Don't encrypt trailer, fixes fuzz issue 15983

Ordinarily the trailer doesn't contain any strings, so this is usually
a non-issue, but if the trailer contains strings, linearizing and
encrypting with object streams would include encrypted strings in the
trailer, which would blow out the padding because encrypted strings
are longer than their cleartext counterparts.

2 files changed, 9 insertions, 2 deletions

diff --git a/fuzz/qpdf_extra/15983.fuzz b/fuzz/qpdf_extra/15983.fuzz
new file mode 100644
index 00000000..7e3366df
--- /dev/null
+++ b/fuzz/qpdf_extra/15983.fuzz
diff --git a/libqpdf/QPDFWriter.cc b/libqpdf/QPDFWriter.cc
index f5fa2bc9..116d493f 100644
--- a/libqpdf/QPDFWriter.cc
+++ b/libqpdf/QPDFWriter.cc
@@ -1341,7 +1341,11 @@ QPDFWriter::writeTrailer(trailer_e which, int size, bool xref_stream,
                          qpdf_offset_t prev, int linearization_pass)
 {
     QPDFObjectHandle trailer = getTrimmedTrailer();
-    if (! xref_stream)
+    if (xref_stream)
+    {
+        this->m->cur_data_key.clear();
+    }
+    else
     {
 	writeString("trailer <<");
     }
@@ -3320,7 +3324,10 @@ QPDFWriter::writeLinearized()
 		if (this->m->pipeline->getCount() != first_xref_end)
                 {
                     throw std::logic_error(
-                        "insufficient padding for first pass xref stream");
+                        "insufficient padding for first pass xref stream; "
+                        "first_xref_end=" +
+                        QUtil::int_to_string(first_xref_end) +
+                        "; endpos=" + QUtil::int_to_string(endpos));
                 }
 	    }
 	    writeString("\n");