A script to calculate an HOTP code

hotp.rb

#!/usr/bin/env ruby

require 'base32'
require 'openssl'

#  Script to calculate HOTP so I don't have to use my phone
class HOTP

  def initialize(original_secret, counter = 0)
    secret = Base32.decode(original_secret)
    counter_bytestring = counter_to_bytestring(counter)
    hmac = OpenSSL::HMAC.digest('SHA1', secret, counter_bytestring)

    token_base = hmac_to_token_base(hmac)
    @token = token_base % (10**6)
  end


  def password
    md = sprintf("%06d", @token).match(/(\d{3})(\d{3})/)
    "#{md[1]} #{md[2]}"
  end


  private
  def hmac_to_token_base(hmac)
    offset = hmac[-1].ord & 0xf
    [24, 16, 8, 0].each_with_index.reduce(0) do |sum, (bitshift, index)|
      mask = bitshift == 24 ? 0x7f : 0xff
      sum |= (hmac[offset + index].ord & mask) << bitshift
    end
  end


  def counter_to_bytestring(counter, wordsize = 8)
    shifts = (wordsize.pred).downto(0).each.map{ |position| position * 8 }
    shifts.map do |shift|
      mask = 0xff << shift
      ((counter & mask) >> shift).chr
    end.join
  end
end

secret, counter, code_length = ARGV
counter = counter.to_i
code_length = code_length.to_i

hotp = HOTP.new(secret, counter)
puts hotp.password