ROT13. The ROT13 algorithm encodes plain text. In this algorithm, the letters A-Z and a-z are rotated 13 places. This function can be written in the VB.NET language.

Some notes. ROT13 scrambles the text but is easily reversible. It can be implemented with If and ElseIf in VB.NET code—this is likely the simplest approach.

Example. We test to make sure the Rot13 function, when called twice, correctly reverses the operation. In the Rot13 function, we receive one String parameter and return a new String value.

Module Module1
    Sub Main()
        ' Use Rot13.
        Console.WriteLine("The apartment")
        Console.WriteLine(Rot13("The apartment"))
        Console.WriteLine(Rot13(Rot13("The apartment")))
    End Sub

    Public Function Rot13(ByVal value As String) As String
        ' Could be stored as integers directly.
        Dim lowerA As Integer = Asc("a"c)
        Dim lowerZ As Integer = Asc("z"c)
        Dim lowerM As Integer = Asc("m"c)

        Dim upperA As Integer = Asc("A"c)
        Dim upperZ As Integer = Asc("Z"c)
        Dim upperM As Integer = Asc("M"c)

        ' Convert to character array.
        Dim array As Char() = value.ToCharArray

        ' Loop over string.
        Dim i As Integer
        For i = 0 To array.Length - 1

            ' Convert to integer.
            Dim number As Integer = Asc(array(i))

            ' Shift letters.
            If ((number >= lowerA) AndAlso (number <= lowerZ)) Then
                If (number > lowerM) Then
                    number -= 13
                Else
                    number += 13
                End If
            ElseIf ((number >= upperA) AndAlso (number <= upperZ)) Then
                If (number > upperM) Then
                    number -= 13
                Else
                    number += 13
                End If
            End If

            ' Convert to character.
            array(i) = Chr(number)
        Next i

        ' Return string.
        Return New String(array)
    End Function
End Module
The apartment
Gur ncnegzrag
The apartment

A summary. The ROT13 algorithm is easily broken and won't stop others from reading your important secrets. ROT13 is helpful for learning how to write string-manipulation code.