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Google Authenticator

google authenticator, google authenticator for windows
Google Authenticator is an application that implements two-step verification services using the Time-based One-time Password Algorithm TOTP and HMAC-based One-time Password Algorithm HOTP, for authenticating users of mobile applications by Google The service implements algorithms specified in RFC 6238 and RFC 4226

Authenticator provides a six- to eight-digit one-time password which users must provide in addition to their username and password to log in to Google services or other sites The Authenticator can also generate codes for third-party applications, such as password managers or file hosting services Previous versions of the software were open-sourced but subsequent releases are proprietary


  • 1 Typical use case
  • 2 Implementations
  • 3 Technical description
    • 31 Pseudocode for one-time password OTP
    • 32 Pseudocode for event or counter OTP
  • 4 Open Source status on Android
  • 5 References
  • 6 External links

Typical use case

Typically, a user installs the Authenticator app on a smartphone To log in to a site or service that uses two-factor authentication, the user provides user name and password to the site and runs the Authenticator app The app displays an additional six-digit one-time password and transmits it to the site, which asks the user what that password is The user enters it, thus authenticating the user's identity

For this to work, a set-up operation has to be performed ahead of time:the site provides a shared secret key to the user over a secure channel, to be stored in the Authenticator app This secret key will be used for all future logins to the site

With this kind of two-factor authentication, mere knowledge of username and password is not sufficient to break into a user's account The attacker also needs knowledge of the shared secret key or physical access to the device running the Authenticator app An alternative route of attack is a man-in-the-middle attack:if the computer used for the login process is compromised by a trojan, then username, password and one-time password can be captured by the trojan, which can then initiate its own login session to the site or monitor and modify the communication between user and site


Google provides Android, BlackBerry, and iOS versions of Authenticator Several third party implementations are available

  • Windows Phone 75/8/81/10:Microsoft Authenticator Virtual TokenFactor
  • Windows Mobile:Google Authenticator for Windows Mobile
  • Java CLI:Authenticatorjar
  • Java GUI:JAuth FXAuth
  • J2ME:gauthj2me lwuitgauthj2me Mobile-OTP Chinese only totp-me
  • Palm OS:gauthj2me
  • Python:onetimepass
  • PHP:GoogleAuthenticatorphp
  • Ruby:rotp, twofu
  • Rails:active_model_otp third party implementation
  • webOS:GAuth
  • Windows:gauth4win MOS Authenticator WinAuth
  • NET:TwoStepsAuthenticator
  • HTML5:html5-google-authenticator
  • MeeGo/Harmattan Nokia N9:GAuth
  • Sailfish OS:SGAuth, SailOTP
  • Apache:Google Authenticator Apache Module
  • PAM:Google Pluggable Authentication Module oauth-pam
  • Backend:LinOTP Management Backend implemented in python
  • Chrome/Chrome OS:Authenticator
  • iOS:OTP Auth
  • privacyIDEA Authentication System

Technical description

The service provider generates an 80-bit secret key for each user whereas RFC 4226 §4 requires 128 bits and recommends 160 bits This is provided as a 16, 26 or 32 character base32 string or as a QR code The client creates an HMAC-SHA1 using this secret key The message that is HMAC-ed can be:

  • the number of 30-second periods having elapsed since the Unix epoch TOTP; or
  • the counter that is incremented with each new code HOTP

A portion of the HMAC is extracted and converted to a six-digit code

Pseudocode for one-time password OTP

function GoogleAuthenticatorCodestring secretkey := base32decodesecretmessage := floorcurrent Unix time / 30hash := HMAC-SHA1key, messageoffset := last nibble of hashtruncatedHash := hash//4 bytes starting at the offsetSet the first bit of truncatedHash to zero//remove the most significant bitcode := truncatedHash mod 1000000pad code with 0 until length of code is 6return code

Pseudocode for event or counter OTP

function GoogleAuthenticatorCodestring secretkey := base32decodesecretmessage := counter encoded on 8 byteshash := HMAC-SHA1key, messageoffset := last nibble of hashtruncatedHash := hash//4 bytes starting at the offsetSet the first bit of truncatedHash to zero//remove the most significant bitcode := truncatedHash mod 1000000pad code with 0 until length of code is 6return code

Open Source status on Android

The Authenticator app as available on Google's Android app market is proprietary Although now Google has moved the source for their Authenticator app to its own GitHub repository, in the past this was explained on the project's development page:

"This open source project allows you to download the code that powered version 221 of the application Subsequent versions contain Google-specific workflows that are not part of the project"

An independent fork of the Android version of the software named OTP Authenticator has been created, which was based on the last version of the open source code that had been provided by Google, prior to their move to github Another Open Source fork named FreeOTP is maintained by Red Hat


  1. ^ "Google Is Making Your Account Vastly More Secure With Two-Step Authentication - TechCrunch" TechCrunch 2010-09-20 Retrieved 2016-03-12 
  2. ^ "GitHub - google/google-authenticator:Open source version of Google Authenticator except the Android app" GitHub Google These implementations support the HMAC-Based One-time Password HOTP algorithm specified in RFC 4226 and the Time-based One-time Password TOTP algorithm specified in RFC 6238 
  3. ^ a b c Willis, Nathan 22 January 2014"FreeOTP multi-factor authentication" LWNnet Retrieved 10 August 2015
  4. ^ https://playgooglecom/store/apps/detailsid=comgoogleandroidappsauthenticator2 A
  5. ^ "Google Authenticator" App Store 
  6. ^ "Authenticator" 4 April 2013 
  7. ^ "Virtual TokenFactor" 26 February 2012 
  8. ^ "Google Authenticator for Windows Mobile" XDA Developers 
  9. ^ "http://blog dot jamesdotcuff dot net" 
  10. ^ "mclamp/JAuth" GitHub 
  11. ^ "kamenitxan/FXAuth" GitHub 
  12. ^ "gauthj2me - Google Authentification in Java Mobile, j2me - Google Project Hosting" 
  13. ^ "lwuitgauthj2me - Google Authenticator for J2ME phones - Google Project Hosting" 
  14. ^ "chunlinyao / mobile-otp — Bitbucket" 
  15. ^ "totp-me - TOTP for Java ME - Google authenticator" 
  16. ^ "gauthprc - gauthj2me - Google Authenticator for Palm OS converted from java - Google Authentification in Java Mobile, j2me - Google Project Hosting" 
  17. ^ "tadeck/onetimepass" GitHub 
  18. ^ "chregu/GoogleAuthenticatorphp" GitHub 
  19. ^ "rotp - RubyGemsorg - your community gem host" 
  20. ^ "ukazap/twofu" GitHub 
  21. ^ "heapsource/active_model_otp" GitHub 
  22. ^ "GAuth" 
  23. ^ "gauth4win - Google Authenticator for windows - Google Project Hosting" 
  24. ^ "MOS Authenticator Home" 
  25. ^ "winauth - Windows Authenticator for Battlenet / World of Warcraft / Guild Wars 2 / Glyph / WildStar / Google / Bitcoin - Google Project Hosting" 
  26. ^ "glacasa/TwoStepsAuthenticator" GitHub 
  27. ^ "gbraad/html5-google-authenticator" GitHub 
  28. ^ Techtransit "Nokia Store:Download GAuth and many other games, wallpaper, ringtones and mobile apps on your Nokia phone" 
  29. ^ "SGAuth" 
  30. ^ "SailOTP" 
  31. ^ "google-authenticator-apache-module - Apache Module for Two-Factor Authentication via Google Authenticator - Google Project Hosting" 
  32. ^ a b "google-authenticator - Two-step verification - Google Project Hosting" 
  33. ^ "oauth-pam - PAM for use with OAuth Websites - Google Project Hosting" 
  34. ^ "Authenticator" 
  35. ^ "OTP Auth" App Store 
  36. ^ https://toolsietforg/html/c#section-4
  37. ^ "kaie/otp-authenticator-android" GitHub 
  38. ^ "FreeOTP" 

External links

  • Google Authenticator on Google Help
  • Google Authenticator legacy source code on GitHub
  • Google Authenticator implementation in Python on Stack Overflow
  • Authenticator Android package at the F-Droid repository

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Google Authenticator

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