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/*-------------------------------------------------------------------------
*
* pg_strong_random.c
* generate a cryptographically secure random number
*
* Our definition of "strong" is that it's suitable for generating random
* salts and query cancellation keys, during authentication.
*
* Note: this code is run quite early in postmaster and backend startup;
* therefore, even when built for backend, it cannot rely on backend
* infrastructure such as elog() or palloc().
*
* Copyright (c) 1996-2021, PostgreSQL Global Development Group
*
* IDENTIFICATION
* src/port/pg_strong_random.c
*
*-------------------------------------------------------------------------
*/
#include "c.h"
#include <fcntl.h>
#include <unistd.h>
#include <sys/time.h>
/*
* pg_strong_random & pg_strong_random_init
*
* Generate requested number of random bytes. The returned bytes are
* cryptographically secure, suitable for use e.g. in authentication.
*
* Before pg_strong_random is called in any process, the generator must first
* be initialized by calling pg_strong_random_init().
*
* We rely on system facilities for actually generating the numbers.
* We support a number of sources:
*
* 1. OpenSSL's RAND_bytes()
* 2. Windows' CryptGenRandom() function
* 3. /dev/urandom
*
* Returns true on success, and false if none of the sources
* were available. NB: It is important to check the return value!
* Proceeding with key generation when no random data was available
* would lead to predictable keys and security issues.
*/
#ifdef USE_OPENSSL
#include <openssl/rand.h>
void
pg_strong_random_init(void)
{
/*
* Make sure processes do not share OpenSSL randomness state. This is no
* longer required in OpenSSL 1.1.1 and later versions, but until we drop
* support for version < 1.1.1 we need to do this.
*/
RAND_poll();
}
bool
pg_strong_random(void *buf, size_t len)
{
int i;
/*
* Check that OpenSSL's CSPRNG has been sufficiently seeded, and if not
* add more seed data using RAND_poll(). With some older versions of
* OpenSSL, it may be necessary to call RAND_poll() a number of times. If
* RAND_poll() fails to generate seed data within the given amount of
* retries, subsequent RAND_bytes() calls will fail, but we allow that to
* happen to let pg_strong_random() callers handle that with appropriate
* error handling.
*/
#define NUM_RAND_POLL_RETRIES 8
for (i = 0; i < NUM_RAND_POLL_RETRIES; i++)
{
if (RAND_status() == 1)
{
/* The CSPRNG is sufficiently seeded */
break;
}
RAND_poll();
}
if (RAND_bytes(buf, len) == 1)
return true;
return false;
}
#elif WIN32
#include <wincrypt.h>
/*
* Cache a global crypto provider that only gets freed when the process
* exits, in case we need random numbers more than once.
*/
static HCRYPTPROV hProvider = 0;
void
pg_strong_random_init(void)
{
/* No initialization needed on WIN32 */
}
bool
pg_strong_random(void *buf, size_t len)
{
if (hProvider == 0)
{
if (!CryptAcquireContext(&hProvider,
NULL,
MS_DEF_PROV,
PROV_RSA_FULL,
CRYPT_VERIFYCONTEXT | CRYPT_SILENT))
{
/*
* On failure, set back to 0 in case the value was for some reason
* modified.
*/
hProvider = 0;
}
}
/* Re-check in case we just retrieved the provider */
if (hProvider != 0)
{
if (CryptGenRandom(hProvider, len, buf))
return true;
}
return false;
}
#else /* not USE_OPENSSL or WIN32 */
/*
* Without OpenSSL or Win32 support, just read /dev/urandom ourselves.
*/
void
pg_strong_random_init(void)
{
/* No initialization needed */
}
bool
pg_strong_random(void *buf, size_t len)
{
int f;
char *p = buf;
ssize_t res;
f = open("/dev/urandom", O_RDONLY, 0);
if (f == -1)
return false;
while (len)
{
res = read(f, p, len);
if (res <= 0)
{
if (errno == EINTR)
continue; /* interrupted by signal, just retry */
close(f);
return false;
}
p += res;
len -= res;
}
close(f);
return true;
}
#endif
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