Class: PWN::SDR::Decoder::WiFi::DSSSIQ

Inherits:
Object
  • Object
show all
Defined in:
lib/pwn/sdr/decoder/wifi.rb

Overview

IEEE 802.11b 18.2/18.4 long-preamble DBPSK/Barker 1 Mbps only. https://www.ieee802.org/11/Documents/DocumentArchives/1999_docs/90845b_p80211b-draft3.1.pdf Integer 11 MHz chip-rate multiples; frequency-centred IQ. No OFDM/CCK, DQPSK, short preamble, resampling, equalizer or clock-drift tracking.

Constant Summary collapse

BARKER = 
[1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1].freeze
SYNC = 
(([1] * 96) + [0xf3a0].pack('v').unpack1('b*').chars.map(&:to_i)).freeze

Instance Method Summary collapse

Constructor Details

#initialize(rate:) ⇒ DSSSIQ

Returns a new instance of DSSSIQ.

Raises:

  • (ArgumentError) 


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# File 'lib/pwn/sdr/decoder/wifi.rb', line 151

def initialize(rate:)
  @rate = Integer(rate)
  raise ArgumentError, 'DSSS requires 11, 22 or 44 MHz IQ' unless [11_000_000, 22_000_000, 44_000_000].include?(@rate)

  @chips = BARKER.flat_map { |chip| [chip] * (@rate / 11_000_000) }
  @width = @chips.length
  @window = []
  @lanes = Array.new(@width) { {} }
  @count = 0
  @last_frame = -@width
  @reassembler = MACReassembler.new
end

Instance Method Details

#feed_iq(samples, rate: nil) ⇒ Object

Raises:

  • (ArgumentError) 


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# File 'lib/pwn/sdr/decoder/wifi.rb', line 164

def feed_iq(samples, rate: nil)
  raise ArgumentError, 'sample rate changed midstream' if rate && (rate.to_f - @rate).abs.positive?
  raise ArgumentError, 'interleaved IQ pairs required' unless samples.length.even?

  samples.each_slice(2) do |i, q|
    @count += 1
    @window << [i, q]
    @window.shift if @window.length > @width
    next unless @window.length == @width

    real = 0.0
    imag = 0.0
    power = 0.0
    @window.each_with_index do |(wi, wq), index|
      real += wi * @chips[index]
      imag += wq * @chips[index]
      power += (wi * wi) + (wq * wq)
    end
    lane = @lanes[@count % @width]
    if power < 1e-8 || ((real * real) + (imag * imag)) < 0.65 * power * @width
      lane.clear
      next
    end
    previous = lane[:previous]
    lane[:previous] = [real, imag]
    next unless previous

    scrambled = ((real * previous[0]) + (imag * previous[1])).negative? ? 1 : 0
    history = lane.fetch(:history, 0)
    bit = scrambled ^ ((history >> 3) & 1) ^ ((history >> 6) & 1)
    lane[:history] = ((history << 1) | scrambled) & 127
    frame = consume(lane, bit)
    next unless frame && @count - @last_frame >= @width

    @last_frame = @count
    frame = @reassembler.feed(frame, sample_time: @count.to_f / @rate)
    yield frame if block_given?
  end
end

#flush ⇒ Object 



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# File 'lib/pwn/sdr/decoder/wifi.rb', line 204

def flush
  @window.clear
  @lanes.each(&:clear)
  @reassembler.clear
end