All you need to do is look at the entries in the file.
Assuming you are using SharpZipLib:
public static bool IsPasswordProtected(string path)
{
using (FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read))
{
using (ZipInputStream zip = new ZipInputStream(fs))
{
ZipEntry entry = zip.GetNextEntry();
return entry.IsCrypted;
}
}
}
Should do it.
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