ถอดรหัส Base64 ใน C# — คู่มือ

using System;
using System.Text;

// Connection string เก็บเป็น Base64 ใน Kubernetes secret
string encoded = "cmVkaXM6Ly9jYWNoZS1wcm9kLmludGVybmFsOjYzNzkvc2Vzc2lvbi1zdG9yZQ==";

byte[] decodedBytes = Convert.FromBase64String(encoded);
string connectionString = Encoding.UTF8.GetString(decodedBytes);

Console.WriteLine(connectionString);
// redis://cache-prod.internal:6379/session-store

using System;
using System.Text;

string original = "postgres://db-admin:Kx8!mQ@db-prod.us-east-1.internal:5432/orders";

// Encode
string encoded = Convert.ToBase64String(Encoding.UTF8.GetBytes(original));
Console.WriteLine(encoded);
// cG9zdGdyZXM6Ly9kYi1hZG1pbjpLeDghbVFAZGItcHJvZC51cy1lYXN0LTEuaW50ZXJuYWw6NTQzMi9vcmRlcnM=

// Decode
byte[] decoded = Convert.FromBase64String(encoded);
string recovered = Encoding.UTF8.GetString(decoded);

Console.WriteLine(recovered == original); // True

using System;
using System.Text;

// Bytes เดียวกัน, encoding ต่างกัน — ผลลัพธ์ต่างกัน
byte[] decoded = Convert.FromBase64String("w7bDvMOk");

Console.WriteLine(Encoding.UTF8.GetString(decoded));    // öüä  (ถูกต้อง)
Console.WriteLine(Encoding.ASCII.GetString(decoded));   // ??????  (สูญหาย)
Console.WriteLine(Encoding.Latin1.GetString(decoded));  // öüä  (mojibake)

using System;
using System.Text;

static class Base64Helper
{
    public static string? DecodeToString(string encoded)
    {
        if (string.IsNullOrWhiteSpace(encoded))
            return null;

        // ลบ whitespace ที่ decoder ของ .NET ปฏิเสธ
        string cleaned = encoded
            .Replace("
", "")
            .Replace("
", "")
            .Replace(" ", "")
            .Trim();

        try
        {
            byte[] bytes = Convert.FromBase64String(cleaned);
            return Encoding.UTF8.GetString(bytes);
        }
        catch (FormatException)
        {
            return null; // หรือโยน domain-specific exception
        }
    }
}

// การใช้งาน
string? decoded = Base64Helper.DecodeToString("  cmVkaXM6Ly9jYWNoZQ==  \n");
Console.WriteLine(decoded); // redis://cache

using System;

// บาง API ส่ง GUID เป็น Base64 ขนาด 22 ตัวอักษรแทน hex string 36 ตัวอักษร
string compactGuid = "C0HqetxMckKlZw4CssPUeQ==";
byte[] guidBytes = Convert.FromBase64String(compactGuid);
Guid recovered = new Guid(guidBytes);

Console.WriteLine(recovered);
// 7aea41c0-4cdc-4272-a567-0e02b2c3d479

using System;
using System.Text;
using System.Text.Json;

// JSON payload ที่ encode เป็น Base64 จาก message queue
string encoded = "eyJzZXJ2aWNlIjoicGF5bWVudC1nYXRld2F5IiwicmVnaW9uIjoiZXUtd2VzdC0xIiwicmVwbGljYXMiOjR9";

byte[] jsonBytes = Convert.FromBase64String(encoded);
string json = Encoding.UTF8.GetString(jsonBytes);
Console.WriteLine(json);
// {"service":"payment-gateway","region":"eu-west-1","replicas":4}

// Deserialize เป็น record
var deployEvent = JsonSerializer.Deserialize<DeployEvent>(jsonBytes);
Console.WriteLine($"{deployEvent!.Service} in {deployEvent.Region}");
// payment-gateway in eu-west-1

record DeployEvent(string Service, string Region, int Replicas);

using System;

// SHA-256 hash เก็บเป็น Base64
string hashBase64 = "n4bQgYhMfWWaL+qgxVrQFaO/TxsrC4Is0V1sFbDwCgg=";
byte[] hashBytes = Convert.FromBase64String(hashBase64);
string hex = Convert.ToHexString(hashBytes);

Console.WriteLine(hex);
// 9F86D081884C7D659A2FEAA0C55AD015A3BF4F1B2B0B822CD15D6C15B0F00A08

using System;
using System.Text;

string userInput = "eyJob3N0IjoiMTAuMC4xLjUwIiwicG9ydCI6ODQ0M30=";

// Stack-allocate สำหรับ payload ขนาดเล็ก (< 1KB เป็น rule of thumb ที่ดี)
Span<byte> buffer = stackalloc byte[256];

if (Convert.TryFromBase64String(userInput, buffer, out int bytesWritten))
{
    string result = Encoding.UTF8.GetString(buffer[..bytesWritten]);
    Console.WriteLine(result);
    // {"host":"10.0.1.50","port":8443}
}
else
{
    Console.WriteLine("Invalid Base64 input — skipping");
}

using System;

static bool IsValidBase64(string input)
{
    // คำนวณขนาดสูงสุดที่ decode แล้ว
    Span<byte> buffer = stackalloc byte[((input.Length + 3) / 4) * 3];
    return Convert.TryFromBase64String(input, buffer, out _);
}

// การใช้งานใน API controller
Console.WriteLine(IsValidBase64("eyJob3N0IjoiMTAuMC4xLjUwIn0=")); // True
Console.WriteLine(IsValidBase64("not!!valid!!base64"));              // False
Console.WriteLine(IsValidBase64(""));                                // True (ว่างคือ valid)

using System;
using System.IO;

string inputPath = "tls-cert.pem.b64";
string outputPath = "tls-cert.pem";

try
{
    string encoded = File.ReadAllText(inputPath).Trim();
    // ลบ line break — decoder ของ .NET ปฏิเสธ
    encoded = encoded.Replace("
", "").Replace("
", "");

    byte[] decoded = Convert.FromBase64String(encoded);
    File.WriteAllBytes(outputPath, decoded);

    Console.WriteLine($"Decoded {decoded.Length} bytes -> {outputPath}");
}
catch (IOException ex)
{
    Console.Error.WriteLine($"File error: {ex.Message}");
}
catch (FormatException ex)
{
    Console.Error.WriteLine($"Invalid Base64: {ex.Message}");
}

using System;
using System.Net.Http;
using System.Text;
using System.Text.Json;
using System.Threading.Tasks;

var client = new HttpClient();
client.DefaultRequestHeaders.Add("Authorization", "Bearer sk-prod-9f8e7d6c");

try
{
    var response = await client.GetAsync("https://api.example.com/secrets/db-password");
    response.EnsureSuccessStatusCode();

    string body = await response.Content.ReadAsStringAsync();

    // API ส่งกลับ: {"name":"db-password","value":"cG9zdGdyZXM6eGs5bVAycVI=","version":3}
    using var doc = JsonDocument.Parse(body);
    string encodedValue = doc.RootElement.GetProperty("value").GetString()!;

    byte[] decoded = Convert.FromBase64String(encodedValue);
    string secret = Encoding.UTF8.GetString(decoded);

    Console.WriteLine($"Secret: {secret}");
    // Secret: postgres:xk9mP2qR
}
catch (HttpRequestException ex)
{
    Console.Error.WriteLine($"HTTP error: {ex.Message}");
}
catch (FormatException ex)
{
    Console.Error.WriteLine($"Base64 decode failed: {ex.Message}");
}

# PowerShell (ติดตั้งมาใน Windows, ใช้ได้บน Linux/macOS ด้วย)
[System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String("eyJob3N0IjoiMTAuMC4xLjUwIn0="))
# {"host":"10.0.1.50"}

# คำสั่ง base64 ของ Linux / macOS
echo "eyJob3N0IjoiMTAuMC4xLjUwIn0=" | base64 --decode
# {"host":"10.0.1.50"}

# macOS ใช้ -D แทน --decode
echo "eyJob3N0IjoiMTAuMC4xLjUwIn0=" | base64 -D

# dotnet-script (ติดตั้ง: dotnet tool install -g dotnet-script)
echo 'Console.WriteLine(System.Text.Encoding.UTF8.GetString(Convert.FromBase64String("eyJob3N0IjoiMTAuMC4xLjUwIn0=")));' | dotnet-script eval

# Decode และ pretty-print JSON ด้วย jq
echo "eyJob3N0IjoiMTAuMC4xLjUwIiwicG9ydCI6ODQ0M30=" | base64 --decode | jq .

using System;
using System.Buffers.Text;
using System.Text;

// จำลอง raw UTF-8 bytes จาก HTTP request body
byte[] utf8Input = Encoding.UTF8.GetBytes("eyJob3N0IjoiMTAuMC4xLjUwIiwicG9ydCI6ODQ0M30=");

byte[] output = new byte[Base64.GetMaxDecodedFromUtf8Length(utf8Input.Length)];

OperationStatus status = Base64.DecodeFromUtf8(
    utf8Input, output, out int bytesConsumed, out int bytesWritten);

if (status == OperationStatus.Done)
{
    string result = Encoding.UTF8.GetString(output.AsSpan(0, bytesWritten));
    Console.WriteLine(result);
    // {"host":"10.0.1.50","port":8443}
    Console.WriteLine($"Consumed: {bytesConsumed}, Written: {bytesWritten}");
    // Consumed: 44, Written: 31
}
else
{
    Console.WriteLine($"Decode failed: {status}");
    // ค่าที่เป็นไปได้: InvalidData, DestinationTooSmall, NeedMoreData
}

using System;
using System.Buffers.Text;
using System.Text;

// input buffer จะถูกเขียนทับด้วย decoded bytes
byte[] data = Encoding.UTF8.GetBytes("c2VydmVyLWNvbmZpZw==");

OperationStatus status = Base64.DecodeFromUtf8InPlace(data, out int bytesWritten);

if (status == OperationStatus.Done)
{
    Console.WriteLine(Encoding.UTF8.GetString(data.AsSpan(0, bytesWritten)));
    // server-config
}

using System;
using System.Text;
using Spectre.Console;
using Spectre.Console.Json;

string encoded = "eyJob3N0IjoiMTAuMC4xLjUwIiwicG9ydCI6ODQ0MywibWF4Q29ubiI6MTAwfQ==";
byte[] decoded = Convert.FromBase64String(encoded);
string json = Encoding.UTF8.GetString(decoded);

// Pretty-print พร้อม syntax highlighting ใน terminal
AnsiConsole.Write(new JsonText(json));
// แสดง JSON สีสัน:
// {
//   "host": "10.0.1.50",
//   "port": 8443,
//   "maxConn": 100
// }

using System.IO;
using System.Security.Cryptography;

string inputPath = "database-export.sql.b64";
string outputPath = "database-export.sql";

using var inputStream = new FileStream(inputPath, FileMode.Open, FileAccess.Read);
using var transform = new FromBase64Transform(FromBase64TransformMode.IgnoreWhiteSpaces);
using var base64Stream = new CryptoStream(inputStream, transform, CryptoStreamMode.Read);
using var outputStream = new FileStream(outputPath, FileMode.Create, FileAccess.Write);

base64Stream.CopyTo(outputStream);

Console.WriteLine($"Decoded {new FileInfo(outputPath).Length} bytes -> {outputPath}");

using System.IO;
using System.Security.Cryptography;

async Task DecodeStreamAsync(Stream inputStream, string outputPath)
{
    using var transform = new FromBase64Transform(FromBase64TransformMode.IgnoreWhiteSpaces);
    using var base64Stream = new CryptoStream(inputStream, transform, CryptoStreamMode.Read);
    await using var outputStream = new FileStream(
        outputPath, FileMode.Create, FileAccess.Write, FileShare.None,
        bufferSize: 81920, useAsync: true);

    await base64Stream.CopyToAsync(outputStream);
}

// การใช้งานใน endpoint:
// await DecodeStreamAsync(Request.Body, "uploaded-file.bin");

using System;
using System.Text;

static string DecodeJwtPayload(string token)
{
    string[] parts = token.Split('.');
    if (parts.Length != 3)
        throw new ArgumentException($"Invalid JWT: expected 3 segments, got {parts.Length}");

    // นำ payload (segment ที่สอง)
    string payload = parts[1];

    // แทนตัวอักษร URL-safe ด้วย Base64 มาตรฐาน
    payload = payload.Replace('-', '+').Replace('_', '/');

    // เติม padding ให้เป็นจำนวนคูณ 4
    switch (payload.Length % 4)
    {
        case 2: payload += "=="; break;
        case 3: payload += "=";  break;
    }

    byte[] bytes = Convert.FromBase64String(payload);
    return Encoding.UTF8.GetString(bytes);
}

// ทดสอบด้วย token จริง
string token = "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9"
    + ".eyJzdWIiOiJ1c3ItNjcyIiwiaXNzIjoiYXV0aC5leGFtcGxlLmNvbSIsImV4cCI6MTc0MTk1NjgwMCwicm9sZXMiOlsiYWRtaW4iLCJiaWxsaW5nIl19"
    + ".SIGNATURE_PLACEHOLDER";

Console.WriteLine(DecodeJwtPayload(token));
// {"sub":"usr-672","iss":"auth.example.com","exp":1741956800,"roles":["admin","billing"]}

using System;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;

record JwtPayload(
    [property: JsonPropertyName("sub")] string Subject,
    [property: JsonPropertyName("iss")] string Issuer,
    [property: JsonPropertyName("exp")] long Expiration,
    [property: JsonPropertyName("roles")] string[] Roles
);

// หลังจาก DecodeJwtPayload() ด้านบน
string json = DecodeJwtPayload(token);
var claims = JsonSerializer.Deserialize<JwtPayload>(json)!;

Console.WriteLine($"User: {claims.Subject}");
Console.WriteLine($"Issuer: {claims.Issuer}");
Console.WriteLine($"Expires: {DateTimeOffset.FromUnixTimeSeconds(claims.Expiration):u}");
Console.WriteLine($"Roles: {string.Join(", ", claims.Roles)}");
// User: usr-672
// Issuer: auth.example.com
// Expires: 2025-03-14 12:00:00Z
// Roles: admin, billing