RND(4)

NAME
rnd - in kernel entropy collection and random number generation

SYNOPSIS
pseudo-device rnd

DESCRIPTION
The rnd pseudo-device uses event timing information collected from many devices, and mixes this into an entropy pool. This pool is stirred with a cryptographically strong hash function when data is extracted from the pool.

INTERNAL ENTROPY POOL MANAGEMENT
When a hardware event occurs (such as completion of a hard drive transfer or an interrupt from a network device) a timestamp is generated. This timestamp is compared to the previous timestamp recorded for the device, and the first, second, and third order differentials are calculated.

If any of these differentials is zero, no entropy is assumed to have been gathered. If all are non-zero, one bit is assumed. Next, data is mixed into the entropy pool using an LFSR (linear feedback shift register).

To extract data from the entropy pool, a cryptographically strong hash function is used. The output of this hash is mixed back into the pool using the LFSR, and then folded in half before being returned to the caller.

Mixing the actual hash into the pool causes the next extraction to return a different value, even if no timing events were added to the pool. Folding the data in half prevents the caller to derive the actual hash of the pool, preventing some attacks.

USER ACCESS
User code can obtain random values from the kernel in two ways.

Reading from /dev/random will only return values while sufficient entropy exists in the internal pool. When sufficent entropy does not exist, EAGAIN is returned for non-blocking reads, or the read will block for blocking reads.

Reading from /dev/urandom will return as many values as requested, even when the entropy pool is empty. This data is not as good as reading from /dev/random since when the pool is empty, data is still returned, degenerating to a pseudo-random generator.

Writing to either device will mix the data written into the pool using the LFSR as above, without modifying the entropy estimation for the pool.

RANDOM SOURCE STRUCTURE
Each source has a state structure which the kernel uses to hold the timing information and other state for that source.

    typedef struct {
            char            name[16];
            u_int32_t       last_time;
            u_int32_t       last_delta;
            u_int32_t       last_delta2;
            u_int32_t       total;
            u_int32_t       type;
    	u_int32_t	flags;
    } rndsource_t;

This structure holds the internal representation of a device's timing state. The name field holes the device name, as known to the kernel. The last_time entry is the timestamp of the last time this device generated an event. It is for internal use only, and not in any specific representation. The last_delta and last_delta2 fields hold the last first- and second-order deltas. The total field holds a count of how many bits this device has potentially generated. This is not the same as how many bits were used from it. The type field holds the device type.

Currently, these types are defined:

RND_TYPE_DISK
    The device is a physical hard drive.

RND_TYPE_NET
    The device is a network interface. By default, timing information is collected from this source type, but entropy is not estimated.

RND_TYPE_TAPE
    The device is a tape device.

RND_TYPE_TTY
    The device is a terminal, mouse, or other user input device.

flags is a bitfield.

RND_FLAG_NO_ESTIMATE
    Do not assume any entropy is in the timing information.

RND_FLAG_NO_COLLECT
    Do not even add timing information to the pool.

IOCTL
Various ioctl(2) functions are available to control device behavior, gather statistics, and add data to the entropy pool. These are all defined in the <sys/rnd.h> file, along with the data types and constants.

RNDGETENTCNT
    (u_int32_t) Return the current entropy count (in bits).

RNDGETSRCNUM
    (rndstat_t)

        typedef struct {
                u_int32_t       start;
                u_int32_t       count;
                rndsource_t     source[RND_MAXSTATCOUNT];
        } rndstat_t;

    Return data for sources, starting at start and returning at most count sources.

    The values returned are actual in-kernel snapshots of the entropy status for devices. Leaking the internal timing information will weaken security.

RNDGETSRCNAME
    (rndstat_name_t)

        typedef struct {
                char            name[16];
                rndsource_t     source;
        } rndstat_name_t;

    Return the device state for a named device.

RNDCTL
    (rndctl_t)

        typedef struct {
                char            name[16];
                u_int32_t       type;
                u_int32_t       flags;
                u_int32_t       mask;
        } rndctl_t;

    Change bits in the device state information. If type is 0xff, only the device name stored in name is used. If it is any other value, all devices of type type are altered. This allows all network interfaces to be disabled for entropy collection with one call, for example. The flags and mask work together to change flag bits. The mask field specifies which bits in flags are to be set or cleared.

RNDADDDATA
    (rnddata_t)

        typedef struct {
                u_int32_t       len;
                u_int32_t       entropy;
                u_char          data[RND_POOLWORDS * 4];
        } rnddata_t;

FILES
/dev/random
    Returns ``good'' values only
/dev/urandom
    Always returns data, degenerates to a pseudo-random generator

HISTORY
The random device was first made available in NetBSD 1.3.

AUTHOR
This implementation was written by Michael Graff <explorer@flame.org> using ideas and algorithms gathered from many sources, including the driver written by Ted Ts'o.

SEE ALSO
rndctl(8), rnd(9).