java - 使用 CaesarCipherBreaker 拆分字符串

Question

我如何向此示例添加代码以创建一个 CaesarCipherBreaker 方法，该方法用两个 key 分割加密消息。到目前为止我已经写了这么多:

import edu.duke.*;

public class TestCaesarCipherTwo {
   public int[] countOccurrencesOfLetters(String message) {
        //snippet from lecture
        String alph = "abcdefghijklmnopqrstuvwxyz";
        int[] counts = new int[26];
        for (int k=0; k < message.length(); k++) {
            char ch = Character.toLowerCase(message.charAt(k));
            int dex = alph.indexOf(ch);
            if (dex != -1) {
                counts[dex] += 1;
            }
        }
        return counts;
    }
    public int maxIndex(int[] values) {
        int maxDex = 0;
        for (int k=0; k < values.length; k++) {
            if (values[k] > values[maxDex]) {
                maxDex = k;
            }
        }
        return maxDex;
   }

   public String halfOfString(String message, int start) {
        StringBuilder halfString = new StringBuilder();
        for (int index=start;index < message.length();index += 2) {
            halfString.append(message.charAt(index));
        }
        return halfString.toString();

   }
      public void simpleTests() {
        FileResource fileResource = new FileResource();
        String fileAsString = fileResource.asString();
        CaesarCipherTwoKeys cctk = new CaesarCipherTwoKeys(17, 3);
        String encrypted = cctk.encrypt(fileAsString);
        System.out.println("Encrypted string:\n"+encrypted);
        String decrypted = cctk.decrypt(encrypted);
        System.out.println("Decrypted string:\n"+decrypted);

        String blindDecrypted = breakCaesarCipher(encrypted);
        System.out.println("Decrypted string using breakCaesarCipher():\n"+blindDecrypted);
   }
    public String breakCaesarCipher(String input) {
        int[] freqs = countOccurrencesOfLetters(input);
        int freqDex = maxIndex(freqs);
        int dkey = freqDex - 4;
        if (freqDex < 4) {
            dkey = 26 - (4-freqDex);
        }

        CaesarCipherTwoKeys cctk = new CaesarCipherTwoKeys(dkey);
        return cctk.decrypt(input);
   }
}

警告:我在这一行上也有构造函数错误 CaesarCipherTwoKeys cctk = new CaesarCipherTwoKeys(dkey);说明CaesarCipherTwoKeys in class CaesarCipherTwoKeys cannot be applied to given types; required int,int; found int... 。

我现在的breakCaesarCipher方法只能计算出一个 key ，而不是两个。我应该如何编写一个方法来分割加密字符串，找出用于解密的两个 key 。

Answer 1

如果我正确理解您的代码，您只需调用 halfOfString (两次)即可获取密文的两部分，然后使用通常的方法来破解这两个部分的 Ceaser-Cipher分别。

您的错误似乎是由于双 key 加密需要(毫不奇怪)两个 key 这一事实造成的。您应该将它们都交给构造函数。

public String breakCaesarCipher(String input) {
    String in_0 = halfOfString(input, 0);
    String in_1 = halfOfString(input, 1);
    // Find first key
    // Determine character frequencies in ciphertext
    int[] freqs_0 = countOccurrencesOfLetters(in_0);
    // Get the most common character
    int freqDex_0 = maxIndex(freqs_0);
    // Calculate key such that 'E' would be mapped to the most common ciphertext character
    // since 'E' is expected to be the most common plaintext character
    int dkey_0 = freqDex_0 - 4;
    // Make sure our key is non-negative
    if (dkey_0 < 0) {
        dkey_0 = dkey_0+26;
    }
    // Find second key
    int[] freqs_1 = countOccurrencesOfLetters(in_1);
    int freqDex_1 = maxIndex(freqs_1);
    int dkey_1 = freqDex_1 - 4;
    if (freqDex_1 < 4) {
        dkey_1 = dkey_1+26;
    }

    CaesarCipherTwoKeys cctk = new CaesarCipherTwoKeys(dkey_0, dkey_1);
    return cctk.decrypt(input);
}