# = Secure random number generator interface.
#
# This library is an interface for secure random number generator which is
# suitable for generating session key in HTTP cookies, etc.
#
# It supports following secure random number generators.
#
# * openssl
# * /dev/urandom
# * Win32
#
# == Example
#
# # random hexadecimal string.
# p SecureRandom.hex(10) #=> "52750b30ffbc7de3b362"
# p SecureRandom.hex(10) #=> "92b15d6c8dc4beb5f559"
# p SecureRandom.hex(11) #=> "6aca1b5c58e4863e6b81b8"
# p SecureRandom.hex(12) #=> "94b2fff3e7fd9b9c391a2306"
# p SecureRandom.hex(13) #=> "39b290146bea6ce975c37cfc23"
# ...
#
# # random base64 string.
# p SecureRandom.base64(10) #=> "EcmTPZwWRAozdA=="
# p SecureRandom.base64(10) #=> "9b0nsevdwNuM/w=="
# p SecureRandom.base64(10) #=> "KO1nIU+p9DKxGg=="
# p SecureRandom.base64(11) #=> "l7XEiFja+8EKEtY="
# p SecureRandom.base64(12) #=> "7kJSM/MzBJI+75j8"
# p SecureRandom.base64(13) #=> "vKLJ0tXBHqQOuIcSIg=="
# ...
#
# # random binary string.
# p SecureRandom.random_bytes(10) #=> "\016\t{\370g\310pbr\301"
# p SecureRandom.random_bytes(10) #=> "\323U\030TO\234\357\020\a\337"
# ...

if !defined?(SecureRandom)
  begin
    require 'openssl'
  rescue LoadError
  end

  module SecureRandom
    # SecureRandom.random_bytes generates a random binary string.
    #
    # The argument n specifies the length of the result string.
    #
    # If n is not specified, 16 is assumed.
    # It may be larger in future.
    #
    # If secure random number generator is not available,
    # NotImplementedError is raised.
    def self.random_bytes(n=nil)
      n ||= 16

      if defined? OpenSSL::Random
        return OpenSSL::Random.random_bytes(n)
      end

      if !defined?(@has_urandom) || @has_urandom
        flags = File::RDONLY
        flags |= File::NONBLOCK if defined? File::NONBLOCK
        flags |= File::NOCTTY if defined? File::NOCTTY
        flags |= File::NOFOLLOW if defined? File::NOFOLLOW
        begin
          File.open("/dev/urandom", flags) {|f|
            unless f.stat.chardev?
              raise Errno::ENOENT
            end
            @has_urandom = true
            ret = f.readpartial(n)
            if ret.length != n
              raise NotImplementedError,
                "Unexpected partial read from random device"
            end
            return ret
          }
        rescue Errno::ENOENT
          @has_urandom = false
        end
      end

      if !defined?(@has_win32)
        begin
          require 'Win32API'

          crypt_acquire_context = Win32API.new(
            "advapi32", "CryptAcquireContext", 'PPPII', 'L'
          )
          @crypt_gen_random = Win32API.new(
            "advapi32", "CryptGenRandom", 'LIP', 'L'
          )

          hProvStr = " " * 4
          prov_rsa_full = 1
          crypt_verifycontext = 0xF0000000

          if crypt_acquire_context.call(
              hProvStr, nil, nil, prov_rsa_full, crypt_verifycontext) == 0
            raise SystemCallError,
              "CryptAcquireContext failed: #{lastWin32ErrorMessage}"
          end
          @hProv, = hProvStr.unpack('L')

          @has_win32 = true
        rescue LoadError
          @has_win32 = false
        end
      end
      if @has_win32
        bytes = " " * n
        if @crypt_gen_random.call(@hProv, bytes.size, bytes) == 0
          raise SystemCallError,
            "CryptGenRandom failed: #{lastWin32ErrorMessage}"
        end
        return bytes
      end

      raise NotImplementedError, "No random device"
    end

    # SecureRandom.hex generates a random hex string.
    #
    # The argument n specifies the length of the random length.
    # The length of the result string is twice of n.
    #
    # If n is not specified, 16 is assumed.
    # It may be larger in future.
    #
    # If secure random number generator is not available,
    # NotImplementedError is raised.
    def self.hex(n=nil)
      random_bytes(n).unpack("H*")[0]
    end

    # SecureRandom.base64 generates a random base64 string.
    #
    # The argument n specifies the length of the random length.
    # The length of the result string is about 4/3 of n.
    #
    # If n is not specified, 16 is assumed.
    # It may be larger in future.
    #
    # If secure random number generator is not available,
    # NotImplementedError is raised.
    def self.base64(n=nil)
      [random_bytes(n)].pack("m*").delete("\n")
    end

    # SecureRandom.random_number generates a random number.
    #
    # If an positive integer is given as n,
    # SecureRandom.random_number returns an integer:
    # 0 <= SecureRandom.random_number(n) < n.
    #
    # If 0 is given or an argument is not given,
    # SecureRandom.random_number returns an float:
    # 0.0 <= SecureRandom.random_number() < 1.0.
    def self.random_number(n=0)
      if 0 < n
        hex = n.to_s(16)
        hex = '0' + hex if (hex.length & 1) == 1
        bin = [hex].pack("H*")
        first = bin[0..0]
        mask = first.respond_to?(:ord) ? first.ord : first.sum(8)
        mask |= mask >> 1
        mask |= mask >> 2
        mask |= mask >> 4
        begin
          rnd = SecureRandom.random_bytes(bin.length)
          first = rnd[0..0]
          ordinal = first.respond_to?(:ord) ? first.ord : first.sum(8)
          rnd[0..0] = (ordinal & mask).chr
        end until rnd < bin
        rnd.unpack("H*")[0].hex
      else
        # assumption: Float::MANT_DIG <= 64
        i64 = SecureRandom.random_bytes(8).unpack("Q")[0]
        Math.ldexp(i64 >> (64-Float::MANT_DIG), -Float::MANT_DIG)
      end
    end

    # Following code is based on David Garamond's GUID library for Ruby.
    def self.lastWin32ErrorMessage # :nodoc:
      get_last_error = Win32API.new(
        "kernel32", "GetLastError", '', 'L'
      )
      format_message = Win32API.new(
        "kernel32", "FormatMessageA", 'LPLLPLPPPPPPPP', 'L'
      )
      format_message_ignore_inserts = 0x00000200
      format_message_from_system    = 0x00001000

      code = get_last_error.call
      msg = "\0" * 1024
      len = format_message.call(
        format_message_ignore_inserts + format_message_from_system,
        0, code, 0, msg, 1024, nil, nil, nil, nil, nil, nil, nil, nil
      )
      msg[0, len].tr("\r", '').chomp
    end
  end
end