Class: PDF::Reader::AesV2SecurityHandler

Inherits:
Object
  • Object
show all
Defined in:
lib/pdf/reader/aes_v2_security_handler.rb

Overview

Decrypts data using the AESV2 algorithim defined in the PDF spec. Requires a decryption key, which is usually generated by PDF::Reader::StandardKeyBuilder

Instance Method Summary collapse

Constructor Details

#initialize(key) ⇒ AesV2SecurityHandler

Returns a new instance of AesV2SecurityHandler.



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# File 'lib/pdf/reader/aes_v2_security_handler.rb', line 14

def initialize(key)
  @encrypt_key = key
end

Instance Method Details

#decrypt(buf, ref) ⇒ Object

7.6.2 General Encryption Algorithm

Algorithm 1: Encryption of data using the AES-128-CBC algorithm

version == 4 and CFM == AESV2

buf - a string to decrypt ref - a PDF::Reader::Reference for the object to decrypt



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# File 'lib/pdf/reader/aes_v2_security_handler.rb', line 27

def decrypt( buf, ref )
  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.key = Digest::MD5.digest(objKey)[0,length]
  cipher.iv = buf[0..15]
  cipher.update(buf[16..-1]) + cipher.final
end