OpenSSL CPRNG RAND_bytes() and pid wrap-around demo

openssl_rng.c

/* Demo for repeated CPRNG output after pid wrap around
 *
 * Christian Heimes <christian@python.org>
 *
 *
 * $ gcc -lcrypto openssl_rng.c -o openssl_rng
 * $ ./openssl_rng
 * $ head -n1 rng.txt
 * 29104: 76949c6012516ca3
 * $ egrep '2920.+:' rng.txt
 * 29200: 8328d6d9b0f7a71d
 * 29201: a1153ec9674286a3
 * ...
 * 29200: 8328d6d9b0f7a71d
 * 29201: a1153ec9674286a3
 *
 * Link with OpenSSL git:
 * $ gcc -Iinclude -L. -Wl,-rpath,. -lcrypto -DENABLE_ATFORK openssl_rng.c -o openssl_rng
 *
 */
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#include <openssl/crypto.h>
#include <openssl/rand.h>

#define RNGSIZE 8

int
tohex(char *inbuf, char *outbuf, size_t len)
{
    static const char *hexdigits = "0123456789abcdef";
    size_t i, j;
    for (i=j=0; i < len; i++) {
        unsigned char c;
        c = (inbuf[i] >> 4) & 0xf;
        outbuf[j++] = hexdigits[c];
        c = inbuf[i] & 0xf;
        outbuf[j++] = hexdigits[c];
    }
    return 0;
}

int
main(int args, char *argv[])
{
    char buf[RNGSIZE];
    char hex[2*RNGSIZE];
    int i, fd, status;
    pid_t pid;

    fd = open("rng.txt", O_WRONLY | O_CREAT, 0640);
    if (fd < 0) {
        perror("rng.txt");
        return 1;
    }

#if ENABLE_ATFORK
    printf("Enabling OPENSSL_INIT_ATFORK\n");
    OPENSSL_init_crypto(OPENSSL_INIT_ATFORK, NULL);
#else
    printf("OPENSSL_INIT_ATFORK is not enabled\n");
#endif

    /* initialize RNG */
    RAND_bytes(buf, RNGSIZE);

    /* /proc/sys/kernel/pid_max is usually 32768  */
    for (i=0; i < 34000; i++) {
        if (i && (i % 1000) == 0) {
            fprintf(stderr, ".");
        }
        pid = fork();
        if (pid == 0) {
            /* child */
            RAND_bytes(buf, RNGSIZE);
            tohex(buf, hex, sizeof(buf));
            dprintf(fd, "%lu: %.*s\n", getpid(), sizeof(hex), hex);
#if 1
            if (i < 10) {
                fprintf(stderr, "%lu: %.*s\n", getpid(), sizeof(hex), hex);
            }
#endif
            _exit(0);
        } else {
            /* wait for child to terminate */
            waitpid(pid, &status, 0);
        }
    }
    fprintf(stderr, "\nDone, see rng.txt\n");

    close(fd);
    return 0;
}

$ gcc -lcrypto openssl_rng.c -o openssl_rng
$ ./openssl_rng

$ head -n1 rng.txt
29104: 76949c6012516ca3

$ egrep '2920.+:' rng.txt
29200: 8328d6d9b0f7a71d
29201: a1153ec9674286a3
29202: b0211bc6762561e6
29203: ae9814b39264281d
29204: 647062936ec293dc
29205: 38834f27d8fcf702
29206: 519b28497de9db99
29207: 2d73c41467a4a874
29208: 866ad3c4a7b799ef
29209: eb5ed09fc62c0afc
29200: 8328d6d9b0f7a71d
29201: a1153ec9674286a3
29202: b0211bc6762561e6
29203: ae9814b39264281d
29204: 647062936ec293dc
29205: 38834f27d8fcf702
29206: 519b28497de9db99
29207: 2d73c41467a4a874
29208: 866ad3c4a7b799ef
29209: eb5ed09fc62c0afc

$ wc -l rng.txt
34000 rng.txt

$ sort < rng.txt | uniq | wc -l
31505

$ openssl version
OpenSSL 1.0.2g-fips  1 Mar 2016
$ rpm -qa openssl
openssl-1.0.2g-2.fc23.x86_64
$ gcc --version
gcc (GCC) 5.3.1 20160406 (Red Hat 5.3.1-6)
Copyright (C) 2015 Free Software Foundation, Inc.
$ uname -m
x86_64

OpenSSL 1.1.1's OPENSSL_INIT_ATFORK fixes the issue, https://www.openssl.org/blog/blog/2017/08/12/random/