Java 在同一个字符串上返回不同的 CRC32 结果

Question

我正在尝试使用 RC4 和 CRC32 来模拟位翻转攻击，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。 Javadoc 只是针对字节数组的 update() 声明:使用指定的字节数组更新 CRC-32 校验和。，对于 getValue():返回 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() 用于增量计算字节序列的校验和。您的代码在同一 CRC32 对象上计算提供给 update() 的所有密文的串联 的 crc32。

尝试

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