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Add random-state-from-platform' and scm_i_random_bytes_from_platform'

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* libguile/random.c (scm_random_state_from_platform): New procedure.
  (scm_i_random_bytes_from_platform): New internal function.

* libguile/random.h (scm_random_state_from_platform,
  scm_i_random_bytes_from_platform): Add prototypes.

* doc/ref/api-data.texi (Random): Add documentation.
This commit is contained in:
Mark H Weaver 2012-01-18 02:36:17 -05:00
parent 925172cf52
commit d47db067b6
3 changed files with 117 additions and 19 deletions
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101
libguile/random.c
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@ -653,6 +653,107 @@ SCM_DEFINE (scm_random_exp, "random:exp", 0, 1, 0,
}
#undef FUNC_NAME
/* Return a new random-state seeded from the time, date, process ID, an
address from a freshly allocated heap cell, an address from the local
stack frame, and a high-resolution timer if available. This is only
to be used as a last resort, when no better source of entropy is
available. */
static SCM
random_state_of_last_resort (void)
{
SCM state;
SCM time_of_day = scm_gettimeofday ();
SCM sources = scm_list_n
(scm_from_unsigned_integer (SCM_UNPACK (time_of_day)), /* heap addr */
scm_getpid (), /* process ID */
scm_get_internal_real_time (), /* high-resolution process timer */
scm_from_unsigned_integer ((scm_t_bits) &time_of_day), /* stack addr */
scm_car (time_of_day), /* seconds since midnight 1970-01-01 UTC */
scm_cdr (time_of_day), /* microsecond component of the above clock */
SCM_UNDEFINED);
/* Concatenate the sources bitwise to form the seed */
SCM seed = SCM_INUM0;
while (scm_is_pair (sources))
{
seed = scm_logxor (seed, scm_ash (scm_car (sources),
scm_integer_length (seed)));
sources = scm_cdr (sources);
}
/* FIXME The following code belongs in `scm_seed_to_random_state',
and here we should simply do:
return scm_seed_to_random_state (seed);
Unfortunately, `scm_seed_to_random_state' only preserves around 32
bits of entropy from the provided seed. I don't know if it's okay
to fix that in 2.0, so for now we have this workaround. */
{
int i, len;
unsigned char *buf;
len = scm_to_int (scm_ceiling_quotient (scm_integer_length (seed),
SCM_I_MAKINUM (8)));
buf = (unsigned char *) malloc (len);
for (i = len-1; i >= 0; --i)
{
buf[i] = scm_to_int (scm_logand (seed, SCM_I_MAKINUM (255)));
seed = scm_ash (seed, SCM_I_MAKINUM (-8));
}
state = make_rstate (scm_c_make_rstate ((char *) buf, len));
free (buf);
}
return state;
}
/* Attempt to fill buffer with random bytes from /dev/urandom.
Return 1 if successful, else return 0. */
static int
read_dev_urandom (unsigned char *buf, size_t len)
{
size_t res = 0;
FILE *f = fopen ("/dev/urandom", "r");
if (f)
{
res = fread(buf, 1, len, f);
fclose (f);
}
return (res == len);
}
/* Fill a buffer with random bytes seeded from a platform-specific
source of entropy. /dev/urandom is used if available. Note that
this function provides no guarantees about the amount of entropy
present in the returned bytes. */
void
scm_i_random_bytes_from_platform (unsigned char *buf, size_t len)
{
if (read_dev_urandom (buf, len))
return;
else /* FIXME: support other platform sources */
{
/* When all else fails, use this (rather weak) fallback */
SCM random_state = random_state_of_last_resort ();
int i;
for (i = len-1; i >= 0; --i)
buf[i] = scm_to_int (scm_random (SCM_I_MAKINUM (256), random_state));
}
}
SCM_DEFINE (scm_random_state_from_platform, "random-state-from-platform", 0, 0, 0,
(void),
"Construct a new random state seeded from a platform-specific\n\
source of entropy, appropriate for use in non-security-critical applications.")
#define FUNC_NAME s_scm_random_state_from_platform
{
unsigned char buf[32];
if (read_dev_urandom (buf, sizeof(buf)))
return make_rstate (scm_c_make_rstate ((char *) buf, sizeof(buf)));
else
return random_state_of_last_resort ();
}
#undef FUNC_NAME
void
scm_init_random ()
{