diff --git a/src/cipher_analytical_machine/ciphers/simple_substitution.clj b/src/cipher_analytical_machine/ciphers/simple_substitution.clj
new file mode 100644
index 0000000..9d21c98
--- /dev/null
+++ b/src/cipher_analytical_machine/ciphers/simple_substitution.clj
@@ -0,0 +1,47 @@
+(ns cipher-analytical-machine.ciphers.simple-substitution
+  (:require [clojure.string :as cs]
+            [clojure.set :as set]
+            [cipher-analytical-machine.ciphers.caesar :as caesar])
+  (:gen-class))
+
+(defn shuffled-numbers
+  "Generate the shuffled order for integer list."
+  [size]
+  (-> size
+      (take (range))
+      (shuffle)))
+
+(defn generate-substitution-table
+  "Generate the map (char, int) for the substittion."
+  [symbols]
+  (->> (count symbols)
+       (shuffled-numbers)
+       (zipmap symbols)))
+
+(defn find-value-in-table
+  "It uses the substitution table to find the value of a char or a number."
+  [char substitution-table symbols]
+  (get substitution-table char))
+
+(defn encrypt-message
+  "Encrypt a message using the simple substitution cipher. The function is case-insensitive. If a symbol isn't in the symbols list, then it will be removed."
+  [message key substitution-table symbols]
+  (let [max-index (count symbols)]
+    (->> message
+         (cs/lower-case)
+         (caesar/encrypt-message key symbols)
+         (map (fn [char] (find-value-in-table char substitution-table symbols)))
+         (cs/join \,))))
+
+(defn decrypt-message
+  "Decrypt a message using the simple substitution cipher. The function is case-insensitive."
+  [message key substitution-table symbols]
+  (let [substitution-table (set/map-invert substitution-table)
+        max-index (count symbols)
+        message (cs/split message #",")]
+      (->> message
+           (map #(Integer/parseInt %))
+           (map (fn [char] (find-value-in-table char substitution-table symbols)))
+           (caesar/encrypt-message key symbols)
+           (cs/join))))
+
diff --git a/test/cipher_analytical_machine/ciphers/simple_substitution_test.clj b/test/cipher_analytical_machine/ciphers/simple_substitution_test.clj
new file mode 100644
index 0000000..131de20
--- /dev/null
+++ b/test/cipher_analytical_machine/ciphers/simple_substitution_test.clj
@@ -0,0 +1,44 @@
+(ns cipher-analytical-machine.ciphers.simple-substitution
+  (:require
+   [clojure.test :refer :all]
+   [cipher-analytical-machine.ciphers.simple-substitution :refer :all]))
+
+(deftest find-value-in-table-test
+  (let [table {\a 1 \b 2 \c 3}
+        rtable {1 \a 2 \b 3 \c}]
+    (testing "If the symbol is in the table as a key, then the result won't nil."
+      (are [key expected]
+          (= expected (find-value-in-table key table))
+        \a 1
+        \d nil
+        1 nil
+        5 nil))
+
+    (testing "If the digit is in the reversed table as a key, then the result won't nil."
+      (are [key expected]
+          (= expected (find-value-in-table key rtable))
+        \a nil
+        \d nil
+        1 \a
+        5 nil))))
+
+(deftest encrypt-message-text
+  (let [symbols "abc"
+        table {\a 1 \b 2 \c 3}]
+    (testing "The function must encrypt the message and remove unknown symbols."
+      (are [message key expected]
+          (= expected (encrypt-message message key table symbols))
+        "abc" 0 "1,2,3"
+        "abc" 1 "2,3,1"
+        "aDbdc" 0 "1,2,3"))))
+
+(deftest decrypt-message-text
+  (let [symbols "abc"
+        table {\a 1 \b 2 \c 3}]
+    (testing "The function must decrypt the message and remove unknown numbers."
+      (are [message key expected]
+          (= expected (decrypt-message message key table symbols))
+        "1,2,3" 0 "abc"
+        "2,3,1" 1 "abc"
+        "1,12,2,3" 0 "abc"))))
+