Added affine cipher

Author: duaraghav8@gmail

algorithm/category.json

@@ -14,7 +14,8 @@
   "cryptography": {
     "name": "Cryptography",
     "list": {
-      "caesar_cipher": "Caesar Cipher"
+      "caesar_cipher": "Caesar Cipher",
+      "affine_cipher": "Affine Cipher"
     }
   },
   "mst": {

algorithm/cryptography/affine_cipher/basic/code.js

@@ -0,0 +1,82 @@
+/*
+	code assumes that plainText contains ONLY LOWER CASE ALPHABETS
+*/
+
+Number.prototype.mod = function (n) {
+    return ((this%n)+n)%n;
+};
+
+var keys = {a: 5, b: 7},
+	N = 26;
+
+function encrypt (plainText) {
+	var cypherText = '';
+	function cryptAlpha (alpha) {
+		var index = alpha.charCodeAt (0) - 'a'.charCodeAt (0);
+		var result = ((keys.a * index) + keys.b).mod (N);
+
+		logger._print ('Index of ' + alpha + ' = ' + index);
+
+		result += 'a'.charCodeAt (0);
+		return String.fromCharCode (result);
+	}
+
+	logger._print ('Beginning Affine Encryption');
+	logger._print ('Encryption formula: <b>((keys.a * index_of_alphabet) + keys.b) % N</b>');
+	logger._print ('keys.a=' + keys.a + ', keys.b=' + keys.b + ', N=' + N);
+
+	for (var i in plainText) {
+		ptTracer._select (i)._wait ();
+		ptTracer._deselect (i)._wait ();
+
+		cypherText += cryptAlpha (plainText [i]);
+
+		ptTracer._notify (i, cypherText.slice (-1))._wait ();
+		ptTracer._denotify (i)._wait ();
+	}
+
+	return cypherText;
+}
+
+function decrypt (cypherText) {
+	var plainText = '';
+	var aInverse = (function () {
+		for (var i = 1; i < N; i++) {
+			if ( ((keys.a * i).mod (N)) == 1 ) {
+				return i;
+			}
+		}
+	}) ();
+
+	logger._print ('a<sup>-1</sup> = ' + aInverse);
+
+	function decryptAlpha (alpha) {
+		var index = alpha.charCodeAt (0) - 'a'.charCodeAt (0);
+		var result = (aInverse * (index - keys.b)).mod (N);
+
+		logger._print ('Index of ' + alpha + ' = ' + index);
+
+		result += 'a'.charCodeAt (0);
+		return String.fromCharCode (result);
+	}
+
+	logger._print ('Beginning Affine Decryption');
+	logger._print ('Decryption formula: <b>(a<sup>-1</sup> * (index - keys.b)) % N</b>');
+	logger._print ('keys.b=' + keys.b + ', N=' + N);
+
+	for (var i in cypherText) {
+		ctTracer._select (i)._wait ();
+		ctTracer._deselect (i)._wait ();
+
+		plainText += decryptAlpha (cypherText [i]);
+
+		ctTracer._notify (i, plainText.slice (-1))._wait ();
+		ctTracer._denotify (i)._wait ();
+	}
+
+	return plainText;
+}
+
+var cipherText = encrypt (plainText);
+ctTracer._setData (cipherText);
+decrypt (cipherText);

algorithm/cryptography/affine_cipher/basic/data.js

@@ -0,0 +1,18 @@
+function randString (length) {
+	var choices = 'abcdefghijklmnopqrstuvwxyz';
+	var text = '';
+
+	for (var i = 0; i < length; i++) {
+		text += choices [Math.floor (Math.random () * choices.length)];
+	}
+
+	return text;
+}
+
+//var plainText = randString (5);
+var plainText = 'secret';
+var ptTracer = new Array1DTracer ('Encryption');
+var ctTracer = new Array1DTracer ('Decryption');
+var logger = new LogTracer ();
+
+ptTracer._setData (plainText);

algorithm/cryptography/affine_cipher/desc.json

@@ -0,0 +1,17 @@
+{
+  "Affine Cipher": "The affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter.",
+  "Applications": [
+    "Cryptanalysis",
+    "More complex Variations of Affine Cihper are used in practical cryptography"
+  ],
+  "Complexity": {
+    "time": "worst O(N), N = length of plain/cipher text",
+    "space": "worst O(N), to create the new mapping (plain->cipher, cipher->plain)"
+  },
+  "References": [
+    "<a href='http://practicalcryptography.com/ciphers/affine-cipher/'>Practicalcryptography</a>"
+  ],
+  "files": {
+    "basic": "Encrypting and Decrypting a string using affine functions"
+  }
+}