Simple way would be to have two text boxes. One that the user actually enters the number into, another that shows the masked version of that input. Put the masked control over the entry control.
Use TextChanged
or KeyUp
events to track changes and update.
eg
if (textBoxEntry.Text.Length <= 1)
{
textBoxMasked.Text = textBoxEntry.Text;
}
else
{
string lastChar = textBoxEntry.Text.Substring(textBoxEntry.Text.Length-1, 1);
textBoxMasked.Text = lastChar.PadLeft(textBoxEntry.Text.Length, '*');
}
Beyond that you likely have issues with PCI compliance. It isn't enough to just encrypt these values into the database, you need key management policies, key rotation schedules, and heaps more. Hopefully you know all this already, but just mentioning it should you not be aware.