3DES Key Size Matter in C#.Net

Question

Below Code is Working Fine in c#.NET

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));

My questions regarding above are...

1. How can I specify KeySize? if i use des.KeySize= 128 or 192 or 256 it gives

Specified key is not a valid size for this algorithm

1. If I change character length in key by adding more (ex:40 chars). It gives error

Specified key is not a valid size for this algorithm

I want to know why is this happen?

Answer 1

A 3DES key has length 128 or 192 bits. Note that, internally, the algorithm will use only 112 (respectively 168) bits out of those 128 (respectively 192) bits; however, the key itself, as encoded into bytes, stored and exchanged, must have length 16 or 24 bytes. Trying to set a key which does not have one of those two lengths triggers an error, which you observe when you try to use a 40-byte key.

You should not try to set the "key size": you already decide that when you set the key. When you set the TripleDES.Key property, the TripleDES class sees that you give it a 24-byte key, and thus will set itself the KeySize property to 192.

(The output of 3DES encryption is binary, not UTF-8 encoding of a string. Chances are that your final UTF8Encoding.UTF8.GetString(enc) will protest.)