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, - cryptographic 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 generators 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, suitable 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 personalization string (name). The ipl argument specifies the interrupt 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 number 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 generator.

    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 generator 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 implementation 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, 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.