How do you use the ENCRYPTBYASYMKEY function to encrypt data with an asymmetric key?
CarolTh
Posted by CarolTh
Last Updated: July 24, 2024
Encryption Asymmetric
The ENCRYPTBYASYMKEY function in SQL Server is used to encrypt data using an asymmetric key. Asymmetric keys are part of a public/private key pair; data encrypted with a public key can only be decrypted with the corresponding private key, providing a secure method for data encryption. Here’s a step-by-step guide on how to use the ENCRYPTBYASYMKEY function:
Step 1: Create an Asymmetric Key
Before you can encrypt data, you need to create an asymmetric key. You can create a key using the following T-SQL command: 
CREATE ASYMMETRIC KEY MyAsymmetricKey
WITH KEY_USAGE = SIGNATURE;
Step 2: Open the Asymmetric Key
You need to open the key in order to use it. Use the OPEN SYMMETRIC KEY statement: 
OPEN ASYMMETRIC KEY MyAsymmetricKey
DECRYPTION BY PASSWORD = 'YourSecurePassword';
Step 3: Encrypt Data
You can now use the ENCRYPTBYASYMKEY function to encrypt the data. Assuming you have a string that you want to encrypt, you can do it as follows: 
DECLARE @PlainText NVARCHAR(100) = 'Sensitive Data';
DECLARE @EncryptedData VARBINARY(128);

SET @EncryptedData = ENCRYPTBYASYMKEY(
    ASYMKEY_ID('MyAsymmetricKey'), 
    @PlainText
);
Step 4: Store or Use the Encrypted Data
After encrypting the data, you can store it in a table or use it as needed. For example: 
CREATE TABLE EncryptedDataTable (
    Id INT PRIMARY KEY,
    EncryptedData VARBINARY(128)
);

INSERT INTO EncryptedDataTable (Id, EncryptedData) 
VALUES (1, @EncryptedData);
Step 5: Decrypting the Data
To decrypt the data later, you would first open the asymmetric key and use the DECRYPTBYASYMKEY function: 
DECLARE @DecryptedData NVARCHAR(100);
DECLARE @EncryptedDataFromTable VARBINARY(128);

-- Assume you've retrieved the encrypted data from the table.
SELECT @EncryptedDataFromTable = EncryptedData FROM EncryptedDataTable WHERE Id = 1;

-- Decrypting the data
SET @DecryptedData = CAST(DECRYPTBYASYMKEY(
    ASYMKEY_ID('MyAsymmetricKey'), 
    @EncryptedDataFromTable
) AS NVARCHAR(100));

CLOSE ASYMMETRIC KEY MyAsymmetricKey;
Important Notes:
- Security: Always ensure that sensitive data (like passwords for decrypting the key) is handled securely. - Permissions: Ensure that the user executing these commands has the proper permissions to use asymmetric keys. - Performance: Asymmetric encryption is slower than symmetric encryption; consider performance when encrypting large amounts of data. This is a high-level overview, and you may need to adapt it based on your specific requirements and circumstances.
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