Reconfigurable System-on-Chip (SoC) platforms that incorporate hard-core processors surrounded by large amounts of FPGA are today commodities: the reconfigurable logic is often used to speed up execution of applications by implementing critical parts of the code as application-specific coprocessors. Cryptography applications are a good example of coprocessor applications: they are known to benefit significantly from spatial execution in hardware and have an increasing importance for mobile and ubiquitous computing. One of the main limits of FPGA-based coprocessors for these systems is the fact that both the coprocessor hardware description and the software program invoking are inevitably ridden with system details of the specific interface FPGA/processor: this limits significantly design reuse, impacts time-to-market, and makes development more complex. In this paper we present a portable reconfigurable cryptography coprocessor designed for a Virtual Memory Window (VMW) system. A VMW is a generic virtualisation layer composed of a hardware and an operating system components; it lowers the complexity of interfacing, increases portability, and makes it possible for the coprocessor to access the user-space virtual memory. The approach is illustrated here with the IDEA cryptography application running under Linux on a reconfigurable SoC, having its critical function mapped on the FPGA. A significant fraction of the speed-up inherent to hardware execution in the FPGA is preserved, while the hardware and software designs of the cryptography application become perfectly portable.