Scenario 7.3: Crittografia file

Questo scenario descrive la cifratura sicura di file con metodi sicuri Post-Quantum. L'implementazione supporta sia la cifratura basata su password che quella basata su chiave.

Casi d'uso:

• Cifratura backup
• Protezione documenti
• File di configurazione
• Archiviazione

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;
 
using var ctx = PqCryptoContext.Initialize();
 
// File sorgente
string inputFile = "document.pdf";
string outputFile = "document.pdf.enc";
string password = "MySecurePassword123!";
 
// Parametri cifratura
var salt = RandomNumberGenerator.GetBytes(32);
var nonce = RandomNumberGenerator.GetBytes(12);
 
// Derivare chiave da password (Argon2id)
var key = ctx.DeriveKeyArgon2id(
    password: Encoding.UTF8.GetBytes(password),
    salt: salt,
    outputLength: 32,
    iterations: 3,
    memoryKiB: 65536,  // 64 MB
    parallelism: 4
);
 
// Leggere file
var plaintext = File.ReadAllBytes(inputFile);
 
// Cifrare con 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);
 
// Scrivere header + ciphertext
using var output = File.Create(outputFile);
using var writer = new BinaryWriter(output);
 
// Magic number
writer.Write(Encoding.ASCII.GetBytes("PQENC"));
// Versione
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($"File cifrato: {outputFile}");
Console.WriteLine($"  Originale: {plaintext.Length:N0} Bytes");
Console.WriteLine($"  Cifrato: {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);
 
// Verificare magic number
var magic = Encoding.ASCII.GetString(reader.ReadBytes(5));
if (magic != "PQENC")
    throw new InvalidDataException("Formato file non valido");
 
// Verificare versione
var version = reader.ReadByte();
if (version != 1)
    throw new NotSupportedException($"Versione {version} non supportata");
 
// Leggere parametri
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);
 
// Derivare chiave (stessi parametri!)
var key = ctx.DeriveKeyArgon2id(
    password: Encoding.UTF8.GetBytes(password),
    salt: salt,
    outputLength: 32,
    iterations: 3,
    memoryKiB: 65536,
    parallelism: 4
);
 
// Decifrare
var plaintext = new byte[ciphertext.Length];
 
using var aes = new OpenSslAesGcm(key);
aes.Decrypt(nonce, ciphertext, tag, plaintext);
 
// Salvare
File.WriteAllBytes(outputFile, plaintext);
 
Console.WriteLine($"File decifrato: {outputFile}");

public class FileEncryptor
{
    private const int ChunkSize = 64 * 1024;  // Chunk da 64 KB
 
    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);
 
        // Scrivere header
        writer.Write(Encoding.ASCII.GetBytes("PQENC"));
        writer.Write((byte)2);  // Versione 2 = Streaming
 
        // Numero di chunk
        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();
 
            // Nonce univoco per 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 per protezione ordine
            var aad = BitConverter.GetBytes(chunkIndex);
            aes.Encrypt(nonce, chunk, ciphertext, tag, aad);
 
            // Scrivere chunk
            writer.Write(nonce);
            writer.Write(tag);
            writer.Write(ciphertext.Length);
            writer.Write(ciphertext);
 
            chunkIndex++;
        }
 
        Console.WriteLine($"Cifrato: {chunkIndex} chunk");
    }
 
    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);
 
        // Leggere header
        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);
 
        // Derivare chiave cifratura file
        var fileKey = ctx.DeriveKey(
            sharedSecret,
            outputLength: 32,
            info: Encoding.UTF8.GetBytes("file-encryption-key")
        );
 
        // Cifrare file
        var encryptor = new FileEncryptor();
        var tempFile = Path.GetTempFileName();
        encryptor.EncryptLargeFile(inputPath, tempFile, fileKey);
 
        // Output con 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);
 
        // Allegare file cifrato
        using var encryptedContent = File.OpenRead(tempFile);
        encryptedContent.CopyTo(output);
 
        File.Delete(tempFile);
    }
}

PQENC v1 (Single-Shot):
+----------------------------------+
| Magic: "PQENC" (5 bytes)         |
| Versione: 0x01 (1 byte)          |
| Lunghezza Salt (4 bytes)         |
| Salt (variabile)                 |
| Lunghezza Nonce (4 bytes)        |
| Nonce (12 bytes)                 |
| Lunghezza Tag (4 bytes)          |
| Tag (16 bytes)                   |
| Lunghezza Ciphertext (4 bytes)   |
| Ciphertext (variabile)           |
+----------------------------------+

PQENC v2 (Streaming):
+----------------------------------+
| Magic: "PQENC" (5 bytes)         |
| Versione: 0x02 (1 byte)          |
| Totale Chunk (4 bytes)           |
+----------------------------------+
| Chunk 0:                         |
|   Nonce (12 bytes)               |
|   Tag (16 bytes)                 |
|   Lunghezza Ciphertext (4 bytes) |
|   Ciphertext (variabile)         |
+----------------------------------+
| Chunk 1: ...                     |
+----------------------------------+

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