How to replace or remove last 500 bytes of a file without rewriting all the file?

Hi everyone,
Usually I only ask for help when I can't find a solution for several days or weeks... And guess what? That just happen!
So, this is what i am trying to do:
I have a program to ZIP folder and protect them with password, then it encrypts the zip file.
That it's working fine, until the user forgets his password.
So, what I want to do is give the user a Recovery Password option for each ZIP file created. I can't use the Windows Registry because the idea is to be able to recover the password in any computer.So i came up with an idea...
In simple terms, this will work like this:
0 - Choose folder to ZIP
1 - Ask user for recover details (date of birth, email etc)
2 - ZIP folder with password
3 - Encrypt ZIP file
4 - Encrypt recover details and convert it to HEX
5 - Add recover details (in HEX) to the end of the ZIP file (last bytes)
6 - Add "5265636F76657244657461696C73" which is the text "RecoverDetails" in HEX
7 - Add "504B0506000000000000000000000000000000000000" this is the final bytes of a ZIP file and will make the Operating System think that is a ZIP file (i know that will give an error when we try to open it.. the ideia is to change the
extension later and use my software to do all the work to access this ZIP/folder again)
So, explaining what it's here, I want to say that I managed how to do all of this so far. The point number 6 will help us to determine where the recover details are in the file, or if they actually exist because user can choose not to use them.
In order to unlock this ZIP and extract it's contents, I need to reverse what I've done. That means, that  need to read only the last 500 bytes (or less if the file is smaller) of the ZIP and remove those extra bytes I added so the program can check
if the user is inputing a correct password, and if so decrypt contents and extract them.
But, if the user insert a wrong password I need to re-add those bytes with the recover details again to the ZIP file.
The second thing is, if the user forgets his password and asks to recover it, a form will be shown asking to insert the recover detail (date of birth, email etc), so we need to reed the last 500 bytes of the ZIP, find the bytes in number 6 and remove the
bytes before number 6, remove bytes in number 6 and number 7, and we will have the recover details to match against the user details input.
I have all done so far with the locking process. But i need help with the unlocking.
I am not sure if it's possible, but this what i am looking for:
Read last 500 bytes of a file, remove the bytes with recover details and save the file. Without reading the whole file, because if we have a 1GB file that will take a very long time. Also, i don't want to "waste" hard drive space creating a new
clone file with 1GB and then delete the original.
And then add them back "in case user fails the password" which should be exactly the same.
This sounds a bit confusing I know, even to me, I am writing and trying to explain this the better I can.. Also my English is not the best..
Here it goes some code to better understanding:
'READ LAST 500 BYTES OF ZIP FILE TO CHECK IF IT CONTAINS RECOVER DETAILS
Dim oFileStream As New FileStream(TextBox_ZIP_to_Protect.Text & ".zip", FileMode.Open, FileAccess.Read)
Dim oBinaryReader As New BinaryReader(oFileStream)
Dim lBytes As Long = oFileStream.Length
oBinaryReader.BaseStream.Position = lBytes - 500
Dim fileData As Byte() = oBinaryReader.ReadBytes(500)
oBinaryReader.Close()
oFileStream.Close()
Dim txtTemp As New System.Text.StringBuilder()
For Each myByte As Byte In fileData
txtTemp.Append(myByte.ToString("X2"))
Next
Dim RecoveryDetailsPass_Holder = txtTemp.ToString()
'Dim Temp_2 = txtTemp.ToString()
'RichTextBox1.Text = txtTemp.ToString()
If txtTemp.ToString.Contains("505245434F47414653") Then
'we have password recovery details(the numbers mean RecoverDetails in HEX)
'next we will get rid of everything before and after of string "cut_at"
Dim mystr As String = RecoveryDetailsPass_Holder 'RichTextBox1.Text
Dim cut_at As String = "505245434F47414653"
Dim x As Integer = InStr(mystr, cut_at)
' Dim string_before As String = mystr.Substring(0, x - 1)
Dim string_after As String = mystr.Substring(x + cut_at.Length - 1)
RecoveryDetailsPass_Holder = RecoveryDetailsPass_Holder.Replace(string_after.ToString, "")
RecoveryDetailsPass_Holder = RecoveryDetailsPass_Holder.Replace("505245434F47414653", "") ' this is RecoverDetails in HEX
RecoveryDetailsPass_Holder = RecoveryDetailsPass_Holder.Replace("504B0506000000000000000000000000000000000000", "") ' this is the bytes of an empty zip file
'AT THIS POINT WE HAVE ONLY THE RECOVER PASSWORD DETAILS (date of birth, email etc) IN THE VARIABLE "RecoveryDetailsPass_Holder"
'////////////////////////////////////////////////////// TO DEBUG
'MsgBox(string_after.ToString & "505245434F47414653")
'InputBox("", "", string_after.ToString)
'\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ TO DEBUG
'Temp_2 = Temp_2.Replace(RecoveryDetailsPass_Holder.ToString, "")
Now that we have the recover details, we need to remove them from ZIP in order to the software try to unzip it with the password provided by the user on the GUI.
If the user needs to recover the password we have the details already in RecoveryDetailsPass_Holder variable and just need to match them against user input details.
If the user fails, we need to put the RecoveryDetailsPass_Holder back on the file.
Any question just ask, it's a bit trick to explain i think, but please ask.
Anyone know how to do this?
Many thanks in advanced.
Nothing is impossible!
@ Portugal
Vote if it's helpfull :)

@ ALL
Thank you very much for you help. I know that if I'm "playing" with bytes you should assume that I know a lot of VB.net, but I don't know that much unfortunately. I am not a beginner but I am still very fresh and I probably do stuff that work but
probably not in the best way...
Anyway, I will explain the idea of this little software I'm making. Once I wanted to create a program to protect folders with password, and I came up with something to change folder permissions to lock access to them, and that actually worked fine and quickly.
However, I managed how to "crack" the protection by going to folder properties, security tab and then give permissions back to my username. So that, to me, wasn't a safer system to protect folders, also I want the ability to use passwords. So I search
and search online for a way to do it, and someone replied (to someone with the same question as me) that the best option would be to create a zip with all contents of the folder, with password and then change the extension from .zip to .whatever and register
the new extension .whatever on the Windows Registry, so that file will have an icon and open with my software.
So I did...The program zips everything, change the extension and I added the encryption to avoid people changing the extension to ZIP or trying to open with 7-Zip or similar and be able to see the protected files names in the .zip/.whatever
Answering to all of you now:
@Armi
"System.IO.FileStream.SetLength"
I know I tried that but I erased the code because it didn't work for some reason, I don't remember why sorry, was long time before I created this post.
The last code I was trying to use was this:
' Set the stream position to the desired location of the stream.
Dim fileStream As IO.FileStream = _
New IO.FileStream(TextBox_ZIP_to_Protect.Text & ".zip", IO.FileMode.Append)
Try
' Set the stream (OFFSET) position to the desired location of the stream.
fileStream.Seek(210, IO.SeekOrigin.Current)
Dim Bytes_do_ZE As Byte() = HexStringToByteArray(Temp_2.ToString)
'Write Characters ASCII
For Each Byte_Do_Zeca As Byte In Bytes_do_ZE
fileStream.WriteByte(Byte_Do_Zeca)
Next
Finally
fileStream.Close()
End Try
and we need this:
Private Shared Function HexStringToByteArray(ByRef strInput As String) As Byte()
Dim length As Integer
Dim bOutput As Byte()
Dim c(1) As Integer
length = strInput.Length / 2
ReDim bOutput(length - 1)
For i As Integer = 0 To (length - 1)
For j As Integer = 0 To 1
c(j) = Asc(strInput.Chars(i * 2 + j))
If ((c(j) >= Asc("0")) And (c(j) <= Asc("9"))) Then
c(j) = c(j) - Asc("0")
ElseIf ((c(j) >= Asc("A")) And (c(j) <= Asc("F"))) Then
c(j) = c(j) - Asc("A") + &HA
ElseIf ((c(j) >= Asc("a")) And (c(j) <= Asc("f"))) Then
c(j) = c(j) - Asc("a") + &HA
End If
Next j
bOutput(i) = (c(0) * &H10 + c(1))
Next i
Return (bOutput)
End Function
That code, as I understand, is to search for the OFFSET of the bytes in the file and start to write from there... That OFFSET should be the beginning of the 500 bytes read on the code before. I got the OFFSET position "210" reading the file with
the HEX editor "HxD - Hexeditor v1.7.7.0" but using the OFFSET won't work because every file, password, recover details and so on, are different and so the file size, changing the OFFSET I
think.
@Reed Kimble
Does that sound like something which might work for you?
Thanks for your help. That might be some solution, however it seams a bit of the same problem where we need to read the bytes again to get the recover details. But, as I said in this post, because this is meant to password protect folders, do you think that
will apply as well?
@Crazypennie
Thanks for your reply.
All this appears really weak. The user has your application since he need it to open the file .... and the code in the application contain the code to read the file without knowing the password. Therefore anyone can read your code and retrieve the
data without the password ... if he knows VB.
The application can only open the file if the user didn't use a password to protect the file. Because the file is encrypted and needs to be unencrypted first.
When the application tries to open/read the file, will need to decrypt it first and then check for a password and do the validation. Also the application is with the code masked/protected which i think it might not be easy for reverse engineering.
- You need to use a web server and a symmetric key encryption
This a good idea, besides I don't know how to implement it. However the idea is to be able to:
1 - Protect a folder anywhere in any Windows computer (portable app)
2 - Recover password details (security question) in any computer, online and offline
And I think we need a computer always connected to the Internet to use that method, right?
@ Mr. Monkeyboy
Thank you very much for your effort.
I just wanted to let you know that the zip method you are using is no longer supported.
I didn't actually knew that. Thanks for letting me know.
Do you require the compressed encrypted files to actually be Zip files or could they just be compressed files that have nothing to do with Zip?
No, it doesn't need to be a .zip extension. I am actually using my own extension. It starts as a Zip but then I changed to my own extension which I have registered on the Windows Registry.
@ ALL
Thanks again to all for trying and spending time helping me.
By the way, I might not be able to answer or try any code during the weekend... It's easter break and family is around. Have a nice easter everyone. :)
Nothing is impossible! Imagination is the limit!

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               FilenameFilter select = new FileListFilter("F", "java");
               //get the contents of the directory
               File[] contents = myDir.listFiles(select);
                //list the contents of the directory
             if(contents != null)
                 System.out.println("\nThe " + contents.length  +
                 " matching item(s) in the directory " + myDir.getName() + " are:\n " );
                 for(int i = 0; i < contents.length; i++)
                   System.out.println("\n" +  contents[i] + " is a " +
                   (contents.isDirectory() ? "directory" : "file\n") +
    " last modified on " + new Date(contents[i].lastModified())
    + "\nthe number of bytes in TryFile3.java are " + myFile.length());
    else {
    System.out.println(myDir.getName() + " is not a directory");
    System.exit(0);
    import java.io.File;
    import java.io.FilenameFilter;
    import java.util.Date;
    public class FileListFilter implements FilenameFilter
    private String name; // file name filter
    private String extension; // File extension filter
    public FileListFilter(String name, String extension)
    this.name = name;
    this.extension = extension;
    // static boolean firstTime = true;
    public boolean accept(File diretory, String filename)
    //the following line of code can be inserted in order to find out who called the method
    // if(firstTime)
    // new Throwable("starting the accept() method").printStackTrace();
    boolean fileOK = true;
    //if there is a name filter specified, check the file name
    if(name != null)
    fileOK &= filename.startsWith(name);
    //if there is an extension filter, check the file extension
    if(extension != null)
    fileOK &= filename.endsWith('.' + extension);
    return fileOK;

    System.out.println("\n" + contents + " is a " +
    (contents.isDirectory() ? "directory" : "file\n") +
    " last modified on " + new Date(contents.lastModified())
    + "\nthe number of bytes in TryFile.java are " + myFile.length());I haven't read any of your italicized code, but perhaps there is a good reason why you have "myFile.length()" and not "contents.length()" in this line of code?

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