class PDF::Reader::StandardSecurityHandler

def self.supports?(encrypt)
  return false if encrypt.nil?
  filter = encrypt.fetch(:Filter, :Standard)
  version = encrypt.fetch(:V, 0)
  algorithm = encrypt.fetch(:CF, {}).fetch(encrypt[:StmF], {}).fetch(:CFM, nil)
  (filter == :Standard) && (encrypt[:StmF] == encrypt[:StrF]) &&
    (version <= 3 || (version == 4 && ((algorithm == :V2) || (algorithm == :AESV2))))
end

def auth_owner_pass(pass)
  md5 = Digest::MD5.digest(pad_pass(pass))
  if @revision > 2 then
    50.times { md5 = Digest::MD5.digest(md5) }
    keyBegins = md5[0, key_length]
    #first iteration decrypt owner_key
    out = @owner_key
    #RC4 keyed with (keyBegins XOR with iteration #) to decrypt previous out
    19.downto(0).each { |i| out=RC4.new(xor_each_byte(keyBegins,i)).decrypt(out) }
  else
    out = RC4.new( md5[0, 5] ).decrypt( @owner_key )
  end
  # c) check output as user password
  auth_user_pass( out )
end

def auth_user_pass(pass)
  keyBegins = make_file_key(pass)
  if @revision >= 3
    #initialize out for first iteration
    out = Digest::MD5.digest(PassPadBytes.pack("C*") + @file_id)
    #zero doesn't matter -> so from 0-19
    20.times{ |i| out=RC4.new(xor_each_byte(keyBegins, i)).encrypt(out) }
    pass = @user_key[0, 16] == out
  else
    pass = RC4.new(keyBegins).encrypt(PassPadBytes.pack("C*")) == @user_key
  end
  pass ? keyBegins : nil
end

def build_standard_key(pass)
  encrypt_key   = auth_owner_pass(pass)
  encrypt_key ||= auth_user_pass(pass)
  raise PDF::Reader::EncryptedPDFError, "Invalid password (#{pass})" if encrypt_key.nil?
  encrypt_key
end

def decrypt( buf, ref )
  case @cfm
    when :AESV2
      decrypt_aes128(buf, ref)
    else
      decrypt_rc4(buf, ref)
  end
end

def decrypt_aes128( 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

def decrypt_rc4( 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 ) }
  length = objKey.length < 16 ? objKey.length : 16
  rc4 = RC4.new( Digest::MD5.digest(objKey)[0,length] )
  rc4.decrypt(buf)
end

def initialize(opts = {})
  @key_length    = opts[:key_length].to_i/8
  @revision      = opts[:revision].to_i
  @owner_key     = opts[:owner_key]
  @user_key      = opts[:user_key]
  @permissions   = opts[:permissions].to_i
  @encryptMeta   = opts.fetch(:encrypted_metadata, true)
  @file_id       = opts[:file_id] || ""
  @encrypt_key   = build_standard_key(opts[:password] || "")
  @cfm           = opts[:cfm]
  if @key_length != 5 && @key_length != 16
    msg = "StandardSecurityHandler only supports 40 and 128 bit\
           encryption (#{@key_length * 8}bit)"
    raise ArgumentError, msg
  end
end

def make_file_key( user_pass )
  # a) if there's a password, pad it to 32 bytes, else, just use the padding.
  @buf  = pad_pass(user_pass)
  # c) add owner key
  @buf << @owner_key
  # d) add permissions 1 byte at a time, in little-endian order
  (0..24).step(8){|e| @buf << (@permissions >> e & 0xFF)}
  # e) add the file ID
  @buf << @file_id
  # f) if revision >= 4 and metadata not encrypted then add 4 bytes of 0xFF
  if @revision >= 4 && !@encryptMeta
    @buf << [0xFF,0xFF,0xFF,0xFF].pack('C*')
  end
  # b) init MD5 digest + g) finish the hash
  md5 = Digest::MD5.digest(@buf)
  # h) spin hash 50 times
  if @revision >= 3
    50.times {
      md5 = Digest::MD5.digest(md5[0, @key_length])
    }
  end
  # i) n = key_length revision >= 3, n = 5 revision == 2
  if @revision < 3
    md5[0, 5]
  else
    md5[0, @key_length]
  end
end

def pad_pass(p="")
  if p.nil? || p.empty?
    PassPadBytes.pack('C*')
  else
    p[0, 32] + PassPadBytes[0, 32-p.length].pack('C*')
  end
end

def xor_each_byte(buf, int)
  buf.each_byte.map{ |b| b^int}.pack("C*")
end