3.3. Ring Oscillator Sources

If an integrated circuit is being designed or field-programmed, an odd number of gates can be connected in series to produce a free-running ring oscillator. By sampling a point in the ring at a fixed frequency (for example, one determined by a stable crystal oscillator), some amount of entropy can be extracted due to variations in the free-running oscillator timing. It is possible to increase the rate of entropy by XOR'ing sampled values from a few ring oscillators with relatively prime lengths. It is sometimes recommended that an odd number of rings be used so that, even if the rings somehow become synchronously locked to each other, there will still be sampled bit transitions. Another possible source to sample is the output of a noisy diode.

Sampled bits from such sources will have to be heavily de-skewed, as disk rotation timings must be (see Section 4). An engineering study would be needed to determine the amount of entropy being produced depending on the particular design. In any case, these can be good sources whose cost is a trivial amount of hardware by modern standards.

As an example, IEEE 802.11i suggests the circuit below, with due attention in the design to isolation of the rings from each other and from clocked circuits to avoid undesired synchronization, etc., and with extensive post processing [IEEE_802.11i].

        |\     |\                |\
    +-->| >0-->| >0-- 19 total --| >0--+-------+
    |   |/     |/                |/    |       |
    |                                  |       |
    +----------------------------------+       V
                                            +-----+
        |\     |\                |\         |     | output
    +-->| >0-->| >0-- 23 total --| >0--+--->| XOR |------>
    |   |/     |/                |/    |    |     |
    |                                  |    +-----+
    +----------------------------------+      ^ ^
                                              | |
        |\     |\                |\           | |
    +-->| >0-->| >0-- 29 total --| >0--+------+ |
    |   |/     |/                |/    |        |
    |                                  |        |
    +----------------------------------+        |
                                                |
        Other randomness, if available ---------+