How to validate that a file is a password protected ZIP file, using C#

Question

Given a path to a file, how can I validate that the file is a password-protected zip file?

i.e., how would I implement this function?

bool IsPasswordProtectedZipFile(string pathToFile)

I don't need to unzip the file -- I just need to verify that it's a ZIP and has been protected with some password.

Thanks

Answer 1

Using SharpZipLib, the following code works. And by works I mean entry.IsCrypted returns true or false based on whether or not there is a password for the first entry in the zip file.

var file = @"c:\testfile.zip";
FileStream fileStreamIn = new FileStream(file, FileMode.Open, FileAccess.Read);
ZipInputStream zipInStream = new ZipInputStream(fileStreamIn);
ZipEntry entry = zipInStream.GetNextEntry();
Console.WriteLine("IsCrypted: " + entry.IsCrypted);

There's a simple tutorial on using SharpZipLib on CodeProject.

Thus a simple implementation looks something like:

public static bool IsPasswordProtectedZipFile(string path)
{
    using (FileStream fileStreamIn = new FileStream(path, FileMode.Open, FileAccess.Read))
    using (ZipInputStream zipInStream = new ZipInputStream(fileStreamIn))
    {
        ZipEntry entry = zipInStream.GetNextEntry();
        return entry.IsCrypted;
    }
}

Note there's no real error handling or anything...

Answer 2

In ZIP archives, the password is not placed on the file, but on the individual entries within the file. A zip can contain some entries encrypted and some not. Here's some example code to check for encryption on entries in DotNetZip:

int encryptedEntries = 0;
using (var zip = ZipFile.Read(nameOfZipFile)) 
{
    // check a specific, named entry: 
    if (zip["nameOfEntry.doc"].UsesEncryption)
       Console.WriteLine("Entry 'nameOfEntry.doc' uses encryption"); 

    // check all entries: 
    foreach (var e in zip)
    {
       if (e.UsesEncryption)
       {
           Console.WriteLine("Entry {0} uses encryption", e.FileName); 
           encryptedEntries++; 
       }
    }
}

if (encryptedEntries > 0) 
    Console.WriteLine("That zip file uses encryption on {0} entrie(s)", encryptedEntries); 

If you'd prefer, you can use LINQ:

private bool ZipUsesEncryption(string archiveToRead)
{
    using (var zip = ZipFile.Read(archiveToRead))
    {
        var selection = from e in zip.Entries
            where e.UsesEncryption
            select e;

        return selection.Count > 0;
    }
}

Answer 3

At this point in the .NET Framework maturity you will need to use a 3rd party tool. There are many commercial libraries that can be Googled. I'm suggesting one free one from Microsoft's Codeplex website DotNetZip. The front page states "the library supports zip passwords".