Cifrado de columnas de SQL Server

Usted puede crear un Certificate y luego un Symmetric Key que utiliza el certificado:

CREATE CERTIFICATE YourCertificate
 WITH SUBJECT = 'Encrypted Content';
GO

CREATE SYMMETRIC KEY SSN_Key
  WITH ALGORITHM = AES_256
   ENCRYPTION BY CERTIFICATE YourCertificate;
GO

Para descifrar: puede usar DecryptByKey:

El ejemplo de MSDN es:

SELECT CardNumber, CardNumber_Encrypted AS 'Encrypted card number',
   CONVERT(nvarchar,
   DecryptByKey(CardNumber_Encrypted, 1 , 
   HashBytes('SHA1', CONVERT(varbinary, CreditCardID)))) AS 'Decrypted card number'  
FROM Sales.CreditCard;

http://msdn.microsoft.com/en-us/library/ms179331(v=sql.105).aspx

Eche un vistazo a la publicación de MSDN Cómo: cifrar una columna de datos

En primer lugar: crea tu clave asym usando este código:

USE [yourDB]    
GO    
CREATE ASYMMETRIC KEY ClaveAsym    
WITH ALGORITHM = RSA_2048 ---->Take a look in this type of algorithms    
ENCRYPTION BY PASSWORD = 'yourKey'
GO

Recuerda esto, debes declarar una variable siempre que quieras desencriptar o encriptar:

DECLARE @KEYID INT SET @KEYID = AsymKey_ID('ClaveAsym')

Descifrando datos

SELECT columnA, CAST(DecryptByAsymKey(@keyid, columnUser, N'yourKey') AS VARCHAR(100)), CONVERT(VARCHAR(MAX), DECRYPTBYASYMKEY(@keyId, columnPass, N'yourKey')) FROM yourTable

Cifrar datos

DECLARE @User VARCHAR(MAX), @pass VARCHAR(MAX)    
SET @User = 'yourUser'    
sET @pass = 'yourPass'    
DECLARE @KEYID INT SET @KEYID = AsymKey_ID('ClaveAsym')    
INSERT INTO yourTable( User, passw) VALUES EncryptByAsymKey (@keyid, @User ), EncryptByAsymKey (@keyid, @pass))