Merge pull request #8346 from Calinou/random-number-generation-csprng

Document cryptographically secure random number generation

tutorials/math/random_number_generation.rst

@@ -9,7 +9,9 @@ Godot.
 
 After giving you a brief overview of useful functions that generate random
 numbers, you will learn how to get random elements from arrays, dictionaries,
-and how to use a noise generator in GDScript.
+and how to use a noise generator in GDScript. Lastly, we'll take a look at
+cryptographically secure random number generation and how it differs from
+typical random number generation.
 
 .. note::
 
@@ -17,6 +19,9 @@ and how to use a noise generator in GDScript.
     `pseudorandom number generators
     <https://en.wikipedia.org/wiki/Pseudorandom_number_generator>`__ (PRNGs).
 
+    Godot internally uses the `PCG Family <https://www.pcg-random.org/>`__
+    of pseudorandom number generators.
+
 Global scope versus RandomNumberGenerator class
 -----------------------------------------------
 
@@ -34,6 +39,14 @@ the RandomNumberGenerator class.
 The randomize() method
 ----------------------
 
+.. note::
+
+    Since Godot 4.0, the random seed is automatically set to a random value when
+    the project starts. This means you don't need to call ``randomize()`` in
+    ``_ready()`` anymore to ensure that results are random across project runs.
+    However, you can still use ``randomize()`` if you want to use a specific
+    seed number, or generate it using a different method.
+
 In global scope, you can find a :ref:`randomize()
 <class_@GlobalScope_method_randomize>` method. **This method should be called only
 once when your project starts to initialize the random seed.** Calling it
@@ -473,3 +486,48 @@ terrain. Godot provides :ref:`class_fastnoiselite` for this, which supports
             GD.Print(_noise.GetNoise1D(i));
         }
     }
+
+Cryptographically secure pseudorandom number generation
+-------------------------------------------------------
+
+So far, the approaches mentioned above are **not** suitable for
+*cryptographically secure* pseudorandom number generation (CSPRNG). This is fine
+for games, but this is not sufficient for scenarios where encryption,
+authentication or signing is involved.
+
+Godot offers a :ref:`class_Crypto` class for this. This class can perform
+asymmetric key encryption/decryption, signing/verification, while also
+generating cryptographically secure random bytes, RSA keys, HMAC digests, and
+self-signed :ref:`class_X509Certificate`\ s.
+
+The downside of :abbr:`CSPRNG (Cryptographically secure pseudorandom number generation)`
+is that it's much slower than standard pseudorandom number generation. Its API
+is also less convenient to use. As a result,
+:abbr:`CSPRNG (Cryptographically secure pseudorandom number generation)`
+should be avoided for gameplay elements.
+
+Example of using the Crypto class to generate 2 random integers between ``0``
+and ``2^32 - 1`` (inclusive):
+
+::
+
+    var crypto := Crypto.new()
+    # Request as many bytes as you need, but try to minimize the amount
+    # of separate requests to improve performance.
+    # Each 32-bit integer requires 4 bytes, so we request 8 bytes.
+    var byte_array := crypto.generate_random_bytes(8)
+
+    # Use the ``decode_u32()`` method from PackedByteArray to decode a 32-bit unsigned integer
+    # from the beginning of `byte_array`. This method doesn't modify `byte_array`.
+    var random_int_1 := byte_array.decode_u32(0)
+    # Do the same as above, but with an offset of 4 bytes since we've already decoded
+    # the first 4 bytes previously.
+    var random_int_2 := byte_array.decode_u32(4)
+
+    prints("Random integers:", random_int_1, random_int_2)
+
+.. seealso::
+
+    See :ref:`class_PackedByteArray`'s documentation for other methods you can
+    use to decode the generated bytes into various types of data, such as
+    integers or floats.