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title: "DECRYPTBYKEY (Transact-SQL) | Microsoft Docs"
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# DECRYPTBYKEY (Transact-SQL)
[!INCLUDE[tsql-appliesto-ss2008-asdb-xxxx-xxx_md](../../includes/tsql-appliesto-ss2008-asdb-xxxx-xxx-md.md)]

  Decrypts data by using a symmetric key.  
  
 ![Topic link icon](../../database-engine/configure-windows/media/topic-link.gif "Topic link icon") [Transact-SQL Syntax Conventions](../../t-sql/language-elements/transact-sql-syntax-conventions-transact-sql.md)  
  
## Syntax  
  
```  
  
DecryptByKey ( { 'ciphertext' | @ciphertext }   
    [ , add_authenticator, { authenticator | @authenticator } ] )  
```  
  
## Arguments  
 *ciphertext*  
 Is data that has been encrypted with the key. *ciphertext* is **varbinary**.  
  
 **@ciphertext**  
 Is a variable of type **varbinary** that contains data that has been encrypted with the key.  
  
 *add_authenticator*  
 Indicates whether an authenticator was encrypted together with the plaintext. Must be the same value passed to EncryptByKey when encrypting the data. *add_authenticator* is **int**.  
  
 *authenticator*  
 Is data from which to generate an authenticator. Must match the value that was supplied to EncryptByKey. *authenticator* is **sysname**.  
  
 **@authenticator**  
 Is a variable that contains data from which to generate an authenticator. Must match the value that was supplied to EncryptByKey.  
  
## Return Types  
 **varbinary** with a maximum size of 8,000 bytes.  
  
## Remarks  
 DecryptByKey uses a symmetric key. This symmetric key must already be open in the database. There can be multiple keys open at the same time. You do not have to open the key immediately before decrypting the cipher text.  
  
 Symmetric encryption and decryption is relatively fast, and is suitable for working with large amounts of data.  
  
## Permissions  
 Requires the symmetric key to have been opened in the current session. For more information, see [OPEN SYMMETRIC KEY (Transact-SQL)](../../t-sql/statements/open-symmetric-key-transact-sql.md).  
  
## Examples  
  
### A. Decrypting by using a symmetric key  
 The following example decrypts ciphertext by using a symmetric key.  
  
```  
-- First, open the symmetric key with which to decrypt the data.  
OPEN SYMMETRIC KEY SSN_Key_01  
   DECRYPTION BY CERTIFICATE HumanResources037;  
GO  
  
-- Now list the original ID, the encrypted ID, and the   
-- decrypted ciphertext. If the decryption worked, the original  
-- and the decrypted ID will match.  
SELECT NationalIDNumber, EncryptedNationalID   
    AS 'Encrypted ID Number',  
    CONVERT(nvarchar, DecryptByKey(EncryptedNationalID))   
    AS 'Decrypted ID Number'  
    FROM HumanResources.Employee;  
GO  
```  
  
### B. Decrypting by using a symmetric key and an authenticating hash  
 The following example decrypts data that was encrypted together with an authenticator.  
  
```  
-- First, open the symmetric key with which to decrypt the data  
OPEN SYMMETRIC KEY CreditCards_Key11  
   DECRYPTION BY CERTIFICATE Sales09;  
GO  
  
-- Now list the original card number, the encrypted card number,  
-- and the decrypted ciphertext. If the decryption worked,   
-- the original number will match the decrypted number.  
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;  
GO  
  
```  
  
## See Also  
 [ENCRYPTBYKEY (Transact-SQL)](../../t-sql/functions/encryptbykey-transact-sql.md)   
 [CREATE SYMMETRIC KEY (Transact-SQL)](../../t-sql/statements/create-symmetric-key-transact-sql.md)   
 [ALTER SYMMETRIC KEY (Transact-SQL)](../../t-sql/statements/alter-symmetric-key-transact-sql.md)   
 [DROP SYMMETRIC KEY (Transact-SQL)](../../t-sql/statements/drop-symmetric-key-transact-sql.md)   
 [Encryption Hierarchy](../../relational-databases/security/encryption/encryption-hierarchy.md)   
 [Choose an Encryption Algorithm](../../relational-databases/security/encryption/choose-an-encryption-algorithm.md)