key_GUID

Is the GUID of the key to be used to encrypt the cleartext. uniqueidentifier.

' cleartext '

Is the data that is to be encrypted with the key.

@cleartext

Is a variable of type nvarchar, char, varchar, binary, varbinary, or nchar that contains data that is to be encrypted with the key.

add_authenticator

Indicates whether an authenticator will be encrypted together with the cleartext. Must be 1 when using an authenticator. int.

@add_authenticator

Indicates whether an authenticator will be encrypted together with the cleartext. Must be 1 when using an authenticator. int.

authenticator

Is the data from which to derive an authenticator. sysname.

@authenticator

Is a variable that contains data from which to derive an authenticator.

varbinary with a maximum size of 8,000 bytes.

EncryptByKey uses a symmetric key. This key must be open. If the symmetric key is already open in the current session, you do not have to open it again in the context of the query.

The authenticator helps you deter whole-value substitution of encrypted fields. For example, consider the following table of payroll data.

Without breaking the encryption, a hacker can infer significant information from the context in which the ciphertext is stored. Because a Chief Financial Officer is paid more than a Copy Room Assistant, it follows that the value encrypted as M0x8900f56543 must be greater than the value encrypted as Fskj%7^edhn00. If so, any user with ALTER permission on the table can give the Copy Room Assistant a raise by replacing the data in his Base_Pay field with a copy of the data stored in the Base_Pay field of the Chief Financial Officer. This whole-value substitution attack bypasses encryption altogether.

Whole-value substitution attacks can be thwarted by adding contextual information to the plaintext before encrypting it. This contextual information is used to verify that the plaintext data has not been moved.

If an authenticator parameter is specified when data is encrypted, the same authenticator is required to decrypt the data by using DecryptByKey. At encryption time, a hash of the authenticator is encrypted together with the plaintext. At decryption time, the same authenticator must be passed to DecryptByKey. If the two do not match, the decryption will fail. This indicates that the value has been moved since it was encrypted. We recommend using the primary key of the table in which the result will be stored as the value of this parameter.

Symmetric encryption and decryption is relatively fast, and is suitable for working with large amounts of data.

Important:
Using the SQL Server encryption functions that use the ANSI_PADDING OFF setting could cause data loss because of implicit conversions. For more information about ANSI_PADDING, see SET ANSI_PADDING (Transact-SQL).

The functionality illustrated in the following examples relies on keys and certificates created in How To: Encrypt a Column of Data.

A. Encrypting a string with a symmetric key

USE AdventureWorks;
GO

-- Create a column in which to store the encrypted data.
ALTER TABLE HumanResources.Employee
    ADD EncryptedNationalIDNumber varbinary(128); 
GO

-- Open the symmetric key with which to encrypt the data.
OPEN SYMMETRIC KEY SSN_Key_01
   DECRYPTION BY CERTIFICATE HumanResources037;

-- Encrypt the value in column NationalIDNumber with symmetric key
-- SSN_Key_01. Save the result in column EncryptedNationalIDNumber.
UPDATE HumanResources.Employee
SET EncryptedNationalIDNumber
    = EncryptByKey(Key_GUID('SSN_Key_01'), NationalIDNumber);
GO

B. Encrypting a record together with an authentication value

USE AdventureWorks;

-- Create a column in which to store the encrypted data.
ALTER TABLE Sales.CreditCard. 
    ADD CardNumber_Encrypted varbinary(128); 
GO

-- Open the symmetric key with which to encrypt the data.
OPEN SYMMETRIC KEY CreditCards_Key11
    DECRYPTION BY CERTIFICATE Sales09;

-- Encrypt the value in column CardNumber with symmetric 
-- key CreditCards_Key11.
-- Save the result in column CardNumber_Encrypted.  
UPDATE Sales.CreditCard
SET CardNumber_Encrypted = EncryptByKey(Key_GUID('CreditCards_Key11'), 
    CardNumber, 1, CONVERT( varbinary, CreditCardID) );
GO

Reference

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