class PDF::Reader::AesV2SecurityHandler

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

Public Class Methods

new(key) click to toggle source

: (String) -> void

# File lib/pdf/reader/aes_v2_security_handler.rb, line 15
def initialize(key)
  @encrypt_key = key
end

Public Instance Methods

decrypt( buf, ref ) click to toggle source

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

# File lib/pdf/reader/aes_v2_security_handler.rb, line 33
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 Instance Methods

internal_decrypt(buf, ref, padding = true) click to toggle source

: (String, PDF::Reader::Reference, ?bool) -> String

# File lib/pdf/reader/aes_v2_security_handler.rb, line 56
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