如何用Java编写哈希解码程序还原加密邮箱地址？

我需要编写Java代码处理哈希编码，实现加密邮箱地址的解码，以下是我写的代码，请帮忙做技术层面的确认或指导：

import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class EmailDecoder {
    public static void main(String[] args) {
        String encodedEmail = "f894e71e1551d1833a977df952d0cc9de44a1f9669fbf97d51309a2c6574d5eaa746cd" +
                "eb9ee1a5dfc771d280d33e5672e48a0b49e999dc7d88f3e7073040596e98687c4d1" +
                "730f3ac2fb2fe4f3e2fba56af628f412d7fdd02405f755a7a9ed59f2fd71f28ca8fc7500f" +
                ".....5098b5499d";

        String decodedEmail = decodeEmail(encodedEmail);
        System.out.println(decodedEmail);
    }

    public static String decodeEmail(String encodedEmail) {
        StringBuilder decodedEmail = new StringBuilder();
        int length = encodedEmail.length();

        for (int i = 0; i < length; i += 64) {
            String substring = encodedEmail.substring(i, Math.min(i + 64, length));
            String reversed = reverseHash(substring);
            String character = reversed.substring(reversed.length() - 2);
            decodedEmail.append(character);
        }

        return decodedEmail.toString();
    }

    public static String reverseHash(String hash) {
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            byte[] bytes = new BigInteger(1, hexToBytes(hash)).toByteArray();
            byte[] digest = md.digest(bytes);
            StringBuilder sb = new StringBuilder();

            for (byte b : digest) {
                sb.append(String.format("%02x", b));
            }

            return sb.toString();
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }

        return null;
    }

    public static byte[] hexToBytes(String hex) {
        byte[] bytes = new byte[hex.length() / 2];

        for (int i = 0; i < bytes.length; i++) {
            int index = i * 2;
            int j = Integer.parseInt(hex.substring(index, index + 2), 16);
            bytes[i] = (byte) j;
        }

        return bytes;
    }
}

技术问题分析与改进建议 #

1. 核心功能缺失：十六进制转字符 #

当前代码仅将哈希结果的最后两位十六进制字符串拼接，但未将其解析为对应的ASCII字符，最终输出的是十六进制序列而非邮箱地址。需要添加转换步骤：

// 替换decodeEmail方法中的decodedEmail.append(character);
char emailChar = (char) Integer.parseInt(character, 16);
decodedEmail.append(emailChar);

2. BigInteger转换导致数据失真 #

reverseHash中通过BigInteger转换字节数组会额外添加0x00前缀（当原字节最高位为1时），导致MD5计算的原始数据与加密时的输入不一致，直接使用hexToBytes的返回值即可：

// 替换reverseHash中的byte[] bytes = new BigInteger(1, hexToBytes(hash)).toByteArray();
byte[] bytes = hexToBytes(hash);

3. 异常处理存在空指针风险 #

reverseHash捕获异常后返回null，后续调用reversed.substring会触发NullPointerException，建议抛出运行时异常以明确错误：

catch (NoSuchAlgorithmException e) {
    throw new RuntimeException("MD5算法不可用", e);
}

4. 加密规则的强依赖性 #

当前解码逻辑假设邮箱每个字符对应64位十六进制加密段，且加密规则是对该段原始数据做MD5哈希后取最后两位作为原字符的十六进制。若实际加密规则与此不符，解码逻辑将完全失效，需确保解码逻辑与加密逻辑严格对称。

5. 边界场景处理 #

当加密字符串长度不是64的整数倍时，需确认剩余片段是否为完整的单个字符加密段，否则会导致末尾字符解码错误。

修正后的完整代码 #

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class EmailDecoder {
    public static void main(String[] args) {
        String encodedEmail = "f894e71e1551d1833a977df952d0cc9de44a1f9669fbf97d51309a2c6574d5eaa746cd" +
                "eb9ee1a5dfc771d280d33e5672e48a0b49e999dc7d88f3e7073040596e98687c4d1" +
                "730f3ac2fb2fe4f3e2fba56af628f412d7fdd02405f755a7a9ed59f2fd71f28ca8fc7500f" +
                ".....5098b5499d";

        String decodedEmail = decodeEmail(encodedEmail);
        System.out.println(decodedEmail);
    }

    public static String decodeEmail(String encodedEmail) {
        StringBuilder decodedEmail = new StringBuilder();
        int length = encodedEmail.length();

        for (int i = 0; i < length; i += 64) {
            String substring = encodedEmail.substring(i, Math.min(i + 64, length));
            String reversedHash = reverseHash(substring);
            String charHex = reversedHash.substring(reversedHash.length() - 2);
            char emailChar = (char) Integer.parseInt(charHex, 16);
            decodedEmail.append(emailChar);
        }

        return decodedEmail.toString();
    }

    public static String reverseHash(String hash) {
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            byte[] bytes = hexToBytes(hash);
            byte[] digest = md.digest(bytes);
            StringBuilder sb = new StringBuilder();

            for (byte b : digest) {
                sb.append(String.format("%02x", b));
            }

            return sb.toString();
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException("MD5算法不可用", e);
        }
    }

    public static byte[] hexToBytes(String hex) {
        byte[] bytes = new byte[hex.length() / 2];

        for (int i = 0; i < bytes.length; i++) {
            int index = i * 2;
            int value = Integer.parseInt(hex.substring(index, index + 2), 16);
            bytes[i] = (byte) value;
        }

        return bytes;
    }
}

内容的提问来源于stack exchange，提问作者berry.11