C＃.Net中的3DES密钥大小问题

byte[] key = Encoding.ASCII.GetByte(\"012345678901234567890123\"); //24characters        
byte[] plainText = Encoding.ASCII.GetBytes(\"lasaa\"); 
TripleDES des = TripleDES.Create();
des.Key = key;
des.Mode = CipherMode.CBC;
ICryptoTransform ic = des.CreateEncryptor();
byte[] enc = ic.TransformFinalBlock(plainText, 0, plainText.Length);
MessageBox.Show(UTF8Encoding.UTF8.GetString(enc));

des.KeySize=

128

192

256

TripleDES.Key

TripleDES

UTF8Encoding.UTF8.GetString(enc)

byte[] keyArray;
SHA512CryptoServiceProvider hash = new SHA512CryptoServiceProvider();
keyArray = hash.ComputeHash(UTF8Encoding.UTF8.GetBytes(\"someProperlySaltedKey\"));
byte[] trimmedBytes = new byte[24];
Buffer.BlockCopy(keyArray, 0, trimmedBytes, 0, 24);
keyArray = trimmedBytes;
TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
tdes.Key = keyArray;