Universally composable zero-knowledge arguments and commitments from signature cards

In the Universal Composability framework many cryptographic tasks cannot be built from scratch. Additional \helping" functionalities are needed to realise zero-knowledge or bit commitment. However, all the additional functionalities presented in the literature so far have to be specially designed as a \helping" functionality and cannot directly serve any other purpose without endangering the universal composability. In this work, we introduce the concept of catalysts. Informally a functionality C is a catalyst for a functionality F if F can be implemented given the primitive C and the functionality C can still directly be used by other applications without any additional precautions. We prove that catalysts exist for zero-knowledge and bit commitment. And, what is more, we show that a signature card, which is in accordance with the German law [Bun01] can be used as such a catalyst.