How do I get an Ascii to warp to a certain value after it has past 122? - vb.net

Question

I am trying to write an encryption program. The problem I am facing is that I am converting the text to ascii and then adding on the offset. However when it goes past the letter 'z' I want it to warp back to 'a' and go from there.
Sub enc()
Text = TextBox1.Text
finalmessage = ""
letters = Text.ToCharArray
offset = ComboBox1.SelectedItem
For x = LBound(letters) To UBound(letters)
finalmessage = finalmessage + Chr(Asc(letters(x)) + offset)
Next
TextBox2.Text = finalmessage
End Sub

Answer 1

I guess to make it easy to decode afterwards, you should to it somewhat in the line of base64 encoding, first encoding everything to a normalized binary string, then encode in the range you want (since using binary, it has to be something that fits with 2^X).
To match your range, i used a baseset of 32, and a simple encoding decoding example (a bit more verbose that it should be, perhaps)
Module Module1
Dim encodeChars As String = "abcdefghijklmnopqrstuvwxyzABCDEF" ' use 32 as a base
Function Encode(text As String) As String
Dim bitEncoded As String = ""
Dim outputMessage As String = ""
For Each ch As Char In text.ToCharArray()
Dim i As Integer = Convert.ToByte(ch)
bitEncoded &= Convert.ToString(i, 2).PadLeft(8, "0"c)
Next
While bitEncoded.Length Mod 5 <> 0
bitEncoded &= "0"
End While
For position As Integer = 0 To bitEncoded.Length - 1 Step 5
Dim range As String = bitEncoded.Substring(position, 5)
Dim index As Integer = Convert.ToInt32(range, 2)
outputMessage &= encodeChars(index).ToString()
Next
Return outputMessage
End Function
Function Decode(encodedText As String) As String
Dim bitEncoded As String = ""
Dim outputMessage As String = ""
For Each ch In encodedText
Dim index As Integer = encodeChars.IndexOf(ch)
If index < 0 Then
Throw New FormatException("Invalid character in encodedText!")
End If
bitEncoded &= Convert.ToString(index, 2).PadLeft(5, "0"c)
Next
' strip the extra 0's
While bitEncoded.Length Mod 8 <> 0
bitEncoded = bitEncoded.Substring(0, bitEncoded.Length - 1)
End While
For position As Integer = 0 To bitEncoded.Length - 1 Step 8
Dim range As String = bitEncoded.Substring(position, 8)
Dim index As Integer = Convert.ToInt32(range, 2)
outputMessage &= Chr(index).ToString()
Next
Return outputMessage
End Function
Sub Main()
Dim textToEncode As String = "This is a small test, with some special characters! Just testing..."
Dim encodedText As String = Encode(textToEncode)
Dim decodedText As String = Decode(encodedText)
Console.WriteLine(textToEncode)
Console.WriteLine(encodedText)
Console.WriteLine(decodedText)
If Not String.Equals(decodedText, textToEncode) Then
Console.WriteLine("Encoding / decoding failed!")
Else
Console.WriteLine("Encoding / decoding completed succesfully!")
End If
Console.ReadLine()
End Sub
End Module
this then gives the following output?
This is a small test, with some special characters! Just testing...
krugsCzanfzsayjaonwwcBdmebAgkCBufqqhoAlunaqhgBBnmuqhgCdfmnuwcBbamnugcCtbmnAgkCtteeqeuDltoqqhizltoruwCzzofyxa
This is a small test, with some special characters! Just testing...
Encoding / decoding completed succesfully!