diff --git a/src/cipher_analytical_machine/caesar.clj b/src/cipher_analytical_machine/caesar.clj
new file mode 100644
index 0000000..7ac2285
--- /dev/null
+++ b/src/cipher_analytical_machine/caesar.clj
@@ -0,0 +1,37 @@
+(ns cipher-analytical-machine.caesar
+  (:require [clojure.string :as cs])
+  (:gen-class))
+
+(defn calculate-char-index
+  "Calculate the index of a char."
+  [char symbols]
+  (cs/index-of symbols char))
+
+(defn encrypt-index
+  "Calculates new index of a character. Uses its index, the key and max-index (exclusive)."
+  [index key max-index]
+  (-> index
+       (+ key)
+       (mod max-index)))
+
+(defn encrypt-char
+  "Calculates new index of a character. Uses a char from symbols, the key and max-index (exclusive)."
+  [char key symbols max-index]
+  (get symbols
+       (-> char
+           (calculate-char-index symbols)
+           (encrypt-index key max-index))))
+
+(defn encrypt-message
+  "Encrypt a message using the Caesar cipher."
+  [message key symbols]
+  (let [max-index (count symbols)]
+    (cs/join
+     (map (fn [char] (encrypt-char char key symbols max-index))
+          message))))
+
+(defn decrypt-message
+  "Decrypt the ciphtext using the Caesar cipher."
+  [ciphertext key symbols]
+  (encrypt-message ciphertext (- key) symbols))
+
diff --git a/test/cipher_analytical_machine/caesar_test.clj b/test/cipher_analytical_machine/caesar_test.clj
new file mode 100644
index 0000000..bdb4b8d
--- /dev/null
+++ b/test/cipher_analytical_machine/caesar_test.clj
@@ -0,0 +1,64 @@
+(ns cipher-analytical-machine.caesar-test
+  (:require
+   [clojure.test :refer :all]
+   [cipher-analytical-machine.caesar :refer :all]
+  ))
+
+(deftest calculate-char-index-test
+  (let [symbols "abc123%$"]
+    (testing "Letter 'b' in symbols"
+      (is (= 1 (calculate-char-index \b symbols))))
+
+    (testing "Letter 'd' does not in symbols"
+      (is (= nil (calculate-char-index \d symbols))))
+
+    (testing "Digit '1' in symbols"
+      (is (= 3 (calculate-char-index \1 symbols))))
+
+    (testing "Digit '0' does not in symbols"
+      (is (= nil (calculate-char-index \0 symbols))))
+
+    (testing "Symbol '?' does not in symbols"
+      (is (= nil (calculate-char-index \? symbols))))))
+
+(deftest encrypt-index-test
+  (testing "The new index is in the range."
+    (is (= 2 (encrypt-index 0 2 33))))
+
+  (testing "The new index that equal to max-index."
+    (is (= 0 (encrypt-index 30 3 33))))
+
+  (testing "The new index that greater than max-index."
+    (is (= 1 (encrypt-index 30 4 33))))
+
+  (testing "The new index that lower than 0."
+    (is (= 32 (encrypt-index 0 -1 33)))))
+
+(deftest encrypt-char-test
+  (let [symbols "abc"
+        max-index (count symbols)]
+    (testing "Letter 'a' is encrypted to 'b'"
+      (is (= \b (encrypt-char \a 1 symbols max-index))))
+
+    (testing "Letter 'b' is encrypted to 'a'"
+      (is (= \a (encrypt-char \b 2 symbols max-index))))
+
+    (testing "Letter 'b' is decryped to 'a'"
+      (is (= \a (encrypt-char \b -1 symbols max-index))))))
+
+(deftest encrypt-message-text
+  (let [symbols "abc"]
+    (testing "The message 'abc' is encrypted to 'bca'"
+      (is (= "bca" (encrypt-message "abc" 1 symbols))))
+
+    (testing "The message 'bca' is decrypted to 'abc'"
+      (is (= "abc" (encrypt-message "bca" -1 symbols))))))
+
+(deftest decrypt-message-text
+  (let [symbols "abc"]
+    (testing "The message 'bca' is decrypted to 'abc'"
+      (is (= "abc" (decrypt-message "bca" 1 symbols)))
+
+    (testing "The message 'abc' is encrypted to 'bca'"
+      (is (= "bca" (decrypt-message "abc" -1 symbols)))))))
+
diff --git a/test/cipher_analytical_machine/core_test.clj b/test/cipher_analytical_machine/core_test.clj
index 8141c86..cfc3ab0 100644
--- a/test/cipher_analytical_machine/core_test.clj
+++ b/test/cipher_analytical_machine/core_test.clj
@@ -2,6 +2,3 @@
   (:require [clojure.test :refer :all]
             [cipher-analytical-machine.core :refer :all]))
 
-(deftest a-test
-  (testing "FIXME, I fail."
-    (is (= 0 1))))