# Autokey Cipher | Symmetric Ciphers

Autokey Cipher is a polyalphabetic substitution cipher. It is closely related to the Vigenere cipher but uses a different method of generating the key. It was invented by Blaise de Vigenère in 1586. In general, more secure than the Vigenere cipher. Example-1:

```Plaintext = "HELLO"
Autokey = N
Ciphertext = "ULPWZ" ```

Example-2:

```Plaintext = "GEEKSFORGEEKS"
Autokey = P
Ciphertext = "VKIOCXTFXKIOC" ```

In this cipher, the key is a stream of subkeys which is used to encrypt the corresponding character in the plaintext. As shown, the autokey is added at the first of the subkeys.

```Let's explain Example 1:

Given plain text is : H E L L O
Key is              : N H E L L

Let's encrypt:

Plain Text(P)       : H   E   L   L   O
Corresponding Number: 7   4   11  11  14
Key(K)              : N   H   E   L   L
Corresponding Number: 13  7   4   11  11
---------------------
Applying the formula: 20  11  15  22  25

Corresponding
Letters are         : U    L   P   W   Z

Hence Ciphertext is: ULPWZ

Let's decrypt:

Cipher Text(C)      : U   L   P   W   Z
Key(K)              : N   H   E   L   L
---------------------
Applying the formula: H   E   L   L   O

Hence Plaintext is: HELLO ```

Here’s the java code for Autokey Cipher.

## Java

 `// A JAVA program to illustrate ` `// Autokey Cipher Technique ` ` `  `// Importing required library ` `import` `java.lang.*; ` `import` `java.util.*; ` ` `  `public` `class` `AutoKey { ` ` `  `    ``private` `static` `final` `String alphabet = ``"ABCDEFGHIJKLMNOPQRSTUVWXYZ"``; ` ` `  `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``String msg = ``"HELLO"``; ` `        ``String key = ``"N"``; ` ` `  `        ``// This if statement is all about java regular expression ` `        ``// [] for range ` `        ``// // Extra \ is used to escape one \ ` `        ``// \\d acts as delimiter ` `        ``// ? once or not at all ` `        ``// . Any character (may or may not match line terminators) ` `        ``if` `(key.matches(``"[-+]?\\d*\\.?\\d+"``)) ` `            ``key = ``""` `+ alphabet.charAt(Integer.parseInt(key)); ` `        ``String enc = autoEncryption(msg, key); ` ` `  `        ``System.out.println(``"Plaintext : "` `+ msg); ` `        ``System.out.println(``"Encrypted : "` `+ enc); ` `        ``System.out.println(``"Decrypted : "` `+ autoDecryption(enc, key)); ` `    ``} ` ` `  `    ``public` `static` `String autoEncryption(String msg, String key) ` `    ``{ ` `        ``int` `len = msg.length(); ` ` `  `        ``// generating the keystream ` `        ``String newKey = key.concat(msg); ` `        ``newKey = newKey.substring(``0``, newKey.length() - key.length()); ` `        ``String encryptMsg = ``""``; ` ` `  `        ``// applying encryption algorithm ` `        ``for` `(``int` `x = ``0``; x < len; x++) { ` `            ``int` `first = alphabet.indexOf(msg.charAt(x)); ` `            ``int` `second = alphabet.indexOf(newKey.charAt(x)); ` `            ``int` `total = (first + second) % ``26``; ` `            ``encryptMsg += alphabet.charAt(total); ` `        ``} ` `        ``return` `encryptMsg; ` `    ``} ` ` `  `    ``public` `static` `String autoDecryption(String msg, String key) ` `    ``{ ` `        ``String currentKey = key; ` `        ``String decryptMsg = ``""``; ` ` `  `        ``// applying decryption algorithm ` `        ``for` `(``int` `x = ``0``; x < msg.length(); x++) { ` `            ``int` `get1 = alphabet.indexOf(msg.charAt(x)); ` `            ``int` `get2 = alphabet.indexOf(currentKey.charAt(x)); ` `            ``int` `total = (get1 - get2) % ``26``; ` `            ``total = (total < ``0``) ? total + ``26` `: total; ` `            ``decryptMsg += alphabet.charAt(total); ` `            ``currentKey += alphabet.charAt(total); ` `        ``} ` `        ``return` `decryptMsg; ` `    ``} ` `} `

```Output:

Plaintext : HELLO
Encrypted : ULPWZ
Decrypted : HELLO

```

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