Function verschlüsseln$(Text1$, schlüssel$)
zeichennummer=1
laenge=Len(Text1$)
   For a=1 To laenge
      asccodenummer=Asc(Mid(schlüssel,zeichennummer,1))
      If asccodenummer>255 Then asccodenummer=asccodenummer-255
      zeichen=Asc(Mid(Text1$,a,1))+asccodenummer
      newcode$=newcode$+zeichen+",+,"
      zeichennummer=zeichennummer+1
      If zeichennummer>Len(schlüssel$) Then zeichennummer=1
   Next
End Function

Function verschlüsseln$(Text1$, schlüssel$)
zeichennummer=1
laenge=Len(Text1$)
   For a=1 To laenge
      asccodenummer=Asc(Mid(schlüssel,zeichennummer,1))
      If asccodenummer>255 Then asccodenummer=asccodenummer-255
      zeichen=Asc(Mid(Text1$,a,1))+asccodenummer
      newbuchstabe$=Chr(zeichen)
      newcode$=newcode$+newbuchstabe$
      zeichennummer=zeichennummer+1
      If zeichennummer>Len(schlüssel$) Then zeichennummer=1
   Next
End Function

If asccodenummer>255 Then asccodenummer=asccodenummer-255

Function verschlüsseln$(Text1$, schlüssel$)
zeichennummer=1
laenge=Len(Text1$)
   For a=1 To laenge
      asccodenummer=Asc(Mid(schlüssel,zeichennummer,1))
      zeichen=Asc(Mid(Text1$,a,1))+asccodenummer
      If zeichen>255 Then zeichen=zeichen-255
      newbuchstabe$=Chr(zeichen)
      newcode$=newcode$+newbuchstabe$
      zeichennummer=zeichennummer+1
      If zeichennummer>Len(schlüssel$) Then zeichennummer=1
   Next
Return newcode$
End Function

Function Encrypt$(Plain$,Key$)
   PlainLen = Len(Plain$)
   KeyLen = Len(Key$)
   PrevChar = 0
   For I = 1 To PlainLen
      KeyChar = Asc(Mid(Key$,(I-1) Mod KeyLen + 1,1))
      PlainChar = Asc(Mid(Plain$,I,1))
      CipherChar = (PlainChar + KeyChar + PrevChar) Mod 256
      Cipher$ = Cipher$ + Chr(CipherChar)
      PrevChar = CipherChar
   Next
   Return Cipher$
End Function

Function Decrypt$(Cipher$,Key$)
   CipherLen = Len(Cipher$)
   KeyLen = Len(Key$)
   PrevChar = 0
   For I = 1 To CipherLen
      KeyChar = Asc(Mid(Key$,(I-1) Mod KeyLen + 1,1))
      CipherChar = Asc(Mid(Cipher$,I,1))
      PlainChar = (CipherChar - KeyChar - PrevChar + 512) Mod 256
      Plain$ = Plain$ + Chr(PlainChar)
      PrevChar = CipherChar
   Next
   Return Plain$
End Function

; Zum Vergleich:
Print Encrypt("Verschlüsselung","abc")
Print Encrypt("verschlüsselung","abc")

Zeile 1
Zeile 2
Zeile 3
Zeile 4
Zeile 5
Zeile 6

Zeile 1
Zeile 2
Zeile ³?
Zeile 4
Zeiìå 5
]Žh’@e#_

var$ = Encrypt("Zeile 1"+Chr(13)+Chr(10)+"Zeile 2"+Chr(13)+Chr(10)+"Zeile 3"+Chr(13)+Chr(10)+"Zeile 4"+Chr(13)+Chr(10)+"Zeile 5"+Chr(13)+Chr(10)+"Zeile 6","abc")
var$ = Decrypt(var$,"abc")

Print var$

Global window=CreateWindow("Verschlüsselung",ClientWidth(Desktop())/2-250,ClientHeight(Desktop())/2-180,500,400,0,1)

Global text1label=CreateLabel("Text 1",10,10,200,15,window)
Global text1=CreateTextArea(10,30,480,100,window)

Global schlüssel=CreateLabel("Schlüssel",10,140,200,15,window)
Global schlfield=CreateTextField(10,165,480,20,window)

Global button=CreateButton("Verschlüsseln",110,200,80,25,window)
Global button2=CreateButton("Entschlüsseln",310,200,80,25,window)

Global Text2label=CreateLabel("Text 2",10,235,200,15,window)
Global text2=CreateTextArea(10,255,480,100,window)

Repeat
   Select WaitEvent()
   Case $803
      End
   Case $401   
      If EventSource()=button Then
         SetTextAreaText text2,Encrypt$(TextAreaText(text1),TextFieldText(schlfield))
      EndIf
      If EventSource()=button2 Then
         SetTextAreaText text2, Decrypt$(TextAreaText(text1),TextFieldText(schlfield))
      EndIf
   End Select
Forever

Function Encrypt$(Plain$,Key$)
   PlainLen = Len(Plain$)
   KeyLen = Len(Key$)
   PrevChar = 0
   For I = 1 To PlainLen
      KeyChar = Asc(Mid(Key$,(I-1) Mod KeyLen + 1,1))
      PlainChar = Asc(Mid(Plain$,I,1))
      CipherChar = (PlainChar + KeyChar + PrevChar) Mod 256
      Cipher$ = Cipher$ + Chr(CipherChar)
      PrevChar = CipherChar
   Next
   Return Cipher$
End Function

Function Decrypt$(Cipher$,Key$)
   CipherLen = Len(Cipher$)
   KeyLen = Len(Key$)
   PrevChar = 0
   For I = 1 To CipherLen
      KeyChar = Asc(Mid(Key$,(I-1) Mod KeyLen + 1,1))
      CipherChar = Asc(Mid(Cipher$,I,1))
      PlainChar = (CipherChar - KeyChar - PrevChar + 512) Mod 256
      Plain$ = Plain$ + Chr(PlainChar)
      PrevChar = CipherChar
   Next
   Return Plain$
End Function

SeedRnd MilliSecs()

t1$ = "timmeTheOnly"
k$ = "ich"
s = 0

t2$ = encrypt$(t1$,k$,s)

t3$ = decrypt$(t2$,k$,s)

Graphics 1024,768,32,2

Print t1$
Print t2$
Print t3$

WaitKey

Function encrypt$(t$,k$,s%)
   Local ret$, char$, tl, kl, pos
   
   tl = Len(t$)
   kl = Len(k$)
   
   For i = 1 To Len(t$)
      pos = (pos + 1) Mod kl
      
      char$ = Chr$(Asc(Mid$(t$,i,1)) Xor Asc(Mid$(k$,pos+1,1)))
      
      ret$ = ret$ + char$
      
      For j = 1 To s
         ret$ = ret$ + Chr$(Rand(0,255))
      Next
   Next
   
   Return ret$
End Function

Function decrypt$(t$,k$,s%)
   Local ret$, char$, tl, kl, pos
   
   tl = Len(t$)
   kl = Len(k$)
   
   For i = 1 To Len(t$)
      If cnt = 0
         pos = (pos + 1) Mod kl
         
         char$ = Chr$(Asc(Mid$(t$,i,1)) Xor Asc(Mid$(k$,pos+1,1)))
         
         ret$ = ret$ + char$
      EndIf
      
      cnt = (cnt + 1) Mod(s+1)
   Next
   
   Return ret$
End Function