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

Question

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?

Answer 1

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)