diff --git a/project.clj b/project.clj
index 116b5f3..a9445d9 100644
--- a/project.clj
+++ b/project.clj
@@ -6,7 +6,8 @@
   :dependencies [[org.clojure/clojure "1.11.1"]
                  [org.clojure/tools.cli "1.0.219"]
                  [org.apache.tika/tika-core "1.28.5"]
-                 [org.apache.tika/tika-langdetect "1.28.5"]]
+                 [org.apache.tika/tika-langdetect "1.28.5"]
+                 [cheshire "5.12.0"]]
   :main ^:skip-aot cipher-analytical-machine.core
   :java-source-paths ["src/main/java"]
   :target-path "target/%s"
diff --git a/src/cipher_analytical_machine/parsers/parsers.clj b/src/cipher_analytical_machine/parsers/parsers.clj
new file mode 100644
index 0000000..e0ef1dd
--- /dev/null
+++ b/src/cipher_analytical_machine/parsers/parsers.clj
@@ -0,0 +1,9 @@
+(ns cipher-analytical-machine.parsers.parsers
+  (:require [cheshire.core :as cc])
+  (:gen-class))
+
+(defn unsigned-int?
+  [str]
+  (if (re-matches #"\d+" str)
+    true false))
+
diff --git a/src/cipher_analytical_machine/parsers/simple_substitution.clj b/src/cipher_analytical_machine/parsers/simple_substitution.clj
new file mode 100644
index 0000000..1b3ca1e
--- /dev/null
+++ b/src/cipher_analytical_machine/parsers/simple_substitution.clj
@@ -0,0 +1,44 @@
+(ns cipher-analytical-machine.parsers.simple-substitution
+  (:require
+   [cipher-analytical-machine.ciphers.simple-substitution :as ss]
+   [cipher-analytical-machine.parsers.parsers :as ps]
+   [cheshire.core :as cc])
+  (:gen-class))
+
+(defn encode-key-and-substitution-table-to-json
+  [key substitution-table]
+  (-> {"key" key}
+      (assoc "table" substitution-table)
+      (cc/generate-string)))
+
+(defn decode-key
+  "Returns the first character if the string is a char. Return the number if the string is a number."
+  [str]
+  (if (ps/unsigned-int? str)
+    (Integer/parseInt str)
+    (first str)))
+
+(defn decode-pair
+  "Decode a numbers as a string and a char as a string"
+  [acc [key value]]
+  (if (ps/unsigned-int? key)
+    (assoc acc (decode-key key) (decode-key value))
+    (assoc acc (decode-key key) value)))
+
+(defn decode-substitution-table
+  [substitution-table-map-from-json]
+  (reduce decode-pair {} substitution-table-map-from-json))
+
+(defn decode-key-and-substitution-table-from-json
+  [json]
+  (let [data (cc/parse-string json)]
+    (assoc data "table" (decode-substitution-table (get data "table")))))
+
+(defn generate-table-or-decode-json
+  "It generate the table and return map with the key and the table if the argument is a key. Else it decode the json."
+  [key-or-json symbols]
+  (if (ps/unsigned-int? key-or-json)
+    (-> {"key" key-or-json}
+        (assoc "table" (ss/generate-substitution-table symbols)))
+    (decode-key-and-substitution-table-from-json key-or-json)))
+
diff --git a/test/cipher_analytical_machine/parsers/parsers_test.clj b/test/cipher_analytical_machine/parsers/parsers_test.clj
new file mode 100644
index 0000000..ab723ef
--- /dev/null
+++ b/test/cipher_analytical_machine/parsers/parsers_test.clj
@@ -0,0 +1,15 @@
+(ns cipher-analytical-machine.parsers.parsers-test
+  (:require
+   [clojure.test :refer :all]
+   [cipher-analytical-machine.parsers.parsers :refer :all]))
+
+(deftest unsigned-int?-test
+  (testing ""
+    (are [str expected]
+        (= expected
+           (unsigned-int? str))
+      "9" true
+      "10" true
+      "-10" false
+      "abc" false)))
+
diff --git a/test/cipher_analytical_machine/parsers/simple_substitution_test.clj b/test/cipher_analytical_machine/parsers/simple_substitution_test.clj
new file mode 100644
index 0000000..74e5478
--- /dev/null
+++ b/test/cipher_analytical_machine/parsers/simple_substitution_test.clj
@@ -0,0 +1,41 @@
+(ns cipher-analytical-machine.parsers.simple-substitution-test
+  (:require
+   [clojure.test :refer :all]
+   [cipher-analytical-machine.parsers.simple-substitution :refer :all]))
+
+(deftest encode-key-and-substitution-table-to-json-test
+  (testing "The encoder must parse the map and create a json."
+    (are [key table expected-json]
+        (= expected-json
+           (encode-key-and-substitution-table-to-json key table))
+      1 {0 \a 1 \b 2 \c} "{\"key\":1,\"table\":{\"0\":\"a\",\"1\":\"b\",\"2\":\"c\"}}"
+      2 {\a 0 \b 1 \c 2} "{\"key\":2,\"table\":{\"a\":0,\"b\":1,\"c\":2}}")))
+
+(deftest decode-key-and-substitution-table-from-json-test
+  (testing "The decoder must parse the json and create a map."
+    (are [json expected-data]
+        (= expected-data
+           (decode-key-and-substitution-table-from-json json))
+      "{\"key\":1, \"table\":{\"0\":\"a\",\"1\":\"b\",\"2\":\"c\"}}" {"key" 1 "table" {0 \a 1 \b 2 \c}}
+      "{\"key\":2, \"table\":{\"a\":0,\"b\":1,\"c\":2}}" {"key" 2 "table" {\a 0 \b 1 \c 2}})))
+
+(deftest generate-table-or-decode-json-test
+  (let [symbols "abc"]
+    (testing "The function decodes the json."
+      (are [json expected-data]
+          (= expected-data
+             (generate-table-or-decode-json json symbols))
+        "{\"key\":1, \"table\":{\"0\":\"a\",\"1\":\"b\",\"2\":\"c\"}}" {"key" 1 "table" {0 \a 1 \b 2 \c}}
+        "{\"key\":2, \"table\":{\"a\":0,\"b\":1,\"c\":2}}" {"key" 2 "table" {\a 0 \b 1 \c 2}}))
+
+    (testing "The function generates the table, because the argument is a key."
+      (are [json expected-data]
+          (let [data (generate-table-or-decode-json json symbols)]
+            (and (= (get expected-data "key")
+                    (get data "key"))
+                 (= (keys (get expected-data "table"))
+                    (keys (get data "table"))))
+             (generate-table-or-decode-json json symbols))
+        "1" {"key" 1 "table" {0 \a 1 \b 2 \c}}
+        "2" {"key" 2 "table" {\a 0 \b 1 \c 2}}))))
+