add four square cipher

cryptography/FourSquare_cipher/JavaScript/FourSquare_cipher.js

@@ -0,0 +1,28 @@
+function encrypt(messageInput, keyword) {
+    var alphabet = 'abcdefghijklmnopqrstuvwxyz';
+    var messageOutput = "";
+
+    var pos = 0;
+    while (pos < messageInput.length) {
+        var m1 = messageInput[pos];
+        var m2 = '';
+
+        if (pos + 1 < messageInput.length) {
+            m2 = messageInput[pos + 1];
+            pos += 2;
+        } else {
+            m2 = 'Q'  // some dummy letter
+            pos += 1;
+        }
+
+        var idx1 = alphabet.indexOf(m1);  // upper-left table
+        var idx2 = alphabet.indexOf(m2);  // lower-right table
+        var c1 = keyword[0][(5 * Math.floor(idx1 / 5)) + idx2 % 5];
+        var c2 = keyword[1][(5 * Math.floor(idx2 / 5)) + idx1 % 5];
+
+        messageOutput = messageOutput.concat(c1);
+        messageOutput = messageOutput.concat(c2);
+    }
+
+    return messageOutput;
+}