Add Random::System

Allows to generate random numbers using a secure source provided by
the system. It actually uses the same source as SecureRandom.

spec/std/random/system_spec.cr

@@ -0,0 +1,10 @@
+require "spec"
+require "random/system"
+
+describe "Random::System" do
+  rng = Random::System.new
+
+  it "returns random numbers from the secure system source" do
+    typeof(rng.next_u).should eq(UInt32)
+  end
+end

src/lib_c/amd64-unknown-openbsd/c/stdlib.cr

@@ -7,6 +7,7 @@ lib LibC
     rem : Int
   end
 
+  fun arc4random() : UInt32
   fun arc4random_buf(x0 : Void*, x1 : SizeT) : Void
   fun atof(x0 : Char*) : Double
   fun div(x0 : Int, x1 : Int) : DivT

src/random/system.cr

@@ -0,0 +1,17 @@
+require "sys/random"
+
+# Generates random numbers from a secure source of the system.
+#
+# For example `arc4random` is used on OpenBSD, whereas on Linux it uses
+# `getrandom` (if the kernel supports it) and fallbacks on reading from
+# `/dev/urandom` on UNIX systems.
+class Random::System
+  include Random
+
+  def initialize
+  end
+
+  def next_u
+    Sys::Random.random_number
+  end
+end

src/sys/linux/random.cr

@@ -30,6 +30,21 @@ module Sys
       end
     end
 
+    # Returns a random UInt32.
+    def self.random_number : UInt32
+      init unless @@initialized
+
+      if @@getrandom_available
+        buf = uninitialized UInt8[sizeof(UInt32)]
+        getrandom(buf.to_slice)
+        buf.to_unsafe.as(UInt32*).value
+      elsif urandom = @@urandom
+        urandom.read_bytes(UInt32)
+      else
+        raise "Failed to access secure source to generate random bytes!"
+      end
+    end
+
     # Reads n random bytes using the Linux `getrandom(2)` syscall.
     private def self.getrandom(buf)
       # getrandom(2) may only read up to 256 bytes at once without being

src/sys/random.cr

@@ -1,7 +1,14 @@
+# :nodoc:
 module Sys
+  # :nodoc:
   module Random
     # Fills *buffer* with random bytes from a secure source.
     # def self.random_bytes(buffer : Bytes) : Nil
+
+    # Returns a random UInt32 from a secure source.
+    #
+    # This method is optional.
+    # def self.random_number : UInt32
   end
 end
 

src/sys/unix/arc4random.cr

@@ -8,5 +8,9 @@ module Sys
     def self.random_bytes(buf : Bytes) : Nil
       LibC.arc4random_buf(buf.to_unsafe.as(Void*), buf.size)
     end
+
+    def self.random_number : UInt32
+      LibC.arc4random()
+    end
   end
 end