How to shorten string to specific length according to textBox1.Length c#

I need to know how to shorten a string to a certain length, I tried string.Trim() but it is not working. Sorry I'm still a beginner.

Thanks

Edit

I'm trying to cut off padding from decrypted text. For example: A user inputs text, the user encrypts and decrypts text. On the decrypted text there is padding left...