Generate Google Authenticator codes in Ruby (requires base32 gem)

totp.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'base32'
require 'openssl'

int = 30

now = Time.now.to_i / int
key = Base32.decode File.read('secret.key').strip
sha = OpenSSL::Digest::Digest.new('sha1')

(-1..1).each do |x|
  bytes  = [ now + x ].pack('>q').reverse
  hmac   = OpenSSL::HMAC.digest(sha, key.to_s, bytes)
  offset = hmac[-1] & 0x0F
  hash   = hmac[offset...offset + 4]

  code   = hash.reverse.unpack('L')[0]
  code  &= 0x7FFFFFFF
  code  %= 1000000

  puts code
end