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. 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. Google Authenticator uses the time-based One-time Password Algorithm as defined in RFC 6238.
Contributions by Edoderoo, F, and Darranc.