of encryption before removing the old form of encryption.
To change the owner of a symmetric key, use ALTER AUTHORIZATION.
The RC4 algorithm is only supported for backward compatibility. New material can only be encrypted using RC4 or RC4_128 when the database is in compatibility level 90 or 100. (Not recommended.) Use a newer algorithm such as one of the AES algorithms instead. In SQL Server 2012 material encrypted using RC4 or RC4_128 can be decrypted in any compatibility level.
Requires ALTER permission on the symmetric key. If adding encryption by a certificate or asymmetric key, requires VIEW DEFINITION permission on the certificate or asymmetric key. If dropping encryption by a certificate or asymmetric key, requires CONTROL permission on the certificate or asymmetric key.
The following example changes the encryption method that is used to protect a symmetric key. The symmetric key JanainaKey043 is encrypted using certificate Shipping04 when the key was created. Because the key can never be stored unencrypted, in this example, encryption is added by password, and then encryption is removed by certificate.
CREATE SYMMETRIC KEY JanainaKey043 WITH ALGORITHM = AES_256 ENCRYPTION BY CERTIFICATE Shipping04; -- Open the key. OPEN SYMMETRIC KEY JanainaKey043 DECRYPTION BY CERTIFICATE Shipping04 WITH PASSWORD = '<enterStrongPasswordHere>'; -- First, encrypt the key with a password. ALTER SYMMETRIC KEY JanainaKey043 ADD ENCRYPTION BY PASSWORD = '<enterStrongPasswordHere>'; -- Now remove encryption by the certificate. ALTER SYMMETRIC KEY JanainaKey043 DROP ENCRYPTION BY CERTIFICATE Shipping04; CLOSE SYMMETRIC KEY JanainaKey043;