多因素认证设计#

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.nio.ByteBuffer;
import java.nio.charset.StandardCharsets;

public class TotpGenerator {
    
    private static final int TIME_STEP = 30;
    private static final int PASSWORD_LENGTH = 6;
    private static final String ALGORITHM = "HmacSHA1";
    
    /**
     * 生成 TOTP 密码
     * @param secret Base32 编码的密钥
     * @param time 当前 Unix 时间戳（秒）
     * @return 6 位数字 TOTP
     */
    public static String generateTotp(String secret, long time) {
        byte[] key = base32Decode(secret);
        long counter = time / TIME_STEP;
        
        // 将计数器转换为 8 字节大端序
        byte[] counterBytes = ByteBuffer.allocate(8)
            .putLong(counter)
            .array();
        
        // 计算 HMAC-SHA1
        byte[] hash = hmacSha1(key, counterBytes);
        
        // 动态截断
        int offset = hash[hash.length - 1] & 0x0F;
        int binary = ((hash[offset] & 0x7F) << 24)
                | ((hash[offset + 1] & 0xFF) << 16)
                | ((hash[offset + 2] & 0xFF) << 8)
                | (hash[offset + 3] & 0xFF);
        
        // 取模得到 6 位数字
        int otp = binary % (int) Math.pow(10, PASSWORD_LENGTH);
        return String.format("%0" + PASSWORD_LENGTH + "d", otp);
    }
    
    private static byte[] hmacSha1(byte[] key, byte[] data) {
        try {
            Mac mac = Mac.getInstance(ALGORITHM);
            mac.init(new SecretKeySpec(key, ALGORITHM));
            return mac.doFinal(data);
        } catch (Exception e) {
            throw new RuntimeException("HMAC-SHA1 计算失败", e);
        }
    }
    
    private static byte[] base32Decode(String input) {
        // 简化的 Base32 解码实现
        // 实际应使用 Apache Commons Codec 或 Google Guava
        return new byte[0];
    }
    
    /**
     * 验证 TOTP
     * 考虑前后一个时间窗口的偏移，允许时间偏差
     */
    public static boolean verifyTotp(String secret, String token, long time) {
        // 检查前后各 1 个时间窗口
        for (int i = -1; i <= 1; i++) {
            String expected = generateTotp(secret, time + i * TIME_STEP);
            if (constantTimeEquals(token, expected)) {
                return true;
            }
        }
        return false;
    }
    
    // 常数时间比较，防止时序攻击
    private static boolean constantTimeEquals(String a, String b) {
        if (a.length() != b.length()) return false;
        int result = 0;
        for (int i = 0; i < a.length(); i++) {
            result |= a.charAt(i) ^ b.charAt(i);
        }
        return result == 0;
    }
}