How do you use the DECRYPTBYKEY function to decrypt data with a symmetric key?
JackBrn
Posted by JackBrn
Last Updated: July 27, 2024
decrypt key
The DECRYPTBYKEY function in SQL Server is used to decrypt data that has been encrypted using a symmetric key. To use DECRYPTBYKEY, you first need to ensure that you've opened the symmetric key with the OPEN SYMMETRIC KEY statement. Below is a general guide on how to use DECRYPTBYKEY.
Steps to Decrypt Data using DECRYPTBYKEY
1. Open the Symmetric Key: You must open the symmetric key that was used to encrypt the data. You can do this with the OPEN SYMMETRIC KEY command. 2. Decrypt the Data: Use the DECRYPTBYKEY function to decrypt the encrypted data. 3. Close the Symmetric Key: After you finish your operations, you should close the symmetric key using CLOSE SYMMETRIC KEY.
Example
Here’s an example that demonstrates how to decrypt data using a symmetric key: 1. Creating a Symmetric Key and Encrypting Data: This is just for context; you need to ensure you already have encrypted data. 
-- Create a database and a table to store encrypted data
   CREATE DATABASE EncryptionDemo;
   GO

   USE EncryptionDemo;
   GO

   CREATE TABLE SensitiveData (ID INT PRIMARY KEY, EncryptedValue VARBINARY(MAX));

   -- Create a symmetric key
   CREATE SYMMETRIC KEY MySymmetricKey
       WITH ALGORITHM = AES_256
       ENCRYPTION BY PASSWORD = 'StrongPassword123';

   -- Open the symmetric key
   OPEN SYMMETRIC KEY MySymmetricKey
       DECRYPTION BY PASSWORD = 'StrongPassword123';

   -- Insert encrypted data
   DECLARE @PlainText NVARCHAR(100) = 'Secret Data';
   INSERT INTO SensitiveData (ID, EncryptedValue)
   VALUES (1, ENCRYPTBYKEY(KEY_GUID('MySymmetricKey'), @PlainText));

   -- Close the symmetric key
   CLOSE SYMMETRIC KEY MySymmetricKey;
2. Decrypting the Data: 
-- Open the symmetric key again
   OPEN SYMMETRIC KEY MySymmetricKey
       DECRYPTION BY PASSWORD = 'StrongPassword123';

   -- Decrypt the data
   SELECT ID, CONVERT(NVARCHAR(100), DECRYPTBYKEY(EncryptedValue)) AS DecryptedValue
   FROM SensitiveData;

   -- Close the symmetric key
   CLOSE SYMMETRIC KEY MySymmetricKey;
Important Notes
- Key Management: Always manage your keys securely. Ensure you protect the key password and the keys themselves. - Permissions: Make sure the user has the appropriate permissions to open the symmetric key and access the encrypted data. - Error Handling: Consider implementing error handling in production scenarios, especially when dealing with encryption and decryption operations. - Data Types: The data type returned by DECRYPTBYKEY is VARBINARY. You may need to cast or convert it to the appropriate format (e.g., using CONVERT function) to use the decrypted text effectively. This should provide a basic idea of how to use the DECRYPTBYKEY function to decrypt data that has been encrypted with a symmetric key in SQL Server.
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