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@

2 files changed, 84 insertions, 3 deletions

diff --git a/src/lib/libc/crypt/arc4random.3 b/src/lib/libc/crypt/arc4random.3
index 31da5ec7ec..d32ea4a951 100644
--- a/src/lib/libc/crypt/arc4random.3
+++ b/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
diff --git a/src/lib/libc/crypt/arc4random.c b/src/lib/libc/crypt/arc4random.c
index 8604548f3f..bbe42bd204 100644
--- a/src/lib/libc/crypt/arc4random.c
+++ b/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>