diff --git a/encrypt.rb b/encrypt.rb
new file mode 100644
index 0000000..ad64bcc
--- /dev/null
+++ b/encrypt.rb
@@ -0,0 +1,23 @@
+# def encrypted_message(message, key)
+#   message_splitted = message.split("")
+#   message_splitted.map! do |letter|
+#     new_number = letter.ord + key
+#     new_number.chr
+#   end
+#   message_splitted.join
+# end
+
+require_relative 'encryption_engine'
+
+key = 13
+message = "This is my secret"
+
+# do your encryption here
+
+# puts encrypted_message(message, key)
+
+engine = EncryptionEngine.new
+output = engine.encrypt(message, key)
+puts output  # outputs "Zl Zrffntr"
+output2 = engine.decrypt(output, key)
+puts output2 # outputs "My Message"
diff --git a/encryption_engine.rb b/encryption_engine.rb
new file mode 100644
index 0000000..b015b48
--- /dev/null
+++ b/encryption_engine.rb
@@ -0,0 +1,36 @@
+class EncryptionEngine
+
+  def encrypt(string, key)
+    message_splitted = string.split("")
+    result = message_splitted.map do |letter|
+      new_number = letter.ord + key
+      # if rotation of ascii number is greater than (Z|z).ord : -26 (back to (A|a))
+      new_number -= 26 if letter.ord + key > max_AZ(letter)
+      # keep space and punctuation unchanged
+      new_number = letter.ord if /\W+/.match(letter)
+      new_number.chr
+    end
+    result.join
+  end
+
+  # max_AZ refer to "Z".ord if letter is capitalized, "z" if not
+  def max_AZ(letter)
+    if letter.ord >= 65 && letter.ord <= 90
+      return 90
+    else
+      return 122
+    end
+  end
+
+  def decrypt(string, key)
+    message_splitted = string.split("")
+    result = message_splitted.map do |letter|
+      new_number = letter.ord - key
+      new_number += 26 if letter.ord - key < max_AZ(letter) - 26
+      new_number = letter.ord if /\W+/.match(letter)
+      new_number.chr
+    end
+    result.join
+  end
+
+end
diff --git a/test/encryption_engine_test.rb b/test/encryption_engine_test.rb
new file mode 100644
index 0000000..2e191dd
--- /dev/null
+++ b/test/encryption_engine_test.rb
@@ -0,0 +1,16 @@
+require 'minitest/autorun'
+require_relative '../encryption_engine'
+
+class EncryptionEngineTest < Minitest::Test
+  def test_it_encrypts_using_rot13
+    engine = EncryptionEngine.new
+    output = engine.encrypt("My Message", 13)
+    assert_equal "Zl Zrffntr", output
+  end
+
+  def test_it_decrypts_using_rot13
+    engine = EncryptionEngine.new
+    output = engine.decrypt("Guvf vf zl frperg", 13)
+    assert_equal "This is my secret", output
+  end
+end