arc4randbytes(9) - Manual pages

NAME

cprng, cprng_strong_create, cprng_strong_destroy, cprng_strong, cprng_strong32, cprng_strong64, cprng_fast, cprng_fast32, cprng_fast64, — cryptographic pseudorandom number generators

SYNOPSIS

#include <sys/cprng.h>

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

void
cprng_strong_destroy(cprng_strong_t *cprng);

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

uint32_t
cprng_strong32(void);

uint64_t
cprng_strong64(void);

size_t
cprng_fast(void *buf, size_t len);

uint32_t
cprng_fast32(void);

uint64_t
cprng_fast64(void);

#define CPRNG_MAX_LEN   524288

DESCRIPTION

The cprng family of functions provide cryptographic pseudorandom number generators automatically seeded from the kernel entropy pool. All applications in the kernel requiring random data or random choices should use the cprng_strong family of functions, unless performance constraints demand otherwise.

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 applications require higher throughput or lower per-request latency than the cprng_strong family of functions provide. If in doubt, choose cprng_strong.

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 generator 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 rnd_extract_data(9) functions.

FUNCTIONS

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 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)), 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, during cprng_strong_create(), only partial entropy for the generator is available from the entropy pool.
CPRNG_REKEY_ANY: Suppress a warning message to the console if, during cprng_strong() after the generator has been exhausted and must be reseeded, only partial entropy for the generator is available from the entropy pool.
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 generator before reseeding to the number of bits in its seed, so that it approximates the information-theoretic entropy of its seed. Otherwise, the generator 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 generator may cause reseeding.

cprng_strong_create() may sleep to allocate memory.

cprng_strong_destroy(cprng)

Destroy cprng.

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 CPRNG_MAX_LEN.

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.

cprng_strong32()

Generate 32 bits using the kern_cprng strong generator.

cprng_strong32() does not sleep.

cprng_strong64()

Generate 64 bits using the kern_cprng strong generator.

cprng_strong64() does not sleep.

cprng_fast(buf, len)

Fill memory location buf with len bytes from the fast generator.

cprng_fast() does not sleep.

cprng_fast32()

Generate 32 bits using the fast generator.

cprng_fast32() does not sleep.

cprng_fast64()

Generate 64 bits using the fast generator.

cprng_fast64() does not sleep.

SECURITY MODEL

The cprng family of functions provide the following security properties:

An attacker who has seen some outputs of any of the cprng functions 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.

CODE REFERENCES

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.

HISTORY

The cprng family of functions first appeared in NetBSD 6.0.

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