~~NOTOC~~
====== Scenario 7.1: Hybrid Encryption ======

<WRAP round info>
**Category:** [[.:start|Encryption]] \\
**Complexity:** **** (High) \\
**Prerequisites:** ML-KEM key pair, classical key pair \\
**Estimated Time:** 20-30 minutes
</WRAP>

----

===== Description =====

This scenario describes **hybrid encryption** with classical (RSA/ECDH) and Post-Quantum (ML-KEM) algorithms. Hybrid encryption combines both methods to protect against both classical and quantum attacks.

**Concept:**
  * **Classical key** - Protection against current threats
  * **PQ key** - Protection against future quantum computers
  * **Combined key** - Security as long as one of them is secure

----

===== Workflow =====

<mermaid>
flowchart LR
    subgraph Sender
        DATA[Data] --> SYM[AES-256-GCM]
        KEY1[RSA/ECDH Key Exchange] --> KDF
        KEY2[ML-KEM Key Encapsulation] --> KDF
        KDF[HKDF Combine] --> SYM
    end

    subgraph Recipient
        SYM2[AES-256-GCM Decrypt]
        KDF2[HKDF Combine] --> SYM2
        DEC1[RSA/ECDH Decrypt] --> KDF2
        DEC2[ML-KEM Decapsulate] --> KDF2
    end

    SYM --> |Ciphertext| SYM2

    style KDF fill:#e8f5e9
    style KDF2 fill:#e8f5e9
</mermaid>

----

===== Code Example (C#) =====

<code csharp>
using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;

using var ctx = PqCryptoContext.Initialize();

// Recipient keys (both algorithms)
var recipientEcdh = ECDiffieHellman.Create(ECCurve.NamedCurves.nistP384);
var recipientMlKem = ctx.GenerateKeyPair(PqAlgorithm.MlKem768);

// === ENCRYPTION ===

// 1. Classical key exchange (ECDH)
using var senderEcdh = ECDiffieHellman.Create(ECCurve.NamedCurves.nistP384);
var ecdhSharedSecret = senderEcdh.DeriveKeyMaterial(recipientEcdh.PublicKey);

// 2. PQ key encapsulation (ML-KEM)
var (mlKemCiphertext, mlKemSharedSecret) = ctx.Encapsulate(recipientMlKem.PublicKey);

// 3. Combine keys via HKDF
var combinedSecret = CombineSecrets(ecdhSharedSecret, mlKemSharedSecret);
var encryptionKey = ctx.DeriveKey(
    combinedSecret,
    outputLength: 32,
    salt: null,
    info: Encoding.UTF8.GetBytes("hybrid-encryption-v1")
);

// 4. Encrypt data
var plaintext = Encoding.UTF8.GetBytes("Secret message");
var nonce = RandomNumberGenerator.GetBytes(12);
var ciphertext = new byte[plaintext.Length];
var tag = new byte[16];

using var aes = new OpenSslAesGcm(encryptionKey);
aes.Encrypt(nonce, plaintext, ciphertext, tag);

// 5. Assemble encrypted message
var encryptedMessage = new HybridEncryptedMessage
{
    EcdhPublicKey = senderEcdh.PublicKey.ExportSubjectPublicKeyInfo(),
    MlKemCiphertext = mlKemCiphertext,
    Nonce = nonce,
    Ciphertext = ciphertext,
    Tag = tag
};

Console.WriteLine($"Encrypted: {encryptedMessage.Ciphertext.Length} bytes");
</code>

----

===== Decryption =====

<code csharp>
// === DECRYPTION ===

using var ctx = PqCryptoContext.Initialize();

// Load recipient private keys
var recipientEcdh = ctx.LoadEcdhPrivateKey("recipient-ecdh.key.pem", "Password!");
var recipientMlKem = ctx.LoadPrivateKey("recipient-mlkem.key.pem", "Password!");

// 1. Classical key exchange
var senderEcdhPubKey = ECDiffieHellman.Create();
senderEcdhPubKey.ImportSubjectPublicKeyInfo(encryptedMessage.EcdhPublicKey, out _);
var ecdhSharedSecret = recipientEcdh.DeriveKeyMaterial(senderEcdhPubKey.PublicKey);

// 2. PQ key decapsulation
var mlKemSharedSecret = ctx.Decapsulate(recipientMlKem, encryptedMessage.MlKemCiphertext);

// 3. Combine keys
var combinedSecret = CombineSecrets(ecdhSharedSecret, mlKemSharedSecret);
var decryptionKey = ctx.DeriveKey(
    combinedSecret,
    outputLength: 32,
    salt: null,
    info: Encoding.UTF8.GetBytes("hybrid-encryption-v1")
);

// 4. Decrypt data
var decrypted = new byte[encryptedMessage.Ciphertext.Length];

using var aes = new OpenSslAesGcm(decryptionKey);
aes.Decrypt(
    encryptedMessage.Nonce,
    encryptedMessage.Ciphertext,
    encryptedMessage.Tag,
    decrypted
);

var message = Encoding.UTF8.GetString(decrypted);
Console.WriteLine($"Decrypted: {message}");
</code>

----

===== Secret Combination (HKDF) =====

<code csharp>
private static byte[] CombineSecrets(byte[] secret1, byte[] secret2)
{
    // Concat and hash - simple but secure method
    // Alternative: Parallel HKDF and XOR
    var combined = new byte[secret1.Length + secret2.Length];
    Buffer.BlockCopy(secret1, 0, combined, 0, secret1.Length);
    Buffer.BlockCopy(secret2, 0, combined, secret1.Length, secret2.Length);

    // HKDF Extract
    return HKDF.Extract(HashAlgorithmName.SHA256, combined);
}
</code>

----

===== Message Format =====

<code csharp>
public class HybridEncryptedMessage
{
    // Header
    public string Version { get; set; } = "1.0";
    public string Algorithm { get; set; } = "ECDH-P384+ML-KEM-768/AES-256-GCM";

    // Key Encapsulation
    public byte[] EcdhPublicKey { get; set; }   // ECDH ephemeral public key
    public byte[] MlKemCiphertext { get; set; } // ML-KEM ciphertext

    // Encrypted Content
    public byte[] Nonce { get; set; }           // 12 bytes
    public byte[] Ciphertext { get; set; }      // Variable
    public byte[] Tag { get; set; }             // 16 bytes

    // Serialization
    public byte[] Serialize()
    {
        using var ms = new MemoryStream();
        using var writer = new BinaryWriter(ms);

        writer.Write(Version);
        writer.Write(Algorithm);
        writer.Write(EcdhPublicKey.Length);
        writer.Write(EcdhPublicKey);
        writer.Write(MlKemCiphertext.Length);
        writer.Write(MlKemCiphertext);
        writer.Write(Nonce.Length);
        writer.Write(Nonce);
        writer.Write(Ciphertext.Length);
        writer.Write(Ciphertext);
        writer.Write(Tag.Length);
        writer.Write(Tag);

        return ms.ToArray();
    }
}
</code>

----

===== Algorithm Combinations =====

^  Combination  ^  Classical  ^  PQ  ^  Security Level  ^
| Standard | ECDH P-384 | ML-KEM-768 | 192-bit hybrid |
| High Security | ECDH P-521 | ML-KEM-1024 | 256-bit hybrid |
| Legacy Support | RSA-4096 + ECDH P-256 | ML-KEM-512 | 128-bit hybrid |
| Minimal | X25519 | ML-KEM-512 | 128-bit hybrid |

----

===== Industry-Specific Requirements =====

^  Industry  ^  Min. Security  ^  Recommended Combination  ^
| **Financial Sector** | 192-bit | ECDH P-384 + ML-KEM-768 |
| **Healthcare** | 128-bit | ECDH P-256 + ML-KEM-512 |
| **Government** | 256-bit | ECDH P-521 + ML-KEM-1024 |
| **Energy** | 192-bit | ECDH P-384 + ML-KEM-768 |

----

===== Related Scenarios =====

^  Relationship  ^  Scenario  ^  Description  ^
| **Component** | [[.:key_encapsulation|7.2 Key Encapsulation]] | ML-KEM details |
| **Application** | [[.:file_encryption|7.3 File Encryption]] | Practical usage |
| **Related** | [[en:int:pqcrypt:szenarien:schluessel:generierung|11.1 Key Generation]] | Create keys |

----

<< [[.:start|<- Encryption Overview]] | [[en:int:pqcrypt:szenarien:start|^ Scenarios]] | [[.:key_encapsulation|7.2 Key Encapsulation ->]] >>

{{tag>scenario encryption hybrid ml-kem ecdh}}

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