totp.rb

class OneTimePasswordGenerator

  def initialize(pass_code_length = 6)
    @pin_modulo = 10 ** pass_code_length
  end

  def get_code(base64_secret, time = nil)
    time = (Time.current.to_i / 30).floor unless time

    secret = Base64.decode64 base64_secret
    time = [time].pack('N').rjust 8, "\0"

    hash = OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha1'), secret, time)
    offset = hash[-1].ord & 0xF

    truncated_hash = hash_to_int(hash, offset) & 0x7FFFFFFF
    (truncated_hash % @pin_modulo).to_s.rjust 6, '0'
  end

  private

    def hash_to_int(bytes, start)
      bytes.slice(start, bytes.size).slice(0, 4).unpack('N').first
    end
end

p OneTimePasswordGenerator.new.get_code 'base64EncodedSecret'