Question

I want to test if a zip has a particular password in vb.net. How can I create a function like check_if_zip_pass(file, pass) As Boolean?

I can’t seem to find anything in the .net framework that does this already, unless I’m missing something incredibly obvious.

This method should NOT extract the files, only return True if the attempted pass is valid and False if not.

Answer 1

Use a 3rd party library, like DotNetZip. Keep in mind that passwords in zipfiles are applied to entries, not to the entire zip file. So your test doesn’t quite make sense.

One reason WinZip may refuse to unpack the zipfile is that the very first entry is protected with a password. It could be the case that only some entries are protected by password, and some are not. It could be that different passwords are used on different entries. You’ll have to decide what you want to do about these possibilities.

One option is to suppose that only one password is used on any entries in the zipfile that are encrypted. (This is not required by the zip specification) In that case, below is some sample code to check the password. There is no way to check a password without doing the decryption. So this code decrypts and extracts into Stream.Null.

public bool CheckZipPassword(string filename, string password)
{
    bool success = false;
    try
    {
        using (ZipFile zip1 = ZipFile.Read(filename))
        {
            var bitBucket = System.IO.Stream.Null;
            foreach (var e in zip1)
            {
                if (!e.IsDirectory && e.UsesEncryption)
                {
                    e.ExtractWithPassword(bitBucket, password);
                }
            }
        }
        success = true;    
    }
    catch(Ionic.Zip.BadPasswordException) { }
    return success;
}

Whoops! I think in C#. In VB.NET this would be:

    Public Function CheckZipPassword(filename As String, password As String) As System.Boolean
        Dim success As System.Boolean = False
        Try
            Using zip1 As ZipFile = ZipFile.Read(filename)
                Dim bitBucket As System.IO.Stream = System.IO.Stream.Null
                Dim e As ZipEntry
                For Each e in zip1
                    If (Not e.IsDirectory) And e.UsesEncryption Then
                        e.ExtractWithPassword(bitBucket, password)
                    End If
                Next
             End Using
             success = True
        Catch ex As Ionic.Zip.BadPasswordException
        End Try
        Return success
    End Function