HMAC-based one-time password

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HMAC-based one-time password (HOTP) is a one-time password (OTP) algorithm based on the hash-based message authentication code (HMAC) cryptographic algorithm and is a basis for the commonly used Time-based one-time password (TOTP) algorithm which extends upon it. The HOTP algorithm is a freely available open standard and often forms part of multi-factor authentication protocols such as the Open Authentication initiative (OATH) challenge-response algorithm.. At setup time a client and server share a secret and other initialization settings. In future exchanges when a client attempts to access a server a challenge is sent from the server to the client and the client then uses the previously exchanged secret and computes a value (a one time password) to be used as the server's challenge response. 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).