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


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 appli-
     cations 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 applica-
     tions 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 gen-
     erator can also be created by calling cprng_strong_create().

     The cprng functions may be used in soft interrupt context, except for
     cprng_strong_create() and cprng_strong_destroy() which are allowed only
     at IPL_NONE in thread context; 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 Hash_DRBG with SHA-256 as
           the hash function.

           The name argument is used to ``personalize'' the Hash_DRBG accord-
           ing 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 gener-
           ator (see spl(9)), and must be no higher than IPL_SOFTSERIAL.

           The flags argument must be zero.

           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(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.  flags must be zero.

     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 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.


CODE REFERENCES

     The cprng_strong functions are implemented in sys/kern/subr_cprng.c, and
     use the NIST SP 800-90A Hash_DRBG implementation in
     sys/crypto/nist_hash_drbg.  The cprng_fast functions are implemented in
     sys/crypto/cprng_fast/cprng_fast.c, and use the ChaCha8 stream cipher.


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.

     Daniel J. Bernstein, ChaCha, a variant of Salsa20,
     http://cr.yp.to/papers.html#chacha, 2008-01-28, Document ID:
     4027b5256e17b9796842e6d0f68b0b5e.


HISTORY

     The cprng family of functions first appeared in NetBSD 6.0.

NetBSD 10.1                     August 16, 2020                    NetBSD 10.1