OTP algorithms in Ruby

hotp.rb

require 'openssl'

def hotp(secret, counter, digits = 6)
  hash = OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha1'), secret, int_to_bytestring(counter))
  "%0#{digits}i" % (truncate(hash) % 10**digits)
end

def truncate(string)
  offset = string.bytes.last & 0xf
  partial = string.bytes[offset..offset+3]
  partial.pack("C*").unpack("N").first & 0x7fffffff
end

def int_to_bytestring(int, padding = 8)
  result = []
  until int == 0
    result << (int & 0xFF).chr
    int >>= 8
  end
  result.reverse.join.rjust(padding, 0.chr)
end

hotp_test_values.txt

Appendix D - HOTP Algorithm: Test Values

   The following test data uses the ASCII string
   "12345678901234567890" for the secret:

   Secret = 0x3132333435363738393031323334353637383930

   Table 1 details for each count, the intermediate HMAC value.

   Count    Hexadecimal HMAC-SHA-1(secret, count)
   0        cc93cf18508d94934c64b65d8ba7667fb7cde4b0
   1        75a48a19d4cbe100644e8ac1397eea747a2d33ab
   2        0bacb7fa082fef30782211938bc1c5e70416ff44
   3        66c28227d03a2d5529262ff016a1e6ef76557ece
   4        a904c900a64b35909874b33e61c5938a8e15ed1c
   5        a37e783d7b7233c083d4f62926c7a25f238d0316
   6        bc9cd28561042c83f219324d3c607256c03272ae
   7        a4fb960c0bc06e1eabb804e5b397cdc4b45596fa
   8        1b3c89f65e6c9e883012052823443f048b4332db
   9        1637409809a679dc698207310c8c7fc07290d9e5

   Table 2 details for each count the truncated values (both in
   hexadecimal and decimal) and then the HOTP value.

                     Truncated
   Count    Hexadecimal    Decimal        HOTP
   0        4c93cf18       1284755224     755224
   1        41397eea       1094287082     287082
   2         82fef30        137359152     359152
   3        66ef7655       1726969429     969429
   4        61c5938a       1640338314     338314
   5        33c083d4        868254676     254676
   6        7256c032       1918287922     287922
   7         4e5b397         82162583     162583
   8        2823443f        673399871     399871
   9        2679dc69        645520489     520489

totp.rb

require 'hotp'

def totp(secret, digits = 6, step = 30, initial_time = 0)
  steps = (Time.now.to_i - initial_time) / step

  hotp(secret, steps, digits)
end