view dbrandom.c @ 994:5c5ade336926

Prefer stronger algorithms in algorithm negotiation. Prefer diffie-hellman-group14-sha1 (2048 bit) over diffie-hellman-group1-sha1 (1024 bit). Due to meet-in-the-middle attacks the effective key length of three key 3DES is 112 bits. AES is stronger and faster then 3DES. Prefer to delay the start of compression until after authentication has completed. This avoids exposing compression code to attacks from unauthenticated users. (github pull request #9)
author Fedor Brunner <fedor.brunner@azet.sk>
date Fri, 23 Jan 2015 23:00:25 +0800
parents 220f55d540ae
children a1e79ffa5862
line wrap: on
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/*
 * Dropbear - a SSH2 server
 * 
 * Copyright (c) 2002,2003 Matt Johnston
 * All rights reserved.
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE. */

#include "includes.h"
#include "buffer.h"
#include "dbutil.h"
#include "bignum.h"
#include "dbrandom.h"


/* this is used to generate unique output from the same hashpool */
static uint32_t counter = 0;
/* the max value for the counter, so it won't integer overflow */
#define MAX_COUNTER 1<<30 

static unsigned char hashpool[SHA1_HASH_SIZE] = {0};
static int donerandinit = 0;

#define INIT_SEED_SIZE 32 /* 256 bits */

/* The basic setup is we read some data from /dev/(u)random or prngd and hash it
 * into hashpool. To read data, we hash together current hashpool contents,
 * and a counter. We feed more data in by hashing the current pool and new
 * data into the pool.
 *
 * It is important to ensure that counter doesn't wrap around before we
 * feed in new entropy.
 *
 */

/* Pass len=0 to hash an entire file */
static int
process_file(hash_state *hs, const char *filename,
		unsigned int len, int prngd)
{
	static int already_blocked = 0;
	int readfd;
	unsigned int readcount;
	int ret = DROPBEAR_FAILURE;

#ifdef DROPBEAR_PRNGD_SOCKET
	if (prngd)
	{
		readfd = connect_unix(filename);
	}
	else
#endif
	{
		readfd = open(filename, O_RDONLY);
	}

	if (readfd < 0) {
		goto out;
	}

	readcount = 0;
	while (len == 0 || readcount < len)
	{
		int readlen, wantread;
		unsigned char readbuf[4096];
		if (!already_blocked && !prngd)
		{
			int res;
			struct timeval timeout;
			fd_set read_fds;

 			timeout.tv_sec  = 2;
 			timeout.tv_usec = 0;

			FD_ZERO(&read_fds);
			FD_SET(readfd, &read_fds);
			res = select(readfd + 1, &read_fds, NULL, NULL, &timeout);
			if (res == 0)
			{
				dropbear_log(LOG_WARNING, "Warning: Reading the randomness source '%s' seems to have blocked.\nYou may need to find a better entropy source.", filename);
				already_blocked = 1;
			}
		}

		if (len == 0)
		{
			wantread = sizeof(readbuf);
		} 
		else
		{
			wantread = MIN(sizeof(readbuf), len-readcount);
		}

#ifdef DROPBEAR_PRNGD_SOCKET
		if (prngd)
		{
			char egdcmd[2];
			egdcmd[0] = 0x02;	/* blocking read */
			egdcmd[1] = (unsigned char)wantread;
			if (write(readfd, egdcmd, 2) < 0)
			{
				dropbear_exit("Can't send command to egd");
			}
		}
#endif

		readlen = read(readfd, readbuf, wantread);
		if (readlen <= 0) {
			if (readlen < 0 && errno == EINTR) {
				continue;
			}
			if (readlen == 0 && len == 0)
			{
				/* whole file was read as requested */
				break;
			}
			goto out;
		}
		sha1_process(hs, readbuf, readlen);
		readcount += readlen;
	}
	ret = DROPBEAR_SUCCESS;
out:
	close(readfd);
	return ret;
}

void addrandom(char * buf, unsigned int len)
{
	hash_state hs;

	/* hash in the new seed data */
	sha1_init(&hs);
	/* existing state (zeroes on startup) */
	sha1_process(&hs, (void*)hashpool, sizeof(hashpool));

	/* new */
	sha1_process(&hs, buf, len);
	sha1_done(&hs, hashpool);
}

static void write_urandom()
{
#ifndef DROPBEAR_PRNGD_SOCKET
	/* This is opportunistic, don't worry about failure */
	unsigned char buf[INIT_SEED_SIZE];
	FILE *f = fopen(DROPBEAR_URANDOM_DEV, "w");
	if (!f) {
		return;
	}
	genrandom(buf, sizeof(buf));
	fwrite(buf, sizeof(buf), 1, f);
	fclose(f);
#endif
}

/* Initialise the prng from /dev/urandom or prngd. This function can
 * be called multiple times */
void seedrandom() {
		
	hash_state hs;

	pid_t pid;
	struct timeval tv;
	clock_t clockval;

	/* hash in the new seed data */
	sha1_init(&hs);
	/* existing state */
	sha1_process(&hs, (void*)hashpool, sizeof(hashpool));

#ifdef DROPBEAR_PRNGD_SOCKET
	if (process_file(&hs, DROPBEAR_PRNGD_SOCKET, INIT_SEED_SIZE, 1) 
			!= DROPBEAR_SUCCESS) {
		dropbear_exit("Failure reading random device %s", 
				DROPBEAR_PRNGD_SOCKET);
	}
#else
	/* non-blocking random source (probably /dev/urandom) */
	if (process_file(&hs, DROPBEAR_URANDOM_DEV, INIT_SEED_SIZE, 0) 
			!= DROPBEAR_SUCCESS) {
		dropbear_exit("Failure reading random device %s", 
				DROPBEAR_URANDOM_DEV);
	}
#endif

	/* A few other sources to fall back on. 
	 * Add more here for other platforms */
#ifdef __linux__
	/* Seems to be a reasonable source of entropy from timers. Possibly hard
	 * for even local attackers to reproduce */
	process_file(&hs, "/proc/timer_list", 0, 0);
	/* Might help on systems with wireless */
	process_file(&hs, "/proc/interrupts", 0, 0);

	process_file(&hs, "/proc/loadavg", 0, 0);
	process_file(&hs, "/proc/sys/kernel/random/entropy_avail", 0, 0);

	/* Mostly network visible but useful in some situations.
	 * Limit size to avoid slowdowns on systems with lots of routes */
	process_file(&hs, "/proc/net/netstat", 4096, 0);
	process_file(&hs, "/proc/net/dev", 4096, 0);
	process_file(&hs, "/proc/net/tcp", 4096, 0);
	/* Also includes interface lo */
	process_file(&hs, "/proc/net/rt_cache", 4096, 0);
	process_file(&hs, "/proc/vmstat", 0, 0);
#endif

	pid = getpid();
	sha1_process(&hs, (void*)&pid, sizeof(pid));

	/* gettimeofday() doesn't completely fill out struct timeval on 
	   OS X (10.8.3), avoid valgrind warnings by clearing it first */
	memset(&tv, 0x0, sizeof(tv));
	gettimeofday(&tv, NULL);
	sha1_process(&hs, (void*)&tv, sizeof(tv));

	clockval = clock();
	sha1_process(&hs, (void*)&clockval, sizeof(clockval));

	/* When a private key is read by the client or server it will
	 * be added to the hashpool - see runopts.c */

	sha1_done(&hs, hashpool);

	counter = 0;
	donerandinit = 1;

	/* Feed it all back into /dev/urandom - this might help if Dropbear
	 * is running from inetd and gets new state each time */
	write_urandom();
}

/* return len bytes of pseudo-random data */
void genrandom(unsigned char* buf, unsigned int len) {

	hash_state hs;
	unsigned char hash[SHA1_HASH_SIZE];
	unsigned int copylen;

	if (!donerandinit) {
		dropbear_exit("seedrandom not done");
	}

	while (len > 0) {
		sha1_init(&hs);
		sha1_process(&hs, (void*)hashpool, sizeof(hashpool));
		sha1_process(&hs, (void*)&counter, sizeof(counter));
		sha1_done(&hs, hash);

		counter++;
		if (counter > MAX_COUNTER) {
			seedrandom();
		}

		copylen = MIN(len, SHA1_HASH_SIZE);
		memcpy(buf, hash, copylen);
		len -= copylen;
		buf += copylen;
	}
	m_burn(hash, sizeof(hash));
}

/* Generates a random mp_int. 
 * max is a *mp_int specifying an upper bound.
 * rand must be an initialised *mp_int for the result.
 * the result rand satisfies:  0 < rand < max 
 * */
void gen_random_mpint(mp_int *max, mp_int *rand) {

	unsigned char *randbuf = NULL;
	unsigned int len = 0;
	const unsigned char masks[] = {0xff, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f};

	const int size_bits = mp_count_bits(max);

	len = size_bits / 8;
	if ((size_bits % 8) != 0) {
		len += 1;
	}

	randbuf = (unsigned char*)m_malloc(len);
	do {
		genrandom(randbuf, len);
		/* Mask out the unrequired bits - mp_read_unsigned_bin expects
		 * MSB first.*/
		randbuf[0] &= masks[size_bits % 8];

		bytes_to_mp(rand, randbuf, len);

		/* keep regenerating until we get one satisfying
		 * 0 < rand < max    */
	} while (mp_cmp(rand, max) != MP_LT);
	m_burn(randbuf, len);
	m_free(randbuf);
}