Google authenticator

Typically, users must first install the application on their mobile device. In order to enter the site or use the services of the service, you need to enter a username and password, run the Authenticator application and enter the generated one-time password in a special field.

To do this, the site provides a shared secret key to the user, which must be saved in the Google Authenticator application. This secret key will be used for all future site logins.

With two-step authentication, just knowing the username / password is not enough to crack the account. An attacker must also know the secret key or have physical access to the device with Google Authenticator. An alternative way is the MITM attack: if the user's computer is infected with a trojan , then the user name, password and one-time code can be intercepted, then initiate their own login session on the site or track and change information between the user and the site.

Google Authenticator is provided on Android , ^[5] BlackBerry , and iOS ^[6] . Several third-party implementations are also available:

The service provider generates an 80-bit secret key for each user (although RFC 4226 § 4 requires a minimum of 128 bits and recommends 160 bits). ^{[37] The} key is provided as a 16-, 26-, 32-digit base32 encoded string or as a QR code . Using the secret key, the client creates an HMAC - SHA1 from:

• the number of 30 second intervals since the beginning of the " UNIX era " for the TOTP variant
• counter, which increases with each new code for the HOTP option.

Then, the HMAC part is retrieved and converted to a 6-digit code.

Pseudo-code for Time-based 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

• Google Authenticator on Google Help
• Google Authenticator legacy source code on GitHub
• Google Authenticator implementation in Python on Stack Overflow