Class: DRM::Unwrapper

Inherits:
Object
  • Object
show all
Defined in:
lib/drm/unwrapper.rb

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(read_proc) ⇒ Unwrapper

new unwrapper for a DRM-wrapped file read_proc(length, offset) reads length bytes at offset from the DRM-wrapped file



4
5
6
7
8
9
10
 
# File 'lib/drm/unwrapper.rb', line 4

def initialize(read_proc)
  @data_offset = [0].pack('N').length
  metadata_lenstr = read_proc.call @data_offset, 0
  metadata_str = read_proc.call metadata_lenstr.unpack('N').first, @data_offset
  @data_offset += metadata_str.length
  @metadata = DRM::Metadata.new_from_yaml_str metadata_str
end

Instance Attribute Details

#metadata ⇒ Object (readonly)

the DRM metadata for the wrapped file



13
14
15
 
# File 'lib/drm/unwrapper.rb', line 13

def metadata
  @metadata
end

Instance Method Details

#unwrap(start_byte, end_byte, read_proc, write_proc, subkey_proc) ⇒ Object

unwraps bytes start_byte..end_byte from the content of a DRM-wrapped file read_proc(length, offset) reads length bytes at offset from the file to be wrapped write_proc(data) appends the given data to the file containing the wrapped content subkey_proc(metadata, index) produces the indexth DRM subkey for the contents identified by metadata



19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
 
# File 'lib/drm/unwrapper.rb', line 19

def unwrap(start_byte, end_byte, read_proc, write_proc, subkey_proc)
  block_size = @metadata.block_size
  start_block = start_byte / block_size
  end_block = end_byte / block_size

  in_offset = @data_offset + start_block * block_size
  old_ski = nil
  subkey = nil
  start_block.upto(end_block) do |block_index|
    # read
    crypted_data = read_proc.call block_size, in_offset
    in_offset += crypted_data.length
    # decrypt
    ski = @metadata.subkey_index(block_index)
    subkey = subkey_proc.call @metadata, ski unless ski == old_ski
    old_ski = ski
    block_data = @metadata.decrypt_block crypted_data, block_index, subkey
    # write
    write_proc.call block_data[((block_index == start_block) ? start_byte % block_size : 0)..((block_index == end_block) ? end_byte % block_size : block_size - 1)]
  end
end