Rijndael 属对称加密，对称加密在加密和解密时都使用相同的密钥。2000 年 10 月，NIST 选择 Rijndael（发音为 "Rhine dale"）作为 AES 算法，用以取代 DES。

Rijndael 的名称空间是：

System.Security.Cryptography

byte[] plaintextBuffer = System.Text.Encoding.UTF8.GetBytes("明文");
 
//加密
Rijndael rijndael = Rijndael.Create();
ICryptoTransform transform = rijndael.CreateEncryptor();
byte[] cipherTextBuffer = transform.TransformFinalBlock(plaintextBuffer, 0, plaintextBuffer.Length);
lbl.Text = Convert.ToBase64String(cipherTextBuffer) + "<br />";
transform.Dispose();
 
//解密
Rijndael rijndael2 = Rijndael.Create();
ICryptoTransform transform2 = rijndael2.CreateDecryptor(rijndael.Key, rijndael.IV);
byte[] decryption = transform2.TransformFinalBlock(cipherTextBuffer, 0, cipherTextBuffer.Length);
lbl.Text += System.Text.Encoding.UTF8.GetString(decryption) + "<br />";
transform2.Dispose();

解密时，使用加密的 Key 和 IV。