~~NOTOC~~
====== Scenario 7.2: Key Encapsulation (ML-KEM) ======

<WRAP round info>
**Category:** [[.:start|Encryption]] \\
**Complexity:** *** (Medium) \\
**Prerequisites:** ML-KEM key pair \\
**Estimated Time:** 15-20 minutes
</WRAP>

----

===== Description =====

This scenario describes **Key Encapsulation** with ML-KEM (Module-Lattice-based Key Encapsulation Mechanism, NIST FIPS 203). ML-KEM is a Post-Quantum Key Encapsulation Mechanism (KEM) that is secure against quantum computer attacks.

**KEM vs. Key Exchange:**
^  Aspect  ^  Key Exchange (DH)  ^  Key Encapsulation (KEM)  ^
| Interactivity | Both parties active | Only sender active |
| Key generation | Jointly computed | Randomly generated |
| Application | TLS handshake | Encryption, hybrid TLS |

----

===== ML-KEM Parameter Sets =====

^  Parameter  ^  Security  ^  Public Key  ^  Ciphertext  ^  Shared Secret  ^
| **ML-KEM-512** | 128-bit | 800 bytes | 768 bytes | 32 bytes |
| **ML-KEM-768** | 192-bit | 1,184 bytes | 1,088 bytes | 32 bytes |
| **ML-KEM-1024** | 256-bit | 1,568 bytes | 1,568 bytes | 32 bytes |

<WRAP round tip>
**Recommendation:** ML-KEM-768 for most applications (BSI recommends at least 192-bit).
</WRAP>

----

===== Workflow =====

<mermaid>
flowchart LR
    subgraph Recipient
        GEN[KeyGen] --> PK[Public Key]
        GEN --> SK[Secret Key]
    end

    subgraph Sender
        PK --> ENC[Encapsulate]
        ENC --> CT[Ciphertext]
        ENC --> SS1[Shared Secret]
    end

    subgraph Recipient2[Recipient]
        CT --> DEC[Decapsulate]
        SK --> DEC
        DEC --> SS2[Shared Secret]
    end

    SS1 -.->|equal| SS2

    style SS1 fill:#e8f5e9
    style SS2 fill:#e8f5e9
</mermaid>

----

===== Code Example: Generate Key Pair =====

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

using var ctx = PqCryptoContext.Initialize();

// Generate ML-KEM-768 key pair
using var mlKemKeyPair = ctx.GenerateKeyPair(PqAlgorithm.MlKem768);

// Export public key (for sender)
var publicKeyPem = mlKemKeyPair.PublicKey.ToPem();
File.WriteAllText("recipient-mlkem.pub.pem", publicKeyPem);

// Save private key encrypted
mlKemKeyPair.ToEncryptedPemFile("recipient-mlkem.key.pem", "SecurePassword!");

Console.WriteLine("ML-KEM-768 key pair generated:");
Console.WriteLine($"  Public Key: {mlKemKeyPair.PublicKey.GetRawData().Length} bytes");
Console.WriteLine($"  Algorithm OID: {mlKemKeyPair.PublicKey.Oid.Value}");
</code>

----

===== Code Example: Encapsulation (Sender) =====

<code csharp>
using var ctx = PqCryptoContext.Initialize();

// Load recipient public key
var recipientPublicKey = ctx.LoadPublicKey("recipient-mlkem.pub.pem");

// Perform key encapsulation
var (ciphertext, sharedSecret) = ctx.Encapsulate(recipientPublicKey);

Console.WriteLine($"Encapsulation completed:");
Console.WriteLine($"  Ciphertext: {ciphertext.Length} bytes");
Console.WriteLine($"  Shared Secret: {sharedSecret.Length} bytes");
Console.WriteLine($"  Shared Secret (hex): {Convert.ToHexString(sharedSecret)}");

// Send ciphertext to recipient
File.WriteAllBytes("ciphertext.bin", ciphertext);

// Use shared secret for encryption
var encryptionKey = ctx.DeriveKey(
    sharedSecret,
    outputLength: 32,
    info: Encoding.UTF8.GetBytes("aes-encryption-key")
);
</code>

----

===== Code Example: Decapsulation (Recipient) =====

<code csharp>
using var ctx = PqCryptoContext.Initialize();

// Load private key
var privateKey = ctx.LoadPrivateKey("recipient-mlkem.key.pem", "SecurePassword!");

// Receive ciphertext
var ciphertext = File.ReadAllBytes("ciphertext.bin");

// Perform decapsulation
var sharedSecret = ctx.Decapsulate(privateKey, ciphertext);

Console.WriteLine($"Decapsulation completed:");
Console.WriteLine($"  Shared Secret: {sharedSecret.Length} bytes");
Console.WriteLine($"  Shared Secret (hex): {Convert.ToHexString(sharedSecret)}");

// Derive same key as sender
var decryptionKey = ctx.DeriveKey(
    sharedSecret,
    outputLength: 32,
    info: Encoding.UTF8.GetBytes("aes-encryption-key")
);
</code>

----

===== Wrapper Class for ML-KEM =====

<code csharp>
public class MlKemService
{
    private readonly PqCryptoContext _ctx;

    public MlKemService()
    {
        _ctx = PqCryptoContext.Initialize();
    }

    public (byte[] PublicKey, byte[] PrivateKey) GenerateKeyPair(MlKemParameterSet paramSet = MlKemParameterSet.MlKem768)
    {
        var algorithm = paramSet switch
        {
            MlKemParameterSet.MlKem512 => PqAlgorithm.MlKem512,
            MlKemParameterSet.MlKem768 => PqAlgorithm.MlKem768,
            MlKemParameterSet.MlKem1024 => PqAlgorithm.MlKem1024,
            _ => throw new ArgumentException("Invalid parameter set")
        };

        using var keyPair = _ctx.GenerateKeyPair(algorithm);
        return (
            keyPair.PublicKey.GetRawData(),
            keyPair.PrivateKey.GetRawData()
        );
    }

    public (byte[] Ciphertext, byte[] SharedSecret) Encapsulate(byte[] publicKey)
    {
        using var pubKey = _ctx.ImportPublicKey(publicKey);
        return _ctx.Encapsulate(pubKey);
    }

    public byte[] Decapsulate(byte[] privateKey, byte[] ciphertext)
    {
        using var privKey = _ctx.ImportPrivateKey(privateKey);
        return _ctx.Decapsulate(privKey, ciphertext);
    }
}

public enum MlKemParameterSet
{
    MlKem512,
    MlKem768,
    MlKem1024
}
</code>

----

===== Error Handling =====

<code csharp>
try
{
    var sharedSecret = ctx.Decapsulate(privateKey, ciphertext);
}
catch (CryptographicException ex) when (ex.Message.Contains("decapsulation"))
{
    // Decapsulation failed - ciphertext tampered or wrong key
    Console.WriteLine("Decapsulation failed: Ciphertext invalid");
    throw new SecurityException("Integrity check failed", ex);
}
catch (ArgumentException ex)
{
    // Wrong format
    Console.WriteLine($"Format error: {ex.Message}");
    throw;
}
</code>

<WRAP round important>
**Security Note:** ML-KEM is IND-CCA2 secure. On decapsulation failures, a deterministic but unpredictable value is returned (implicit rejection). This prevents chosen-ciphertext attacks.
</WRAP>

----

===== Industry-Specific Parameters =====

^  Industry  ^  Recommended Parameter  ^  Rationale  ^
| **Standard IT** | ML-KEM-768 | Good balance |
| **Financial Sector** | ML-KEM-768/1024 | Regulatory requirements |
| **Healthcare** | ML-KEM-768 | Long-term data protection |
| **Government** | ML-KEM-1024 | Highest security |
| **IoT/Embedded** | ML-KEM-512 | Resource-constrained |

----

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

^  Relationship  ^  Scenario  ^  Description  ^
| **Application** | [[.:hybrid_encryption|7.1 Hybrid Encryption]] | Combination with classical |
| **Application** | [[.:file_encryption|7.3 File Encryption]] | Practical usage |
| **Related** | [[en:int:pqcrypt:szenarien:schluessel:generierung|11.1 Key Generation]] | Key generation |
| **Related** | [[en:int:pqcrypt:szenarien:tls:hybrid_tls|10.4 Hybrid TLS]] | TLS with ML-KEM |

----

<< [[.:hybrid_encryption|<- 7.1 Hybrid Encryption]] | [[.:start|^ Encryption Overview]] | [[.:file_encryption|7.3 File Encryption ->]] >>

{{tag>scenario encryption ml-kem kem post-quantum fips203}}

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