dev/getentropy/getentropy-cpucounter.c

0001 /* SPDX-License-Identifier: BSD-2-Clause */
0002
0003 /*
0004  * Copyright (c) 2017 embedded brains GmbH & Co. KG
0005  *
0006  * Redistribution and use in source and binary forms, with or without
0007  * modification, are permitted provided that the following conditions
0008  * are met:
0009  * 1. Redistributions of source code must retain the above copyright
0010  *    notice, this list of conditions and the following disclaimer.
0011  * 2. Redistributions in binary form must reproduce the above copyright
0012  *    notice, this list of conditions and the following disclaimer in the
0013  *    documentation and/or other materials provided with the distribution.
0014  *
0015  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
0016  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
0017  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
0018  * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
0019  * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
0020  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
0021  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
0022  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
0023  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
0024  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
0025  * POSSIBILITY OF SUCH DAMAGE.
0026  */
0027
0028 /*
0029  * ATTENTION: THIS IS A VERY LIMITED ENTROPY SOURCE.
0030  *
0031  * This implementation uses a time-based value for it's entropy. The only thing
0032  * that makes it random are interrupts from external sources. Don't use it if
0033  * you need for example a strong crypto.
0034  */
0035
0036 #include <sys/param.h>
0037 #include <unistd.h>
0038 #include <string.h>
0039 #include <rtems/bsd.h>
0040 #include <rtems/counter.h>
0041 #include <rtems/sysinit.h>
0042
0043 static uint32_t state;
0044
0045 int getentropy(void *ptr, size_t n)
0046 {
0047   uint8_t *dest = ptr;
0048
0049   state ^= rtems_counter_read();
0050   state *= 25169206;
0051   state += 1679610226;
0052
0053   while (n > 0) {
0054     size_t m;
0055
0056     m = MIN(n, sizeof(state));
0057     memcpy(dest, &state, m);
0058     n -= m;
0059     dest += m;
0060     state *= 85236167;
0061     state += 30557471;
0062   }
0063
0064   return 0;
0065 }
0066
0067 static void getentropy_init(void)
0068 {
0069   struct bintime bt;
0070
0071   rtems_bsd_bintime(&bt);
0072   state = (uint32_t) bt.frac;
0073   state ^= (uint32_t) (bt.frac >> 32);
0074   state ^= (uint32_t) bt.sec;
0075   state ^= (uint32_t) (bt.sec >> 32);
0076 }
0077
0078 RTEMS_SYSINIT_ITEM(
0079   getentropy_init,
0080   RTEMS_SYSINIT_DEVICE_DRIVERS,
0081   RTEMS_SYSINIT_ORDER_LAST_BUT_5
0082 );