C# 敏感資訊遮罩工具：手機號、身分證、信用卡號通用實作

常見遮罩規則

通用遮罩引擎

using System;
using System.Text.RegularExpressions;

public static class MaskHelper
{
    /// 

    /// 依規則遮罩字串
    /// 
    /// 原始字串
    /// 保留前 N 碼
    /// 保留後 N 碼
    /// 遮罩字元 (預設 '*')
    /// 最小遮罩長度 (不足則不遮罩)
    public static string Mask(string input, int keepStart, int keepEnd, char maskChar = '*', int minMask = 1)
    {
        if (string.IsNullOrEmpty(input)) return input;
        int len = input.Length;
        int maskLen = len - keepStart - keepEnd;
        if (maskLen < minMask) return input; // 太短不遮罩

        var sb = new StringBuilder(len);
        sb.Append(input.Substring(0, keepStart));
        sb.Append(maskChar, maskLen);
        sb.Append(input.Substring(len - keepEnd));
        return sb.ToString();
    }

    /// 

    /// 手機號遮罩 (自動偵測台/陸)
    /// 
    public static string MaskPhone(string phone)
    {
        if (string.IsNullOrEmpty(phone)) return phone;
        phone = Regex.Replace(phone, @"\D", ""); // 移除非數字
        if (phone.StartsWith("09") && phone.Length == 10) // 台灣
            return Mask(phone, 3, 3);
        if (phone.StartsWith("1") && phone.Length == 11) // 中國大陸
            return Mask(phone, 3, 4);
        return phone; // 無法識別格式
    }

    /// 

    /// Email 遮罩
    /// 
    public static string MaskEmail(string email)
    {
        if (string.IsNullOrEmpty(email)) return email;
        int at = email.IndexOf('@');
        if (at <= 1) return email;
        return email[0] + new string('*', at - 1) + email.Substring(at);
    }

    /// 

    /// 信用卡遮罩 (保留前4後4，每4碼空格)
    /// 
    public static string MaskCreditCard(string card)
    {
        if (string.IsNullOrEmpty(card)) return card;
        card = Regex.Replace(card, @"\D", "");
        if (card.Length != 16) return card;
        return $"{card.Substring(0,4)} **** **** {card.Substring(12,4)}";
    }
}

資料庫層遮罩 (SQL Server)

-- 檢視表/函數遮罩
CREATE FUNCTION dbo.MaskPhone(@phone VARCHAR(20))
RETURNS VARCHAR(20)
AS BEGIN
    DECLARE @clean VARCHAR(20) = REPLACE(REPLACE(REPLACE(@phone, '-', ''), ' ', ''), '+886', '0');
    IF LEN(@clean) = 10 AND @clean LIKE '09%'
        RETURN LEFT(@clean, 3) + REPLICATE('*', 4) + RIGHT(@clean, 3);
    IF LEN(@clean) = 11 AND @clean LIKE '1%'
        RETURN LEFT(@clean, 3) + REPLICATE('*', 4) + RIGHT(@clean, 4);
    RETURN @clean;
END

進階：屬性標記自動遮罩 (ASP.NET Core / DTO)

[AttributeUsage(AttributeTargets.Property)]
public class MaskAttribute : Attribute
{
    public int KeepStart { get; set; }
    public int KeepEnd { get; set; }
    public char MaskChar { get; set; } = '*';
}

public class UserDto
{
    public string Name { get; set; }
    
    [Mask(KeepStart = 3, KeepEnd = 3)]
    public string Phone { get; set; }

    [Mask(KeepStart = 1, KeepEnd = 0)] // Email 特殊處理
    public string Email { get; set; }
}

// 序列化時自動遮罩 (System.Text.Json)
public class MaskJsonConverter : JsonConverter
{
    public override string Read(...) => throw new NotSupportedException();
    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
    {
        var prop = options.GetPropertyMetadata(); // 需自行實作取得屬性
        if (prop?.GetCustomAttribute() is { } attr)
            value = MaskHelper.Mask(value, attr.KeepStart, attr.KeepEnd, attr.MaskChar);
        writer.WriteStringValue(value);
    }
}

總結

敏感資訊遮罩核心在於 **`KeepStart` + `KeepEnd` + `MaskChar` 參數化**，配合 **Regex 正規化輸入**、**最小遮罩長度保護**、**屬性標記自動化**。前端/後端/資料庫三層同步實施，並建立**遮罩規則表**統一管理，即可滿足個資法 (GDPR/PDPA) 合規與資安稽核需求。