Add an arc4random implementation for use by evdns

Previously, evdns was at the mercy of the user for providing a good
entropy source; without one, it would be vulnerable to various
active attacks.

This patch adds a port of OpenBSD's arc4random() calls to Libevent
[port by Chris Davis], and wraps it up a little bit so we can use it
more safely.

1 files changed, 358 insertions, 0 deletions

diff --git a/arc4random.c b/arc4random.c
new file mode 100644
index 00000000..2353626e
--- /dev/null
+++ b/arc4random.c
@@ -0,0 +1,358 @@
+/* Portable arc4random.c based on arc4random.c from OpenBSD.
+ * Portable version by Chris Davis, adapted for Libevent by Nick Mathewson
+ *
+ * Note that in Libevent, this file isn't compiled directly.  Instead,
+ * it's included from evutil_rand.c
+ */
+
+/*
+ * Copyright (c) 1996, David Mazieres <dm@uun.org>
+ * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * Arc4 random number generator for OpenBSD.
+ *
+ * This code is derived from section 17.1 of Applied Cryptography,
+ * second edition, which describes a stream cipher allegedly
+ * compatible with RSA Labs "RC4" cipher (the actual description of
+ * which is a trade secret).  The same algorithm is used as a stream
+ * cipher called "arcfour" in Tatu Ylonen's ssh package.
+ *
+ * Here the stream cipher has been modified always to include the time
+ * when initializing the state.  That makes it impossible to
+ * regenerate the same random sequence twice, so this can't be used
+ * for encryption, but will generate good random numbers.
+ *
+ * RC4 is a registered trademark of RSA Laboratories.
+ */
+
+#ifndef ARC4RANDOM_EXPORT
+#define ARC4RANDOM_EXPORT
+#endif
+
+#ifndef ARC4RANDOM_NO_INCLUDES
+#ifdef WIN32
+#include <wincrypt.h>
+#else
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/param.h>
+#include <sys/time.h>
+#endif
+#include <limits.h>
+#include <stdlib.h>
+#include <string.h>
+#endif
+
+/* Add platform entropy 32 bytes (256 bits) at a time. */
+#define ADD_ENTROPY 32
+
+/* Re-seed from the platform RNG after generating this many bytes. */
+#define BYTES_BEFORE_RESEED 1600000
+
+struct arc4_stream {
+	unsigned char i;
+	unsigned char j;
+	unsigned char s[256];
+};
+
+static int rs_initialized;
+static struct arc4_stream rs;
+static pid_t arc4_stir_pid;
+static int arc4_count;
+static int arc4_seeded_ok;
+
+static inline unsigned char arc4_getbyte(void);
+
+static inline void
+arc4_init(void)
+{
+	int     n;
+
+	for (n = 0; n < 256; n++)
+		rs.s[n] = n;
+	rs.i = 0;
+	rs.j = 0;
+}
+
+static inline void
+arc4_addrandom(const unsigned char *dat, int datlen)
+{
+	int     n;
+	unsigned char si;
+
+	rs.i--;
+	for (n = 0; n < 256; n++) {
+		rs.i = (rs.i + 1);
+		si = rs.s[rs.i];
+		rs.j = (rs.j + si + dat[n % datlen]);
+		rs.s[rs.i] = rs.s[rs.j];
+		rs.s[rs.j] = si;
+	}
+	rs.j = rs.i;
+}
+
+#ifndef WIN32
+static ssize_t
+read_all(int fd, unsigned char *buf, size_t count)
+{
+	size_t numread = 0;
+	ssize_t result;
+
+	while (numread < count) {
+		result = read(fd, buf+numread, count-numread);
+		if (result<0)
+			return -1;
+		else if (result == 0)
+			break;
+		numread += result;
+	}
+
+	return (ssize_t)numread;
+}
+#endif
+
+/* This is adapted from Tor's crypto_seed_rng() */
+static int
+arc4_seed(void)
+{
+	unsigned char buf[ADD_ENTROPY];
+
+	/* local variables */
+#ifdef WIN32
+	static int provider_set = 0;
+	static HCRYPTPROV provider;
+#else
+	static const char *filenames[] = {
+		"/dev/srandom", "/dev/urandom", "/dev/random", NULL
+	};
+	int fd, i;
+	size_t n;
+#endif
+
+#ifdef WIN32
+	if (!provider_set) {
+		if (!CryptAcquireContext(&provider, NULL, NULL, PROV_RSA_FULL,
+		    CRYPT_VERIFYCONTEXT)) {
+			if ((unsigned long)GetLastError() != (unsigned long)NTE_BAD_KEYSET)
+				return -1;
+		}
+		provider_set = 1;
+	}
+	if (!CryptGenRandom(provider, sizeof(buf), buf))
+		return -1;
+	arc4_addrandom(buf, sizeof(buf));
+	memset(buf, 0, sizeof(buf));
+	arc4_seeded_ok = 1;
+	return 0;
+#else
+	for (i = 0; filenames[i]; ++i) {
+		fd = open(filenames[i], O_RDONLY, 0);
+		if (fd<0)
+			continue;
+		n = read_all(fd, buf, sizeof(buf));
+		close(fd);
+		if (n != sizeof(buf))
+			return -1;
+		arc4_addrandom(buf, sizeof(buf));
+		memset(buf, 0, sizeof(buf));
+		arc4_seeded_ok = 1;
+		return 0;
+	}
+
+	return -1;
+#endif
+}
+
+static void
+arc4_stir(void)
+{
+	int     i;
+
+	if (!rs_initialized) {
+		arc4_init();
+		rs_initialized = 1;
+	}
+
+	arc4_seed();
+
+	/*
+	 * Discard early keystream, as per recommendations in
+	 * "Weaknesses in the Key Scheduling Algorithm of RC4" by
+	 * Scott Fluhrer, Itsik Mantin, and Adi Shamir.
+	 * http://www.wisdom.weizmann.ac.il/~itsik/RC4/Papers/Rc4_ksa.ps
+	 *
+	 * Ilya Mironov's "(Not So) Random Shuffles of RC4" suggests that
+	 * we drop at least 2*256 bytes, with 12*256 as a conservative
+	 * value.
+	 *
+	 * RFC4345 says to drop 6*256.
+	 *
+	 * At least some versions of this code drop 4*256, in a mistaken
+	 * belief that "words" in the Fluhrer/Mantin/Shamir paper refers
+	 * to processor words.
+	 *
+	 * We add another sect to the cargo cult, and choose 12*256.
+	 */
+	for (i = 0; i < 12*256; i++)
+		(void)arc4_getbyte();
+	arc4_count = BYTES_BEFORE_RESEED;
+}
+
+static void
+arc4_stir_if_needed(void)
+{
+	pid_t pid = getpid();
+
+	if (arc4_count <= 0 || !rs_initialized || arc4_stir_pid != pid)
+	{
+		arc4_stir_pid = pid;
+		arc4_stir();
+	}
+}
+
+static inline unsigned char
+arc4_getbyte(void)
+{
+	unsigned char si, sj;
+
+	rs.i = (rs.i + 1);
+	si = rs.s[rs.i];
+	rs.j = (rs.j + si);
+	sj = rs.s[rs.j];
+	rs.s[rs.i] = sj;
+	rs.s[rs.j] = si;
+	return (rs.s[(si + sj) & 0xff]);
+}
+
+static inline unsigned int
+arc4_getword(void)
+{
+	unsigned int val;
+
+	val = arc4_getbyte() << 24;
+	val |= arc4_getbyte() << 16;
+	val |= arc4_getbyte() << 8;
+	val |= arc4_getbyte();
+
+	return val;
+}
+
+#ifndef ARC4RANDOM_NOSTIR
+ARC4RANDOM_EXPORT int
+arc4random_stir(void)
+{
+	int val;
+	_ARC4_LOCK();
+	val = arc4_stir();
+	_ARC4_UNLOCK();
+	return val;
+}
+#endif
+
+#ifndef ARC4RANDOM_NOADDRANDOM
+ARC4RANDOM_EXPORT void
+arc4random_addrandom(const unsigned char *dat, int datlen)
+{
+	int j;
+	_ARC4_LOCK();
+	if (!rs_initialized)
+		arc4_stir();
+	for (j = 0; j < datlen; j += 256) {
+		/* arc4_addrandom() ignores all but the first 256 bytes of
+		 * its input.  We want to make sure to look at ALL the
+		 * data in 'dat', just in case the user is doing something
+		 * crazy like passing us all the files in /var/log. */
+		arc4_addrandom(dat + j, datlen - j);
+	}
+	_ARC4_UNLOCK();
+}
+#endif
+
+#ifndef ARC4RANDOM_NORANDOM
+ARC4RANDOM_EXPORT unsigned int
+arc4random(void)
+{
+	unsigned int val;
+	_ARC4_LOCK();
+	arc4_count -= 4;
+	arc4_stir_if_needed();
+	val = arc4_getword();
+	_ARC4_UNLOCK();
+	return val;
+}
+#endif
+
+ARC4RANDOM_EXPORT void
+arc4random_buf(void *_buf, size_t n)
+{
+	unsigned char *buf = _buf;
+	_ARC4_LOCK();
+	arc4_stir_if_needed();
+	while (n--) {
+		if (--arc4_count <= 0)
+			arc4_stir();
+		buf[n] = arc4_getbyte();
+	}
+	_ARC4_UNLOCK();
+}
+
+#ifndef ARC4RANDOM_NOUNIFORM
+/*
+ * Calculate a uniformly distributed random number less than upper_bound
+ * avoiding "modulo bias".
+ *
+ * Uniformity is achieved by generating new random numbers until the one
+ * returned is outside the range [0, 2**32 % upper_bound).  This
+ * guarantees the selected random number will be inside
+ * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
+ * after reduction modulo upper_bound.
+ */
+ARC4RANDOM_EXPORT unsigned int
+arc4random_uniform(unsigned int upper_bound)
+{
+	unsigned int r, min;
+
+	if (upper_bound < 2)
+		return 0;
+
+#if (UINT_MAX > 0xffffffffUL)
+	min = 0x100000000UL % upper_bound;
+#else
+	/* Calculate (2**32 % upper_bound) avoiding 64-bit math */
+	if (upper_bound > 0x80000000)
+		min = 1 + ~upper_bound;		/* 2**32 - upper_bound */
+	else {
+		/* (2**32 - (x * 2)) % x == 2**32 % x when x <= 2**31 */
+		min = ((0xffffffff - (upper_bound * 2)) + 1) % upper_bound;
+	}
+#endif
+
+	/*
+	 * This could theoretically loop forever but each retry has
+	 * p > 0.5 (worst case, usually far better) of selecting a
+	 * number inside the range we need, so it should rarely need
+	 * to re-roll.
+	 */
+	for (;;) {
+		r = arc4random();
+		if (r >= min)
+			break;
+	}
+
+	return r % upper_bound;
+}
+#endif