diff --git a/specs/CipherSpecs/CaesarCipherSpec.hs b/specs/CipherSpecs/CaesarCipherSpec.hs
new file mode 100644
index 0000000..b50fe12
--- /dev/null
+++ b/specs/CipherSpecs/CaesarCipherSpec.hs
@@ -0,0 +1,25 @@
+{-# LANGUAGE ScopedTypeVariables #-}
+module CipherSpecs.CaesarCipherSpec where
+
+import Test.Hspec
+import Test.QuickCheck
+import Ciphers.CaesarCipher
+import Data.Char (toLower)
+
+spec :: Spec 
+spec = do
+  describe "encrypt" $ do
+    it "does nothing when alphabet is empty" $ property $
+      encrypt "hello" 5 [] == "hello"
+
+    it "does nothing when key is 0" $ property $
+      encrypt "hello" 0 ['a'..'z'] == "hello"
+
+    it "does not transform characters not found in the provided alphabet" $ property $
+      encrypt "h3llo" 1 ['a'..'z'] == "i3mmp"
+
+    describe "decrypt" $ do
+      it "returns the original input when given the same key and alphabet as encrypt" $ property $
+        forAll arbitrary $
+          \(asciiInput :: ASCIIString, key :: Int) -> let input = map toLower $ getASCIIString asciiInput
+            in decrypt (encrypt input key ['a'..'z']) key ['a'..'z'] == input
diff --git a/src/Ciphers/CaesarCipher.hs b/src/Ciphers/CaesarCipher.hs
new file mode 100644
index 0000000..e17840f
--- /dev/null
+++ b/src/Ciphers/CaesarCipher.hs
@@ -0,0 +1,22 @@
+module Ciphers.CaesarCipher(encrypt,decrypt) where 
+
+import Data.List (elemIndex)
+import Data.Maybe (fromMaybe)
+
+-- | Encrypting a string maps each character to the character `n` elements
+-- after it in the given alphabet, where `n`is the provided key
+encrypt :: String -> Int -> String -> String
+encrypt = helper (+)
+
+-- | Decrypting a string maps each character to the character `n` elements
+-- before it in the given alphabet, where `n` is the provided key
+decrypt :: String -> Int -> String -> String
+decrypt = helper (-)
+
+helper :: (Int -> Int -> Int) -> String -> Int -> String -> String
+helper _ input 0 _ = input
+helper _ input _ [] = input
+helper op input key alphabet = map (\x -> fromMaybe x (mappedChar $ elemIndex x alphabet)) input
+  where
+    len = length alphabet
+    mappedChar = fmap (\c -> alphabet !! (c `op` key `mod` len))
diff --git a/src/Ciphers/Rot13.hs b/src/Ciphers/Rot13.hs
new file mode 100644
index 0000000..1c60ebd
--- /dev/null
+++ b/src/Ciphers/Rot13.hs
@@ -0,0 +1,16 @@
+module Ciphers.Rot13 where
+
+import Data.Char (isAsciiLower, isAsciiUpper)
+
+-- See https://en.wikipedia.org/wiki/ROT13
+
+-- | "Rotates" each character in a string by 13.
+-- Note: This only rotates alphabetic characters, any other character is left
+-- as-is
+dencrypt :: String -> String
+dencrypt = map rotate
+
+rotate :: Char -> Char
+rotate c | isAsciiLower c = ([c..'z'] ++ ['a'..'z']) !! 13
+         | isAsciiUpper c = ([c..'Z'] ++ ['A'..'Z']) !! 13
+         | otherwise = c