网络安全-2.3

2.3实现加解密程序。

替代密码:

替代密码算法的原理是使用替代法进行加密，就是将明文中的字符用其它字符替代后形成密文。例如:明文字母a，b，c，d ，用D，E，F，G做对应替换后形成密文。

替代密码包括多种类型，如单表替代密码，多明码替代密码，多字母替代密码，多表替代密码等。下面我们介绍一种典型的单表替代密码，恺撒(caesar)密码，又叫循环移位密码。它的加密方法，就是将明文中的每个字母用此字符在字母表中后面第k个字母替代。它的加密过程可以表示为下面的函数:

E(m)=(m+k) mod n

其中:m为明文字母在字母表中的位置数；n为字母表中的字母个数；k为密钥；E(m)为密文字母在字母表中对应的位置数。

例如，对于明文字母H，其在字母表中的位置数为8，设k=4，则按照上式计算出来的密文为L:

E(8) = (m+k) mod n= (8+4) mod 26 = 12 = L

源代码（java实现）：

import java.util.Scanner;

public class MyTiHuan {

        public final int smalla = 97;

        public final int bigA = 65;

        public final int smallz = 122;

        public final int bigZ = 90;

        public final int space = 32;

        public final int smallZero = 96;

        public final int bigZero = 64;

        /**

         * 判断是否为小写字母

         *

         * @param a

         * @return

         */

        public boolean isSmall(char a) {

            if (a >= 97 && a <= 122) {

                return true;

            } else

                return false;

        }

            /**

         * 判断是否为大写字母

         *

         * @param A

         * @return

         */

        public boolean isBig(char A) {

            if (A >= 65 && A <= 90) {

                return true;

            } else

                return false;

        }

        /**

         * 判断是否为空格

         *

         * @param spa

         * @return

         */

        public boolean isSpace(char spa) {

            if (spa == space) {

                return true;

            } else

                return false;

        }

        /**

         * 检查非法字符

         *

         * @param a

         * @return

         */

        public boolean isPass(char a) {

            if (a == space) {

                return true;

            } else if (a >= smalla && a <= smallz) {

                return true;

            } else if (a >= bigA && a <= bigZ) {

                return true;

            }

            return false;

        }

    /**

         *

         * 加密

         *

         * @param mingwen

         *            明文

         * @param yiwei

         *            右移的位数

         * @return 密文

         *

         */

        public String enCode(String mingwen, int yiwei) {

            StringBuffer sBuff = new StringBuffer();

            char[] mw = mingwen.toCharArray();

            System.out.println("加密中----");

            for (int i = 0; i < mw.length; i++) {

                char temp = 'a';

                char c = mw[i];

                if (isPass(c)) {

                   if (isSmall(c)) {

                       temp = (char) (c + yiwei);

                       if (!isPass(temp)) {

                           temp = (char) (temp - smallz + smallZero);

                       }

                   } else if (isBig(c)) {

                       temp = (char) (c + yiwei);

                       if (!isPass(temp)) {

                           temp = (char) (temp - bigZ + bigZero);

                       }

                   } else if (isSpace(c)) {

                       temp = c;

                   }

                } else {

                   return sBuff.append("输入中带有非法字符！").toString();

                }

                sBuff.append(temp);   

            }

            System.out.println("加密成功！");

            return sBuff.toString();

        }

        /**

         * 解密

         *

         * @param miwen

         *            密文

         * @param yiwei

         *            左移的位数

         * @return 明文

         */

        public String deCode(String miwen, int yiwei) {

            StringBuffer sBuff = new StringBuffer();

            char[] mw = miwen.toCharArray();

            System.out.println("解密中----");

            for (int i = 0; i < mw.length; i++) {

                char temp = 'a';

                char c = mw[i];

                if (isPass(c)) {

                   if (isSmall(c)) {

                       temp = (char) (c - yiwei);

                       if (!isPass(temp)) {

                           temp = (char) (smallz - smallZero + temp);

                       }

                   } else if (isBig(c)) {

                       temp = (char) (c - yiwei);

                       if (!isPass(temp)) {

                           temp = (char) (bigZ - bigZero + temp);

                       }

                   } else if (isSpace(c)) {

                       temp = c;

                   }

                } else {

                   return sBuff.append("密文中带有非法字符！").toString();

                }

                sBuff.append(temp);

            }

            System.out.println("解密成功！");

            return sBuff.toString();

        }

        public static void main(String[] args) {

            MyTiHuan mt = new MyTiHuan();

            System.out.println("请输入明文：");

            Scanner scan = new Scanner(System.in);

            String mingw = scan.nextLine();

            String miw = mt.enCode(mingw, 3);

            System.out.println("密文为：");

            System.out.println(miw);

            mingw = mt.deCode(miw, 3);

            System.out.println("明文为：");

            System.out.println(mingw);

        }

}

运行结果：