Problem Statement

Implement a Caesar cipher, both encoding and decoding. The key is an integer from 1 to 25. This cipher rotates (either towards left or right) the letters of the alphabet (A to Z). The encoding replaces each letter with the 1st to 25th next letter in the alphabet (wrapping Z to A). So key 2 encrypts "HI" to "JK", but key 20 encrypts "HI" to "BC". This simple "mono-alphabetic substitution cipher" provides almost no security, because an attacker who has the encoded message can either use frequency analysis to guess the key, or just try all 25 keys. Caesar cipher is identical to Vigenère cipher with a key of length 1. Also, Rot-13 is identical to Caesar cipher with key 13.

Let's start with the solution:

TextWindow.Write("Enter a 1-25 number key (-ve number to decode): ") 
key = TextWindow.ReadNumber()
TextWindow.Write("Enter message: ") 
message = text.ConvertToUpperCase(TextWindow.Read())
caeser = ""
For n = 1 To Text.GetLength(message)
  letter = Text.GetSubText(message,n,1)
  code = Text.GetCharacterCode(letter)
  If code = 32 Then
    newCode = 32
  Else
    newCode = code + key
    If newCode > 90 Then
      newCode = newCode - 26
    ElseIf newCode < 65 then
      newCode = newCode + 26
    EndIf
  EndIf
  codeLetter = Text.GetCharacter(newCode)
  caeser = Text.Append(caeser,codeLetter)
EndFor
TextWindow.WriteLine(message)
TextWindow.WriteLine(caeser)