pg_strong_random.c

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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) 1996-2026, 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().  Initialization is a no-
 * op for all supported randomness sources, it is kept to maintain backwards
 * compatibility with extensions.
 *
 * 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)
{
    /* No initialization needed */
}
 
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