Conventional cryptography deals with the encryption and decryption of traditional textual data. The advent of networked multimedia systems will make continuous media streams, such as real-time audio and video, increasingly pervasive in future computing and communications environments. It is thus important to secure networked continuous media from potential eavesdroppers. We consider the process of real-time encryption and decryption for video streams. We implement a software-only security-enhanced MPEG player. The security-enhanced player implements a protection hierarchy by specializing the encryption scheme based on MPEG's coding sequences. Encryption may be performed on only I frames (intra-frames), on I and P frames (forward predicted frames), or on all I, P and B frames (bidirectional predicted frames). Increased protection incurs more overhead as more encryption is done. Our security-enhanced MPEG player incurs small average overheads in terms of achievable frame rate compared with the unmodified MPEG player depending on the MPEG frame size, encoding format and encryption method used, with speeds fast enough for most multimedia Internet applications. This is demonstrated by its integration with Vosaic, a real-time multimedia WWW browser. We also observe that increased compression actually results in less cryptographic overhead, due to the fact that more compression means less data, as well as longer dependencies between MPEG frames. Our work shows that video streams can also be encrypted and decrypted while satisfying the real-time requirements of the present-day Internet.