rfc:improve_predictable_prng_random

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rfc:improve_predictable_prng_random [2017/02/02 03:45] – yohgakirfc:improve_predictable_prng_random [2018/03/01 23:13] (current) – RFC is Under Discussion carusogabriel
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   * Date: 2017-02-01   * Date: 2017-02-01
   * Author: Yasuo Ohgaki <yohgaki@ohgaki.net>   * Author: Yasuo Ohgaki <yohgaki@ohgaki.net>
-  * Status: Draft+  * Status: Under Discussion
   * First Published at: http://wiki.php.net/rfc/improve_predictable_prng_random   * First Published at: http://wiki.php.net/rfc/improve_predictable_prng_random
  
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 <code php> <code php>
 // We need the same random numbers here // We need the same random numbers here
-mt_srand(1234); +srand(1234); 
 for ($i=0; $i < 10; $i++) { for ($i=0; $i < 10; $i++) {
    // Use my PRNG state    // Use my PRNG state
-   $my_rand[] = mt_rand(); +   $my_rand[] = rand(); 
 } }
  
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 </code> </code>
  
-**This is not limited to specific request that calls mt_srand($some_value), but applies to consecutive requests.**+**Above code worked as it should. PHP 7.1 broke this code.** Similarly, shuffle()/etc are broken by PHP 7.1. 
 + 
 +<code php> 
 +// We need the same random numbers here 
 +mt_srand(1234);  
 +for ($i=0; $i < 10; $i++) { 
 +   // Use my PRNG state 
 +   $my_rand[] = mt_rand();  
 +} 
 + 
 +// Somewhere later in code AND/OR even other requests 
 + 
 +// We need to shuffle randomly 
 +shuffle($my_random_array); // This is NOT RANDOM at all 
 +</code> 
 + 
 +**These behaviors are not limited to specific request that calls mt_srand($some_value)/srand($some_value), but applies to consecutive requests.**
  
 PHP should have system and user PRNG state to resolve this behavior. PHP should have system and user PRNG state to resolve this behavior.
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 ==== Rack of Reseeding ==== ==== Rack of Reseeding ====
  
-Reseeding is important for PRNG to mitigate guessed random value. Since MT rand is predictable PRNG, using the same PRNG state allows to guess random value. Current PHP only supports very weak initialization and keeps using the same PRNG state once it is initialized. This behavior makes trivial to guess  MT rand generated random numbers.+Reseeding is important for PRNG to mitigate guessed random value. Since MT rand is predictable PRNG, using the same PRNG state allows to guess next random value easily. Current PHP only supports very weak initialization and keeps using the same PRNG state once it is initialized. This behavior makes trivial to guess  MT rand generated random numbers.
  
 To resolve this issue, PHP should reseed MT rand when state is used certain number of times. To resolve this issue, PHP should reseed MT rand when state is used certain number of times.
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 <code php> <code php>
-  int mt_rand([RandomState $seed_object]) +  int mt_rand([RandomMT $seed_object]) 
-  int mt_rand(int $min, int $max [, RandomState $seed_object]) +  int mt_rand(int $min, int $max [, RandomMT $seed_object]) 
-  int rand([RandomState $seed_object]) +  int rand([Random $seed_object]) 
-  int rand(int $min, int $max [, RandomState $seed_object]) +  int rand(int $min, int $max [, Random $seed_object]) 
-  bool shuffle(array &$arr [, RandomSatate $seed_object]);+  bool shuffle(array &$arr [, Random $seed_object]);
 </code> </code>
  
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     public function getBytes(int $length); // Raw bytes     public function getBytes(int $length); // Raw bytes
     public function getString(int $length, int $bits = 6); // String [0-9a-zA-Z,-]+     public function getString(int $length, int $bits = 6); // String [0-9a-zA-Z,-]+
-    public function seed($seed = NULL); // No use with CS RNG, return TRUE always. +    public function seed($seed = NULL); // No use with CS RNG, raise exception. 
-    public function getState(); // Return string representation PRNG state. No use with CS RNG, return NULL. +    public function getState(); // Return string representation PRNG state. No use with CS RNG, raise exception. 
-    public function setState(string $state); // Set PRNG state. No use with CS RNG, return TRUE always. +    public function setState(string $state); // Set PRNG state. No use with CS RNG, raise exception. 
-    public function getCount(); // No use with CS RNG, return 0 always. +    public function getCount(); // No use with CS RNG, raise exception. 
-    public function getReseedCycle(); // No use with CS RNG, return 0 always. +    public function getReseedCycle(); // No use with CS RNG, raise exception. 
-    public function setReseedCycle(int $count); // No use with CS RNG, return TRUE always.+    public function setReseedCycle(int $count); // No use with CS RNG, raise exception.
 } }
  
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     public function __construct($seed = NULL) {     public function __construct($seed = NULL) {
       $this->seed($seed);       $this->seed($seed);
 +    }
 +
 +    private reseed() {
 +      $this->count++;
 +      if ($this->reseed && !($this->count % $this->reseed)) {
 +        $this->seed();
 +        $this->count = 1;
 +      }
     }     }
          
     public function getInt($min = NULL, $max = NULL) {     public function getInt($min = NULL, $max = NULL) {
       assert($min <= $max);       assert($min <= $max);
 +      $this->reseed();
       if ($min && $max) {       if ($min && $max) {
         return mt_rand($min, $max);         return mt_rand($min, $max);
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     public function getBytes(int $length) {     public function getBytes(int $length) {
-      // Return raw random bytes +      // Return raw random bytes. 3 out of 4 bytes are used not to disclose full PRNG state
     }     }
          
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         // Update state by user seed         // Update state by user seed
         mt_srand($seed);         mt_srand($seed);
 +        $this->reseed = 0;
       } else {       } else {
         // Seed by system generated random value         // Seed by system generated random value
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 </code> </code>
  
-uint32_t BG(mt_rand_is_seeded) is used for already seeded flag and counter. Upper 16 bits are used for seeded flag, lower 16 bits are used for counters. Therefore, max reseed count is 2^16.+uint32_t BG(mt_rand_is_seeded) is used for already seeded flag and counter. MSB is used for seeded flag, the rest bits are used for counters. Therefore, max reseed count is 2^31.
  
  
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 mt_srand()/srand() returned nothing previously. mt_srand()/srand() returned nothing previously.
  
-If users want static random values, they have to use RandomStatus object to get it. Use of mt_srand()/srand() would be rare in general.+If users want static random values, they have to use Random object to get certain random sequence. i.e. Call mt_srand()/srand() for Random object, then use it with functions, rand()/mt_rand()/shuffle()/etc. Use of mt_srand()/srand() would be rare in general.
  
 ===== Proposed PHP Version(s) ===== ===== Proposed PHP Version(s) =====
rfc/improve_predictable_prng_random.1486007101.txt.gz · Last modified: 2017/09/22 13:28 (external edit)

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