• Úvod do OTP – http://blog.agilebits.com/2012/12/20/doing-the-two-step-until-the-end-of-time/
• Oficiální Google projekt – https://code.google.com/p/google-authenticator/
• Nasazení pro SSH loginy – http://linuxdrops.com/use-two-step-verification-for-ssh-using-google-authenticator/#
• libpam-google-authenticator – http://packages.debian.org/wheezy/libpam-google-authenticator
• Timebased http://en.wikipedia.org/wiki/Time-based_One-time_Password_Algorithm
• RFCčka
  • TOTP: Time-Based One-Time Password Algorithm – http://tools.ietf.org/html/rfc6238
  • HOTP: An HMAC-Based One-Time Password Algorithm – http://tools.ietf.org/html/rfc4226

• OTP Sharp [C#] – https://bitbucket.org/devinmartin/otp-sharp/wiki/Home
• KeeOtp [C#] – https://bitbucket.org/devinmartin/keeotp/src/2c8ffea05856b65376d0e61ae58c7d3c87687ee1/KeeOtp?at=default
• Postup pro PHP [PHP] – http://www.idontplaydarts.com/2011/07/google-totp-two-factor-authentication-for-php/
• OTPHP [PHP] – https://github.com/lelag/otphp

function GoogleAuthenticatorCode(string secret)
   key := base32decode(secret)
   message := current Unix time ÷ 30
   hash := HMAC-SHA1(key, message)
   offset := last nibble of hash
   truncatedHash := hash[offset..offset+3]  //4 bytes starting at the offset
   Set the first bit of truncatedHash to zero  //remove the most significant bit 
   code := truncatedHash mod 1000000
   pad code with 0 until length of code is 6
   return code 

podrobnosti na http://en.wikipedia.org/wiki/Google_Authenticator