[Gllug] Questions arising from the random discussions...

[Walter Stanish]

> So, where does the stream for /dev/random and /dev/urandom come from and
> what's the difference?
>
> Pointers and continued ramblings most welcome

It may be of interest to read Documentation/hw_random.txt in /usr/src/linux

        The hw_random framework is software that makes use of a
        special hardware feature on your CPU or motherboard,
        a Random Number Generator (RNG).  The software has two parts:
        a core providing the /dev/hw_random character device and its
        sysfs support, plus a hardware-specific driver that plugs
        into that core.

        To make the most effective use of these mechanisms, you
        should download the support software as well.  Download the
        latest version of the "rng-tools" package from the
        hw_random driver's official Web site:

                http://sourceforge.net/projects/gkernel/

        Those tools use /dev/hw_random to fill the kernel entropy pool,
        which is used internally and exported by the /dev/urandom and
        /dev/random special files.

Theory of operation:

        CHARACTER DEVICE.  Using the standard open()
        and read() system calls, you can read random data from
        the hardware RNG device.  This data is NOT CHECKED by any
        fitness tests, and could potentially be bogus (if the
        hardware is faulty or has been tampered with).  Data is only
        output if the hardware "has-data" flag is set, but nevertheless
        a security-conscious person would run fitness tests on the
        data before assuming it is truly random.

        The rng-tools package uses such tests in "rngd", and lets you
        run them by hand with a "rngtest" utility.

        /dev/hw_random is char device major 10, minor 183.

        CLASS DEVICE.  There is a /sys/class/misc/hw_random node with
        two unique attributes, "rng_available" and "rng_current".  The
        "rng_available" attribute lists the hardware-specific drivers
        available, while "rng_current" lists the one which is currently
        connected to /dev/hw_random.  If your system has more than one
        RNG available, you may change the one used by writing a name from
        the list in "rng_available" into "rng_current".


- Walter
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