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Google Authenticator

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Google Authenticator is an open-source 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 Android[1], BlackBerry and iOS,[2] versions of Authenticator. Several third party implementations are available.

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 implements the time-based One-time Password Algorithm as defined in IETF RFC 6238, and the HMAC-based One Time Password Algorithm as defined in IETF RFC 4226.

Usage

Google Authenticator can be used for the following websites:

References

  1. ^ Android version of Google Authenticator
  2. ^ iOS version of Google Authenticator
  3. ^ Added security for your App.net account
  4. ^ Enabling Multifactor Authentication
  5. ^ Dropbox with Google Authenticator
  6. ^ LastPass with Google Authenticator
  7. ^ Registro.br HOTP/TOTP Token
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