Encryption program string error

Question

So, im making a simple encryption program, this is the algorithm:

1. Exemple:char:a;key:10000
2. Subtract ASCII number from CONTROL number 1000 - 97 = 9903
3. Convert result to Hexadecimal and then into a string 9903 = 26AF
4. Split Hex result into groups of two digits 26, AF
5. Convert split Hex values to ASCII 26 = 38 (&) AF = 175 (¯) Your result would be: &¯

The problem is that sometimes(code below), depending on the encryption key, the encryption results come out all worn and the decryption function wont work. I have done tests and i know that the problem lies on the encryption function, but i don't know where. Here's the code:

Option Strict On

Imports System.Numerics

Public Class MainF

    'tick for random keys
    Dim objRandom As New System.Random(CType(System.DateTime.Now.Ticks Mod System.Int32.MaxValue, Integer))   

    Private Function AsciiToChars(ByVal nums As IEnumerable(Of Integer)) As Char()
        'converts ASCII value to char
        Return (From c In nums Select Chr(c)).ToArray
    End Function

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        'set random key
        KeyE.Text = CStr(GetRandomNumber(10000, 99999))
        KeyD.Text = KeyE.Text
    End Sub

    Public Function GetRandomNumber(Optional ByVal Low As Integer = 1, Optional ByVal High As Integer = 100) As Integer

        'generate a randomic number, for the random key
        Return objRandom.Next(Low, High + 1)
    End Function

    Private Sub GenKey_Click(sender As Object, e As EventArgs) Handles GenKey.Click

        'renew key
        KeyE.Text = CStr(GetRandomNumber(10000, 99999))
        Dim tmptxt As String
        tmptxt = En.Text
        En.Text = ""
        En.Text = tmptxt
        KeyD.Text = KeyE.Text
        tmptxt = String.Empty
    End Sub

    Sub En_TextChanged(sender As Object, e As EventArgs) Handles En.TextChanged

        'encrypt
        If (String.IsNullOrEmpty(En.Text)) Then
            Enres.Text = ""
        Else
            Dim key As Integer = CInt(KeyE.Text)
            Dim chars() As Char = En.Text.ToCharArray
            Dim ints(chars.Length) As Integer
            Dim hex(chars.Length) As String
            Dim fex As String = ""
            Dim Mlist As New List(Of String)

            For Loop0 As Integer = 0 To chars.Length - 1
                ints(Loop0) = key - Asc(chars(Loop0))
                hex(Loop0) = Conversion.Hex(ints(Loop0))
                fex &= hex(Loop0)
            Next

            If fex.Length Mod 2 <> 0 Then 'Mod returns the remainder of a division calculation. It will be 0 if the number is even.
                fex = "0" & fex 'This will change "6AF" to "06AF"
            End If

            For x As Integer = 0 To fex.Length - 1 Step 2
                Mlist.Add(fex.Substring(x, 2))
            Next

            Dim fdec(CInt((fex.Length - 2) / 2)) As Integer
            Dim fstr As String

            For y As Integer = 0 To fdec.Length - 1
                fdec(y) = CInt(Val("&H" & Mlist(y)))
            Next

            fstr = AsciiToChars(fdec)
            Enres.Text = fstr
        End If
    End Sub

    Private Sub De_TextChanged(sender As Object, e As EventArgs) Handles De.TextChanged

        'decrypt
        If (String.IsNullOrEmpty(De.Text)) Then
            DeRes.Text = ""
        Else
            Dim final As String = ""
            Dim key As Integer
            key = CInt(KeyD.Text)
            Dim FSTR As String = De.Text
            Dim chars() As Char = FSTR.ToCharArray
            Dim hexsub(chars.Length) As String
            Dim ints(chars.Length) As String
            Dim finalhex As String

            For loop1 As Integer = 0 To chars.Length - 1
                ints(loop1) = CStr(Asc(chars(loop1)))
                hexsub(loop1) = Hex(ints(loop1))
            Next          

            finalhex = Join(hexsub, String.Empty)

            If finalhex.Length Mod 4 = 0 Then
                Dim newlist As New List(Of String)

                For x As Integer = 0 To finalhex.Length - 1 Step 4
                    newlist.Add(finalhex.Substring(x, 4))
                Next

                Dim sourceNUM(newlist.Count) As Int32
                Dim finalascii(newlist.Count) As Int32
                Dim finalchar(newlist.Count) As Char
                key = CInt(KeyD.Text)

                For b As Int32 = 0 To newlist.Count - 1
                    sourceNUM(b) = Convert.ToInt32(newlist(b), 16)
                    finalascii(b) = key - sourceNUM(b)

                    If finalascii(b) >= 32 And finalascii(b) <= 255 Then
                        finalchar(b) = Chr(finalascii(b))
                        final &= finalchar(b)
                    Else : final = "Invalid Input"
                    End If
                Next

                DeRes.Text = final
            Else
                DeRes.Text = ""
            End If
        End If
    End Sub

    Private Sub KeyE_TextChanged(sender As Object, e As KeyEventArgs) Handles KeyE.KeyDown

        'future idea, enter key will run encription sub
        If e.KeyCode = Keys.Enter Then
            e.SuppressKeyPress = True
        End If
    End Sub
End Class

Answer 1

The problem is that sometimes(code below), depending on the encryption key, the encryption results come out all worn and the decryption function wont work.

Your main problem that I can find is that sometimes your passing undisplayable codes to the encrypted string, which then treats it as null, which in turn will make the decryption algorithm fail

You might need to figure out what range of keys will give bad values and filter those keys out or use a byte array instead of a string