Google Authenticator: Difference between revisions

Google Authenticator is an application that implements TOTP security tokens from RFC6238 in mobile apps made by Google, sometimes branded "two-step authentication". The Authenticator provides a six to eight digit one-time password users must provide in addition to their username and password to log into 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 source.

Typically, users will install the Authenticator app on their smartphone. To log into a site or service that uses two-factor authentication, they provide user name and password to the site and run the Authenticator app which produces an additional six-digit one-time password. The user provides this to the site, the site checks it for correctness and authenticates the user.

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 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,^[1] BlackBerry and iOS,^[2] versions of Authenticator. Several third party implementations are available.

The service provider generates an 80-bit secret key for each user. This is provided as a 16, 24 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; or
• the counter that is incremented with each new code.

A portion of the HMAC is extracted and converted to a 6 digit code.

 function GoogleAuthenticatorCode(string secret)
     key := base32decode(secret)
     message := floor(current Unix time / 30)
     hash := HMAC-SHA1(key, message)
     offset := value of 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 

 function GoogleAuthenticatorCode(string secret)
     key := base32decode(secret)
     message := counter encoded on 8 bytes
     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 

The Authenticator app as available on Google's Android app market is proprietary, as explained on the project's development page:

"This open source project allows you to download the code that powered version 2.21 of the application. Subsequent versions contain Google-specific workflows that are not part of the project."^[3]

An independent fork of the Android version of the software named OTP Authenticator^[4] has been created, which is based on the last version of the open source code that had been provided by Google. Another Open Source fork named FreeOTP^[5] has been published by Red Hat.

Google Authenticator can be used for the following websites and applications:

• Google Authenticator on Google Help
• Google Authenticator open source project on Google Code
• Google Authenticator implementation in Python on Stack Overflow
• Google Authenticator Android package at the F-Droid repository