Implements PDF encryption (password protection and permissions) as specified in the PDF Reference, version 1.3, section 3.5 “Encryption”.
Flags in the permissions word, numbered as LSB = 1
Encrypts the given string under the given key, also requiring the object ID and generation number of the reference. See Algorithm 3.1.
# File lib/prawn/security.rb, line 110 def self.encrypt_string(str, key, id, gen) # Convert ID and Gen number into little-endian truncated byte strings id = [id].pack('V')[0,3] gen = [gen].pack('V')[0,2] extended_key = "#{key}#{id}#{gen}" # Compute the RC4 key from the extended key and perform the encryption rc4_key = Digest::MD5.digest(extended_key)[0, 10] RC4.new(rc4_key).encrypt(str) end
Encrypts the document, to protect confidential data or control modifications to the document. The encryption algorithm used is detailed in the PDF Reference 1.3, section 3.5 “Encryption”, and it is implemented by all major PDF readers.
options
can contain the following:
:user_password
Password required to open the document. If this is omitted or empty, no password will be required. The document will still be encrypted, but anyone can read it.
:owner_password
Password required to make modifications to the document or change or
override its permissions. If this is set to :random
, a random
password will be used; this can be useful if you never want users to be
able to override the document permissions.
:permissions
A hash mapping permission symbols (see below) to true
or
false
. True means “permitted”, and false means “not
permitted”. All permissions default to true
.
The following permissions can be specified:
:print_document
Print document.
:modify_contents
Modify contents of document (other than text annotations and interactive form fields).
:copy_contents
Copy text and graphics from document.
:modify_annotations
Add or modify text annotations and interactive form fields.
Deny printing to everyone, but allow anyone to open without a password:
encrypt_document :permissions => { :print_document => false }, :owner_password => :random
Set a user and owner password on the document, with full permissions for both the user and the owner:
encrypt_document :user_password => 'foo', :owner_password => 'bar'
Set no passwords, grant all permissions (This is useful because the default in some readers, if no permissions are specified, is “deny”):
encrypt_document
The encryption used is weak; the key is password-derived and is limited to 40 bits, due to US export controls in effect at the time the PDF standard was written.
There is nothing technologically requiring PDF readers to respect the permissions embedded in a document. Many PDF readers do not.
In short, you have no security at all against a moderately motivated person. Don't use this for anything super-serious. This is not a limitation of Prawn, but is rather a built-in limitation of the PDF format.
# File lib/prawn/security.rb, line 88 def encrypt_document(options={}) Prawn.verify_options [:user_password, :owner_password, :permissions], options @user_password = options.delete(:user_password) || "" @owner_password = options.delete(:owner_password) || @user_password if @owner_password == :random # Generate a completely ridiculous password @owner_password = (1..32).map{ rand(256) }.pack("c*") end self.permissions = options.delete(:permissions) || {} # Shove the necessary entries in the trailer and enable encryption. state.trailer[:Encrypt] = encryption_dictionary state.encrypt = true state.encryption_key = user_encryption_key end
Provides the values for the trailer encryption dictionary.
# File lib/prawn/security.rb, line 124 def encryption_dictionary { :Filter => :Standard, # default PDF security handler :V => 1, # "Algorithm 3.1", PDF reference 1.3 :R => 2, # Revision 2 of the algorithm :O => ByteString.new(owner_password_hash), :U => ByteString.new(user_password_hash), :P => permissions_value } end
The O (owner) value in the encryption dictionary. Algorithm 3.3.
# File lib/prawn/security.rb, line 185 def owner_password_hash @owner_password_hash ||= begin key = Digest::MD5.digest(pad_password(@owner_password))[0, 5] RC4.new(key).encrypt(pad_password(@user_password)) end end
Pads or truncates a password to 32 bytes as per Alg 3.2.
# File lib/prawn/security.rb, line 169 def pad_password(password) password = password[0, 32] password + PasswordPadding[0, 32 - password.length] end
# File lib/prawn/security.rb, line 141 def permissions=(perms={}) @permissions ||= FullPermissions perms.each do |key, value| unless PermissionsBits[key] raise ArgumentError, "Unknown permission :#{key}. Valid options: " + PermissionsBits.keys.map { |k| k.inspect }.join(", ") end # 0-based bit number, from LSB bit_position = PermissionsBits[key] - 1 if value # set bit @permissions |= (1 << bit_position) else # clear bit @permissions &= ~(1 << bit_position) end end end
# File lib/prawn/security.rb, line 160 def permissions_value @permissions || FullPermissions end
# File lib/prawn/security.rb, line 174 def user_encryption_key @user_encryption_key ||= begin md5 = Digest::MD5.new md5 << pad_password(@user_password) md5 << owner_password_hash md5 << [permissions_value].pack("V") md5.digest[0, 5] end end
The U (user) value in the encryption dictionary. Algorithm 3.4.
# File lib/prawn/security.rb, line 193 def user_password_hash RC4.new(user_encryption_key).encrypt(PasswordPadding) end