Java解密加密字符串：解析给定encode函数的工作原理与逆向实现

Hey there! Let's break down this encryption function step by step, then work out how to reverse it properly.

加密函数的工作原理 #

First, let's unpack what the encode function actually does—it has two key steps:

1. 单个字符的转换逻辑 #

The line c += c+i is a shorthand for c = c + c + i, which simplifies to:

加密后字符的ASCII值 = 原字符ASCII值 × 2 + 当前索引i

For example, if the original character is 'a' (ASCII 97) at index i=0, the encrypted character becomes 97*2 + 0 = 194, which corresponds to the character Â.

2. 整体字符串反转 #

After processing every character in the input message, the entire sequence of transformed characters is reversed. So if the transformed sequence is [A, B, C], the final encrypted string becomes [C, B, A].

解密的思路与实现 #

To decrypt, we need to reverse the encryption steps in reverse order (since the last step of encryption was reversing, we start with that):

解密步骤 #

1. Reverse the encrypted string first: This restores the sequence of transformed characters to the order they were in before the final encryption step.
2. Calculate the original character for each position:
   From the encryption formula c_new = 2*c_old + i, we can rearrange to get the original character:

   原字符ASCII值 = (加密后字符的ASCII值 - 当前索引i) ÷ 2

   • Important: (c_new - i) must be even—if not, the input encrypted string is invalid (since encryption always produces a value where this subtraction results in an even number).

Java解密代码实现 #

public static String decode(String encryptedMessage) {
    // Step 1: Reverse the encrypted string to get back the transformed character sequence
    StringBuilder reversedTransformed = new StringBuilder(encryptedMessage).reverse();
    StringBuilder originalMessage = new StringBuilder();
    
    for (int i = 0; i < reversedTransformed.length(); i++) {
        char cNew = reversedTransformed.charAt(i);
        // Calculate original character using the reversed formula
        int originalCharCode = (cNew - i) / 2;
        
        // Optional: Validate the result to catch invalid input
        if (2 * originalCharCode + i != cNew) {
            throw new IllegalArgumentException("Invalid encrypted string at index " + i);
        }
        
        originalMessage.append((char) originalCharCode);
    }
    
    return originalMessage.toString();
}

Test Example #

Let's test with the original string "hello":

• Encrypted string (from the encode function) is "âÛÚËÐ"
• Run decode("âÛÚËÐ"):
  1. Reverse to get "ÐËÚÛâ"
  2. For each index:
     • i=0: (208 - 0)/2 = 104 → 'h'
     • i=1: (203 - 1)/2 = 101 → 'e'
     • i=2: (218 - 2)/2 = 108 → 'l'
     • i=3: (219 - 3)/2 = 108 → 'l'
     • i=4: (226 - 4)/2 = 111 → 'o'
  3. Returns the original string "hello"

内容的提问来源于stack exchange，提问作者Dookoto_Sea