bug #470, use a dedicated lib to generate random bytes

1 files changed, 191 insertions, 0 deletions

diff --git a/include/random_compat/random_int.php b/include/random_compat/random_int.php
new file mode 100644
index 000000000..fd3ef87a2
--- /dev/null
+++ b/include/random_compat/random_int.php
@@ -0,0 +1,191 @@
+<?php
+/**
+ * Random_* Compatibility Library 
+ * for using the new PHP 7 random_* API in PHP 5 projects
+ * 
+ * The MIT License (MIT)
+ * 
+ * Copyright (c) 2015 Paragon Initiative Enterprises
+ * 
+ * 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.
+ */
+
+/**
+ * Fetch a random integer between $min and $max inclusive
+ * 
+ * @param int $min
+ * @param int $max
+ * 
+ * @throws Exception
+ * 
+ * @return int
+ */
+function random_int($min, $max)
+{
+    /**
+     * Type and input logic checks
+     * 
+     * If you pass it a float in the range (~PHP_INT_MAX, PHP_INT_MAX)
+     * (non-inclusive), it will sanely cast it to an int. If you it's equal to
+     * ~PHP_INT_MAX or PHP_INT_MAX, we let it fail as not an integer. Floats 
+     * lose precision, so the <= and => operators might accidentally let a float
+     * through.
+     */
+    
+    try {
+        $min = RandomCompat_intval($min);
+    } catch (TypeError $ex) {
+        throw new TypeError(
+            'random_int(): $min must be an integer'
+        );
+    }
+
+    try {
+        $max = RandomCompat_intval($max);
+    } catch (TypeError $ex) {
+        throw new TypeError(
+            'random_int(): $max must be an integer'
+        );
+    }
+    
+    /**
+     * Now that we've verified our weak typing system has given us an integer,
+     * let's validate the logic then we can move forward with generating random
+     * integers along a given range.
+     */
+    if ($min > $max) {
+        throw new Error(
+            'Minimum value must be less than or equal to the maximum value'
+        );
+    }
+
+    if ($max === $min) {
+        return $min;
+    }
+
+    /**
+     * Initialize variables to 0
+     * 
+     * We want to store:
+     * $bytes => the number of random bytes we need
+     * $mask => an integer bitmask (for use with the &) operator
+     *          so we can minimize the number of discards
+     */
+    $attempts = $bits = $bytes = $mask = $valueShift = 0;
+
+    /**
+     * At this point, $range is a positive number greater than 0. It might
+     * overflow, however, if $max - $min > PHP_INT_MAX. PHP will cast it to
+     * a float and we will lose some precision.
+     */
+    $range = $max - $min;
+
+    /**
+     * Test for integer overflow:
+     */
+    if (!is_int($range)) {
+
+        /**
+         * Still safely calculate wider ranges.
+         * Provided by @CodesInChaos, @oittaa
+         * 
+         * @ref https://gist.github.com/CodesInChaos/03f9ea0b58e8b2b8d435
+         * 
+         * We use ~0 as a mask in this case because it generates all 1s
+         * 
+         * @ref https://eval.in/400356 (32-bit)
+         * @ref http://3v4l.org/XX9r5  (64-bit)
+         */
+        $bytes = PHP_INT_SIZE;
+        $mask = ~0;
+
+    } else {
+
+        /**
+         * $bits is effectively ceil(log($range, 2)) without dealing with 
+         * type juggling
+         */
+        while ($range > 0) {
+            if ($bits % 8 === 0) {
+               ++$bytes;
+            }
+            ++$bits;
+            $range >>= 1;
+            $mask = $mask << 1 | 1;
+        }
+        $valueShift = $min;
+    }
+
+    /**
+     * Now that we have our parameters set up, let's begin generating
+     * random integers until one falls between $min and $max
+     */
+    do {
+        /**
+         * The rejection probability is at most 0.5, so this corresponds
+         * to a failure probability of 2^-128 for a working RNG
+         */
+        if ($attempts > 128) {
+            throw new Exception(
+                'random_int: RNG is broken - too many rejections'
+            );
+        }
+
+        /**
+         * Let's grab the necessary number of random bytes
+         */
+        $randomByteString = random_bytes($bytes);
+        if ($randomByteString === false) {
+            throw new Exception(
+                'Random number generator failure'
+            );
+        }
+
+        /**
+         * Let's turn $randomByteString into an integer
+         * 
+         * This uses bitwise operators (<< and |) to build an integer
+         * out of the values extracted from ord()
+         * 
+         * Example: [9F] | [6D] | [32] | [0C] =>
+         *   159 + 27904 + 3276800 + 201326592 =>
+         *   204631455
+         */
+        $val = 0;
+        for ($i = 0; $i < $bytes; ++$i) {
+            $val |= ord($randomByteString[$i]) << ($i * 8);
+        }
+
+        /**
+         * Apply mask
+         */
+        $val &= $mask;
+        $val += $valueShift;
+
+        ++$attempts;
+        /**
+         * If $val overflows to a floating point number,
+         * ... or is larger than $max,
+         * ... or smaller than $min,
+         * then try again.
+         */
+    } while (!is_int($val) || $val > $max || $val < $min);
+
+    return (int) $val;
+}