~~NOTOC~~ ====== Scenario 7.3: File Encryption ====== **Category:** [[.:start|Encryption]] \\ **Complexity:** *** (Medium) \\ **Prerequisites:** Key material or password \\ **Estimated Time:** 15-20 minutes ---- ===== Description ===== This scenario describes **secure file encryption** with Post-Quantum secure methods. The implementation supports both password-based and key-based encryption. **Use cases:** * Backup encryption * Document protection * Configuration files * Archiving ---- ===== Workflow ===== flowchart TD FILE[Original file] --> COMPRESS[Optional: Compress] COMPRESS --> CHUNK[Split into chunks] CHUNK --> ENCRYPT[AES-256-GCM per chunk] subgraph Key Derivation PWD[Password] --> KDF[Argon2id] KDF --> KEY[Encryption Key] end KEY --> ENCRYPT ENCRYPT --> OUTPUT[Encrypted file] style KEY fill:#e8f5e9 style OUTPUT fill:#e3f2fd ---- ===== Code Example: Encrypt File =====


using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;

using var ctx = PqCryptoContext.Initialize();

// Source file
string inputFile = "document.pdf";
string outputFile = "document.pdf.enc";
string password = "MySecurePassword123!";

// Encryption parameters
var salt = RandomNumberGenerator.GetBytes(32);
var nonce = RandomNumberGenerator.GetBytes(12);

// Derive key from password (Argon2id)
var key = ctx.DeriveKeyArgon2id(
    password: Encoding.UTF8.GetBytes(password),
    salt: salt,
    outputLength: 32,
    iterations: 3,
    memoryKiB: 65536,  // 64 MB
    parallelism: 4
);

// Read file
var plaintext = File.ReadAllBytes(inputFile);

// Encrypt with 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);

// Write header + ciphertext
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($"File encrypted: {outputFile}");
Console.WriteLine($"  Original: {plaintext.Length:N0} bytes");
Console.WriteLine($"  Encrypted: {output.Length:N0} bytes");

---- ===== Code Example: Decrypt File =====


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);

// Check magic number
var magic = Encoding.ASCII.GetString(reader.ReadBytes(5));
if (magic != "PQENC")
    throw new InvalidDataException("Invalid file format");

// Check version
var version = reader.ReadByte();
if (version != 1)
    throw new NotSupportedException($"Version {version} not supported");

// Read parameters
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);

// Derive key (same parameters!)
var key = ctx.DeriveKeyArgon2id(
    password: Encoding.UTF8.GetBytes(password),
    salt: salt,
    outputLength: 32,
    iterations: 3,
    memoryKiB: 65536,
    parallelism: 4
);

// Decrypt
var plaintext = new byte[ciphertext.Length];

using var aes = new OpenSslAesGcm(key);
aes.Decrypt(nonce, ciphertext, tag, plaintext);

// Save
File.WriteAllBytes(outputFile, plaintext);

Console.WriteLine($"File decrypted: {outputFile}");

---- ===== Streaming for Large Files =====


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);

        // Write header
        writer.Write(Encoding.ASCII.GetBytes("PQENC"));
        writer.Write((byte)2);  // Version 2 = Streaming

        // Number of 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();

            // Unique nonce 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 for order protection
            var aad = BitConverter.GetBytes(chunkIndex);
            aes.Encrypt(nonce, chunk, ciphertext, tag, aad);

            // Write chunk
            writer.Write(nonce);
            writer.Write(tag);
            writer.Write(ciphertext.Length);
            writer.Write(ciphertext);

            chunkIndex++;
        }

        Console.WriteLine($"Encrypted: {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);

        // Read 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);
        }
    }
}

---- ===== Hybrid Encryption with ML-KEM =====


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);

        // Derive file encryption key
        var fileKey = ctx.DeriveKey(
            sharedSecret,
            outputLength: 32,
            info: Encoding.UTF8.GetBytes("file-encryption-key")
        );

        // Encrypt file
        var encryptor = new FileEncryptor();
        var tempFile = Path.GetTempFileName();
        encryptor.EncryptLargeFile(inputPath, tempFile, fileKey);

        // Output with 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);

        // Append encrypted file
        using var encryptedContent = File.OpenRead(tempFile);
        encryptedContent.CopyTo(output);

        File.Delete(tempFile);
    }
}

---- ===== File Format Specification =====


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: ...                    |
+---------------------------------+

---- ===== Related Scenarios ===== ^ Relationship ^ Scenario ^ Description ^ | **Component** | [[.:key_encapsulation|7.2 Key Encapsulation]] | ML-KEM for recipients | | **Related** | [[.:hybrid_encryption|7.1 Hybrid Encryption]] | Hybrid concept | | **Related** | [[en:int:pqcrypt:szenarien:verwaltung:backup|4.4 Backup]] | Backup encryption | ---- << [[.:key_encapsulation|<- 7.2 Key Encapsulation]] | [[.:start|^ Encryption Overview]] | [[en:int:pqcrypt:szenarien:start|-> All Scenarios]] >> {{tag>scenario encryption file aes-gcm argon2id}} ---- //Wolfgang van der Stille @ EMSR DATA d.o.o. - Post-Quantum Cryptography Professional//