TOTP authentication script that uses the ~/.google_authenticator file in the user's home directory. Based on https://moocode.com/posts/5-simple-two-factor-ssh-authentication

two_factor_ssh.rb

#!/usr/bin/env ruby
require 'rubygems'
require 'rotp'

# use ~/.google_authenticator to store the secret for
# compatibility with the google authenticator pam lib

secret_file = "#{ENV['HOME']}/.google_authenticator"
begin
  secret = File.open(secret_file).read
rescue
  abort "Could not open #{secret_file}"
end

# prompt the user for their validation code

STDERR.write "Enter the validation code: "
until validation_code = STDIN.gets.strip
  sleep 1
end

# check the validation code is correct

security = ROTP::TOTP.new(secret)

if security.verify(validation_code)
  Kernel.exec ENV['SSH_ORIGINAL_COMMAND'] || ENV['SHELL']
else
  abort "Invalid"
end