Signing a PDF with multiple revisions results in malformed xref table

[mpldr]

Signing a PDF with multiple revisions results in malformed xref table

[vanbroup]

I can confirm this happens with some files but have not been able to identify the cause yet.

[vanbroup]

Signing fails if the document has more than one xref table (revisions).

[vanbroup]

This item count includes all objects of the document:

pdfsign/sign/pdfxref.go

Line 32 in 4fb6faf

new_xref_size := "xref\n0 " + strconv.FormatInt(context.PDFReader.XrefInformation.ItemCount+4, 10) + "\n"

But only the (latest?) xref table entries are included in the xref new table.

Using the correct count for the xref does not seem to fix the problem.

Adding a new referencing xref table instead of including all items from the last table seems to be a cleaner approach but has not resulted in any success for now.

The ReaderXrefInformation only contains information about the entire document and probably needs to be updated to contain information about each xref table or stream:
https://github.com/digitorus/pdf/blob/master/read.go#L92

[dhernandez]

This code at the writeXrefTable function makes the trick, but it probably is not the best option:

func (context *SignContext) writeXrefTable() error {
	context.InputFile.Seek(context.PDFReader.XrefInformation.StartPos, 0)
	scanner := bufio.NewScanner(context.InputFile)
	scanner.Split(bufio.ScanWords)
	scanner.Scan() // This should be xref string
	scanner.Scan()
	first_object_id := scanner.Text()
	scanner.Scan()
	xref_items := scanner.Text()
	xref_items_i, _ := strconv.Atoi(xref_items)

	xref_size := fmt.Sprintf("xref\n%s %s\n", first_object_id, xref_items)
	new_xref_size := fmt.Sprintf("xref\n%s %s\n", first_object_id, fmt.Sprint(xref_items_i+4))
...
}