With the proliferation of physical attacks the implicit assumptions made in traditional security models no longer reflect the real world. To address this issue, a number of new security models, e.g. Algorithmic Tamper-Proof Security, have been proposed. In this work, we take another step and identify the cryptographic properties of a particular family of physical functions, termed as Physically Unclonable Functions (PUFs), that exploit physical phenomena at deep-submicron and nano-scale level. PUFs provide low-cost tamper-evident and tamper-resiliant implementations. Motivated by this fact, we specically describe a general method for constructing Pseudorandom Functions (PRFs) from a class of PUFs. We provide a formal model for certain types of PUFs that build the basis for PRFs, which we call PUF-PRFs. Furthermore, we show experimentally that some real world PUF instantiations (e.g.,SRAM PUFs) satisfy the model. This strongly indicates that PUF-PRFs can indeed be physically realized. Finally, we describe two symmetric ciphers that use PUF-PRFs as building blocks.