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# coding: utf-8
# typed: strict
# frozen_string_literal: true
require 'digest/md5'
class PDF::Reader
# Decrypts data using the AESV2 algorithim defined in the PDF spec. Requires
# a decryption key, which is usually generated by PDF::Reader::StandardKeyBuilder
#
class AesV2SecurityHandler
#: (String) -> void
def initialize(key)
@encrypt_key = key
end
##7.6.2 General Encryption Algorithm
#
# Algorithm 1: Encryption of data using the AES-128-CBC algorithm
#
# version == 4 and CFM == AESV2
#
# used to decrypt PDF streams (buf). Input data should be in bytesizes of
# a multiple of 16, anything else is an error. The first 16 bytes are the initialization
# vector, so any input of exactly 16 bytes decrypts to an empty string
#
# buf - a string to decrypt
# ref - a PDF::Reader::Reference for the object to decrypt
#
#: (String, PDF::Reader::Reference) -> String
def decrypt( buf, ref )
if buf.bytesize % 16 > 0
raise PDF::Reader::MalformedPDFError.new("Ciphertext not a multiple of 16")
elsif buf.bytesize == 16
return ""
else
begin
internal_decrypt(buf, ref)
rescue OpenSSL::Cipher::CipherError
# If we failed to decrypt it might be a padding error, so try again
# and assume no padding in the ciphertext. This will "suceed" but might
# return garbage if the key is incorrect but that's OK - well before this
# class is used we have confirmed the user provided key is correct so if
# this works without error we can be confident the returned plaintext is
# correct
internal_decrypt(buf, ref, false)
end
end
end
private
#: (String, PDF::Reader::Reference, ?bool) -> String
def internal_decrypt(buf, ref, padding = true)
objKey = @encrypt_key.dup
(0..2).each { |e| objKey << (ref.id >> e*8 & 0xFF ) }
(0..1).each { |e| objKey << (ref.gen >> e*8 & 0xFF ) }
objKey << 'sAlT' # Algorithm 1, b)
length = objKey.length < 16 ? objKey.length : 16
cipher = OpenSSL::Cipher.new("AES-#{length << 3}-CBC")
cipher.decrypt
cipher.padding = 0 unless padding
cipher.key = Digest::MD5.digest(objKey)[0,length]
cipher.iv = buf[0..15]
cipher.update(buf[16..-1]) + cipher.final
end
end
end
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