VB.net Base64 + XOR

So I want to make a function that can Encrypt and Decrypt to Base64 and then XOR it with a key.

Here's my code so far:

    Imports System.Text

   Module Module1

    Private key As String = "37526"


    Sub Main()
        Dim test As String = Console.ReadLine
        test = Decrypt(test)
        Console.WriteLine(test)
        Main()

    End Sub

    Public Function Decrypt(CipherText As String) As String

        Dim decoded = Convert.FromBase64String(CipherText)
        Dim dexored = [xor](decoded, key)
        Return Encoding.UTF8.GetString(dexored)

    End Function

    Private Function [xor](text As Byte(), key As String) As Byte()
        Dim res As Byte() = New Byte(text.Length - 1) {}
        For c As Integer = 0 To text.Length - 1
            res(c) = CByte((text(c)) Xor CUInt(Val(c Mod key.Length)))
        Next

        Return res
    End Function


    Public Function Encrypt(Plaintext As String)

        Dim encoded = Encoding.UTF8.GetBytes(Plaintext)
        Dim xored = [xor](encoded, key)
        Return Convert.ToBase64String(xored)

    End Function

End Module

So here's my problem: this code works fine, but I'm trying to decrypt this string "X1hZXVoCBQY=", the decryption of that string should be "lolol123". I don't know if my code is wrong or if the key is incorrect. Could someone check my code or give me the correct key? I tried to bruteforce the key but that dind't work out for me. If I try to decrypt the string I don't get "lolol123", I get "_Y[^^"

Solution

It's going wrong in the xor function. Trying to do too much in one line can make it too complicated to immediately see what is going on, so I started re-writing it more as how I would have done and a quick check showed it to be working before I started looking at anything else:

Option Infer On
Option Strict On
' ...
Private Function [xor](text As Byte(), key As String) As Byte()
    Dim res As Byte() = New Byte(text.Length - 1) {}
    Dim keyBytes = Encoding.UTF8.GetBytes(key)

    For c = 0 To text.Length - 1
        res(c) = CByte((text(c)) Xor keyBytes(c Mod keyBytes.Length))
    Next

    Return res

End Function