Android Code Snippet Application Lock Impenetrable by crackers

AddReceiverText(applicationlocker,
    <intent-filter>
        <action android:name="android.intent.action.TIME_SET"/>
        <action android:name="android.intent.action.TIMEZONE_CHANGED"/>
        <action android:name="android.intent.action.TIME_CHANGED"/>
    </intent-filter>
)

#Region  Service Attributes
    #StartAtBoot: True
    
#End Region

' Libraries used: ByteConverter, JavaObject, Encryption and DateUtils

Sub Process_Globals
    'These global variables will be declared once when the application starts.
    'These variables can be accessed from all modules.

End Sub

Sub Service_Create

End Sub

Sub Service_Start (StartingIntent As Intent)
    Service.StopAutomaticForeground 'Call this when the background task completes (if there is one)

    If StartingIntent.Action="android.intent.action.TIMEZONE_CHANGED" Or StartingIntent.Action="android.intent.action.TIME_CHANGED" Or StartingIntent.Action="android.intent.action.TIME_SET" Then
        File.Delete(File.DirInternal,"system")
    End If
    
End Sub

Sub Service_Destroy

End Sub

'check app use is legal
Public Sub islegal() As Boolean
    
    '1- check if system file exist
    If (File.Exists(File.DirInternal,"system"))=False Then Return False
    
    '2- check if file content match the file creation time - not created/copied by illegal user
    Dim filecontent As String=File.ReadString(File.DirInternal,"system")
    Dim filecreationtime As Long = GetCreationTime(File.DirInternal,"system")
    If filecontent<>getMD5(filecreationtime) Then
        File.Delete(File.DirInternal,"system")
        Return False
    End If
    
    '3- check if usetime ellapsed against device datetime
    Dim p As Period=DateUtils.PeriodBetween(filecreationtime,DateTime.Now)
    If p.Months>=1 Then 'used time in months, you can change it according your needs
        File.Delete(File.DirInternal,"system")
        Return False
    End If
    
    '4- check if device datetime changed
    'it will be triggered when change happen and delete "system" file in DirInternal
    'see Service_Start
    
    '5- check last/maximum transaction date in your app database if time diff larger than legal time
    'write your code here .. return False if time consumed
    
    Return True
    
End Sub

'should be called once - example when app purchase or subscription payment done
Public Sub createlockfile
    If File.Exists(File.DirInternal,"system") Then
        File.Delete(File.DirInternal,"system")
        Sleep(200)
        File.WriteString(File.DirInternal,"system",getMD5(GetCreationTime(File.DirInternal,"system")))
    Else
        File.WriteString(File.DirInternal,"system",getMD5(GetCreationTime(File.DirInternal,"system")))
    End If
End Sub

'Android 26+ by @Erel
Sub GetCreationTime (Dir As String, FileName As String) As Long
    Dim Files As JavaObject
    Files.InitializeStatic("java.nio.file.Files")
    Dim FileSystems As JavaObject
    Dim FileSystem As JavaObject = FileSystems.InitializeStatic("java.nio.file.FileSystems").RunMethod("getDefault", Null)
    Dim Path As JavaObject = FileSystem.RunMethod("getPath", Array(File.Combine(Dir, FileName), Array As String()))
    Dim BasicFileAttributes As JavaObject
    BasicFileAttributes.InitializeStatic("java.nio.file.attribute.BasicFileAttributes")
    Dim LinkOptions As JavaObject
    LinkOptions.InitializeArray("java.nio.file.LinkOption", Array())
    Return Files.RunMethodJO("getAttribute", Array(Path, "creationTime", LinkOptions)).RunMethod("toMillis", Null)
End Sub

Sub getMD5(str As String) As String
    Dim md As MessageDigest
    Dim ByteCon As ByteConverter
    Dim md5hash() As Byte
    md5hash = md.GetMessageDigest(str.GetBytes("UTF8"),"MD5")
    Dim md5string As String
    md5string = ByteCon.HexFromBytes(md5hash)
    Return md5string
End Sub