SIP authentication based on HOTP

Current authentications with a secret key are based on the Challenge-Response paradigm. This mechanism avoids the clear transmission of the password in the network. Most protocols in Internet applications integrate this mechanism to authenticate a client before providing a service. Unfortunately this method in the protocol design increases the protocolar exchanges. This contribution is equivalent to Challenge-Response model but it permits to reduce the handshakes. Based on HMAC One-Time-Password (HOTP), the challenge is implicit in the user request. This solution has same security properties of a Challenge-Response authentication and become integrated naturally into Session Initiation Protocol (SIP); SIP is nowadays most popular Voice over IP protocol. Integrated in the existing SIP parameter, the solution doesn't change the SIP signaling, thus permitting a total interoperability between equipments modified or not.