We propose a new architecture for a content protection system that conceals confidential data and algorithms in an FPGA as electrical circuits. This architecture is designed for a client-server type on-line contents distribution services. The key component of this architecture is the digital content security circuit that is dynamically configured on the FPGA and performs the signal processing such as certification and decryption for replaying digital contents. The digital content security circuit is composed of two different circuits. One is the content-specific circuit that is built from the configuration data generated by the server. This circuit is specialized for each item of digital content. The other is the terminal built-in circuit that is uniquely programmed and implemented for each terminal, and is not open for the others. The content-specific circuit properly works only if it is combined with the terminal built-in circuit of the authorized client. We prototype and demonstrate a proof-of-concept model of the FPGA-based content protection system applicable to embedded consumer electronics such as set top boxes and cell phones.