Google Authenticator

Google Authenticator is a software based two-step authentication token developed by Google. The Authenticator provides a six digit number users must provide in addition to their username and password to log in to Google services. The Authenticator can also generate codes for third party applications, such as LastPass or Dropbox.

Implementation

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

• Windows Phone 7: Authenticator
• Windows Mobile: Google Authenticator for Windows Mobile
• Java CLI: Authenticator.jar
• Java GUI: JAuth
• J2ME: gauthj2me lwuitgauthj2me Mobile-OTP
• PalmOS: gauthj2me
• webOS: GAuth
• Windows: gauth4win
• HTML5: html5-google-authenticator
• MeeGo/Harmattan (Nokia N9): GAuth

Technical description

Google generates an 80-bit secret key for each user. This is provided as a 16 character base32 string or as a QR code. The client creates a HMAC-SHA1 using this secret key, with the message being the number of 30 second periods having elapsed since the Unix epoch. A portion of the HMAC is extracted and converted to a 6 digit code.

Pseudocode

 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 

Technology

Google Authenticator uses the time-based One-time Password Algorithm as defined in RFC 6238.

External links

• Google Authenticator on Google Help
• Google Authenticator implementation in Python on Stack Overflow