HMAC-based One-time Password Algorithm

HMAC-based One-time Password algorithm (HOTP) is a one-time password (OTP) algorithm based on HMAC. It is a cornerstone of the Initiative for Open Authentication (OATH).

HOTP was published as an informational IETF RFC 4226 in December 2005, documenting the algorithm along with a Java implementation. Since then, the algorithm has been adopted by many companies worldwide (see below). The HOTP algorithm is a freely available open standard.

Let:

Then HOTP(K, C) is mathematically defined by

The mask 0x7FFFFFFF sets the result's most significant bit to zero. This avoids problems if the result is interpreted as a signed number as some processors do.

For HOTP to be useful for an individual to input to a system, the result must be converted into a HOTP value, a 6–8 digits number that is implementation dependent.

HOTP can be used to authenticate a user in a system via an authentication server. Also, if some more steps are carried out (the server calculates subsequent OTP value and sends/displays it to the user who checks it against subsequent OTP value calculated by their token), the user can also authenticate the validation server.

Both hardware and software tokens are available from various vendors, for some of them see references below. Hardware tokens implementing OATH HOTP tend to be significantly cheaper than their competitors based on proprietary algorithms. As of 2010, OATH HOTP hardware tokens can be purchased for a marginal price. Some products can be used for strong passwords as well as OATH HOTP.

Software tokens are available for (nearly) all major mobile/smartphone platforms (J2ME,Android,iPhone,BlackBerry,Maemo,Mac OS X,Windows Mobile).

...
Wikipedia