Google Authenticator: Difference between revisions

Google Authenticator is a software based two-factor 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

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

Google generates a 80-bit secret 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.

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

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

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