How to decrypt a simpleXor Base64encoded string - vb.net

Question

I am new with this encryption stuff and also i am looking into a pervious engineers code. The encrypt works find but i would like a decryptor, i am not sure what or how to start
There is a PasswordEncryption class which looks as below
Public Class PasswordEncryption
Public Shared arrBase64EncMap(64) As String
Public Shared arrBase64DecMap(127) As Integer
Const BASE_64_MAP_INIT As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
Public Shared Function EncryptionPassword() As String
Return "F2oWrB4sA3feEh1tz"
End Function
Public Shared Function simpleXor(ByVal strIn As String, ByVal strKey As String) As String
Dim iInIndex As Integer
Dim iKeyIndex As Integer
Dim strReturn As String
If Len(strIn) = 0 Or Len(strKey) = 0 Then
simpleXor = ""
Exit Function
End If
iInIndex = 1
iKeyIndex = 1
strReturn = ""
'** Step through the plain text source XORing the character at each point with the next character in the key **
'** Loop through the key characters as necessary **
Do While iInIndex <= Len(strIn)
strReturn = strReturn & Chr(Asc(Mid(strIn, iInIndex, 1)) Xor Asc(Mid(strKey, iKeyIndex, 1)))
iInIndex = iInIndex + 1
If iKeyIndex = Len(strKey) Then iKeyIndex = 0
iKeyIndex = iKeyIndex + 1
Loop
simpleXor = strReturn
End Function
Public Shared Function base64Encode(ByVal strPlain As String) As String
Dim iLoop As Integer
Dim iBy3 As Integer
Dim strReturn As String
Dim iIndex As Integer
Dim iFirst As Integer
Dim iSecond As Integer
Dim iiThird As Integer
If strPlain.Length = 0 Then
base64Encode = ""
Exit Function
End If
'** Set up Base64 Encoding and Decoding Maps for when we need them **
For iLoop = 0 To Len(BASE_64_MAP_INIT) - 1
arrBase64EncMap(iLoop) = Mid(BASE_64_MAP_INIT, iLoop + 1, 1)
Next
For iLoop = 0 To Len(BASE_64_MAP_INIT) - 1
arrBase64DecMap(Asc(arrBase64EncMap(iLoop))) = iLoop
Next
'** Work out rounded down multiple of 3 bytes length for the unencoded text **
iBy3 = (strPlain.Length \ 3) * 3
strReturn = ""
'** For each 3x8 byte chars, covert them to 4x6 byte representations in the Base64 map **
iIndex = 1
Do While iIndex <= iBy3
iFirst = Asc(Mid(strPlain, iIndex + 0, 1))
iSecond = Asc(Mid(strPlain, iIndex + 1, 1))
iiThird = Asc(Mid(strPlain, iIndex + 2, 1))
strReturn = strReturn & arrBase64EncMap((iFirst \ 4) And 63)
strReturn = strReturn & arrBase64EncMap(((iFirst * 16) And 48) + ((iSecond \ 16) And 15))
strReturn = strReturn & arrBase64EncMap(((iSecond * 4) And 60) + ((iiThird \ 64) And 3))
strReturn = strReturn & arrBase64EncMap(iiThird And 63)
iIndex = iIndex + 3
Loop
'** Handle any trailing characters not in groups of 3 **
'** Extend to multiple of 3 characters using = signs as per RFC **
If iBy3 < strPlain.Length Then
iFirst = Asc(Mid(strPlain, iIndex + 0, 1))
strReturn = strReturn & arrBase64EncMap((iFirst \ 4) And 63)
If (strPlain.Length Mod 3) = 2 Then
iSecond = Asc(Mid(strPlain, iIndex + 1, 1))
strReturn = strReturn & arrBase64EncMap(((iFirst * 16) And 48) + ((iSecond \ 16) And 15))
strReturn = strReturn & arrBase64EncMap((iSecond * 4) And 60)
Else
strReturn = strReturn & arrBase64EncMap((iFirst * 16) And 48)
strReturn = strReturn & "="
End If
strReturn = strReturn & "="
End If
'** Return the encoded result string **
base64Encode = strReturn
End Function
End Class
I have a simple form which has two buttons Encrypt and Decrypt and couple of textboxes.
The click event of the encrypt button does the following
Private Sub btnEncrypt_Click(sender As Object, e As EventArgs) Handles btnEncrypt.Click
If tbxPassword.Text IsNot Nothing Then
Dim PasswordEncryption As PasswordEncryption = New PasswordEncryption()
Dim strXOR As String = PasswordEncryption.simpleXor(Strings.Left(tbxPassword.Text.ToString().Trim, 20), PasswordEncryption.EncryptionPassword)
Dim encryptedPassword = PasswordEncryption.base64Encode(strXOR)
tbxResult.Text = encryptedPassword
Else
MessageBox.Show("No action can be performed")
End If
End Sub
I have the following code in the decrypt button but it does not bring my original string back
Private Sub btnDecrypt_Click(sender As Object, e As EventArgs) Handles btnDecrypt.Click
If tbxPassword.Text IsNot Nothing Then
Dim PasswordEncryption As PasswordEncryption = New PasswordEncryption()
Dim strXOR As String = PasswordEncryption.simpleXor(Strings.Left(tbxPassword.Text.ToString().Trim, 20), PasswordEncryption.EncryptionPassword)
Dim decryptedPassword = PasswordEncryption.base64Encode(strXOR)
tbxResult.Text = decryptedPassword
Else
MessageBox.Show("No action can be performed")
End If
What is that i need to implement to get the original string back. Thank you guys!

Answer 1

Ok guys i have figured it out from the following link
https://www.codingforums.com/archive/index.php/t-28425.html
I have included the decode method and as per comment from MarkL I have first Base64Decoded the string and then performed simpleXOR and i got the original string back.
My decrypt method looks like this
Private Sub btnDecrypt_Click(sender As Object, e As EventArgs) Handles btnDecrypt.Click
If tbxPassword.Text IsNot Nothing Then
Dim PasswordEncryption As PasswordEncryption = New PasswordEncryption()
' Dim strXOR As String = PasswordEncryption.simpleXor(Strings.Left(tbxPassword.Text.ToString().Trim, 20), PasswordEncryption.EncryptionPassword)
Dim base64DecodedValue = PasswordEncryption.base64Decode(tbxPassword.Text)
tbxResult.Text = PasswordEncryption.simpleXor(Strings.Left(base64DecodedValue.Trim, 20), PasswordEncryption.EncryptionPassword)
Else
MessageBox.Show("No action can be performed")
End If
End Sub