CPRNG(9)               NetBSD Kernel Developer's Manual               CPRNG(9)

NAME
     cprng, cprng_strong_create, cprng_strong, cprng_strong32, cprng_strong64,
     cprng_strong_getflags, cprng_strong_setflags, cprng_strong_ready,
     cprng_strong_destroy, cprng_fast, cprng_fast32, cprng_fast64, -- crypto-
     graphic pseudo-random number generators

SYNOPSIS
     #include <sys/cprng.h>

     cprng_strong_t
     cprng_strong_create(const char *const name, int ipl, int flags);

     void
     cprng_strong_destroy(cprng_strong_t *cprng);

     size_t
     cprng_strong(cprng_strong_t *const cprng, void *buf, size_t len,
         int blocking);

     size_t
     cprng_fast(void *buf, size_t len);

     uint32_t
     cprng_strong32(void);

     uint64_t
     cprng_strong64(void);

     uint32_t
     cprng_fast32(void);

     uint32_t
     cprng_fast64(void);

     int
     cprng_strong_getflags(cprng_strong_t *const cprng);

     void
     cprng_strong_setflags(cprng_strong_t *const cprng, int flags);

     #define CPRNG_MAX_LEN   524288

     typedef struct _cprng_strong {
             kmutex_t        mtx;
             kcondvar_t      cv;
             struct selinfo  selq;
             NIST_CTR_DRBG   drbg;
             int             flags;
             char            name[16];
             int             reseed_pending;
             rndsink_t       reseed;
     } cprng_strong_t;

DESCRIPTION
     The cprng family of functions supply randomness to callers within the
     NetBSD kernel.  They replace the arc4random(9) and rnd_extract_data(9)
     functions for this purpose.  The cprng functions provide stream genera-
     tors automatically keyed (and if necessary rekeyed) from the kernel
     entropy pool.  The NetBSD kernel no longer supports direct reading from
     the kernel entropy pool; all access is mediated by the cprng functions.

     The ``strong'' family of functions supply cryptographically strong random
     numbers suitable for keying crypto systems and similar purposes.  Calls
     to rnd_extract_data(9) should be replaced with calls to cprng_strong.

     The ``fast'' family of functions supply less strong random numbers, suit-
     able for initialization vectors, nonces in certain protocols, and other
     similar purposes, using a faster but less secure stream-cipher generator.
     stream-cipher generator.  Calls to arc4random(9) should be replaced with
     calls to cprng_fast32, and calls to arc4randbytes(9) should be replaced
     with calls to cprng_fast.

     A single instance of the cprng_fast generator serves the entire kernel.
     A single, well-known instance of the cprng_strong generator, kern_cprng,
     may be used by any in-kernel caller, but new separately-keyed instances
     of the cprng_strong generator can also be created by calling
     cprng_strong_create.

FUNCTIONS
     cprng_strong_create(name, ipl, flags)

           Create an instance of the cprng_strong generator.  This generator
           implements the NIST SP 800-90 CTR_DRBG with AES128 as the block
           transform.  The name argument is used to "personalize" the CTR_DRBG
           according to the standard, so that its initial state will depend
           both on keying material from the entropy pool and also on the per-
           sonalization string (name).  The ipl argument specifies the inter-
           rupt priority level for the mutex which will serialize access to
           the new instance of the generator (see spl(9)).  The flags argument
           controls the behavior of the generator:

           CPRNG_INIT_ANY   Perform initial keying of the generator from the
                            entropy pool even if the current estimate of
                            entropy in the pool is less than the required num-
                            ber of key bits for the generator.

           CPRNG_REKEY_ANY  When rekeying of the generator is required, key
                            the generator from the entropy pool even if the
                            current estimate of entropy in the pool is less
                            than the required number of key bits for the gen-
                            erator.

           CPRNG_USE_CV     Perform a cv_broadcast(9) operation on the "cv"
                            member of the returned cprng_strong_t each time
                            the generator is successfully rekeyed.  If this
                            flag is set, the generator will sleep when
                            rekeying is needed, and will therefore always
                            return the requested number of bytes.

           Creation will succeed even if key material for the generator is not
           available.  In this case, the first request to read from the gener-
           ator may cause rekeying.

     cprng_strong_destroy(cprng)

           Destroy an instance of the cprng_strong generator.

     cprng_strong(cprng, buf, len, blocking)

           Fill memory location buf with len bytes from the generator cprng.
           The blocking argument controls the blocking/non-blocking behavior
           of the generator: if it is set to FNONBLOCK, the generator may
           return less than len bytes if it requires rekeying.  If the
           CPRNG_USE_CV flag is set on the generator, the caller can wait on
           cprng->cv for notification that the generator can again supply
           bytes.  A maximum of CPRNG_MAX_LEN bytes may be requested at once;
           this is a restriction of the CTR_DRBG specification.

     cprng_strong32(cprng)

           Generate 32 bits using cprng_strong generator cprng.

     cprng_strong64(cprng)

           Generate 64 bits using cprng_strong generator cprng.

     cprng_strong_getflags(cprng)

           Get the flags currently in use by generator cprng.

     cprng_strong_setflags(cprng, flags)
           Set the flags on generator cprng to flags.

     cprng_fast(buf, len)
           Fill memory location buf with len bytes from the fast generator.

     cprng_fast32()
           Generate 32 bits using the fast generator.

     cprng_fast64()
           Generate 64 bits using the fast generator.

CODE REFERENCES
     The cprng API is implemented by sys/kern/subr_cprng.c and
     sys/sys/cprng.h.  The ``strong'' generator uses the CTR_DRBG implementa-
     tion in sys/crypto/nist_ctr_drbg.  The ``fast'' generator uses the
     arc4random implementation in sys/lib/libkern/arc4random.c.

SEE ALSO
     condvar(9), rnd(9), spl(9)

     Elaine Barker and John Kelsey, Recommendation for Random Number
     Generation Using Deterministic Random Bit Generators (Revised), National
     Institute of Standards and Technology, 2011, NIST Special Publication
     800-90A, Rev 1.

HISTORY
     The cprng family of functions first appeared in NetBSD 6.0.

NetBSD 6.0                     December 17, 2011                    NetBSD 6.0

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