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CPRNG(9) NetBSD Kernel Developer's Manual CPRNG(9)
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Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
cprng, cprng_strong_create, cprng_strong_destroy, cprng_strong,
cprng_strong32, cprng_strong64, cprng_fast, cprng_fast32, cprng_fast64,
-- cryptographic pseudorandom number generators
cprng_strong_create(const char *name, int ipl, int flags);
cprng_strong(cprng_strong_t *cprng, void *buf, size_t len, int flags);
cprng_fast(void *buf, size_t len);
#define CPRNG_MAX_LEN 524288
The cprng family of functions provide cryptographic pseudorandom number
generators automatically seeded from the kernel entropy pool. All appli-
cations in the kernel requiring random data or random choices should use
the cprng_strong family of functions, unless performance constraints
The cprng_fast family of functions may be used in applications that can
tolerate exposure of past random data, such as initialization vectors or
transaction ids that are sent over the internet anyway, if the applica-
tions require higher throughput or lower per-request latency than the
cprng_strong family of functions provide. If in doubt, choose
A single instance of the fast generator serves the entire kernel. A
well-known instance of the strong generator, kern_cprng, may be used by
any in-kernel caller, but separately seeded instances of the strong gen-
erator can also be created by calling cprng_strong_create().
The cprng functions may be used at interrupt priority level IPL_VM or
below, except for cprng_strong_create() and cprng_strong_destroy() which
are allowed only at IPL_NONE; see spl(9).
The cprng functions replace the legacy arc4random(9) and
cprng_strong_create(name, ipl, flags)
Create an instance of the cprng_strong generator. This generator
currently implements the NIST SP 800-90A CTR_DRBG with AES-128 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 seed
material from the entropy pool and also on the personalization
The ipl argument specifies the interrupt priority level for the
mutex which will serialize access to the new instance of the gener-
ator (see spl(9)), and must be no higher than IPL_VM.
The flags argument controls the behavior of the generator:
CPRNG_INIT_ANY Suppress a warning message to the console if, dur-
ing cprng_strong_create(), only partial entropy
for the generator is available from the entropy
CPRNG_REKEY_ANY Suppress a warning message to the console if, dur-
ing cprng_strong() after the generator has been
exhausted and must be reseeded, only partial
entropy for the generator is available from the
CPRNG_USE_CV Make cprng_strong() sleep if the generator has not
been seeded with full entropy until full entropy
is available. Otherwise, cprng_strong() will
never sleep when passed this generator.
CPRNG_HARD Limit the number of bits of output from the gener-
ator before reseeding to the number of bits in its
seed, so that it approximates the information-the-
oretic entropy of its seed. Otherwise, the gener-
ator may provide many more bits of output than it
was seeded with.
Creation will succeed even if full entropy for the generator is not
available. In this case, the first request to read from the gener-
ator may cause reseeding.
cprng_strong_create() may sleep to allocate memory.
cprng_strong_destroy() may sleep.
cprng_strong(cprng, buf, len, flags)
Fill memory location buf with up to len bytes from the generator
cprng, and return the number of bytes. len must be at most
If cprng was created with the CPRNG_USE_CV flag and has been
exhausted, then cprng_strong() may sleep until full entropy can be
obtained from the entropy pool to reseed it. However, if flags
includes the FNONBLOCK flag, then cprng_strong() will immediately
return zero in this case instead.
If cprng was created with the CPRNG_HARD flag, then cprng_strong()
will return at most as many bytes as are left from its seed size
since the last reseeding.
If cprng was created with neither the CPRNG_USE_CV flag nor the
CPRNG_HARD flag, then cprng_strong() is guaranteed to return as
many bytes as requested, up to CPRNG_MAX_LEN, without sleeping.
Generate 32 bits using the kern_cprng strong generator.
cprng_strong32() does not sleep.
Generate 64 bits using the kern_cprng strong generator.
cprng_strong64() does not sleep.
Fill memory location buf with len bytes from the fast generator.
cprng_fast() does not sleep.
Generate 32 bits using the fast generator.
cprng_fast32() does not sleep.
Generate 64 bits using the fast generator.
cprng_fast64() does not sleep.
The cprng family of functions provide the following security properties:
· An attacker who has seen some outputs of any of the cprng func-
tions cannot predict past or future unseen outputs.
· An attacker who has compromised kernel memory cannot predict past
outputs of the cprng_strong functions. However, such an attacker
may be able to predict past outputs of the cprng_fast functions.
The second property is sometimes called ``backtracking resistance'',
``forward secrecy'', or ``key erasure'' in the cryptography literature.
The cprng_strong functions provide backtracking resistance; the
cprng_fast functions do not.
The cprng_strong functions are implemented in sys/kern/subr_cprng.c, and
use the NIST SP 800-90A CTR_DRBG implementation in
sys/crypto/nist_ctr_drbg. The cprng_fast functions are implemented in
sys/crypto/cprng_fast/cprng_fast.c, and use the ChaCha8 stream cipher.
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.
Daniel J. Bernstein, ChaCha, a variant of Salsa20,
http://cr.yp.to/papers.html#chacha, 2008-01-28, Document ID:
The cprng family of functions first appeared in NetBSD 6.0.
NetBSD 7.1.2 February 19, 2015 NetBSD 7.1.2