Question

The Asc function in VB takes a character and return an integer that represents that character. It does not matter what the integer representing the character actually is, but what matters is this: Asc("a") is 1 less than Asc("b"), so that: x=Asc("a") thisLetter = x+1 # thisLetter is the Asc("b") This is a powerful fact that is used in encryption techniques, so data transferred over the web is 'deciphered so it is unreadable to others. To decipher data, we take a string and change it into another string by adding a constant to its asc (short for ascii) value. To cipher and decipher data, we need two functions: one to take a string, and change it to something else. For this we need to use the asc function, and add the 'key' to result in a new string. For example, say x is one letter to be ciphered, and the key is 3. We can use: Newx=asc(x)+3 Newx will be an integer. To find out what letter that integer represents you can use the chr function as in: chr(x). Write a function cipher that takes a string and a key (integer). The function ciphers the string into another string and returns the new string. Note that when we reach 'z', and we want to add the key, we must 'roll' into the alphabet one more time, hence asc(''z''+3 should give us asc(''c''). Write another function to decipher (do the opposite of the previous function), given a string and a key, it returns the deciphered string. In the form, write an event handler for a button to get a string, and an integer as input, then call the cipher function and show its output. Then call the decipher function and display its output. The decipher output should match the original string.

Answer 1

The below program would help to provide an idea on how to solve the stated problem

================================VB code=====================================

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text.RegularExpressions
Imports System.Integer

Namespace StringCipherDecipher
Public Module Program
  
' The below function will check if the string has valid characters
Function CheckCharacters(ByVal WordToCheck As String)
Dim i As Integer
For i = 0 To WordToCheck.Length - 1
If Char.IsLetter(WordToCheck.Chars(i)) Then
Return True
End If
Next
Return False
End Function

'Below function will convert each character of the string to a new character adding the key to its Asc value

Function Cipher(ByVal strWord As String, ByVal strKey As Integer)
   Dim Newx As Integer
   Dim NewChar As String
Dim i As Integer

   NewChar=""

   For i = 0 To strWord.Length - 1
If strWord.Chars(i) = "z" Then
   Newx = Asc( "a")+strKey
Else
   Newx = Asc(strWord.Chars(i))+strKey

End If
   NewChar = NewChar + Chr(Newx)
Next
return NewChar
End Function

'Below function will convert each character of the string to a new character subtracting the key to its Asc value, opposite of cipher function

Function Decipher(ByVal strWord As String, ByVal strKey As Integer)
   Dim Newx As Integer
   Dim NewChar As String
Dim i As Integer

   NewChar=""

   For i = 0 To strWord.Length - 1   

If strWord.Chars(i) = "a" Then
   Newx = Asc( "z") - strKey
Else
   Newx = Asc(strWord.Chars(i)) - strKey

End If
   NewChar = NewChar + Chr(Newx)
Next
return NewChar

End Function

'Below function is the event handler that will trigger the input and cipher and decipher of entered string

Private Sub btnCipherDecipher_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles btnCipherDecipher.Click
Dim strWord As String
   Dim strKey As Integer

Dim CipherString As String

Dim DecipherString As String

strWord = InputBox("Enter a string to cipher", "Cipher String") 'Prompting to enter cipher string
strKey = InputBox("Enter the keyword to cipher string", "Cipher String Key") 'Prompting to enter cipher key

   If CheckCharacters(strWord) And IsNumeric(strKey) Then

  CipherString = Cipher(strWord, strKey) 'calling Cipher function passing the entered string with key
MsgBox("Cipher string : " + CipherString )

DecipherString = Decipher(CipherString , strKey) 'calling Decipher function passing the cipher string with key
   MsgBox("Decipher string : " + DecipherString )
Else
MsgBox("You provided an invalid value", _
MsgBoxStyle.OkOnly Or MsgBoxStyle.Information, _
"Cipher string")
End If

End Sub

================================End=========================================================