Class: Rex::RandomIdentifierGenerator

Inherits:
Object
  • Object
show all
Defined in:
lib/rex/random_identifier_generator.rb

Overview

A quick way to produce unique random strings that follow the rules of identifiers, i.e., begin with a letter and contain only alphanumeric characters and underscore.

The advantage of using this class over, say, Text.rand_text_alpha each time you need a new identifier is that it ensures you don’t have collisions.

Examples:

vars = Rex::RandomIdentifierGenerator.new
asp_code = <<-END_CODE
  Sub #{vars[:func]}()
    Dim #{vars[:fso]}
    Set #{vars[:fso]} = CreateObject("Scripting.FileSystemObject")
    ...
  End Sub
  #{vars[:func]}
END_CODE

Defined Under Namespace

Classes: ExhaustedSpaceError

Constant Summary collapse

DefaultOpts =

Default options

{
  # Arbitrary
  :max_length => 12,
  :min_length => 3,
  # This should be pretty universal for identifier rules
  :char_set => Rex::Text::AlphaNumeric+"_",
  :first_char_set => Rex::Text::LowerAlpha
}

Instance Method Summary collapse

Constructor Details

#initialize(opts = {}) ⇒ RandomIdentifierGenerator

Returns a new instance of RandomIdentifierGenerator.

Parameters:

  • opts (Hash) (defaults to: {}) —

    Options, see DefaultOpts for default values

Options Hash (opts):

  • :max_length (Fixnum)
  • :min_length (Fixnum)
  • :char_set (String) 


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# File 'lib/rex/random_identifier_generator.rb', line 42

def initialize(opts={})
  # Holds all identifiers.
  @value_by_name = {}
  # Inverse of value_by_name so we can ensure uniqueness without
  # having to search through the whole list of values
  @name_by_value = {}

  @opts = DefaultOpts.merge(opts)
  if @opts[:min_length] < 1 || @opts[:max_length] < 1 || @opts[:max_length] < @opts[:min_length]
    raise ArgumentError, "Invalid length options"
  end

  # This is really just the maximum number of shortest names. This
  # will still be a pretty big number most of the time, so don't
  # bother calculating the real one, which will potentially be
  # expensive, since we're talking about a 36-digit decimal number to
  # represent the total possibilities for the range of 10- to
  # 20-character identifiers.
  #
  # 26 because the first char is lowercase alpha, (min_length - 1) and
  # not just min_length because it includes that first alpha char.
  @max_permutations = 26 * (@opts[:char_set].length ** (@opts[:min_length]-1))
  # The real number of permutations could be calculated thusly:
  #((@opts[:min_length]-1) .. (@opts[:max_length]-1)).reduce(0) { |a, e|
  #	a + (26 * @opts[:char_set].length ** e)
  #}
end

Instance Method Details

#generate(len = nil) {|String| ... } ⇒ String

Note:

Calling this method with a block that returns only values that this generator already contains will result in an infinite loop.

Create a random string that satisfies most languages’ requirements for identifiers. In particular, with a default configuration, the first character will always be lowercase alpha (unless modified by a block), and the whole thing will contain only a-zA-Z0-9_ characters.

If called with a block, the block will be given the identifier before uniqueness checks. The block’s return value will be the new identifier. Note that the block may be called multiple times if it returns a non-unique value.

Examples:

rig = Rex::RandomIdentifierGenerator.new
const = rig.generate { |val| val.capitalize }
rig.insert(:SOME_CONSTANT, const)
ruby_code = <<-EOC
  #{rig[:SOME_CONSTANT]} = %q^generated ruby constant^
  def #{rig[:my_method]}; ...; end
EOC

Parameters:

  • len (Fixnum) (defaults to: nil) —

    Avoid setting this unless a specific size is necessary. Default is random within range of min .. max

Yields:

  • (String) —

    The identifier before uniqueness checks. This allows you to modify the value and still avoid collisions.

Returns:

  • (String) —

    A string that matches [a-z][a-zA-Z0-9_]*

Raises:



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# File 'lib/rex/random_identifier_generator.rb', line 152

def generate(len=nil)
  raise ArgumentError, "len must be positive integer" if len && len < 1
  raise ExhaustedSpaceError if @value_by_name.length >= @max_permutations

  # pick a random length within the limits
  len ||= rand(@opts[:min_length] .. (@opts[:max_length]))

  ident = ""

  # XXX: Infinite loop if block returns only values we've already
  # generated.
  loop do
    ident  = Rex::Text.rand_base(1, "", @opts[:first_char_set])
    ident << Rex::Text.rand_base(len-1, "", @opts[:char_set])
    if block_given?
      ident = yield ident
    end
    # Try to make another one if it collides with a previously
    # generated one.
    break unless @name_by_value.key?(ident)
  end

  ident
end

#get(name) ⇒ String Also known as: [], init_var

Return a unique random identifier for name, generating a new one if necessary.

Parameters:

  • name (Symbol) —

    A descriptive, intention-revealing name for an identifier. This is what you would normally call the variable if you weren't generating it.

Returns:

  • (String) 


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# File 'lib/rex/random_identifier_generator.rb', line 84

def get(name)
  return @value_by_name[name] if @value_by_name[name]

  @value_by_name[name] = generate
  @name_by_value[@value_by_name[name]] = name

  @value_by_name[name]
end

#store(name, value) ⇒ void

Note:

This should be called before any calls to #get to avoid potential collisions. If you do hit a collision, this method will raise.

This method returns an undefined value.

Add a new identifier. Its name will be checked for uniqueness among previously-generated names.

Parameters:

  • value (String) —

    The identifier that will be returned by subsequent calls to #get with the sane name.

  • name (Symbol) —

    A descriptive, intention-revealing name for an identifier. This is what you would normally call the variable if you weren't generating it.

Raises:

  • RuntimeError if value already exists



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# File 'lib/rex/random_identifier_generator.rb', line 107

def store(name, value)

  case @name_by_value[value]
  when name
    # we already have this value and it is associated with this name
    # nothing to do here
  when nil
    # don't have this value yet, so go ahead and just insert
    @value_by_name[name] = value
    @name_by_value[value] = name
  else
    # then the caller is trying to insert a duplicate
    raise RuntimeError, "Value is not unique!"
  end

  self
end

#to_h ⇒ Hash

Returns the @value_by_name hash

Returns:

  • (Hash) 


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# File 'lib/rex/random_identifier_generator.rb', line 73

def to_h
  return @value_by_name
end