Land #22, Implement XOR with variable-length key

lib/rex/text/xor.rb

@@ -1,28 +1,38 @@
 # -*- coding: binary -*-
 
-module Rex
-  module Text
-
-    # Returns a XOR'd string.
-    #
-    # @param key [String] XOR key.
-    # @param value [String] The string to XOR.
-    # @return [String] An XOR'd string.
-    def self.xor(key, value)
-      xor_key = key.kind_of?(Integer) || key.nil? ? key.to_i : key.to_i(16)
-      unless xor_key.between?(0, 255)
-        raise ArgumentError, 'XOR key should be between 0x00 to 0x0f'
-      end
-
-      buf = ''
-
-      value.each_byte do |byte|
-        xor_byte = byte ^ xor_key
-        buf << [xor_byte].pack('c')
-      end
-
-      buf
+module Rex::Text
+  # XOR a string against a variable-length key
+  #
+  # @param key [String] XOR key
+  # @param value [String] String to XOR
+  # @return [String] XOR'd string
+  def self.xor(key, value)
+    unless key && value
+      raise ArgumentError, 'XOR key and value must be supplied'
     end
 
+    xor_key =
+      case key
+      when String
+        if key.empty?
+          raise ArgumentError, 'XOR key must not be empty'
+        end
+
+        key
+      when Integer
+        unless key.between?(0x00, 0xff)
+          raise ArgumentError, 'XOR key must be between 0x00 and 0xff'
+        end
+
+        # Convert integer to string
+        [key].pack('C')
+      end
+
+    # Get byte arrays for key and value
+    xor_key   = xor_key.bytes
+    xor_value = value.bytes
+
+    # XOR value against cycled key
+    xor_value.zip(xor_key.cycle).map { |v, k| v ^ k }.pack('C*')
   end
-end
+end

spec/rex/text/xor_spec.rb

@@ -18,9 +18,24 @@ describe Rex::Text do
       expect(Rex::Text.xor(xor_key, hello_world_str)).to eq(xor_hello_world_str)
     end
 
-    it 'XORs with a string type key' do
-      xor_key = "0x0f"
-      expect(Rex::Text.xor(xor_key, hello_world_str)).to eq(xor_hello_world_str)
+    it 'XORs with a variable-length key' do
+      xor_key = "\x00\x00\x00\x00\x00\x0c"
+      expect(Rex::Text.xor(xor_key, hello_world_str)).to eq('hello,world')
+    end
+
+    it 'XORs with itself' do
+      xor_key = hello_world_str
+      expect(Rex::Text.xor(xor_key, hello_world_str)).to eq("\x00" * hello_world_str.length)
+    end
+
+    it 'raises an ArgumentError due to a nil key' do
+      bad_key = nil
+      expect { Rex::Text.xor(bad_key, hello_world_str) }.to raise_error(ArgumentError)
+    end
+
+    it 'raises an ArgumentError due to an empty key' do
+      bad_key = ''
+      expect { Rex::Text.xor(bad_key, hello_world_str) }.to raise_error(ArgumentError)
     end
 
     it 'raises an ArgumentError due to an out of range key' do
@@ -29,4 +44,4 @@ describe Rex::Text do
     end
 
   end
-end
+end