Google Authenticator

Google Authenticator implements TOTP security tokens from RFC6238 in mobile apps made by Google, sometimes branded "two-step authentication". The Authenticator provides a six digit one-time password users must provide in addition to their username and password to log into Google services. The Authenticator can also generate codes for third party applications, such as password managers or file hosting services. Some versions of the software are open source.

Implementation

Google provides Android,^[1] BlackBerry and iOS,^[2] versions of Authenticator. See two-step verification for other implementations by third parties.

Technical description

The service provider 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 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.

Pseudocode for Time OTP

 function GoogleAuthenticatorCode(string secret)
     key := base32decode(secret)
     message := floor(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 

Pseudocode for Event/Counter OTP

 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 

Open Source status on Android

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.

Usage

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

References

External links

• Google Authenticator on Google Help
• Google Authenticator open source project on Google Code
• Google Authenticator implementation in Python on Stack Overflow