NetBSD/share/man/man4/rnd.4

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.Dd October 12, 1997
.Dt RND 4
.Os NetBSD
.Sh NAME
.Nm rnd
.Nd in kernel entropy collection and random number generation
.Sh SYNOPSIS
.Cd pseudo-device rnd
.Sh DESCRIPTION
The
.Nm
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.
.Sh 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.
.Pp
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).
.Pp
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.
.Pp
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.
.Sh USER ACCESS
User code can obtain random values from the kernel in two ways.
.Pp
Reading from
.Pa /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.
.Pp
Reading from
.Pa /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
.Pa /dev/random
since when the pool is empty, data is still returned, degenerating to a
pseudo-random generator.
.Pp
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.
.Sh RANDOM SOURCE STRUCTURE
Each source has a state structure which the kernel uses to hold the
timing information and other state for that source.
.Bd -literal -offset indent
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       tyfl;
} rndsource_t;
.Ed
.Pp
This structure holds the internal representation of a device's timing
state.  The
.Va name
field holes the device name, as known to the kernel.  The
.Va 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
.Va last_delta
and
.Va last_delta2
fields hold the last first- and second-order deltas.  The
.Va 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
.Va tyfl
field holds the device flags as well as the device type.  The lower
eight bits hold the device type, and the upper 24 bits are flags
represented as a bitfield.
.Pp
.Bl -tag -width RND_TYPE_DISK
Currently, these types are defined:
.It Dv RND_TYPE_DISK
The device is a physical hard drive.
.It Dv RND_TYPE_NET
The device is a network interface.  By default, timing information is
collected from this source type, but entropy is not estimated.
.It Dv RND_TYPE_TAPE
The device is a tape device.
.It Dv RND_TYPE_TTY
The device is a terminal, mouse, or other user input device.
.El
.Pp
A device can have these flags set:
.Bl -tag -width RND_FLAG_NO_ESTIMATE
.It Dv RND_FLAG_NO_ESTIMATE
Do not assume any entropy is in the timing information.
.It Dv RND_FLAG_NO_COLLECT
Do not even add timing information to the pool.
.El
.Sh IOCTL
Various
.Xr ioctl 2
functions are available to control device behavior, gather statistics,
and add data to the entropy pool.  These are all defined in the
.Aq Pa sys/rnd.h
file, along with the data types and constants.
.Pp
.Bl -tag -width RNDADDTOENTCNT
.It Dv RNDGETENTCNT
.Pq Li "u_int32_t"
Return the current entropy count (in bits).
.It Dv RNDSETENTCNT
.Pq Li "u_int32_t"
Sets the internal entropy count (in bits) to the value given.  This
should be used with caution, as artifically increasing the entropy
count may lead to weakened security.
.It Dv RNDADDTOENTCNT
.Pq Li "u_int32_t"
Increase the entropy count (in bits) by the given amount.  This should
be used with caution, as artifically increasing the entropy count may
lead to weakened security.
.It Dv RNDGETPOOL
.Pq Li "u_char *"
Return the actual contents of the entropy pool.  The area pointed to
in this call should be
.Va "RND_POOLWORDS * 4"
bytes long.  As this is the actual contents of the entropy pool and
not hashes of it, it is extremely important to handle this data with
care.  Leaking any part of this pool would allow an attacker to
predict future random values.
.It Dv RNDGETSRCNUM
.Pq Li "rndstat_t"
.Bd -literal -offset indent
typedef struct {
        u_int32_t       start;
        u_int32_t       count;
        rndsource_t     source[RND_MAXSTATCOUNT];
} rndstat_t;
.Ed
.Pp
Return data for sources, starting at
.Va start
and returning at most
.Va count
sources.
.Pp
The values returned are actual in-kernel snapshots of the entropy
status for devices.  Leaking the internal timing information will
weaken security.
.It Dv RNDGETSRCNAME
.Pq Li "rndstat_name_t"
.Bd -literal -offset indent
typedef struct {
        char            name[16];
        rndsource_t     source;
} rndstat_name_t;
.Ed
.Pp
Return the device state for a named device.
.It Dv RNDCTL
.Pq Li "rndctl_t"
.Bd -literal -offset indent
typedef struct {
        char            name[16];
        u_int32_t       type;
        u_int32_t       flags;
        u_int32_t       mask;
} rndctl_t;
.Ed
.Pp
Change bits in the device state information.  If
.Va type
is 0xff, only the device name stored in
.Va name
is used.  If it is any other value, all devices of type
.Va type
are altered.  This allows all network interfaces to be disabled for
entropy collection with one call, for example. 
The
.Va flags
and
.Va mask
work together to change flag bits.  The
.Va mask
field specifies which bits in
.Va flags
are to be set or cleared.
.It Dv RNDADDDATA
.Pq Li "rnddata_t"
.Bd -literal -offset indent
typedef struct {
        u_int32_t       len;
        u_int32_t       entropy;
        u_char          data[RND_POOLWORDS * 4];
} rnddata_t;
.Ed
.El
.Sh FILES
.Bl -tag -width /dev/urandomx -compact
.It Pa /dev/random
Returns ``good'' values only
.It Pa /dev/urandom
Always returns data, degenerates to a pseudo-random generator
.El
.Sh HISTORY
The random device was first made available in
.Nx 1.3 .
.Sh 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.
.Sh SEE ALSO
.Xr rndctl 8 ,
.Xr rnd 9 .