# = 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"
# ...

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*")
      mask = bin[0].ord
      mask |= mask >> 1
      mask |= mask >> 2
      mask |= mask >> 4
      begin
        rnd = SecureRandom.random_bytes(bin.length)
        rnd[0] = (rnd[0].ord & 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