how can a password-protected PDF file be opened programmatically?

.net, c#, password-protection, pdf

Solution

To open a password protected PDF you will need to develop at least a PDF parser, decryptor and generator. I wouldn't recommend to do that, though. It's nowhere near an easy task to accomplish.

With help of a PDF library everything is much simpler. You might want to try Docotic.Pdf library for the task (disclaimer: I work for the vendor of the library).

Here is a sample for you task:

public static void unprotectPdf(string input, string output)
{
    bool passwordProtected = PdfDocument.IsPasswordProtected(input);
    if (passwordProtected)
    {
        string password = null; // retrieve the password somehow

        using (PdfDocument doc = new PdfDocument(input, password))
        {
            // clear both passwords in order
            // to produce unprotected document
            doc.OwnerPassword = "";
            doc.UserPassword = "";

            doc.Save(output);
        }
    }
    else
    {
        // no decryption is required
        File.Copy(input, output, true);
    }
}

Docotic.Pdf can also extract text (formatted or not) from PDFs. It might be useful for indexing (I guess it's what you are up to because you mentioned Adobe IFilter)

Problem

The Adobe IFilter doesn't provide a mechanism to supply a password to open a password-protected PDF file, so it cannot be used to open password-protected files. I was wondering, is there a relatively straightforward way to programmatically retrieve the actual encrypted data inside the PDF file, decrypt it using a standard cryptography API, and then build a new PDF file with the decrypted data?

Original source