Szenario 7.3: Dateiverschlüsselung

Dieses Szenario beschreibt die sichere Verschlüsselung von Dateien mit Post-Quantum-sicheren Verfahren. Die Implementierung unterstützt sowohl passwortbasierte als auch schlüsselbasierte Verschlüsselung.

Anwendungsfälle:

• Backup-Verschlüsselung
• Dokumentenschutz
• Konfigurationsdateien
• Archivierung

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;
 
using var ctx = PqCryptoContext.Initialize();
 
// Quelldatei
string inputFile = "document.pdf";
string outputFile = "document.pdf.enc";
string password = "MySecurePassword123!";
 
// Verschlüsselungsparameter
var salt = RandomNumberGenerator.GetBytes(32);
var nonce = RandomNumberGenerator.GetBytes(12);
 
// Schlüssel aus Passwort ableiten (Argon2id)
var key = ctx.DeriveKeyArgon2id(
    password: Encoding.UTF8.GetBytes(password),
    salt: salt,
    outputLength: 32,
    iterations: 3,
    memoryKiB: 65536,  // 64 MB
    parallelism: 4
);
 
// Datei einlesen
var plaintext = File.ReadAllBytes(inputFile);
 
// Verschlüsseln mit AES-256-GCM
var ciphertext = new byte[plaintext.Length];
var tag = new byte[16];
 
using var aes = new OpenSslAesGcm(key);
aes.Encrypt(nonce, plaintext, ciphertext, tag);
 
// Header + Ciphertext schreiben
using var output = File.Create(outputFile);
using var writer = new BinaryWriter(output);
 
// Magic number
writer.Write(Encoding.ASCII.GetBytes("PQENC"));
// Version
writer.Write((byte)1);
// Salt
writer.Write(salt.Length);
writer.Write(salt);
// Nonce
writer.Write(nonce.Length);
writer.Write(nonce);
// Tag
writer.Write(tag.Length);
writer.Write(tag);
// Ciphertext
writer.Write(ciphertext.Length);
writer.Write(ciphertext);
 
Console.WriteLine($"Datei verschlüsselt: {outputFile}");
Console.WriteLine($"  Original: {plaintext.Length:N0} Bytes");
Console.WriteLine($"  Verschlüsselt: {output.Length:N0} Bytes");

using var ctx = PqCryptoContext.Initialize();
 
string encryptedFile = "document.pdf.enc";
string outputFile = "document-decrypted.pdf";
string password = "MySecurePassword123!";
 
using var input = File.OpenRead(encryptedFile);
using var reader = new BinaryReader(input);
 
// Magic number prüfen
var magic = Encoding.ASCII.GetString(reader.ReadBytes(5));
if (magic != "PQENC")
    throw new InvalidDataException("Ungültiges Dateiformat");
 
// Version prüfen
var version = reader.ReadByte();
if (version != 1)
    throw new NotSupportedException($"Version {version} nicht unterstützt");
 
// Parameter lesen
var saltLen = reader.ReadInt32();
var salt = reader.ReadBytes(saltLen);
var nonceLen = reader.ReadInt32();
var nonce = reader.ReadBytes(nonceLen);
var tagLen = reader.ReadInt32();
var tag = reader.ReadBytes(tagLen);
var ciphertextLen = reader.ReadInt32();
var ciphertext = reader.ReadBytes(ciphertextLen);
 
// Schlüssel ableiten (gleiche Parameter!)
var key = ctx.DeriveKeyArgon2id(
    password: Encoding.UTF8.GetBytes(password),
    salt: salt,
    outputLength: 32,
    iterations: 3,
    memoryKiB: 65536,
    parallelism: 4
);
 
// Entschlüsseln
var plaintext = new byte[ciphertext.Length];
 
using var aes = new OpenSslAesGcm(key);
aes.Decrypt(nonce, ciphertext, tag, plaintext);
 
// Speichern
File.WriteAllBytes(outputFile, plaintext);
 
Console.WriteLine($"Datei entschlüsselt: {outputFile}");

public class FileEncryptor
{
    private const int ChunkSize = 64 * 1024;  // 64 KB chunks
 
    public void EncryptLargeFile(
        string inputPath,
        string outputPath,
        byte[] key)
    {
        using var ctx = PqCryptoContext.Initialize();
        using var input = File.OpenRead(inputPath);
        using var output = File.Create(outputPath);
        using var writer = new BinaryWriter(output);
 
        // Header schreiben
        writer.Write(Encoding.ASCII.GetBytes("PQENC"));
        writer.Write((byte)2);  // Version 2 = Streaming
 
        // Anzahl Chunks
        var totalChunks = (int)Math.Ceiling((double)input.Length / ChunkSize);
        writer.Write(totalChunks);
 
        var buffer = new byte[ChunkSize];
        var chunkIndex = 0;
 
        while (input.Position < input.Length)
        {
            var bytesRead = input.Read(buffer, 0, ChunkSize);
            var chunk = buffer.AsSpan(0, bytesRead).ToArray();
 
            // Eindeutige Nonce pro Chunk (ChunkIndex + Random)
            var nonce = new byte[12];
            BitConverter.GetBytes(chunkIndex).CopyTo(nonce, 0);
            RandomNumberGenerator.Fill(nonce.AsSpan(4));
 
            var ciphertext = new byte[bytesRead];
            var tag = new byte[16];
 
            using var aes = new OpenSslAesGcm(key);
            // AAD = Chunk-Index für Reihenfolge-Schutz
            var aad = BitConverter.GetBytes(chunkIndex);
            aes.Encrypt(nonce, chunk, ciphertext, tag, aad);
 
            // Chunk schreiben
            writer.Write(nonce);
            writer.Write(tag);
            writer.Write(ciphertext.Length);
            writer.Write(ciphertext);
 
            chunkIndex++;
        }
 
        Console.WriteLine($"Verschlüsselt: {chunkIndex} Chunks");
    }
 
    public void DecryptLargeFile(
        string inputPath,
        string outputPath,
        byte[] key)
    {
        using var input = File.OpenRead(inputPath);
        using var reader = new BinaryReader(input);
        using var output = File.Create(outputPath);
 
        // Header lesen
        var magic = Encoding.ASCII.GetString(reader.ReadBytes(5));
        if (magic != "PQENC") throw new InvalidDataException();
 
        var version = reader.ReadByte();
        if (version != 2) throw new NotSupportedException();
 
        var totalChunks = reader.ReadInt32();
 
        for (int i = 0; i < totalChunks; i++)
        {
            var nonce = reader.ReadBytes(12);
            var tag = reader.ReadBytes(16);
            var ciphertextLen = reader.ReadInt32();
            var ciphertext = reader.ReadBytes(ciphertextLen);
 
            var plaintext = new byte[ciphertextLen];
 
            using var aes = new OpenSslAesGcm(key);
            var aad = BitConverter.GetBytes(i);
            aes.Decrypt(nonce, ciphertext, tag, plaintext, aad);
 
            output.Write(plaintext);
        }
    }
}

public class HybridFileEncryptor
{
    public void EncryptForRecipient(
        string inputPath,
        string outputPath,
        byte[] recipientMlKemPublicKey)
    {
        using var ctx = PqCryptoContext.Initialize();
 
        // ML-KEM Key Encapsulation
        var pubKey = ctx.ImportPublicKey(recipientMlKemPublicKey);
        var (kemCiphertext, sharedSecret) = ctx.Encapsulate(pubKey);
 
        // File Encryption Key ableiten
        var fileKey = ctx.DeriveKey(
            sharedSecret,
            outputLength: 32,
            info: Encoding.UTF8.GetBytes("file-encryption-key")
        );
 
        // Datei verschlüsseln
        var encryptor = new FileEncryptor();
        var tempFile = Path.GetTempFileName();
        encryptor.EncryptLargeFile(inputPath, tempFile, fileKey);
 
        // Output mit KEM Ciphertext
        using var output = File.Create(outputPath);
        using var writer = new BinaryWriter(output);
 
        writer.Write(Encoding.ASCII.GetBytes("PQKEM"));
        writer.Write((byte)1);
        writer.Write(kemCiphertext.Length);
        writer.Write(kemCiphertext);
 
        // Verschlüsselte Datei anhängen
        using var encryptedContent = File.OpenRead(tempFile);
        encryptedContent.CopyTo(output);
 
        File.Delete(tempFile);
    }
}

PQENC v1 (Single-Shot):
┌─────────────────────────────────┐
│ Magic: "PQENC" (5 bytes)        │
│ Version: 0x01 (1 byte)          │
│ Salt Length (4 bytes)           │
│ Salt (variable)                 │
│ Nonce Length (4 bytes)          │
│ Nonce (12 bytes)                │
│ Tag Length (4 bytes)            │
│ Tag (16 bytes)                  │
│ Ciphertext Length (4 bytes)     │
│ Ciphertext (variable)           │
└─────────────────────────────────┘

PQENC v2 (Streaming):
┌─────────────────────────────────┐
│ Magic: "PQENC" (5 bytes)        │
│ Version: 0x02 (1 byte)          │
│ Total Chunks (4 bytes)          │
├─────────────────────────────────┤
│ Chunk 0:                        │
│   Nonce (12 bytes)              │
│   Tag (16 bytes)                │
│   Ciphertext Length (4 bytes)   │
│   Ciphertext (variable)         │
├─────────────────────────────────┤
│ Chunk 1: ...                    │
└─────────────────────────────────┘

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Wolfgang van der Stille @ EMSR DATA d.o.o. - Post-Quantum Cryptography Professional