Add public QPDFWriter::copyEncryptionParameters

Method to copy encryption parameters from another file.  Adapted from
existing code to copy encryption parameters from the original file.

ChangeLog

@@ -1,5 +1,8 @@
 2012-07-14  Jay Berkenbilt  <ejb@ql.org>
 
+	* QPDFWriter: add public copyEncryptionParameters to allow copying
+	encryption parameters from another file.
+
 	* QPDFWriter: detect if the user has inserted an indirect object
 	from another QPDF object and throw an exception directing the user
 	to copyForeignObject.

TODO

@@ -64,10 +64,9 @@ Next
     - Tests through qpdf command line: copy pages from multiple PDFs
       starting with one PDF and also starting with empty.
 
- * (Hopefully) Provide an option to copy encryption parameters from
-   another file.  This would make it possible to decrypt a file,
-   manually work with it, and then re-encrypt it using the original
-   encryption parameters including a possibly unknown owner password.
+ * qpdf commandline: provide an option to copy encryption parameters
+   from another file, specifying file and password.  Search for "Copy
+   encryption parameters" in qpdf.test.
 
 
 Soon

include/qpdf/QPDFWriter.hh

@@ -188,6 +188,11 @@ class QPDFWriter
     QPDF_DLL
     void setPreserveEncryption(bool);
 
+    // Copy encryption parameters from another QPDF object.  If you
+    // want to copy encryption from the object you are writing, call
+    // setPreserveEncryption(true) instead.
+    void copyEncryptionParameters(QPDF&);
+
     // Set up for encrypted output.  Disables stream prefiltering and
     // content normalization.  Note that setting R2 encryption
     // parameters sets the PDF version to at least 1.3, setting R3
@@ -269,7 +274,6 @@ class QPDFWriter
 	int V, int R, int key_len, long P,
 	std::string const& O, std::string const& U,
 	std::string const& id1, std::string const& user_password);
-    void copyEncryptionParameters();
     void setDataKey(int objid);
     int openObject(int objid = 0);
     void closeObject(int objid);

libqpdf/QPDFWriter.cc

@@ -389,10 +389,11 @@ QPDFWriter::setEncryptionParameters(
 }
 
 void
-QPDFWriter::copyEncryptionParameters()
+QPDFWriter::copyEncryptionParameters(QPDF& qpdf)
 {
+    this->preserve_encryption = false;
     generateID();
-    QPDFObjectHandle trailer = this->pdf.getTrailer();
+    QPDFObjectHandle trailer = qpdf.getTrailer();
     if (trailer.hasKey("/Encrypt"))
     {
 	QPDFObjectHandle encrypt = trailer.getKey("/Encrypt");
@@ -410,6 +411,8 @@ QPDFWriter::copyEncryptionParameters()
 	}
 	QTC::TC("qpdf", "QPDFWriter copy encrypt metadata",
 		this->encrypt_metadata ? 0 : 1);
+        this->id1 =
+            trailer.getKey("/ID").getArrayItem(0).getStringValue();
 	setEncryptionParametersInternal(
 	    V,
 	    encrypt.getKey("/R").getIntValue(),
@@ -1625,7 +1628,7 @@ QPDFWriter::write()
 
     if (preserve_encryption)
     {
-	copyEncryptionParameters();
+	copyEncryptionParameters(this->pdf);
     }
 
     if (! this->forced_pdf_version.empty())

qpdf/qtest/qpdf.test

@@ -1369,6 +1369,23 @@ $td->runtest("check output",
 	     {$td->FILE => 'a.pdf'},
 	     {$td->FILE => 'decrypted-crypt-filter.pdf'});
 
+# Copy encryption parameters
+$n_tests += 3;
+$td->runtest("create encrypted file",
+	     {$td->COMMAND =>
+		  "qpdf --encrypt user owner 128 --use-aes=y --extract=n --" .
+                  " minimal.pdf a.pdf"},
+             {$td->STRING => "", $td->EXIT_STATUS => 0});
+$td->runtest("copy encryption parameters",
+             {$td->COMMAND => "test_driver 30 minimal.pdf a.pdf"},
+             {$td->STRING => "test 30 done\n", $td->EXIT_STATUS => 0},
+             $td->NORMALIZE_NEWLINES);
+$td->runtest("checkout encryption",
+	     {$td->COMMAND => "qpdf --show-encryption b.pdf --password=owner"},
+	     {$td->FILE => "copied-encryption.out",
+	      $td->EXIT_STATUS => 0},
+             $td->NORMALIZE_NEWLINES);
+
 show_ntests();
 # ----------
 $td->notify("--- Content Preservation Tests ---");

qpdf/qtest/qpdf/copied-encryption.out

@@ -0,0 +1,12 @@
+R = 4
+P = -20
+User password = user
+extract for accessibility: allowed
+extract for any purpose: not allowed
+print low resolution: allowed
+print high resolution: allowed
+modify document assembly: allowed
+modify forms: allowed
+modify annotations: allowed
+modify other: allowed
+modify anything: allowed

qpdf/test_driver.cc

@@ -1024,6 +1024,17 @@ void runtest(int n, char const* filename1, char const* filename2)
             std::cout << "logic error: " << e.what() << std::endl;
         }
     }
+    else if (n == 30)
+    {
+        assert(filename2 != 0);
+        QPDF encrypted;
+        encrypted.processFile(filename2, "user");
+        QPDFWriter w(pdf, "b.pdf");
+	w.setStaticID(true);
+	w.setStreamDataMode(qpdf_s_preserve);
+        w.copyEncryptionParameters(encrypted);
+	w.write();
+    }
     else
     {
 	throw std::runtime_error(std::string("invalid test ") +