古典密码学之置换

这学期开了门课 叫信息安全技术。

写了个置换的算法。

挺麻烦的。

明文： 
attackpostponeduntiltwoamxyz
密钥：4312567
将明文保存排列成矩阵
4 3 1 2 5 6 7
a  t  t  a c  k  p
o s  t  p o n e
d u  n t   i  l   t
w o a m x y z
然后按照密钥的顺序按列读出就成了密文 ttnaaptmtsuoaodwcoixknlypetz
还可以继续对密文进行再次加密，不过还是不安全 =。=

Java:

package cryptography;

public class PermutationCipher {
	
	//加密
	static String encypt(String plaintext, String key) {

		String cipherText = "";
		String[] cipher = new String[key.length()];
		plaintext = plaintext.trim();

		int row = (plaintext.length()) / (key.length());
		int column = key.length();

		// 将明文和密钥分别保存在char数组和整型变量中
		char[] chars = new char[plaintext.length()];
		plaintext.getChars(0, plaintext.length(), chars, 0);
		int keycode = Integer.parseInt(key);

		char[][] matrix = new char[row][column];

		// 将明文保存在矩阵中
		for (int i = 0; i < row; i++)
			for (int j = 0; j < column; j++) {
				matrix[i][j] = chars[i * column + j];
			}

		int n = keycode;
		int index = 0;
		// 根据密钥译成密文
		for (int m = 0; m < column; m++) {

			index = (int) (n / Math.pow(10, column - m - 1));
			n = (int) (n % Math.pow(10,column - m - 1)) ;
			cipher[index-1] = readColumn(matrix, m);
		}

		for (int i = 0; i < cipher.length; i++)
			cipherText += cipher[i];
		return cipherText;

	}

	static String readColumn(char[][] matrix, int column) {
		String str = "";
		for (int i = 0; i < matrix.length; i++)
			str += matrix[i][column];

		return str;
	}
	//解码
	static String decrypt(String cipherText, String key)
	{
		
		String plainText = "";
		cipherText = cipherText.trim();
		
		int row = (cipherText.length()) / (key.length());
		int column = key.length();
		
		// 将密文和密钥分别保存在char数组和变量中
		char[] chars = new char[cipherText.length()];
		cipherText.getChars(0, cipherText.length(), chars, 0);
		int keycode = Integer.parseInt(key);
		
		//将密文保存在矩阵中
		char[][] matrix = new char[column][row];
		
		for(int i = 0 ; i < column ; i++)
			for(int j = 0 ; j < row ; j++)
				matrix[i][j] = chars[i*row + j];
		
		//根据密钥译出密文
		int index = 0;
		int n = keycode;
		char[][] matrix_plain = new char[row][column];
		
		for(int m = 0 ; m < column ; m++)
		{
			index = (int) (n/Math.pow(10, column - 1 - m));
			n = (int) (n%Math.pow(10, column - m -1));
			writeColumn(matrix_plain,matrix,index-1,m);
		}
		String[] plains = new String[row];
		for(int k = 0 ; k < row ; k++)
		{
			plains[k] = new String(matrix_plain[k]);
			plainText += plains[k];
		}
		return plainText;

		
	}
	static void writeColumn(char[][] matrix_dest , char[][] matrix_s,int dest,int source)
	{
		for (int i = 0 ; i < matrix_dest.length ; i++)
		{
			matrix_dest[i][source] = matrix_s[dest][i];
		}
	}
	
	public static void main(String[] args) {
		String s = "attackpostponeduntiltwoamxyz";
		String key = "4312567";
		String cipher = encypt(s, key);
		String plain  = decrypt(cipher,key);
		
		System.out.println("密文是"+cipher+"明文是"+plain);
	}

}

去掉明文的空格没有实现。算法逻辑有多要优化的地方。