That's the same link provided in the Tcl wiki. I haven't actually used the ISAAC generator since the urandom is considered reliable. However, urandom is only

That's the same link provided in the Tcl wiki. I haven't actually used the ISAAC generator since the urandom is considered reliable. However, urandom is only available on Linux/BSD systems, on Windows alternatives are necessary. From the comments on the ISAAC home page, the ISAAC and ISAAC-64 versions would appear adequate for most uses, but a reader more experienced in CSPRNG analysis might have more information.