diff --git a/lib/sensitive.rb b/lib/sensitive.rb
new file mode 100644
index 0000000..7219426
--- /dev/null
+++ b/lib/sensitive.rb
@@ -0,0 +1,39 @@
+class Sensitive
+  alias_method :eql?, :==
+  alias_method :equal?, :==
+
+  def initialize(v, ch: "*", head: 2, tail: 2)
+    @v = v
+    @ch = ch
+    @head = head
+    @tail = tail
+  end
+
+  def mask(v)
+    "".concat(v[0, @head], @ch * (v.length - (@head + @tail)), v[-@tail, @tail])
+  end
+
+  def unwrap
+    @v
+  end
+
+  def length
+    @v.length
+  end
+
+  def to_s
+    mask @v
+  end
+
+  def inspect
+    "#<#{self.class.name} @v=#{wrap}>"
+  end
+
+  def hash
+    @v.hash
+  end
+
+  def ==(other)
+    other.is_a?(Sensitive) && other.hash == hash
+  end
+end
diff --git a/spec/sensitive_spec.rb b/spec/sensitive_spec.rb
new file mode 100644
index 0000000..a5b91c2
--- /dev/null
+++ b/spec/sensitive_spec.rb
@@ -0,0 +1,26 @@
+require "minitest/autorun"
+
+$LOAD_PATH.unshift File.dirname(__FILE__) + "/../lib"
+
+require "sensitive"
+
+ALPHABET = ('a' .. 'z').reduce(:concat)
+
+describe "Sensitive" do
+  before do
+    @s = Sensitive.new ALPHABET
+  end
+
+  it "test initialize" do
+    _(@s.to_s).must_equal "ab" + "*" * 22 + "yz"
+  end
+
+  it "test initialize" do
+    _(@s.unwrap).must_equal ALPHABET
+  end
+
+  it "test using different mask character" do
+    s = Sensitive.new ALPHABET, ch: "x" 
+    _(s.to_s).must_equal "x"
+  end
+end
diff --git a/test/test_sensitive.rb b/test/test_sensitive.rb
new file mode 100644
index 0000000..6175295
--- /dev/null
+++ b/test/test_sensitive.rb
@@ -0,0 +1,26 @@
+require "minitest/autorun"
+
+$LOAD_PATH.unshift File.dirname(__FILE__) + "/../lib"
+
+require "sensitive"
+
+ALPHABET = ("a".."z").reduce(:concat)
+
+class TestSensitive < Minitest::Test
+  def setup
+    @s = Sensitive.new ALPHABET
+  end
+
+  def test_initialize
+    assert_equal @s.to_s, "ab" + "*" * 22 + "yz"
+  end
+
+  def test_unwrap
+    assert_equal @s.unwrap, ALPHABET
+  end
+
+  def test_using_a_different_mask_character
+    s = Sensitive.new ALPHABET, ch: "x"
+    assert_equal s.to_s, "ab" + "x" * 22 + "yz"
+  end
+end