Aspects of a method and system for command authentication to achieve a secure interface are provided. Command authentication between a host and a slave device in a multimedia system may be achieved by on-the-fly pairing or by an automatic one-time-programming via a security processor. In an on-the-fly pairing scheme, the host may generate a host key based on a host root key and host control words while the slave may generate slave key based the host key, a slave root key and slave control words. The slave key may be stored and later retrieved by the slave device to obtain the host key for authenticating host commands. The host may be disabled from generating and/or passing the host key to the slave. In an automatic one-time programming scheme, the security processor may burn a random number onto a onetime-programmable memory in the host and slave devices for command authentication.