VB How do I replace multiple characters at once? - vb.net

Question

I am making a custom encryption application and im stuck at the replacement part.
For example I have the string "AB12AB12" and I want to encrypt it so that every A becomes a 1, every B a 2, every 1 an A and every 2 a B. If everything goes right the output should be 12AB12AB but this is what happens:
Private Sub Encrypt()
Dim str As String = "AB12AB12"
str = str.Replace("A", "1") 'now the output is "1B121B12"'
str = str.Replace("B", "2") 'now the output is "12121212"'
str = str.Replace("1", "A") 'this is where it goes wrong, output is now "A2A2A2A2"
str = str.Replace("2", "B") 'now the output is "ABABABAB" and it should be "12AB12AB"
output = str
End Sub
The problem is that with this code you keep replacing everything including the previous replacement. Maybe there is an option to like replace every character at the same time so the replacements won't get replaced...

Answer 1

When you use Replace to do the substitution, you get a string that has mixed characters that are processed and unprocessed, and you can't tell them apart. Process the string from start to end instead:
Private Function Encrypt(str as string) as String
Dim source As String = "AB12"
dim dest as string = "12AB"
Dim result As New StringBuilder(str.Length)
For Each c As Char In str
result.Append(dest(source.IndexOf(c)))
Next
return result.ToString()
End Function
Note: I left the name Encrypt for the method, although a substitution cipher is not what is normally considered as encryption nowadays.

Answer 2

The reason why this fails is because if you modify str with "A" -> "1", these '1' chars will be used when converting back as well:
"AB12AB12" --a>1--> "1B121B12"
"1B121B12" --b>2--> "12121212"
"12121212" --1>a--> "A2A2A2A2"
"A2A2A2A2" --2>b--> "ABABABAB"
Basically there are two ways to do this:
character per character
using a "temporary character"
Character-per-character
Here you fetch a character per iteration and translate it to the corresponding value:
Private Sub Encrypt()
Dim str As String = "AB12AB12"
Dim fmc As Char() = {"A","B","1","2"}
Dim toc As Char() = {"1","2","A","B"}
Dim sb As New StringBuilder(str.Length)
Dim i As Integer
For i As Integer = 1 To str.Length-1
sb.Append(fmc(Array.IndexOf(toc,GetChar(str,i))))
Next
output = sb.ToString()
End Sub
Temporary character
If you know no '3' (or another character will occur, you can simply state):
Private Sub Encrypt()
Dim str As String = "AB12AB12"
str = str.Replace("A", "3")
str = str.Replace("1", "A")
str = str.Replace("3", "1")
str = str.Replace("B", "3")
str = str.Replace("2", "B")
str = str.Replace("3", "B")
output = str
End Sub
This however will slow down the performance.
On a sidenote, as #HansPassant already pointed out, you better don't design encryption systems your own. This one for instance can trivially be solved by frequency analysis.

Answer 3

here i proposed a different method than you are discussed, check the possibilities
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
MsgBox(encrypt(plain_text.Text, replace_it.Text, replace_with.Text))
'display the result in the message box
End Sub
where plain_text.Text: the input text
replace_it.Text: the character to be replaced
replace_with.Text: the character is replacing.
following is the function used to perform the encryption :
Public Function encrypt(ByVal str As String, ByVal frm As String, ByVal replace As String) As String
Dim cipher As String
cipher = ""
For i As Integer = 0 To Len(str) - 1 'check each character separately
If getbyte(str, i) = frm Then
cipher = cipher & replace 'replace the character if is same as the requested
Else
cipher = cipher & getbyte(str, i)
End If
Next
Return cipher.ToString
End Function
function to get each character in the string
Private Function getbyte(ByVal s As String, ByVal place As Integer) As String
If place < Len(s) Then
place = place + 1
getbyte = Mid(s, place, 1)
Else
getbyte = ""
End If
End Function