用凯撒密码破解器破解字符串

Question

如何将代码添加到此示例中，以创建CaesarCipherBreaker方法，该方法通过两个密钥分割加密消息。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CaesarCipherTwoKeys cctk = new CaesarCipherTwoKeys(dkey);行也有一个构造函数错误，声明为CaesarCipherTwoKeys in class CaesarCipherTwoKeys cannot be applied to given types; required int,int; found int...。

我现在使用的breakCaesarCipher方法只计算出一个键，而不是两个。我应该如何编写一个方法来分割加密的字符串，找出用于解密的两个密钥。

Answer 1

如果我正确理解您的代码，您只需调用halfOfString（两次）即可获得密文的两个部分，然后使用您通常的方法分别破解两个部分的Ceaser密码。

您的错误似乎是由于双密钥加密需要（毫不奇怪）两个密钥这一事实造成的。您应该将它们都提供给构造函数。

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);
}