Vernam Cipher 或 One Time Pad 算法的实现

One Time Pad 算法也称为Vernam Cipher 。它是一种加密字母纯文本的方法。它是将纯文本转换为密文的替代技术之一。在这种机制中，我们为纯文本的每个字符分配一个数字。

那作业 如下：  

A	B	C	D	E	F	G	H	I	J
0	1	2	3	4	5	6	7	8	9
K	L	M	N	O	P	Q	R	S	T
10	11	12	13	14	15	16	17	18	19
U	V	W	X	Y	Z
20	21	22	23	24	25

密钥与明文的关系：在该算法中，密钥的长度应与明文的长度相等。

例子：

Input: Message = HELLO, Key = MONEY
Output: Cipher – TSYPM, Message – HELLO
Explanation:
    Part 1: Plain text to Ciphertext
        Plain text — H E L L O → 7 4 11 11 14
        Key — M O N E Y → 12 14 13 4 24
        Plain text + key → 19 18 24 15 38 → 19 18 24 15 12 (= 38 – 26)
        Cipher Text → T S Y P M
    Part 2: Ciphertext to Message
        Cipher Text — T S Y P M → 19 18 24 15 12
        Key — M O N E Y→ 12 14 13 4 24
        Cipher text – key → 7 4 11 11 -12 → 7 4 11 11 14
        Message → H E L L O

Input: Message = SAVE, Key = LIFE
Output: Cipher – DIAI
Message – SAVE

下面是 Vernam Cipher 的实现：

Java

// Java program Implementing One Time Pad Algorithm
  
// Importing required classes
import java.io.*;
  
// Main class
public class GFG {
  
    // Method 1
    // Returning encrypted text
    public static String stringEncryption(String text,
                                          String key)
    {
  
        // Initializing cipherText
        String cipherText = "";
  
        // Initialize cipher array of key length
        // which stores the sum of corresponding no.'s
        // of plainText and key.
        int cipher[] = new int[key.length()];
  
        for (int i = 0; i < key.length(); i++) {
            cipher[i] = text.charAt(i) - 'A'
                        + key.charAt(i)
                        - 'A';
        }
  
        // If the sum is greater than 25
        // subtract 26 from it
        // and store that resulting value
        for (int i = 0; i < key.length(); i++) {
            if (cipher[i] > 25) {
                cipher[i] = cipher[i] - 26;
            }
        }
  
        // Converting the no.'s into integers
  
        // Convert these integers to corresponding
        // characters and add them up to cipherText
        for (int i = 0; i < key.length(); i++) {
            int x = cipher[i] + 'A';
            cipherText += (char)x;
        }
  
        // Returning the cipherText
        return cipherText;
    }
  
    // Method 2
    // Returning plain text
    public static String stringDecryption(String s,
                                          String key)
    {
        // Initializing plain text
        String plainText = "";
  
        // Initializing integer array of key length
        // which stores difference
        // of corresponding no.'s of
        // each character of cipherText and key
        int plain[] = new int[key.length()];
  
        // Running for loop for each character
        // subtracting and storing in the array
        for (int i = 0; i < key.length(); i++) {
            plain[i]
                = s.charAt(i) - 'A'
                  - (key.charAt(i) - 'A');
        }
  
        // If the difference is less than 0
        // add 26 and store it in the array.
        for (int i = 0; i < key.length(); i++) {
            if (plain[i] < 0) {
                plain[i] = plain[i] + 26;
            }
        }
  
        // Converting int to corresponding char
        // add them up to plainText
        for (int i = 0; i < key.length(); i++) {
            int x = plain[i] + 'A';
            plainText += (char)x;
        }
  
        // Returning plainText
        return plainText;
    }
  
    // Method 3
    // Main driver method
    public static void main(String[] args)
    {
        // Declaring plain text
        String plainText = "Hello";
  
        // Declaring key
        String key = "MONEY";
  
        // Converting plain text to toUpperCase
        // function call to stringEncryption
        // with plainText and key as parameters
        String encryptedText = stringEncryption(
            plainText.toUpperCase(), key.toUpperCase());
  
        // Printing cipher Text
        System.out.println("Cipher Text - "
                           + encryptedText);
  
        // Calling above method to stringDecryption
        // with encryptedText and key as parameters
        System.out.println(
            "Message - "
            + stringDecryption(encryptedText,
                               key.toUpperCase()));
    }
}

输出：

Cipher Text - TSYPM
Message - HELLO