diff --git a/config.json b/config.json
index c7665a1..7680322 100644
--- a/config.json
+++ b/config.json
@@ -746,6 +746,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "simple-cipher",
+        "name": "Simple Cipher",
+        "uuid": "f55d8d06-21cf-4ac4-a6cc-61ed90ff3588",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "simple-linked-list",
         "name": "Simple Linked List",
diff --git a/exercises/practice/simple-cipher/.busted b/exercises/practice/simple-cipher/.busted
new file mode 100644
index 0000000..86b84e7
--- /dev/null
+++ b/exercises/practice/simple-cipher/.busted
@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}
diff --git a/exercises/practice/simple-cipher/.docs/instructions.md b/exercises/practice/simple-cipher/.docs/instructions.md
new file mode 100644
index 0000000..afd0b57
--- /dev/null
+++ b/exercises/practice/simple-cipher/.docs/instructions.md
@@ -0,0 +1,40 @@
+# Instructions
+
+Create an implementation of the [Vigenère cipher][wiki].
+The Vigenère cipher is a simple substitution cipher.
+
+## Cipher terminology
+
+A cipher is an algorithm used to encrypt, or encode, a string.
+The unencrypted string is called the _plaintext_ and the encrypted string is called the _ciphertext_.
+Converting plaintext to ciphertext is called _encoding_ while the reverse is called _decoding_.
+
+In a _substitution cipher_, each plaintext letter is replaced with a ciphertext letter which is computed with the help of a _key_.
+(Note, it is possible for replacement letter to be the same as the original letter.)
+
+## Encoding details
+
+In this cipher, the key is a series of lowercase letters, such as `"abcd"`.
+Each letter of the plaintext is _shifted_ or _rotated_ by a distance based on a corresponding letter in the key.
+An `"a"` in the key means a shift of 0 (that is, no shift).
+A `"b"` in the key means a shift of 1.
+A `"c"` in the key means a shift of 2, and so on.
+
+The first letter of the plaintext uses the first letter of the key, the second letter of the plaintext uses the second letter of the key and so on.
+If you run out of letters in the key before you run out of letters in the plaintext, start over from the start of the key again.
+
+If the key only contains one letter, such as `"dddddd"`, then all letters of the plaintext are shifted by the same amount (three in this example), which would make this the same as a rotational cipher or shift cipher (sometimes called a Caesar cipher).
+For example, the plaintext `"iamapandabear"` would become `"ldpdsdqgdehdu"`.
+
+If the key only contains the letter `"a"` (one or more times), the shift distance is zero and the ciphertext is the same as the plaintext.
+
+Usually the key is more complicated than that, though!
+If the key is `"abcd"` then letters of the plaintext would be shifted by a distance of 0, 1, 2, and 3.
+If the plaintext is `"hello"`, we need 5 shifts so the key would wrap around, giving shift distances of 0, 1, 2, 3, and 0.
+Applying those shifts to the letters of `"hello"` we get `"hfnoo"`.
+
+## Random keys
+
+If no key is provided, generate a key which consists of at least 100 random lowercase letters from the Latin alphabet.
+
+[wiki]: https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
diff --git a/exercises/practice/simple-cipher/.meta/config.json b/exercises/practice/simple-cipher/.meta/config.json
new file mode 100644
index 0000000..2f1ca1c
--- /dev/null
+++ b/exercises/practice/simple-cipher/.meta/config.json
@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "simple_cipher.moon"
+    ],
+    "test": [
+      "simple_cipher_spec.moon"
+    ],
+    "example": [
+      ".meta/example.moon"
+    ]
+  },
+  "blurb": "Implement the Vigenère cipher, a simple substitution cipher.",
+  "source": "Substitution Cipher at Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Substitution_cipher"
+}
diff --git a/exercises/practice/simple-cipher/.meta/example.moon b/exercises/practice/simple-cipher/.meta/example.moon
new file mode 100644
index 0000000..70bc159
--- /dev/null
+++ b/exercises/practice/simple-cipher/.meta/example.moon
@@ -0,0 +1,27 @@
+KEY_CHARS = [c for c in 'abcdefghijklmnopqrstuvwxyz'\gmatch '.']
+A = string.byte 'a'
+INDEX = {c, string.byte(c) - A for c in *KEY_CHARS}
+
+randomKey = ->
+  table.concat [KEY_CHARS[math.random #KEY_CHARS] for _ = 1, 100]
+
+class SimpleCipher
+  new: (key) =>
+    @Key = key or randomKey!
+
+  key: => @Key
+
+  encipher: (text, direction) =>
+    while #@Key < #text
+      @Key ..= @Key
+
+    idx = 0
+    text\gsub '.', (char) ->
+      idx += 1
+      t = INDEX[char]
+      k = INDEX[@Key\sub idx, idx]
+      i = (t + direction * k) % #KEY_CHARS + 1
+      KEY_CHARS[i]
+
+  encode: (text) => @encipher text, 1
+  decode: (text) => @encipher text, -1
diff --git a/exercises/practice/simple-cipher/.meta/spec_generator.moon b/exercises/practice/simple-cipher/.meta/spec_generator.moon
new file mode 100644
index 0000000..5bbc7f5
--- /dev/null
+++ b/exercises/practice/simple-cipher/.meta/spec_generator.moon
@@ -0,0 +1,59 @@
+{
+  module_name: 'SimpleCipher',
+
+  generate_test: (case, level) ->
+    local lines
+    switch case.uuid
+      when "b8bdfbe1-bea3-41bb-a999-b41403f2b15d"
+        lines = {
+          "cipher = SimpleCipher!"
+          "plaintext = #{quote case.input.plaintext}"
+          "encoded = cipher\\encode plaintext"
+          "expected = cipher\\key!\\sub(1, #plaintext)"
+          "assert.are.equal expected, encoded"
+        }
+      when "3dff7f36-75db-46b4-ab70-644b3f38b81c"
+        lines = {
+          "cipher = SimpleCipher!"
+          "expected = #{quote case.expected}"
+          "ciphertext = cipher\\key!\\sub(1, #expected)"
+          "decoded = cipher\\decode ciphertext"
+          "assert.are.equal expected, decoded"
+        }
+      when "8143c684-6df6-46ba-bd1f-dea8fcb5d265"
+        lines = {
+          "cipher = SimpleCipher!"
+          "plaintext = #{quote case.input.plaintext}"
+          "ciphertext = cipher\\encode plaintext"
+          "decoded = cipher\\decode ciphertext"
+          "assert.are.equal plaintext, decoded"
+        }
+      when "defc0050-e87d-4840-85e4-51a1ab9dd6aa"
+        lines = {
+          "cipher = SimpleCipher!"
+          "key = cipher\\key!"
+          "assert.is.truthy key\\match('^[a-z]+$')"
+        }
+      when "70a16473-7339-43df-902d-93408c69e9d1"
+        lines = {
+          "cipher = SimpleCipher #{quote case.input.key}"
+          "ciphertext = cipher\\encode #{quote case.input.plaintext}"
+          "result = cipher\\decode ciphertext"
+          "assert.are.equal #{quote case.expected}, result"
+        }
+      else
+        switch case.property
+          when "encode"
+            lines = {
+              "cipher = SimpleCipher #{quote case.input.key}"
+              "result = cipher\\encode #{quote case.input.plaintext}"
+              "assert.are.equal #{quote case.expected}, result"
+            }
+          when "decode"
+            lines = {
+              "cipher = SimpleCipher #{quote case.input.key}"
+              "result = cipher\\decode #{quote case.input.ciphertext}"
+              "assert.are.equal #{quote case.expected}, result"
+            }
+    table.concat [indent line, level for line in *lines], '\n'
+}
diff --git a/exercises/practice/simple-cipher/.meta/tests.toml b/exercises/practice/simple-cipher/.meta/tests.toml
new file mode 100644
index 0000000..77e6571
--- /dev/null
+++ b/exercises/practice/simple-cipher/.meta/tests.toml
@@ -0,0 +1,46 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[b8bdfbe1-bea3-41bb-a999-b41403f2b15d]
+description = "Random key cipher -> Can encode"
+
+[3dff7f36-75db-46b4-ab70-644b3f38b81c]
+description = "Random key cipher -> Can decode"
+
+[8143c684-6df6-46ba-bd1f-dea8fcb5d265]
+description = "Random key cipher -> Is reversible. I.e., if you apply decode in a encoded result, you must see the same plaintext encode parameter as a result of the decode method"
+
+[defc0050-e87d-4840-85e4-51a1ab9dd6aa]
+description = "Random key cipher -> Key is made only of lowercase letters"
+
+[565e5158-5b3b-41dd-b99d-33b9f413c39f]
+description = "Substitution cipher -> Can encode"
+
+[d44e4f6a-b8af-4e90-9d08-fd407e31e67b]
+description = "Substitution cipher -> Can decode"
+
+[70a16473-7339-43df-902d-93408c69e9d1]
+description = "Substitution cipher -> Is reversible. I.e., if you apply decode in a encoded result, you must see the same plaintext encode parameter as a result of the decode method"
+
+[69a1458b-92a6-433a-a02d-7beac3ea91f9]
+description = "Substitution cipher -> Can double shift encode"
+
+[21d207c1-98de-40aa-994f-86197ae230fb]
+description = "Substitution cipher -> Can wrap on encode"
+
+[a3d7a4d7-24a9-4de6-bdc4-a6614ced0cb3]
+description = "Substitution cipher -> Can wrap on decode"
+
+[e31c9b8c-8eb6-45c9-a4b5-8344a36b9641]
+description = "Substitution cipher -> Can encode messages longer than the key"
+
+[93cfaae0-17da-4627-9a04-d6d1e1be52e3]
+description = "Substitution cipher -> Can decode messages longer than the key"
diff --git a/exercises/practice/simple-cipher/simple_cipher.moon b/exercises/practice/simple-cipher/simple_cipher.moon
new file mode 100644
index 0000000..0899a61
--- /dev/null
+++ b/exercises/practice/simple-cipher/simple_cipher.moon
@@ -0,0 +1,12 @@
+class SimpleCipher
+  new: () =>
+    error 'Implement the constructor'
+
+  key: =>
+    error 'Implement the "key" method'
+
+  encode: (plaintext) =>
+    error 'Implement the "encode" method'
+
+  decode: (ciphertext) =>
+    error 'Implement the "decode" method'
diff --git a/exercises/practice/simple-cipher/simple_cipher_spec.moon b/exercises/practice/simple-cipher/simple_cipher_spec.moon
new file mode 100644
index 0000000..537c06b
--- /dev/null
+++ b/exercises/practice/simple-cipher/simple_cipher_spec.moon
@@ -0,0 +1,71 @@
+SimpleCipher = require 'simple_cipher'
+
+describe 'simple-cipher', ->
+  describe 'Random key cipher', ->
+    it 'Can encode', ->
+      cipher = SimpleCipher!
+      plaintext = 'aaaaaaaaaa'
+      encoded = cipher\encode plaintext
+      expected = cipher\key!\sub(1, #plaintext)
+      assert.are.equal expected, encoded
+
+    pending 'Can decode', ->
+      cipher = SimpleCipher!
+      expected = 'aaaaaaaaaa'
+      ciphertext = cipher\key!\sub(1, #expected)
+      decoded = cipher\decode ciphertext
+      assert.are.equal expected, decoded
+
+    pending 'Is reversible. I.e., if you apply decode in a encoded result, you must see the same plaintext encode parameter as a result of the decode method', ->
+      cipher = SimpleCipher!
+      plaintext = 'abcdefghij'
+      ciphertext = cipher\encode plaintext
+      decoded = cipher\decode ciphertext
+      assert.are.equal plaintext, decoded
+
+    pending 'Key is made only of lowercase letters', ->
+      cipher = SimpleCipher!
+      key = cipher\key!
+      assert.is.truthy key\match('^[a-z]+$')
+
+  describe 'Substitution cipher', ->
+    pending 'Can encode', ->
+      cipher = SimpleCipher 'abcdefghij'
+      result = cipher\encode 'aaaaaaaaaa'
+      assert.are.equal 'abcdefghij', result
+
+    pending 'Can decode', ->
+      cipher = SimpleCipher 'abcdefghij'
+      result = cipher\decode 'abcdefghij'
+      assert.are.equal 'aaaaaaaaaa', result
+
+    pending 'Is reversible. I.e., if you apply decode in a encoded result, you must see the same plaintext encode parameter as a result of the decode method', ->
+      cipher = SimpleCipher 'abcdefghij'
+      ciphertext = cipher\encode 'abcdefghij'
+      result = cipher\decode ciphertext
+      assert.are.equal 'abcdefghij', result
+
+    pending 'Can double shift encode', ->
+      cipher = SimpleCipher 'iamapandabear'
+      result = cipher\encode 'iamapandabear'
+      assert.are.equal 'qayaeaagaciai', result
+
+    pending 'Can wrap on encode', ->
+      cipher = SimpleCipher 'abcdefghij'
+      result = cipher\encode 'zzzzzzzzzz'
+      assert.are.equal 'zabcdefghi', result
+
+    pending 'Can wrap on decode', ->
+      cipher = SimpleCipher 'abcdefghij'
+      result = cipher\decode 'zabcdefghi'
+      assert.are.equal 'zzzzzzzzzz', result
+
+    pending 'Can encode messages longer than the key', ->
+      cipher = SimpleCipher 'abc'
+      result = cipher\encode 'iamapandabear'
+      assert.are.equal 'iboaqcnecbfcr', result
+
+    pending 'Can decode messages longer than the key', ->
+      cipher = SimpleCipher 'abc'
+      result = cipher\decode 'iboaqcnecbfcr'
+      assert.are.equal 'iamapandabear', result