diff from djm@ committed at his request:

introduce two new APIs for requesting strong random numbers:
arc4random_buf() - fill an arbitrary memory range with random numbers
arc4random_uniform() - return a uniformly distributed random number
below
a specified upper bound, avoiding the bias that comes from a naive
"arc4random() % upper_bound" construction.
these mirror similarly-named functions in the kernel;
lots of discussion deraadt@ mcbride@

src/include/stdlib.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: stdlib.h,v 1.40 2007/09/03 14:37:02 millert Exp $	*/
+/*	$OpenBSD: stdlib.h,v 1.41 2008/03/16 19:47:43 otto Exp $	*/
 /*	$NetBSD: stdlib.h,v 1.25 1995/12/27 21:19:08 jtc Exp $	*/
 
 /*-
@@ -272,6 +272,10 @@ u_int32_t arc4random(void);
 void	arc4random_stir(void);
 void	arc4random_addrandom(unsigned char *, int)
 	__attribute__((__bounded__ (__string__,1,2)));
+u_int32_t arc4random_uniform(u_int32_t);
+void arc4random_buf(void *, size_t)
+	__attribute__((__bounded__ (__string__,1,2)));
+
 #endif /* __BSD_VISIBLE */
 
 __END_DECLS

src/lib/libc/crypt/arc4random.3

@@ -1,4 +1,4 @@
-.\" $OpenBSD: arc4random.3,v 1.22 2007/05/31 19:19:27 jmc Exp $
+.\" $OpenBSD: arc4random.3,v 1.23 2008/03/16 19:47:43 otto Exp $
 .\"
 .\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
 .\" All rights reserved.
@@ -30,7 +30,7 @@
 .\"
 .\" Manual page, using -mandoc macros
 .\"
-.Dd $Mdocdate: May 31 2007 $
+.Dd $Mdocdate: March 16 2008 $
 .Dt ARC4RANDOM 3
 .Os
 .Sh NAME
@@ -43,6 +43,10 @@
 .Ft u_int32_t
 .Fn arc4random "void"
 .Ft void
+.Fn arc4random_buf "void *buf" "size_t nbytes"
+.Ft u_int32_t
+.Fn arc4random_uniform "u_int32_t upper_bound"
+.Ft void
 .Fn arc4random_stir "void"
 .Ft void
 .Fn arc4random_addrandom "u_char *dat" "int datlen"
@@ -73,6 +77,21 @@ versus the fast but poor quality interfaces described in
 and
 .Xr drand48 3 .
 .Pp
+.Fn arc4random_buf
+fills the region
+.Fa buf
+of length
+.Fa nbytes
+with ARC4-derived random data.
+.Pp
+.Fn arc4random_uniform
+will return a uniformly distributed random number less than
+.Fa upper_bound .
+.Fn arc4random_uniform
+is recommended over constructions like
+.Do Li arc4random() % upper_bound Dc
+as it avoids "modulo bias" when the upper bound is not a power of two.
+.Pp
 The
 .Fn arc4random_stir
 function reads data from

src/lib/libc/crypt/arc4random.c

@@ -1,7 +1,8 @@
-/*	$OpenBSD: arc4random.c,v 1.17 2008/01/01 00:43:39 kurt Exp $	*/
+/*	$OpenBSD: arc4random.c,v 1.18 2008/03/16 19:47:43 otto Exp $	*/
 
 /*
  * 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
@@ -34,6 +35,7 @@
  */
 
 #include <fcntl.h>
+#include <limits.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <sys/types.h>
@@ -188,6 +190,66 @@ arc4random(void)
 	return val;
 }
 
+void
+arc4random_buf(void *_buf, size_t n)
+{
+	u_char *buf = (u_char *)_buf;
+	_ARC4_LOCK();
+	if (!rs_initialized || arc4_stir_pid != getpid())
+		arc4_stir();
+	while (n--) {
+		if (--arc4_count <= 0)
+			arc4_stir();
+		buf[n] = arc4_getbyte();
+	}
+	_ARC4_UNLOCK();
+}
+
+/*
+ * 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.
+ */
+u_int32_t
+arc4random_uniform(u_int32_t upper_bound)
+{
+	u_int32_t r, min;
+
+	if (upper_bound < 2)
+		return 0;
+
+#if (ULONG_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;
+}
+
 #if 0
 /*-------- Test code for i386 --------*/
 #include <stdio.h>