Contemporary communication systems are based on modular architectures, where the role of the physical layer is essentially confined to error correction, whereas information security is typically dealt with at the upper layers of the protocol stack. We consider a security architecture that does exactly the opposite: information sequences are first converted to longer channel codewords which are then encrypted using a classical stream cipher. Although this approach requires longer encryption sequences, our analysis shows that the natural randomness of the noisy communication channel can be used effectively against known-plaintext attacks. We also address practical implementation issues in physical-layer encryption and discuss their impact on the system architecture and on the security performance.