Java 对同一字符串返回不同的 CRC32 结果

Question

我正在玩 RC4 和 CRC32 试图模拟位翻转攻击，我对 Java 中 CRC32 的行为摸不着头脑。据我所知，这应该是基于多项式计算的确定性结果。然而，我所看到的是，无论文本是否实际更改，CRC 都会以不可预测的方式更改。 Java文档简单地说明了字节数组上的 update()：用指定的字节数组更新 CRC-32 校验和。，对于 getValue()：Returns CRC-32 值。 这里是否涉及某种我没有考虑的盐或 PRF（我不这么认为）？

输出：

run:
Ciphertext = [B@34e51b72, and CRC = 232697804
Now ciphertext = [B@34e51b72, and CRC = 1990877612
Now ciphertext = [B@34e51b72, and CRC = 1720857375
Now ciphertext = [B@34e51b72, and CRC = 4144065286
Now ciphertext = [B@34e51b72, and CRC = 1992352640
BUILD SUCCESSFUL (total time: 1 second)

代码：

 package rc4_crc32;
import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;
import java.util.zip.CRC32;
public class RC4_CRC32 {
    public static void main(String[] args) throws Exception{ 
        byte[] key, ciphertext;
        CRC32 c = new CRC32();
        javax.crypto.Cipher r;
                 r = Cipher.getInstance("RC4");
                 key = "1@m@L33tH@x0r!".getBytes("ASCII");
                 SecretKeySpec rc4Key = new SecretKeySpec(key, "RC4");
                 r.init(Cipher.ENCRYPT_MODE, rc4Key); 
                 ciphertext = r.update("Secret!".getBytes("ASCII"));                    
                 c.update(ciphertext);                 
                 System.out.println("Ciphertext = " + ciphertext + ", and CRC = " + c.getValue());     
                 ciphertext[0] = (byte)0x2c;
                 c.update(ciphertext);
                 System.out.println("Now ciphertext = " + ciphertext + ", and CRC = " + c.getValue());
                 c.update(ciphertext);
                 System.out.println("Now ciphertext = " + ciphertext + ", and CRC = " + c.getValue());
                 c.update(ciphertext);
                 System.out.println("Now ciphertext = " + ciphertext + ", and CRC = " + c.getValue());
                 c.update(ciphertext);
                 System.out.println("Now ciphertext = " + ciphertext + ", and CRC = " + c.getValue());
    }    
}

Answer 1

update() 用于递增计算字节序列的校验和。您的代码计算了在同一 CRC32 对象上提供给 update() 的所有密文的连接的 crc32。

尝试

c.reset()
c.update(ciphertext);   
System.out.println("Now ciphertext = " + ciphertext + ", and CRC = " + c.getValue());

Java 对同一字符串返回不同的 CRC32 结果

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