name your textboxes as txtCipher_1, txtCipher_2,...etc.
int cIndex;
foreach (Control item in this.Controls)
{
if (item is TextBox)
{
cIndex = Convert.ToInt16((item as TextBox).Name.Split('_')[1]);
(item as TextBox).Text = ((char)(64 + cIndex)).ToString(); ;
}
}
I tested and it works in .NET 3.5