Insert digital signature into existing pdf file

Question

I need to insert a digital signature into already existing pdf files, using a rails application server. (Basically, clients upload pdf files and the server signs them with a

Answer 1

After some research, recurring to the OpenSSL documentation and exploring the Origami solution, i built the code below, and managed to insert a locally generated signature/certificate into a pdf document. Now I just need to figure out how to use this with an external generated certificate (check version 2 below, where i solved it). I've opened a new question where you can find some details on a difficulty i had with OpenSSL and DER encoded certificates.

To develop version 2, i also spent some time wondering how to add an annotation - so the signature becomes visible in Adobe reader - without adding a new page to the document. From origami documentation, i found the get_page method, which solved my last problem on this. I'm using Adobe Reader X, for the record.

Hope you find this useful as I will ;-).

VERSION 1 - Generate certificate and key file, and insert them directly into the document

require 'openssl'

begin
  require 'origami'
rescue LoadError
  ORIGAMIDIR = "C:\RailsInstaller\Ruby1.9.3\lib\ruby\gems\1.9.1\gems\origami-1.2.4\lib"
  $: << ORIGAMIDIR
  require 'origami'
end
include Origami

# Code below is based on documentation available on
# http://www.ruby-doc.org/stdlib-1.9.3/libdoc/openssl/rdoc/OpenSSL.html
key = OpenSSL::PKey::RSA.new 2048

open 'private_key.pem', 'w' do |io| io.write key.to_pem end
open 'public_key.pem', 'w' do |io| io.write key.public_key.to_pem end

cipher = OpenSSL::Cipher::Cipher.new 'AES-128-CBC'
pass_phrase = 'Origami rocks'

key_secure = key.export cipher, pass_phrase

open 'private_key.pem', 'w' do |io|
  io.write key_secure
end

#Create the certificate

name = OpenSSL::X509::Name.parse 'CN=nobody/DC=example'

cert = OpenSSL::X509::Certificate.new
cert.version = 2
cert.serial = 0
cert.not_before = Time.now
cert.not_after = Time.now + 3600

cert.public_key = key.public_key
cert.subject = name


OUTPUTFILE = "test.pdf"

contents = ContentStream.new.setFilter(:FlateDecode)
contents.write OUTPUTFILE,
  :x => 350, :y => 750, :rendering => Text::Rendering::STROKE, :size => 30

pdf = PDF.read('Sample.pdf')


# Open certificate files

#sigannot = Annotation::Widget::Signature.new
#sigannot.Rect = Rectangle[:llx => 89.0, :lly => 386.0, :urx => 190.0, :ury => 353.0]

#page.add_annot(sigannot)

# Sign the PDF with the specified keys
pdf.sign(cert, key, 
  :method => 'adbe.pkcs7.sha1',
  #:annotation => sigannot, 
  :location => "Portugal", 
  :contact => "myemail@email.tt", 
  :reason => "Proof of Concept"
)

# Save the resulting file
pdf.save(OUTPUTFILE)

VERSION 2 - Use existing certificates to sign a pdf document

require 'openssl'

begin
  require 'origami'
rescue LoadError
  ORIGAMIDIR = "C:\RailsInstaller\Ruby1.9.3\lib\ruby\gems\1.9.1\gems\origami-1.2.4\lib"
  $: << ORIGAMIDIR
  require 'origami'
end
include Origami

INPUTFILE = "Sample.pdf"
@inputfile = String.new(INPUTFILE)
OUTPUTFILE = @inputfile.insert(INPUTFILE.rindex("."),"_signed")
CERTFILE = "certificate.pem"
RSAKEYFILE = "private_key.pem"
passphrase = "your passphrase"

key4pem=File.read RSAKEYFILE

key = OpenSSL::PKey::RSA.new key4pem, passphrase
cert = OpenSSL::X509::Certificate.new(File.read CERTFILE)

pdf = PDF.read(INPUTFILE)
page = pdf.get_page(1)

# Add signature annotation (so it becomes visibles in pdf document)

sigannot = Annotation::Widget::Signature.new
sigannot.Rect = Rectangle[:llx => 89.0, :lly => 386.0, :urx => 190.0, :ury => 353.0]

page.add_annot(sigannot)

# Sign the PDF with the specified keys
pdf.sign(cert, key, 
  :method => 'adbe.pkcs7.sha1',
  :annotation => sigannot, 
  :location => "Portugal", 
  :contact => "myemail@email.tt", 
  :reason => "Proof of Concept"
)

# Save the resulting file
pdf.save(OUTPUTFILE)